RFC 2642
Cabletron's VLS Protocol Specification
Pages: 95
Informational
Informational
Part 3 of 4 – Pages 51 to 83
8. Maintaining the Databases
Each switch advertises its state (also known as its link state) by originating switch link advertisements. In addition, the designated switch on each network link advertises the state of the link by originating network link advertisements. As described in Section 7.1, link state advertisements are uniquely identified by their type, link state ID, and advertising switch. Link state advertisements are distributed throughout the switch fabric using a reliable flooding algorithm that ensures that all switches in the fabric are notified of any link state changes.
8.1 Originating Link State Advertisements
A new instance of each link state advertisement is originated any time the state of the switch or link changes. When a new instance of a link state advertisement is originated, its sequence number is incremented, its age is set to zero, and its checksum is calculated. The advertisement is then installed into the local link state database and forwarded out all fully operational interfaces (that is, those interfaces with a state greater than Waiting) for distribution throughout the switch fabric. See Section 8.2.4 for a description of the installation of the advertisement into the link state database and Section 8.2.5 for a description of how advertisements are forwarded. A switch originates a new instance of a link state advertisement as a result of the following events: o The state of one of the switch's interfaces changes such that the contents of the associated switch link advertisement changes. o The designated switch on any of the switch's attached network links changes. The switch originates a new switch link advertisement. Also, if the switch itself is now the designated switch, it originates a new network link advertisement for the link. o One of the switch's neighbor states changes to or from Full. If this changes the contents of the associated switch link advertisement, a new instance is generated. Also, if the switch is the designated switch for the attached network link, it originates a new network link advertisement for the link. Two instances of the same link state advertisement must not be originated within the time period MinLSInterval. Note that this may require that the generation of the second instance to be delayed up to MinLSInterval seconds.8.1.1 Switch Link Advertisements
A switch link advertisement describes the collected states of all functioning links attached to the originating switch -- that is, all attached links with an interface state greater than Down. A switch originates an empty switch link advertisement when it first becomes functional. It then generates a new instance of the advertisement each time one of its interfaces reaches a fully functioning state (Point-to-Point or better).
Each link in the advertisement is assigned a type, based on the state
of interface, as shown in Table 4.
Interface state Link type Description
Point-to-Point 1 Point-to-point link
DS Other* 2 Multi-access link
Backup* 2 Multi-access link
DS** 2 Multi-access link
*If a full adjacency has been formed with the designated
switch.
**If a full adjacency has been formed with at least one
other switch on the link.
Table 4: Link Types in a Switch Link Advertisement
Each link in the advertisement is also assigned a link identifier
based on its link type. In general, this value identifies another
switch that also originates advertisements for the link, thereby
providing a key for accessing other link state advertisements for the
link. The relationship between link type and ID is shown in Table 5.
Type Description Link ID
1 Point-to-point link Switch ID of neighbor switch
2 Multi-access link Switch ID of designated switch
Table 5: Link IDs in a Switch Link Advertisement
In addition to a type and an identifier, the description of each link
specifies the interface ID of the associated network link.
Finally, each link description includes the cost of sending a packet
over the link. This output cost is expressed in the link state
metric and must be greater than zero.
To illustrate the format of a switch link advertisement, consider the
switch fabric shown in Figure 4.
In this example, switch SW1 has 5 neighboring switches (shown as
boxes) distributed over 3 network links (shown as lines). The base
MAC address of each switch is also shown adjacent to each box. On
switch SW1, ports 01 and 02 attach to point-to-point network links,
while port 03 attaches to a multi-access network link with three
attached switches. The interface state of each port is shown next to
the line representing the corresponding link.
