RFC 4271
A Border Gateway Protocol 4 (BGP-4)
Pages: 104
Draft Standard
→ Errata
Obsoletes: 1771
Updated by: 6286 6608 6793 7606 7607 7705 8212 8654 9072 9687
Draft Standard
→ Errata
Obsoletes: 1771
Updated by: 6286 6608 6793 7606 7607 7705 8212 8654 9072 9687
Part 4 of 4 – Pages 75 to 104
9. UPDATE Message Handling
An UPDATE message may be received only in the Established state. Receiving an UPDATE message in any other state is an error. When an UPDATE message is received, each field is checked for validity, as specified in Section 6.3. If an optional non-transitive attribute is unrecognized, it is quietly ignored. If an optional transitive attribute is unrecognized, the Partial bit (the third high-order bit) in the attribute flags octet is set to 1, and the attribute is retained for propagation to other BGP speakers. If an optional attribute is recognized and has a valid value, then, depending on the type of the optional attribute, it is processed locally, retained, and updated, if necessary, for possible propagation to other BGP speakers. If the UPDATE message contains a non-empty WITHDRAWN ROUTES field, the previously advertised routes, whose destinations (expressed as IP prefixes) are contained in this field, SHALL be removed from the Adj-RIB-In. This BGP speaker SHALL run its Decision Process because the previously advertised route is no longer available for use. If the UPDATE message contains a feasible route, the Adj-RIB-In will be updated with this route as follows: if the NLRI of the new route is identical to the one the route currently has stored in the Adj- RIB-In, then the new route SHALL replace the older route in the Adj- RIB-In, thus implicitly withdrawing the older route from service. Otherwise, if the Adj-RIB-In has no route with NLRI identical to the new route, the new route SHALL be placed in the Adj-RIB-In. Once the BGP speaker updates the Adj-RIB-In, the speaker SHALL run its Decision Process.
9.1. Decision Process
The Decision Process selects routes for subsequent advertisement by applying the policies in the local Policy Information Base (PIB) to the routes stored in its Adj-RIBs-In. The output of the Decision Process is the set of routes that will be advertised to peers; the selected routes will be stored in the local speaker's Adj-RIBs-Out, according to policy. The BGP Decision Process described here is conceptual, and does not have to be implemented precisely as described, as long as the implementations support the described functionality and they exhibit the same externally visible behavior. The selection process is formalized by defining a function that takes the attribute of a given route as an argument and returns either (a) a non-negative integer denoting the degree of preference for the route, or (b) a value denoting that this route is ineligible to be installed in Loc-RIB and will be excluded from the next phase of route selection. The function that calculates the degree of preference for a given route SHALL NOT use any of the following as its inputs: the existence of other routes, the non-existence of other routes, or the path attributes of other routes. Route selection then consists of the individual application of the degree of preference function to each feasible route, followed by the choice of the one with the highest degree of preference. The Decision Process operates on routes contained in the Adj-RIBs-In, and is responsible for: - selection of routes to be used locally by the speaker - selection of routes to be advertised to other BGP peers - route aggregation and route information reduction The Decision Process takes place in three distinct phases, each triggered by a different event: a) Phase 1 is responsible for calculating the degree of preference for each route received from a peer. b) Phase 2 is invoked on completion of phase 1. It is responsible for choosing the best route out of all those available for each distinct destination, and for installing each chosen route into the Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each
peer, according to the policies contained in the PIB. Route
aggregation and information reduction can optionally be
performed within this phase.
9.1.1. Phase 1: Calculation of Degree of Preference
The Phase 1 decision function is invoked whenever the local BGP
speaker receives, from a peer, an UPDATE message that advertises a
new route, a replacement route, or withdrawn routes.
The Phase 1 decision function is a separate process,f which completes
when it has no further work to do.
The Phase 1 decision function locks an Adj-RIB-In prior to operating
on any route contained within it, and unlocks it after operating on
all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP
speaker determines a degree of preference as follows:
If the route is learned from an internal peer, either the value of
the LOCAL_PREF attribute is taken as the degree of preference, or
the local system computes the degree of preference of the route
based on preconfigured policy information. Note that the latter
may result in formation of persistent routing loops.
If the route is learned from an external peer, then the local BGP
speaker computes the degree of preference based on preconfigured
policy information. If the return value indicates the route is
ineligible, the route MAY NOT serve as an input to the next phase
of route selection; otherwise, the return value MUST be used as
the LOCAL_PREF value in any IBGP readvertisement.
The exact nature of this policy information, and the computation
involved, is a local matter.
9.1.2. Phase 2: Route Selection
The Phase 2 decision function is invoked on completion of Phase 1.
The Phase 2 function is a separate process, which completes when it
has no further work to do. The Phase 2 process considers all routes
that are eligible in the Adj-RIBs-In.
The Phase 2 decision function is blocked from running while the Phase
3 decision function is in process. The Phase 2 function locks all
Adj-RIBs-In prior to commencing its function, and unlocks them on
completion.
If the NEXT_HOP attribute of a BGP route depicts an address that is
not resolvable, or if it would become unresolvable if the route was
installed in the routing table, the BGP route MUST be excluded from
the Phase 2 decision function.
If the AS_PATH attribute of a BGP route contains an AS loop, the BGP
route should be excluded from the Phase 2 decision function. AS loop
detection is done by scanning the full AS path (as specified in the
AS_PATH attribute), and checking that the autonomous system number of
the local system does not appear in the AS path. Operations of a BGP
speaker that is configured to accept routes with its own autonomous
system number in the AS path are outside the scope of this document.
It is critical that BGP speakers within an AS do not make conflicting
decisions regarding route selection that would cause forwarding loops
to occur.
For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker identifies the route that has:
a) the highest degree of preference of any route to the same set
of destinations, or
b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules
specified in Section 9.1.2.2.
The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being
held in the Loc-RIB. When the new BGP route is installed in the
Routing Table, care must be taken to ensure that existing routes to
the same destination that are now considered invalid are removed from
the Routing Table. Whether the new BGP route replaces an existing
non-BGP route in the Routing Table depends on the policy configured
on the BGP speaker.
The local speaker MUST determine the immediate next-hop address from
the NEXT_HOP attribute of the selected route (see Section 5.1.3). If
either the immediate next-hop or the IGP cost to the NEXT_HOP (where
the NEXT_HOP is resolved through an IGP route) changes, Phase 2 Route
Selection MUST be performed again.
Notice that even though BGP routes do not have to be installed in the Routing Table with the immediate next-hop(s), implementations MUST take care that, before any packets are forwarded along a BGP route, its associated NEXT_HOP address is resolved to the immediate (directly connected) next-hop address, and that this address (or multiple addresses) is finally used for actual packet forwarding. Unresolvable routes SHALL be removed from the Loc-RIB and the routing table. However, corresponding unresolvable routes SHOULD be kept in the Adj-RIBs-In (in case they become resolvable).9.1.2.1. Route Resolvability Condition
As indicated in Section 9.1.2, BGP speakers SHOULD exclude unresolvable routes from the Phase 2 decision. This ensures that only valid routes are installed in Loc-RIB and the Routing Table. The route resolvability condition is defined as follows: 1) A route Rte1, referencing only the intermediate network address, is considered resolvable if the Routing Table contains at least one resolvable route Rte2 that matches Rte1's intermediate network address and is not recursively resolved (directly or indirectly) through Rte1. If multiple matching routes are available, only the longest matching route SHOULD be considered. 2) Routes referencing interfaces (with or without intermediate addresses) are considered resolvable if the state of the referenced interface is up and if IP processing is enabled on this interface. BGP routes do not refer to interfaces, but can be resolved through the routes in the Routing Table that can be of both types (those that specify interfaces or those that do not). IGP routes and routes to directly connected networks are expected to specify the outbound interface. Static routes can specify the outbound interface, the intermediate address, or both. Note that a BGP route is considered unresolvable in a situation where the BGP speaker's Routing Table contains no route matching the BGP route's NEXT_HOP. Mutually recursive routes (routes resolving each other or themselves) also fail the resolvability check. It is also important that implementations do not consider feasible routes that would become unresolvable if they were installed in the Routing Table, even if their NEXT_HOPs are resolvable using the current contents of the Routing Table (an example of such routes
would be mutually recursive routes). This check ensures that a BGP speaker does not install routes in the Routing Table that will be removed and not used by the speaker. Therefore, in addition to local Routing Table stability, this check also improves behavior of the protocol in the network. Whenever a BGP speaker identifies a route that fails the resolvability check because of mutual recursion, an error message SHOULD be logged.9.1.2.2. Breaking Ties (Phase 2)
In its Adj-RIBs-In, a BGP speaker may have several routes to the same destination that have the same degree of preference. The local speaker can select only one of these routes for inclusion in the associated Loc-RIB. The local speaker considers all routes with the same degrees of preference, both those received from internal peers, and those received from external peers. The following tie-breaking procedure assumes that, for each candidate route, all the BGP speakers within an autonomous system can ascertain the cost of a path (interior distance) to the address depicted by the NEXT_HOP attribute of the route, and follow the same route selection algorithm. The tie-breaking algorithm begins by considering all equally preferable routes to the same destination, and then selects routes to be removed from consideration. The algorithm terminates as soon as only one route remains in consideration. The criteria MUST be applied in the order specified. Several of the criteria are described using pseudo-code. Note that the pseudo-code shown was chosen for clarity, not efficiency. It is not intended to specify any particular implementation. BGP implementations MAY use any algorithm that produces the same results as those described here. a) Remove from consideration all routes that are not tied for having the smallest number of AS numbers present in their AS_PATH attributes. Note that when counting this number, an AS_SET counts as 1, no matter how many ASes are in the set. b) Remove from consideration all routes that are not tied for having the lowest Origin number in their Origin attribute.
c) Remove from consideration routes with less-preferred
MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
between routes learned from the same neighboring AS (the
neighboring AS is determined from the AS_PATH attribute).
Routes that do not have the MULTI_EXIT_DISC attribute are
considered to have the lowest possible MULTI_EXIT_DISC value.
This is also described in the following procedure:
for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration
In the pseudo-code above, MED(n) is a function that returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has
no MULTI_EXIT_DISC attribute, the function returns the lowest
possible MULTI_EXIT_DISC value (i.e., 0).
Similarly, neighborAS(n) is a function that returns the
neighbor AS from which the route was received. If the route is
learned via IBGP, and the other IBGP speaker didn't originate
the route, it is the neighbor AS from which the other IBGP
speaker learned the route. If the route is learned via IBGP,
and the other IBGP speaker either (a) originated the route, or
(b) created the route by aggregation and the AS_PATH attribute
of the aggregate route is either empty or begins with an
AS_SET, it is the local AS.
If a MULTI_EXIT_DISC attribute is removed before re-advertising
a route into IBGP, then comparison based on the received EBGP
MULTI_EXIT_DISC attribute MAY still be performed. If an
implementation chooses to remove MULTI_EXIT_DISC, then the
optional comparison on MULTI_EXIT_DISC, if performed, MUST be
performed only among EBGP-learned routes. The best EBGP-
learned route may then be compared with IBGP-learned routes
after the removal of the MULTI_EXIT_DISC attribute. If
MULTI_EXIT_DISC is removed from a subset of EBGP-learned
routes, and the selected "best" EBGP-learned route will not
have MULTI_EXIT_DISC removed, then the MULTI_EXIT_DISC must be
used in the comparison with IBGP-learned routes. For IBGP-
learned routes, the MULTI_EXIT_DISC MUST be used in route
comparisons that reach this step in the Decision Process.