00-00-1d-22-23-c5
+-------+
| SW2 |
+-------+
|
| Point-to-Point
|
| 01
+-------+ Loopback +-------+
| SW3 |----------------| SW1 | 00-00-1d-1f-05-81
+-------+ 02 +-------+
00-00-1d-17-35-a4 | 03
|
| DS Other
|
+--------------------+--------------------+
| | |
| DS Other | Backup | DS
| | |
+-------+ +-------+ +-------+
| SW4 | | SW5 | | SW6 |
+-------+ +-------+ +-------+
00-00-1d-4a-26-b3 00-00-1d-4a-27-1c 00-00-1d-7e-84-2e
Figure 4: Sample Switch Fabric
The switch link advertisement generated by switch SW1 would contain
the following data items:
; switch link advertisement for switch SW1
LS age = 0 ; always true on origination
Options = (T-bit|E-bit) ; options
LS type = 1 ; this is a switch link advert
; SW1's switch ID
Link State ID = 00-00-1d-1f-05-81-00-00-00-00
Advertising switch = 00-00-1d-1f-05-81-00-00-00-00
# links = 2
; link on interface port 1
Link ID = 00-00-1d-22-23-c5-00-00-00-00 ; switch ID
Link Data = 00-00-1d-1f-05-81-00-00-00-01 ; interface ID
Type = 1 ; pt-to-pt link
# other metrics = 0 ; TOS 0 only
TOS 0 metric = 1
; link on interface port 2 is not fully functional
; link on interface port 3
Link ID = 00-00-1d-7e-84-2e-00-00-00-00 ; switch ID of DS
Link Data = 00-00-1d-1f-05-81-00-00-00-03 ; interface ID
Type = 2 ; multi-access
# other metrics = 0 ; TOS 0 only
TOS 0 metric = 2
(See Section 11.2 for a detailed description of the format of a
switch link advertisement.)
8.1.2 Network Link Advertisements
Network link advertisements are used to describe the switches
attached to each multi-access network link.
Note: Network link advertisements are not generated for point-to-
point links.
A network link advertisement is originated by the designated switch
for the associated multi-access link once the switch has established
a full adjacency with at least one other switch on the link. Each
advertisement lists the switch IDs of those switches that are fully
adjacent to the designated switch. The designated switch includes
itself in this list.
To illustrate the format of a network link advertisement, consider
again the switch fabric shown in Figure 4. In this example, network
link advertisements will be generated only by switch SW6, the
designated switch of the multi-access network link between switches
SW1 and switches SW4, SW5, and SW6.
The network link advertisement generated by switch SW6 would contain
the following data items:
; network link advertisement for switch SW6
LS age = 0 ; always true on origination
Options = (T-bit|E-bit) ; options
LS type = 2 ; this is a network link advert
; SW6's switch ID
Link State ID = 00-00-1d-73-84-2e-00-00-00-00
Advertising switch = 00-00-1d-73-84-2e-00-00-00-00
Attached switch = 00-00-1d-7e-84-2e-00-00-00-00
Attached switch = 00-00-1d-4a-26-b3-00-00-00-00
Attached switch = 00-00-1d-1f-05-81-00-00-00-00
Attached switch = 00-00-1d-4a-27-1c-00-00-00-00
(See Section 11.3 for a detailed description of the format of a
network link advertisement.)
8.2 Distributing Link State Advertisements
Link state advertisements are distributed throughout the switch
fabric encapsulated within Link State Update packets. A single Link
State Update packet may contain several distinct advertisements.
To make the distribution process reliable, each advertisement must be
explicitly acknowledged in a Link State Acknowledgment packet. Note,
however, that multiple acknowledgments can be grouped together into a
single Link State Acknowledgment packet. A sending switch retransmits
unacknowledged Link State Update packets at regular intervals until
they are acknowledged.
The remainder of this section is structured as follows:
o Section 8.2.1 presents an overview of the distribution process.
o Section 8.2.2 describes how an incoming Link State Update packet
is processed.
o Section 8.2.3 describes how a Link State Packet is forwarded --
both by the originating switch and an intermediate receiving
switch.
o Section 8.2.4 describes how advertisements are installed into the
local database.
o Section 8.2.5 describes the retransmission of unacknowledged
advertisements.
o Section 8.2.6 describes how advertisements are acknowledged.
8.2.1 Overview
The philosophy behind the distribution of link state advertisements
is based on the concept of adjacencies -- that is, each switch is
only required to remain synchronized with its adjacent neighbors.
When a switch originates a new instance of a link state
advertisement, it formats the advertisement into a Link State Update
packet and floods the packet out each fully operational interface --
that is, each interface with a state greater than Waiting. However,
only those neighbors that are adjacent to the sending switch need to
process the packet.
The sending switch indicates which of its neighbor switches should
process the advertisement by specifying a particular multicast
destination in the network layer address information (see Section
10.3). The sending switch sets the value of the network layer
destination switch ID field according to the state of the interface
over which the packet is sent:
o If the interface state is Point-to-Point, DS, or Backup, the
switch is adjacent to all other switches on the link and all
neighboring switches must process the packet. Therefore, the
destination field is set to the multicast switch ID
AllSPFSwitches.
o If the interface state is DS Other, the switch is only adjacent to
the designated switch and the backup designated switch and only
those two neighboring switches must process the packet.
Therefore, the destination field is set to the multicast switch ID
AllDSwitches.
A similar logic is used when a switch receives a Link State Update
packet containing a new instance of a link state advertisement.
After processing and acknowledging the packet, the receiving switch
forwards the Link State Update packet as
o On the interface over which the original Link State Update packet
was received:
o If the receiving switch is the designated switch for the
attached network link, the packet is forwarded to all other
switches on the link. (The destination field is set to
AllSPFSwitches.) The originating switch will recognize that it
was the advertisement originator and discard the packet.
o If the receiving switch is not the designated switch for the
attached network link, the packet is not sent back out the
interface over which it was received.
o On all other interfaces:
o If the receiving switch is the designated switch for the
attached network link, the packet is forwarded to all switches
on the link. (The destination field is set to AllSPFSwitches.)
o If the receiving switch is neither the designated switch or the
backup designated switch for the attached network link, the
packet is forwarded only to the designated switch and the
backup designated switch. (The destination field is set to
AllDSwitches.)
Each Link State Update packet is forwarded and processed in this
fashion until all switches in the fabric have received notification
of the new instance of the link state advertisement.
8.2.2 Processing an Incoming Link State Update Packet
When the a Link State Update packet is received, it is first
subjected to a number of consistency checks. In particular, the Link
State Update packet is associated with a specific neighbor. If the
state of that neighbor is less than Exchange, the entire Link State
Update packet is discarded.
Each link state advertisement contained in the packet is processed as
follows:
1. Validate the advertisement's link state checksum and type. If the
checksum is invalid or the type is unknown, discard the
advertisement without acknowledging it.
2. If the advertisement's age is equal to MaxAge and there is
currently no instance of the advertisement in the local link state
database, then do the following:
a) Acknowledge the advertisement by sending a Link State
Acknowledgment packet to the sending neighbor (see Section
8.2.6).
b) Purge all outstanding requests for equal or previous instances
of the advertisement from the sending neighbor's Link State
Request list.
c) If the neighbor is Exchange or Loading, install the
advertisement in the link state database (see Section 8.2.4).
Otherwise, discard the advertisement.
3. If the advertisement's age is equal to MaxAge and there is an
instance of the advertisement in the local link state database,
then do the following:
a) If the advertisement is listed in the link state retransmission
list of any neighbor, remove the advertisement from the
retransmission list(s) and delete the database copy of the
advertisement.
b) Discard the received (MaxAge) advertisement without
acknowledging it.
4. If the advertisement's age is less than MaxAge, attempt to locate
an instance of the advertisement in the local link state database.
If there is no database copy of this advertisement, or the
received advertisement is more recent than the database copy (see
Section 7.1.1), do the following:
a) If there is already a database copy, and if the database copy
was installed less than MinLSInterval seconds ago, discard the
new advertisement without acknowledging it.
b) Otherwise, forward the new advertisement out some subset of the
local interfaces (see Section 8.2.3). Note whether the
advertisement was sent back out the receiving interface for
later use by the acknowledgment process.
c) Remove the current database copy from the Link state
retransmission lists of all neighbors.
d) Install the new advertisement in the link state database,
replacing the current database copy. (Note that this may cause
the calculation of the set of best paths to be scheduled. See
Section 9.) Timestamp the new advertisement with the time that
it was received to prevent installation of another instance
within MinLSInterval seconds.
e) Acknowledge the advertisement, if necessary, by sending a Link
State Acknowledgment packet back out the receiving interface.
(See Section 8.2.6.)
f) If the link state advertisement was initially advertised by the
local switch itself, advance the advertisement sequence number
and issue a new instance of the advertisement. (Receipt of a
newer instance of an advertisement means that the local copy of
the advertisement is left over from before the last time the
switch was restarted.)