Including the MULTI_EXIT_DISC of an EBGP-learned route in the
comparison with an IBGP-learned route, then removing the
MULTI_EXIT_DISC attribute, and advertising the route has been
proven to cause route loops.
d) If at least one of the candidate routes was received via EBGP,
remove from consideration all routes that were received via
IBGP.
e) Remove from consideration any routes with less-preferred
interior cost. The interior cost of a route is determined by
calculating the metric to the NEXT_HOP for the route using the
Routing Table. If the NEXT_HOP hop for a route is reachable,
but no cost can be determined, then this step should be skipped
(equivalently, consider all routes to have equal costs).
This is also described in the following procedure.
for m = all routes still under consideration
for n = all routes in still under consideration
if (cost(n) is lower than cost(m))
remove m from consideration
In the pseudo-code above, cost(n) is a function that returns
the cost of the path (interior distance) to the address given
in the NEXT_HOP attribute of the route.
f) Remove from consideration all routes other than the route that
was advertised by the BGP speaker with the lowest BGP
Identifier value.
g) Prefer the route received from the lowest peer address.
9.1.3. Phase 3: Route Dissemination
The Phase 3 decision function is invoked on completion of Phase 2, or
when any of the following events occur:
a) when routes in the Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP
have changed
c) when a new BGP speaker connection has been established
The Phase 3 function is a separate process that completes when it has
no further work to do. The Phase 3 Routing Decision function is
blocked from running while the Phase 2 decision function is in
process.
All routes in the Loc-RIB are processed into Adj-RIBs-Out according
to configured policy. This policy MAY exclude a route in the Loc-RIB
from being installed in a particular Adj-RIB-Out. A route SHALL NOT
be installed in the Adj-Rib-Out unless the destination, and NEXT_HOP described by this route, may be forwarded appropriately by the Routing Table. If a route in Loc-RIB is excluded from a particular Adj-RIB-Out, the previously advertised route in that Adj-RIB-Out MUST be withdrawn from service by means of an UPDATE message (see 9.2). Route aggregation and information reduction techniques (see Section 9.2.2.1) may optionally be applied. Any local policy that results in routes being added to an Adj-RIB-Out without also being added to the local BGP speaker's forwarding table is outside the scope of this document. When the updating of the Adj-RIBs-Out and the Routing Table is complete, the local BGP speaker runs the Update-Send process of 9.2.9.1.4. Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer Reachability Information (NLRI) to another BGP speaker. NLRI overlap occurs when a set of destinations are identified in non-matching multiple routes. Because BGP encodes NLRI using IP prefixes, overlap will always exhibit subset relationships. A route describing a smaller set of destinations (a longer prefix) is said to be more specific than a route describing a larger set of destinations (a shorter prefix); similarly, a route describing a larger set of destinations is said to be less specific than a route describing a smaller set of destinations. The precedence relationship effectively decomposes less specific routes into two parts: - a set of destinations described only by the less specific route, and - a set of destinations described by the overlap of the less specific and the more specific routes The set of destinations described by the overlap represents a portion of the less specific route that is feasible, but is not currently in use. If a more specific route is later withdrawn, the set of destinations described by the overlap will still be reachable using the less specific route. If a BGP speaker receives overlapping routes, the Decision Process MUST consider both routes based on the configured acceptance policy. If both a less and a more specific route are accepted, then the Decision Process MUST install, in Loc-RIB, either both the less and
the more specific routes or aggregate the two routes and install, in Loc-RIB, the aggregated route, provided that both routes have the same value of the NEXT_HOP attribute. If a BGP speaker chooses to aggregate, then it SHOULD either include all ASes used to form the aggregate in an AS_SET, or add the ATOMIC_AGGREGATE attribute to the route. This attribute is now primarily informational. With the elimination of IP routing protocols that do not support classless routing, and the elimination of router and host implementations that do not support classless routing, there is no longer a need to de-aggregate. Routes SHOULD NOT be de-aggregated. In particular, a route that carries the ATOMIC_AGGREGATE attribute MUST NOT be de-aggregated. That is, the NLRI of this route cannot be more specific. Forwarding along such a route does not guarantee that IP packets will actually traverse only ASes listed in the AS_PATH attribute of the route.9.2. Update-Send Process
The Update-Send process is responsible for advertising UPDATE messages to all peers. For example, it distributes the routes chosen by the Decision Process to other BGP speakers, which may be located in either the same autonomous system or a neighboring autonomous system. When a BGP speaker receives an UPDATE message from an internal peer, the receiving BGP speaker SHALL NOT re-distribute the routing information contained in that UPDATE message to other internal peers (unless the speaker acts as a BGP Route Reflector [RFC2796]). As part of Phase 3 of the route selection process, the BGP speaker has updated its Adj-RIBs-Out. All newly installed routes and all newly unfeasible routes for which there is no replacement route SHALL be advertised to its peers by means of an UPDATE message. A BGP speaker SHOULD NOT advertise a given feasible BGP route from its Adj-RIB-Out if it would produce an UPDATE message containing the same BGP route as was previously advertised. Any routes in the Loc-RIB marked as unfeasible SHALL be removed. Changes to the reachable destinations within its own autonomous system SHALL also be advertised in an UPDATE message. If, due to the limits on the maximum size of an UPDATE message (see Section 4), a single route doesn't fit into the message, the BGP speaker MUST not advertise the route to its peers and MAY choose to log an error locally.