5. If the received advertisement is the same instance as the database
copy (as determined by the algorithm described in Section 7.1.1),
do the following:
a) If the advertisement is listed in the neighbor's link state
retransmission list, the local switch is expecting an
acknowledgment for this advertisement. Treat the received
advertisement as an implied acknowledgment, and remove the
advertisement from the link state retransmission list. Note
this implied acknowledgment for later use by the acknowledgment
process (Section 8.2.6).
b) Acknowledge the advertisement, if necessary, by sending a Link
State Acknowledgment packet back out the receiving interface.
(See Section 8.2.6.)
If the database copy of the advertisement is more recent than the
instance just received, do the following:
a) Determine whether the instance is listed in the neighbor link
state request list. If so, an error has occurred in the
database exchange process. Restart the database exchange
process by generating a neighbor BadLSReq event for the sending
neighbor and terminate processing of the Link State Update
packet.
b) Otherwise, generate an unusual event to network management and
discard the advertisement.
8.2.3 Forwarding Link State Advertisements
When a new instance of an advertisement is originated or after an
incoming advertisement has been processed, the switch must decide
over which interfaces and to which neighbors the advertisement will
be forwarded. In some instances, the switch may decide not to
forward the advertisement over a particular interface because it is
able to determine that the neighbors on that attached link have or
will receive the advertisement from another switch on the link.
The decision of whether to forward an advertisement over each of the
switch's interfaces is made as follows:
1. Each neighboring switch attached to the interface is examined to
determine whether it should receive and process the new
advertisement. For each neighbor, the following steps are
executed:
a) If the neighbor state is less than Exchange, the neighbor need
not receive or process the new advertisement.
b) If the neighbor state is Exchange or Loading, examine the link
state request list associated with the neighbor. If an
instance of the new advertisement is on the list, the
neighboring switch already has an instance of the
advertisement. Compare the new advertisement to the neighbor's
copy:
o If the new advertisement is less recent, the neighbor need
not receive or process the new advertisement.
o If the two copies are the same instance, delete the
advertisement from the link state request list. The
neighbor need not receive or process the new advertisement
[7].
o Otherwise, the new advertisement is more recent. Delete the
advertisement from the link state request list. The
neighbor may need to receive and process the new
advertisement.
c) If the new advertisement was received from this neighbor, the
neighbor need not receive or process the advertisement.
d) Add the new advertisement to the link state retransmission list
for the neighbor.
2. The switch must now decide whether to forward the new
advertisement out the interface.
a) If the link state advertisement was not added to any of the
link state retransmission lists for neighbors attached to the
interface, there is no need to forward the advertisement out
the interface.
b) If the new advertisement was received on this interface, and it
was received from either the designated switch or the backup
designated switch, there is no need to forward the
advertisement out the interface. Chances are all neighbors on
the attached network link have also received the advertisement
already.
c) If the new advertisement was received on this interface and the
state of the interface is Point-to-Point, there is no need to
forward the advertisement since the received advertisement was
originated by the neighbor switch.
d) If the new advertisement was received on this interface, and
the interface state is Backup -- that is, the switch itself is
the backup designated switch -- there is no need to forward the
advertisement out the interface. The designated switch will
distribute advertisements on the attached network link.
e) Otherwise, the advertisement must be forwarded out the
interface.
To forward a link state advertisement, the switch first increments
the advertisement's age by InfTransDelay seconds to account for
the transmission time over the link. The switch then copies the
advertisement into a Link State Update packet
Forwarded advertisements are sent to all adjacent switches
associated with the interface. If the interface state is Point-
to-Point, DS, or Backup, the destination switch ID field of the
network layer address information is set to the multicast switch
ID AllSPFSwitches. If the interface state is DS Other, the
destination switch ID field is set to the multicast switch ID
AllDSwitches.
8.2.4 Installing Link State Advertisements in the Database
When a new link state advertisement is installed into the link state
database, as the result of either originating or receiving a new
instance of an advertisement, the switch must determine whether the
best paths need to be recalculated. To make this determination, do
the following:
1. Compare the contents of the new instance with the contents of the
old instance (assuming the older instance is available). Note that
this comparison does not include any data from the link state
header. Differences in fields within the header (such as the
sequence number and checksum, which are guaranteed to be different
in different instances of an advertisement) are of no consequence
when deciding whether or not to recalculate the set of best paths.