9.2.1. Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is, UPDATE messages), in order to limit both the link bandwidth needed to advertise UPDATE messages and the processing power needed by the Decision Process to digest the information contained in the UPDATE messages.9.2.1.1. Frequency of Route Advertisement
The parameter MinRouteAdvertisementIntervalTimer determines the minimum amount of time that must elapse between an advertisement and/or withdrawal of routes to a particular destination by a BGP speaker to a peer. This rate limiting procedure applies on a per- destination basis, although the value of MinRouteAdvertisementIntervalTimer is set on a per BGP peer basis. Two UPDATE messages sent by a BGP speaker to a peer that advertise feasible routes and/or withdrawal of unfeasible routes to some common set of destinations MUST be separated by at least MinRouteAdvertisementIntervalTimer. This can only be achieved by keeping a separate timer for each common set of destinations. This would be unwarranted overhead. Any technique that ensures that the interval between two UPDATE messages sent from a BGP speaker to a peer that advertise feasible routes and/or withdrawal of unfeasible routes to some common set of destinations will be at least MinRouteAdvertisementIntervalTimer, and will also ensure that a constant upper bound on the interval is acceptable. Since fast convergence is needed within an autonomous system, either (a) the MinRouteAdvertisementIntervalTimer used for internal peers SHOULD be shorter than the MinRouteAdvertisementIntervalTimer used for external peers, or (b) the procedure describe in this section SHOULD NOT apply to routes sent to internal peers. This procedure does not limit the rate of route selection, but only the rate of route advertisement. If new routes are selected multiple times while awaiting the expiration of MinRouteAdvertisementIntervalTimer, the last route selected SHALL be advertised at the end of MinRouteAdvertisementIntervalTimer.9.2.1.2. Frequency of Route Origination
The parameter MinASOriginationIntervalTimer determines the minimum amount of time that must elapse between successive advertisements of UPDATE messages that report changes within the advertising BGP speaker's own autonomous systems.
9.2.2. Efficient Organization of Routing Information
Having selected the routing information it will advertise, a BGP speaker may avail itself of several methods to organize this information in an efficient manner.9.2.2.1. Information Reduction
Information reduction may imply a reduction in granularity of policy control - after information is collapsed, the same policies will apply to all destinations and paths in the equivalence class. The Decision Process may optionally reduce the amount of information that it will place in the Adj-RIBs-Out by any of the following methods: a) Network Layer Reachability Information (NLRI): Destination IP addresses can be represented as IP address prefixes. In cases where there is a correspondence between the address structure and the systems under control of an autonomous system administrator, it will be possible to reduce the size of the NLRI carried in the UPDATE messages. b) AS_PATHs: AS path information can be represented as ordered AS_SEQUENCEs or unordered AS_SETs. AS_SETs are used in the route aggregation algorithm described in Section 9.2.2.2. They reduce the size of the AS_PATH information by listing each AS number only once, regardless of how many times it may have appeared in multiple AS_PATHs that were aggregated. An AS_SET implies that the destinations listed in the NLRI can be reached through paths that traverse at least some of the constituent autonomous systems. AS_SETs provide sufficient information to avoid routing information looping; however, their use may prune potentially feasible paths because such paths are no longer listed individually in the form of AS_SEQUENCEs. In practice, this is not likely to be a problem because once an IP packet arrives at the edge of a group of autonomous systems, the BGP speaker is likely to have more detailed path information and can distinguish individual paths from destinations.
9.2.2.2. Aggregating Routing Information
Aggregation is the process of combining the characteristics of several different routes in such a way that a single route can be advertised. Aggregation can occur as part of the Decision Process to reduce the amount of routing information that will be placed in the Adj-RIBs-Out. Aggregation reduces the amount of information that a BGP speaker must store and exchange with other BGP speakers. Routes can be aggregated by applying the following procedure, separately, to path attributes of the same type and to the Network Layer Reachability Information. Routes that have different MULTI_EXIT_DISC attributes SHALL NOT be aggregated. If the aggregated route has an AS_SET as the first element in its AS_PATH attribute, then the router that originates the route SHOULD NOT advertise the MULTI_EXIT_DISC attribute with this route. Path attributes that have different type codes cannot be aggregated together. Path attributes of the same type code may be aggregated, according to the following rules: NEXT_HOP: When aggregating routes that have different NEXT_HOP attributes, the NEXT_HOP attribute of the aggregated route SHALL identify an interface on the BGP speaker that performs the aggregation. ORIGIN attribute: If at least one route among routes that are aggregated has ORIGIN with the value INCOMPLETE, then the aggregated route MUST have the ORIGIN attribute with the value INCOMPLETE. Otherwise, if at least one route among routes that are aggregated has ORIGIN with the value EGP, then the aggregated route MUST have the ORIGIN attribute with the value EGP. In all other cases,, the value of the ORIGIN attribute of the aggregated route is IGP. AS_PATH attribute: If routes to be aggregated have identical AS_PATH attributes, then the aggregated route has the same AS_PATH attribute as each individual route. For the purpose of aggregating AS_PATH attributes, we model each AS within the AS_PATH attribute as a tuple <type, value>, where "type" identifies a type of the path segment the AS
belongs to (e.g., AS_SEQUENCE, AS_SET), and "value" identifies
the AS number. If the routes to be aggregated have different
AS_PATH attributes, then the aggregated AS_PATH attribute SHALL
satisfy all of the following conditions:
- all tuples of type AS_SEQUENCE in the aggregated AS_PATH
SHALL appear in all of the AS_PATHs in the initial set of
routes to be aggregated.
- all tuples of type AS_SET in the aggregated AS_PATH SHALL
appear in at least one of the AS_PATHs in the initial set
(they may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of type AS_SEQUENCE in the aggregated
AS_PATH, which precedes tuple Y in the aggregated AS_PATH,
X precedes Y in each AS_PATH in the initial set, which
contains Y, regardless of the type of Y.
- No tuple of type AS_SET with the same value SHALL appear
more than once in the aggregated AS_PATH.
- Multiple tuples of type AS_SEQUENCE with the same value may
appear in the aggregated AS_PATH only when adjacent to
another tuple of the same type and value.
An implementation may choose any algorithm that conforms to
these rules. At a minimum, a conformant implementation SHALL
be able to perform the following algorithm that meets all of
the above conditions:
- determine the longest leading sequence of tuples (as
defined above) common to all the AS_PATH attributes of the
routes to be aggregated. Make this sequence the leading
sequence of the aggregated AS_PATH attribute.
- set the type of the rest of the tuples from the AS_PATH
attributes of the routes to be aggregated to AS_SET, and
append them to the aggregated AS_PATH attribute.
- if the aggregated AS_PATH has more than one tuple with the
same value (regardless of tuple's type), eliminate all but
one such tuple by deleting tuples of the type AS_SET from
the aggregated AS_PATH attribute.
- for each pair of adjacent tuples in the aggregated AS_PATH,
if both tuples have the same type, merge them together, as
long as doing so will not cause a segment with a length
greater than 255 to be generated.
Appendix F, Section F.6 presents another algorithm that
satisfies the conditions and allows for more complex policy
configurations.
ATOMIC_AGGREGATE:
If at least one of the routes to be aggregated has
ATOMIC_AGGREGATE path attribute, then the aggregated route
SHALL have this attribute as well.
AGGREGATOR:
Any AGGREGATOR attributes from the routes to be aggregated MUST
NOT be included in the aggregated route. The BGP speaker
performing the route aggregation MAY attach a new AGGREGATOR
attribute (see Section 5.1.7).
9.3. Route Selection Criteria
Generally, additional rules for comparing routes among several
alternatives are outside the scope of this document. There are two
exceptions:
- If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than
any other route (provided that the speaker is configured to
accept such routes). If such a route were ever used, a routing
loop could result.
- In order to achieve a successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an
AS SHOULD avoid using unstable routes, and it SHOULD NOT make
rapid, spontaneous changes to its choice of route. Quantifying
the terms "unstable" and "rapid" (from the previous sentence)
will require experience, but the principle is clear. Routes
that are unstable can be "penalized" (e.g., by using the
procedures described in [RFC2439]).
9.4. Originating BGP routes
A BGP speaker may originate BGP routes by injecting routing
information acquired by some other means (e.g., via an IGP) into BGP.
A BGP speaker that originates BGP routes assigns the degree of
preference (e.g., according to local configuration) to these routes
by passing them through the Decision Process (see Section 9.1).
These routes MAY also be distributed to other BGP speakers within the
local AS as part of the update process (see Section 9.2). The
decision of whether to distribute non-BGP acquired routes within an
AS via BGP depends on the environment within the AS (e.g., type of
IGP) and SHOULD be controlled via configuration.
10. BGP Timers
BGP employs five timers: ConnectRetryTimer (see Section 8), HoldTimer (see Section 4.2), KeepaliveTimer (see Section 8), MinASOriginationIntervalTimer (see Section 9.2.1.2), and MinRouteAdvertisementIntervalTimer (see Section 9.2.1.1). Two optional timers MAY be supported: DelayOpenTimer, IdleHoldTimer by BGP (see Section 8). Section 8 describes their use. The full operation of these optional timers is outside the scope of this document. ConnectRetryTime is a mandatory FSM attribute that stores the initial value for the ConnectRetryTimer. The suggested default value for the ConnectRetryTime is 120 seconds. HoldTime is a mandatory FSM attribute that stores the initial value for the HoldTimer. The suggested default value for the HoldTime is 90 seconds. During some portions of the state machine (see Section 8), the HoldTimer is set to a large value. The suggested default for this large value is 4 minutes. The KeepaliveTime is a mandatory FSM attribute that stores the initial value for the KeepaliveTimer. The suggested default value for the KeepaliveTime is 1/3 of the HoldTime. The suggested default value for the MinASOriginationIntervalTimer is 15 seconds. The suggested default value for the MinRouteAdvertisementIntervalTimer on EBGP connections is 30 seconds. The suggested default value for the MinRouteAdvertisementIntervalTimer on IBGP connections is 5 seconds. An implementation of BGP MUST allow the HoldTimer to be configurable on a per-peer basis, and MAY allow the other timers to be configurable. To minimize the likelihood that the distribution of BGP messages by a given BGP speaker will contain peaks, jitter SHOULD be applied to the timers associated with MinASOriginationIntervalTimer, KeepaliveTimer, MinRouteAdvertisementIntervalTimer, and ConnectRetryTimer. A given BGP speaker MAY apply the same jitter to each of these quantities, regardless of the destinations to which the updates are being sent; that is, jitter need not be configured on a per-peer basis.
The suggested default amount of jitter SHALL be determined by multiplying the base value of the appropriate timer by a random factor, which is uniformly distributed in the range from 0.75 to 1.0. A new random value SHOULD be picked each time the timer is set. The range of the jitter's random value MAY be configurable.
Appendix A. Comparison with RFC 1771
There are numerous editorial changes in comparison to [RFC1771] (too many to list here). The following list the technical changes: Changes to reflect the usage of features such as TCP MD5 [RFC2385], BGP Route Reflectors [RFC2796], BGP Confederations [RFC3065], and BGP Route Refresh [RFC2918]. Clarification of the use of the BGP Identifier in the AGGREGATOR attribute. Procedures for imposing an upper bound on the number of prefixes that a BGP speaker would accept from a peer. The ability of a BGP speaker to include more than one instance of its own AS in the AS_PATH attribute for the purpose of inter-AS traffic engineering. Clarification of the various types of NEXT_HOPs. Clarification of the use of the ATOMIC_AGGREGATE attribute. The relationship between the immediate next hop, and the next hop as specified in the NEXT_HOP path attribute. Clarification of the tie-breaking procedures. Clarification of the frequency of route advertisements. Optional Parameter Type 1 (Authentication Information) has been deprecated. UPDATE Message Error subcode 7 (AS Routing Loop) has been deprecated. OPEN Message Error subcode 5 (Authentication Failure) has been deprecated. Use of the Marker field for authentication has been deprecated. Implementations MUST support TCP MD5 [RFC2385] for authentication. Clarification of BGP FSM.