2. If there are no differences in the contents of the two
advertisement instances, there is no need to recalculate the set
of best paths.
3. Otherwise, the set of best paths must be recalculated.
Note also that the older instance of the advertisement must be
removed from the link state database when the new advertisement is
installed. The older instance must also be removed from the link
state retransmission lists of all neighbors.
8.2.5 Retransmitting Link State Advertisements
When a switch sends a link state advertisement to an adjacent
neighbor, it records the advertisement in the neighbor's link state
retransmission list. To ensure the reliability of the distribution
process, the switch continues to periodically retransmit the
advertisements specified in the list until they are acknowledged.
The interval timer used to trigger retransmission of the
advertisements is set to RxmtInterval seconds, as found in the
interface data structure. Note that if this value is too low,
needless retransmissions will ensue. If the value is too high, the
speed with which the databases synchronize across adjacencies may be
affected if there are lost packets.
When the interval timer expires, entries in the retransmission list
are formatted into one or more Link State Update packets. (Remember
that multiple advertisements can fit into a single Link State Update
packet.) The age field of each advertisement is incremented by
InfTransDelay, as found in the interface data structure, before the
advertisement is copied into the outgoing packet.
Link State Update packets containing retransmitted advertisements are
always sent directly to the adjacent switch. That is, the destination
field of the network layer addressing information is set to the
switch ID of the neighboring switch.
If the adjacent switch goes down, retransmissions will continue until
the switch failure is detected and the adjacency is torn down by the
VLSP discovery process. When the adjacency is torn down, the link
state retransmission list is cleared.
8.2.6 Acknowledging Link State Advertisements
Each link state advertisement received by a switch must be acknowledged. In most cases, this is done by sending a Link State Acknowledgment packet. However, acknowledgments can also be done implicitly by sending Link State Update packets (see step 4a of Section 8.2.2). Multiple acknowledgments can be grouped together into a single Link State Acknowledgment packet. Sending an acknowledgment Link State Acknowledgment packets are sent back out the interface over which the advertisement was received. The packet can be sent immediately to the sending neighbor, or it can be delayed and sent when an interval timer expires. o Sending delayed acknowledgments facilitates the formatting of multiple acknowledgments into a single packet. This enables a single packet to send acknowledgments to several neighbors at once by using a multicast switch ID in the destination field of the network layer addressing information (see below). Delaying acknowledgments also randomizes the acknowledgment packets sent by the multiple switches attached to a multi-access network link. Note that the interval used to time delayed acknowledgments must be short (less than RxmtInterval) or needless retransmissions will ensue. Delayed acknowledgments are sent to all adjacent switches associated with the interface. If the interface state is Point-to-Point, DS, or Backup, the destination field of the network layer addressing information is set to the multicast switch ID AllSPFSwitches. If the interface state is DS Other, the destination field is set to the multicast switch ID AllDSwitches. o Immediate acknowledgments are sent directly to a specific neighbor in response to the receipt of duplicate link state advertisements. These acknowledgments are sent immediately when the duplicate is received. The method used to send a Link State Acknowledgment packet -- either delayed or immediate -- depends on the circumstances surrounding the receipt of the advertisement, as shown in Table 6. Note that switches with an interface state of Backup send
acknowledgments differently than other switches because they play
a slightly different role in the distribution process (see Section
8.2.3).
Action taken in state
Circumstances Backup Other states
Advertisement was No ack sent No ack sent
forwarded back out
receiving interface
Advertisement is Delayed ack sent Delayed ack
more recent than if advertisement sent
database copy, but received from DS,
was not forwarded else do nothing
back out receiving
interface
Advertisement was a Delayed ack sent No ack sent
duplicate treated if advertisement
as an implied acknow- received from DS,
ledgment (step 4a of else do nothing
Section 8.2.2)
Advertisement was a Immediate ack Immediate ack
duplicate not treated sent sent
as an implied acknow-
ledgment
Advertisement age Immediate ack Immediate ack
equal to MaxAge and sent sent
no current instance
found in database
Table 6: Sending Link State Acknowledgments
Receiving an acknowledgment
When the a Link State Acknowledgment packet is received, it is
first subjected to a number of consistency checks. In particular,
the packet is associated with a specific neighbor. If the state of
that neighbor is less than Exchange, the entire Link State
Acknowledgment packet is discarded.