Appendix B. Comparison with RFC 1267
All the changes listed in Appendix A, plus the following. BGP-4 is capable of operating in an environment where a set of reachable destinations may be expressed via a single IP prefix. The concept of network classes, or subnetting, is foreign to BGP-4. To accommodate these capabilities, BGP-4 changes the semantics and encoding associated with the AS_PATH attribute. New text has been added to define semantics associated with IP prefixes. These abilities allow BGP-4 to support the proposed supernetting scheme [RFC1518, RFC1519]. To simplify configuration, this version introduces a new attribute, LOCAL_PREF, that facilitates route selection procedures. The INTER_AS_METRIC attribute has been renamed MULTI_EXIT_DISC. A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that certain aggregates are not de-aggregated. Another new attribute, AGGREGATOR, can be added to aggregate routes to advertise which AS and which BGP speaker within that AS caused the aggregation. To ensure that Hold Timers are symmetric, the Hold Timer is now negotiated on a per-connection basis. Hold Timers of zero are now supported.Appendix C. Comparison with RFC 1163
All of the changes listed in Appendices A and B, plus the following. To detect and recover from BGP connection collision, a new field (BGP Identifier) has been added to the OPEN message. New text (Section 6.8) has been added to specify the procedure for detecting and recovering from collision. The new document no longer restricts the router that is passed in the NEXT_HOP path attribute to be part of the same Autonomous System as the BGP Speaker. The new document optimizes and simplifies the exchange of information about previously reachable routes.
Appendix D. Comparison with RFC 1105
All of the changes listed in Appendices A, B, and C, plus the following. Minor changes to the [RFC1105] Finite State Machine were necessary to accommodate the TCP user interface provided by BSD version 4.3. The notion of Up/Down/Horizontal relations presented in RFC 1105 has been removed from the protocol. The changes in the message format from RFC 1105 are as follows: 1. The Hold Time field has been removed from the BGP header and added to the OPEN message. 2. The version field has been removed from the BGP header and added to the OPEN message. 3. The Link Type field has been removed from the OPEN message. 4. The OPEN CONFIRM message has been eliminated and replaced with implicit confirmation, provided by the KEEPALIVE message. 5. The format of the UPDATE message has been changed significantly. New fields were added to the UPDATE message to support multiple path attributes. 6. The Marker field has been expanded and its role broadened to support authentication. Note that quite often BGP, as specified in RFC 1105, is referred to as BGP-1; BGP, as specified in [RFC1163], is referred to as BGP-2; BGP, as specified in RFC 1267 is referred to as BGP-3; and BGP, as specified in this document is referred to as BGP-4.Appendix E. TCP Options that May Be Used with BGP
If a local system TCP user interface supports the TCP PUSH function, then each BGP message SHOULD be transmitted with PUSH flag set. Setting PUSH flag forces BGP messages to be transmitted to the receiver promptly. If a local system TCP user interface supports setting the DSCP field [RFC2474] for TCP connections, then the TCP connection used by BGP SHOULD be opened with bits 0-2 of the DSCP field set to 110 (binary). An implementation MUST support the TCP MD5 option [RFC2385].
Appendix F. Implementation Recommendations
This section presents some implementation recommendations.Appendix F.1. Multiple Networks Per Message
The BGP protocol allows for multiple address prefixes with the same path attributes to be specified in one message. Using this capability is highly recommended. With one address prefix per message there is a substantial increase in overhead in the receiver. Not only does the system overhead increase due to the reception of multiple messages, but the overhead of scanning the routing table for updates to BGP peers and other routing protocols (and sending the associated messages) is incurred multiple times as well. One method of building messages that contain many address prefixes per path attribute set from a routing table that is not organized on a per path attribute set basis is to build many messages as the routing table is scanned. As each address prefix is processed, a message for the associated set of path attributes is allocated, if it does not exist, and the new address prefix is added to it. If such a message exists, the new address prefix is appended to it. If the message lacks the space to hold the new address prefix, it is transmitted, a new message is allocated, and the new address prefix is inserted into the new message. When the entire routing table has been scanned, all allocated messages are sent and their resources are released. Maximum compression is achieved when all destinations covered by the address prefixes share a common set of path attributes, making it possible to send many address prefixes in one 4096-byte message. When peering with a BGP implementation that does not compress multiple address prefixes into one message, it may be necessary to take steps to reduce the overhead from the flood of data received when a peer is acquired or when a significant network topology change occurs. One method of doing this is to limit the rate of updates. This will eliminate the redundant scanning of the routing table to provide flash updates for BGP peers and other routing protocols. A disadvantage of this approach is that it increases the propagation latency of routing information. By choosing a minimum flash update interval that is not much greater than the time it takes to process the multiple messages, this latency should be minimized. A better method would be to read all received messages before sending updates.
Appendix F.2. Reducing Route Flapping
To avoid excessive route flapping, a BGP speaker that needs to withdraw a destination and send an update about a more specific or less specific route should combine them into the same UPDATE message.Appendix F.3. Path Attribute Ordering
Implementations that combine update messages (as described above in Section 6.1) may prefer to see all path attributes presented in a known order. This permits them to quickly identify sets of attributes from different update messages that are semantically identical. To facilitate this, it is a useful optimization to order the path attributes according to type code. This optimization is entirely optional.Appendix F.4. AS_SET Sorting
Another useful optimization that can be done to simplify this situation is to sort the AS numbers found in an AS_SET. This optimization is entirely optional.Appendix F.5. Control Over Version Negotiation
Because BGP-4 is capable of carrying aggregated routes that cannot be properly represented in BGP-3, an implementation that supports BGP-4 and another BGP version should provide the capability to only speak BGP-4 on a per-peer basis.Appendix F.6. Complex AS_PATH Aggregation
An implementation that chooses to provide a path aggregation algorithm retaining significant amounts of path information may wish to use the following procedure: For the purpose of aggregating AS_PATH attributes of two routes, we model each AS as a tuple <type, value>, where "type" identifies a type of the path segment the AS belongs to (e.g., AS_SEQUENCE, AS_SET), and "value" is the AS number. Two ASes are said to be the same if their corresponding <type, value> tuples are the same. The algorithm to aggregate two AS_PATH attributes works as follows: a) Identify the same ASes (as defined above) within each AS_PATH attribute that are in the same relative order within both AS_PATH attributes. Two ASes, X and Y, are said to be in the same order if either:
- X precedes Y in both AS_PATH attributes, or
- Y precedes X in both AS_PATH attributes.
b) The aggregated AS_PATH attribute consists of ASes identified
in (a), in exactly the same order as they appear in the
AS_PATH attributes to be aggregated. If two consecutive
ASes identified in (a) do not immediately follow each other
in both of the AS_PATH attributes to be aggregated, then the
intervening ASes (ASes that are between the two consecutive
ASes that are the same) in both attributes are combined into
an AS_SET path segment that consists of the intervening ASes
from both AS_PATH attributes. This segment is then placed
between the two consecutive ASes identified in (a) of the
aggregated attribute. If two consecutive ASes identified in
(a) immediately follow each other in one attribute, but do
not follow in another, then the intervening ASes of the
latter are combined into an AS_SET path segment. This
segment is then placed between the two consecutive ASes
identified in (a) of the aggregated attribute.
c) For each pair of adjacent tuples in the aggregated AS_PATH,
if both tuples have the same type, merge them together if
doing so will not cause a segment of a length greater than
255 to be generated.
If, as a result of the above procedure, a given AS number appears
more than once within the aggregated AS_PATH attribute, all but
the last instance (rightmost occurrence) of that AS number should
be removed from the aggregated AS_PATH attribute.
Security Considerations
A BGP implementation MUST support the authentication mechanism
specified in RFC 2385 [RFC2385]. The authentication provided by this
mechanism could be done on a per-peer basis.
BGP makes use of TCP for reliable transport of its traffic between
peer routers. To provide connection-oriented integrity and data
origin authentication on a point-to-point basis, BGP specifies use of
the mechanism defined in RFC 2385. These services are intended to
detect and reject active wiretapping attacks against the inter-router
TCP connections. Absent the use of mechanisms that effect these
security services, attackers can disrupt these TCP connections and/or
masquerade as a legitimate peer router. Because the mechanism
defined in the RFC does not provide peer-entity authentication, these
connections may be subject to some forms of replay attacks that will
not be detected at the TCP layer. Such attacks might result in
delivery (from TCP) of "broken" or "spoofed" BGP messages.
The mechanism defined in RFC 2385 augments the normal TCP checksum with a 16-byte message authentication code (MAC) that is computed over the same data as the TCP checksum. This MAC is based on a one- way hash function (MD5) and use of a secret key. The key is shared between peer routers and is used to generate MAC values that are not readily computed by an attacker who does not have access to the key. A compliant implementation must support this mechanism, and must allow a network administrator to activate it on a per-peer basis. RFC 2385 does not specify a means of managing (e.g., generating, distributing, and replacing) the keys used to compute the MAC. RFC 3562 [RFC3562] (an informational document) provides some guidance in this area, and provides rationale to support this guidance. It notes that a distinct key should be used for communication with each protected peer. If the same key is used for multiple peers, the offered security services may be degraded, e.g., due to an increased risk of compromise at one router that adversely affects other routers. The keys used for MAC computation should be changed periodically, to minimize the impact of a key compromise or successful cryptanalytic attack. RFC 3562 suggests a crypto period (the interval during which a key is employed) of, at most, 90 days. More frequent key changes reduce the likelihood that replay attacks (as described above) will be feasible. However, absent a standard mechanism for effecting such changes in a coordinated fashion between peers, one cannot assume that BGP-4 implementations complying with this RFC will support frequent key changes. Obviously, each should key also be chosen to be difficult for an attacker to guess. The techniques specified in RFC 1750 for random number generation provide a guide for generation of values that could be used as keys. RFC 2385 calls for implementations to support keys "composed of a string of printable ASCII of 80 bytes or less." RFC 3562 suggests keys used in this context be 12 to 24 bytes of random (pseudo-random) bits. This is fairly consistent with suggestions for analogous MAC algorithms, which typically employ keys in the range of 16 to 20 bytes. To provide enough random bits at the low end of this range, RFC 3562 also observes that a typical ACSII text string would have to be close to the upper bound for the key length specified in RFC 2385. BGP vulnerabilities analysis is discussed in [RFC4272].