Each acknowledgment contained in the packet is processed as
follows:
o If the advertisement being acknowledged has an instance in the
link state retransmission list for the sending neighbor, do the
following:
o If the acknowledgment is for the same instance as that
specified in the list (as determined by the procedure
described in Section 7.1.1), remove the instance from the
retransmission list.
o Otherwise, log the acknowledgment as questionable.
8.3 Aging the Link State Database
Each link state advertisement has an age field, containing the
advertisement's age, expressed in seconds. When the advertisement is
copied into a Link State Update packet for forwarding out a
particular interface, the age is incremented by InfTransDelay seconds
to account for the transmission time over the link. An
advertisement's age is never incremented past the value MaxAge.
Advertisements with an age of MaxAge are not used to calculate best
paths.
If a link state advertisement's age reaches MaxAge, the switch
flushes the advertisement from the switch fabric by doing the
following:
o Originate a new instance of the advertisement with the age field
set to MaxAge. The distribution process will eventually result in
the advertisement being removed from the retransmission lists of
all switches in the fabric.
o Once the advertisement is no longer contained in the link state
retransmission list of any neighbor and no neighbor is in a state
of Exchange or Loading, remove the advertisement from the local
link state database.
8.3.1 Premature Aging of Advertisements
A link state advertisement can be prematurely flushed from the switch
fabric by forcing its age to MaxAge and redistributing the
advertisement.
A switch that was previously the designated switch for a multi-access
network link but has lost that status due to a failover to the backup
designated switch prematurely ages the network link advertisements it
originated for the link.
Premature aging also occurs when an advertisement's sequence number must wrap -- that is, when the current advertisement instance has a sequence number of 0x7fffffff. In this circumstance, the advertisement is prematurely aged so that the next instance of the advertisement can be originated with a sequence number of 0x80000001 and be recognized as the most recent instance. A switch may only prematurely age those link state advertisements for which it is the advertising switch.9. Calculating the Best Paths
Once an adjacency has been formed and the two switches have synchronized their databases, each switch in the adjacency calculates the best path(s) to all other switches in the fabric, using itself as the root of each path. In this context, "best" path means that path with the lowest total cost metric across all hops. If there are multiple paths with the same (lowest) total cost metric, they are all calculated. Best paths are stored in the area data structure. Paths are calculated using the well-known Dijkstra algorithm. For a detailed description of this algorithm, the reader is referred to [Perlman], or any of a number of standard textbooks dealing with network routing. Note that whenever there is a change in an adjacency relationship, or any change that alters the topology of the switch fabric, the set of best paths must be recalculated.10. Protocol Packets
This section describes VLS protocol packets and link state advertisements.
There are five distinct VLSP packet types, as listed in Table 7.
Type Packet Name Function Description
1 Hello Select DS/Backup DS Section 10.6.1
2 Database Summarize database
Description contents Section 10.6.2
3 LS Request Database download Section 10.6.3
4 LS Update Database update Section 10.6.4
5 LS Ack Flooding acknow-
ledgment Section 10.6.5
Table 7: VLSP Packet Types
All VLSP packets are encapsulated within a standard ISMP packet, with
the VLS packet carried in the ISMP message body. The ISMP packet is
described in Section 10.1.
Since it is important that the link state databases remain
synchronized throughout the switch fabric, processing of both
incoming and outgoing routing protocol packets should take priority
over ordinary data packets. Section 10.2 describes packet
processing.
All VLSP packets begin with network layer addressing information,
described in Section 10.3, followed by a standard header, described
in Section 10.4.
With the exception of Hello packets, all VLSP packets deal with lists
of link state advertisements. The format of a link state
advertisement is described in Section 11.