IANA Considerations
All the BGP messages contain an 8-bit message type, for which IANA has created and is maintaining a registry entitled "BGP Message Types". This document defines the following message types: Name Value Definition ---- ----- ---------- OPEN 1 See Section 4.2 UPDATE 2 See Section 4.3 NOTIFICATION 3 See Section 4.5 KEEPALIVE 4 See Section 4.4 Future assignments are to be made using either the Standards Action process defined in [RFC2434], or the Early IANA Allocation process defined in [RFC4020]. Assignments consist of a name and the value. The BGP UPDATE messages may carry one or more Path Attributes, where each Attribute contains an 8-bit Attribute Type Code. IANA is already maintaining such a registry, entitled "BGP Path Attributes". This document defines the following Path Attributes Type Codes: Name Value Definition ---- ----- ---------- ORIGIN 1 See Section 5.1.1 AS_PATH 2 See Section 5.1.2 NEXT_HOP 3 See Section 5.1.3 MULTI_EXIT_DISC 4 See Section 5.1.4 LOCAL_PREF 5 See Section 5.1.5 ATOMIC_AGGREGATE 6 See Section 5.1.6 AGGREGATOR 7 See Section 5.1.7 Future assignments are to be made using either the Standards Action process defined in [RFC2434], or the Early IANA Allocation process defined in [RFC4020]. Assignments consist of a name and the value. The BGP NOTIFICATION message carries an 8-bit Error Code, for which IANA has created and is maintaining a registry entitled "BGP Error Codes". This document defines the following Error Codes: Name Value Definition ------------ ----- ---------- Message Header Error 1 Section 6.1 OPEN Message Error 2 Section 6.2 UPDATE Message Error 3 Section 6.3 Hold Timer Expired 4 Section 6.5 Finite State Machine Error 5 Section 6.6 Cease 6 Section 6.7
Future assignments are to be made using either the Standards Action process defined in [RFC2434], or the Early IANA Allocation process defined in [RFC4020]. Assignments consist of a name and the value. The BGP NOTIFICATION message carries an 8-bit Error Subcode, where each Subcode has to be defined within the context of a particular Error Code, and thus has to be unique only within that context. IANA has created and is maintaining a set of registries, "Error Subcodes", with a separate registry for each BGP Error Code. Future assignments are to be made using either the Standards Action process defined in [RFC2434], or the Early IANA Allocation process defined in [RFC4020]. Assignments consist of a name and the value. This document defines the following Message Header Error subcodes: Name Value Definition -------------------- ----- ---------- Connection Not Synchronized 1 See Section 6.1 Bad Message Length 2 See Section 6.1 Bad Message Type 3 See Section 6.1 This document defines the following OPEN Message Error subcodes: Name Value Definition -------------------- ----- ---------- Unsupported Version Number 1 See Section 6.2 Bad Peer AS 2 See Section 6.2 Bad BGP Identifier 3 See Section 6.2 Unsupported Optional Parameter 4 See Section 6.2 [Deprecated] 5 See Appendix A Unacceptable Hold Time 6 See Section 6.2 This document defines the following UPDATE Message Error subcodes: Name Value Definition -------------------- --- ---------- Malformed Attribute List 1 See Section 6.3 Unrecognized Well-known Attribute 2 See Section 6.3 Missing Well-known Attribute 3 See Section 6.3 Attribute Flags Error 4 See Section 6.3 Attribute Length Error 5 See Section 6.3 Invalid ORIGIN Attribute 6 See Section 6.3 [Deprecated] 7 See Appendix A Invalid NEXT_HOP Attribute 8 See Section 6.3 Optional Attribute Error 9 See Section 6.3 Invalid Network Field 10 See Section 6.3 Malformed AS_PATH 11 See Section 6.3
Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.Informative References
[RFC904] Mills, D., "Exterior Gateway Protocol formal specification", RFC 904, April 1984. [RFC1092] Rekhter, J., "EGP and policy based routing in the new NSFNET backbone", RFC 1092, February 1989. [RFC1093] Braun, H., "NSFNET routing architecture", RFC 1093, February 1989. [RFC1105] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol (BGP)", RFC 1105, June 1989. [RFC1163] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol (BGP)", RFC 1163, June 1990. [RFC1267] Lougheed, K. and Y. Rekhter, "Border Gateway Protocol 3 (BGP-3)", RFC 1267, October 1991. [RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP- 4)", RFC 1771, March 1995. [RFC1772] Rekhter, Y. and P. Gross, "Application of the Border Gateway Protocol in the Internet", RFC 1772, March 1995. [RFC1518] Rekhter, Y. and T. Li, "An Architecture for IP Address Allocation with CIDR", RFC 1518, September 1993.
[RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC 1519, September 1993. [RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation, selection, and registration of an Autonomous System (AS)", BCP 6, RFC 1930, March 1996. [RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities Attribute", RFC 1997, August 1996. [RFC2439] Villamizar, C., Chandra, R., and R. Govindan, "BGP Route Flap Damping", RFC 2439, November 1998. [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [RFC2796] Bates, T., Chandra, R., and E. Chen, "BGP Route Reflection - An Alternative to Full Mesh IBGP", RFC 2796, April 2000. [RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz, "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000. [RFC3392] Chandra, R. and J. Scudder, "Capabilities Advertisement with BGP-4", RFC 3392, November 2002. [RFC2918] Chen, E., "Route Refresh Capability for BGP-4", RFC 2918, September 2000. [RFC3065] Traina, P., McPherson, D., and J. Scudder, "Autonomous System Confederations for BGP", RFC 3065, February 2001. [RFC3562] Leech, M., "Key Management Considerations for the TCP MD5 Signature Option", RFC 3562, July 2003. [IS10747] "Information Processing Systems - Telecommunications and Information Exchange between Systems - Protocol for Exchange of Inter-domain Routeing Information among Intermediate Systems to Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993. [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", RFC 4272, January 2006 [RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 4020, February 2005.
Editors' Addresses
Yakov Rekhter Juniper Networks EMail: yakov@juniper.net Tony Li EMail: tony.li@tony.li Susan Hares NextHop Technologies, Inc. 825 Victors Way Ann Arbor, MI 48108 Phone: (734)222-1610 EMail: skh@nexthop.com
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