10.1 ISMP Packet Format
All VLSP packets are encapsulated within a standard ISMP packet. ISMP
packets are of variable length and have the following general
structure:
o Frame header
o ISMP packet header
o ISMP message body
10.1.1 Frame Header
ISMP packets are encapsulated within an IEEE 802-compliant frame using a standard header as shown below: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 00 | | + Destination address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 04 | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Source address + 08 | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 12 | Type | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 16 | | + + : : Destination address This 6-octet field contains the Media Access Control (MAC) address of the multicast channel over which all switches in the fabric receive ISMP packets. The destination address of all ISMP packets contain a value of 01-00-1D-00-00-00. Source address This 6-octet field contains the physical (MAC) address of the switch originating the ISMP packet. Type This 2-octet field identifies the type of data carried within the frame. The type field of ISMP packets contains the value 0x81FD.
10.1.2 ISMP Packet Header
The ISMP packet header consists of 6 octets, as shown below: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 00 |///////////////////////////////////////////////////////////////| ://////// Frame header /////////////////////////////////////////: +//////// (14 octets) /////////+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 12 |///////////////////////////////| Version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 16 | ISMP message type | Sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 20 | | + + : : Frame header This 14-octet field contains the frame header. Version This 2-octet field contains the version number of the InterSwitch Message Protocol to which this ISMP packet adheres. This document describes ISMP Version 2.0. ISMP message type This 2-octet field contains a value indicating which type of ISMP message is contained within the message body. Valid values are as follows: 1 (reserved) 2 Interswitch Keepalive messages 3 Interswitch Link State messages 4 Interswitch Spanning Tree BPDU messages and Interswitch Remote Blocking messages 5 Interswitch Resolve and New User messages 6 (reserved) 7 Tag-Based Flood messages 8 Interswitch Tap messages All VLS protocol messages have an ISMP message type of 3.
Sequence number
This 2-octet field contains an internally generated sequence
number used by the various protocol handlers for internal
synchronization of messages.
10.1.3 ISMP Message Body
The ISMP message body is a variable-length field containing the
actual data of the ISMP message. The length and content of this
field are determined by the value found in the message type field.
VLSP packets are contained in the ISMP message body.
10.2 VLSP Packet Processing
Note that with the exception of Hello packets, VLSP packets are sent
only between adjacent neighbors. Therefore, all packets travel a
single hop.
VLSP does not support fragmentation and reassembly of packets.
Therefore, packets containing lists of link state advertisements or
advertisement headers must be formatted such that they contain only
as many advertisements or headers as will fit within the size
constraints of a standard ethernet frame.
When a protocol packet is received by a switch, it must first pass
the following criteria before being accepted for further processing:
o The checksum number must be correct.
o The destination switch ID (as found in the network layer address
information) must be the switch ID of the receiving switch, or one
of the multicast switch IDs AllSPFSwitches or AllDSwitches.
If the destination switch ID is the multicast switch ID
AllDSwitches, the state of the receiving interface must be Point-
to-Point, DS, or Backup.
o The source switch ID (as found in the network layer address
information) must not be that of the receiving switch. (That is,
locally originated packets should be discarded.)
At this point, if the packet is a Hello packet, it is accepted for
further processing.
Since all other packet types are only sent between adjacent neighbors, the packet must have been sent by one of the switch's active neighbors. If the source switch ID matches the switch ID of one of the receiving switch's active neighbors (as stored in the interface data structure associated with the inport interface), the packet is accepted for further processing. Otherwise, the packet is discarded.10.3 Network Layer Address Information
As mentioned in Section 2.2.1, portions of the VLS protocol (as derived from OSPF) are dependent on certain network layer addresses -- in particular, the AllSPFSwitches and AllDSwitches multicast addresses that drive the distribution of link state advertisements throughout the switch fabric. In order to facilitate the implementation of the protocol at the physical MAC layer, network layer address information is encapsulated in the VSLP packets. This information immediately follows the ISMP frame and packet header and immediately precedes the VLSP packet header, as shown below: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | : frame header / ISMP header : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 00 | | : Unused (20 octets) : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 20 | | + Source switch ID + 24 | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 28 | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + 32 | | + Destination switch ID + 36 | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 40 | | : VLSP header : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source switch ID
This 10-octet field contains the switch ID of the sending switch.
Destination switch ID
This 10-octet field contains the switch ID of the packet
destination. The value here is set as follows:
o Hello packets are addressed to the multicast switch ID
AllSPFSwitches.
o The designated switch and the backup designated switch address
initial Link State Update packets and Link State Acknowledgment
packets to the multicast switch ID AllSPFSwitches.
o All other switches address initial Link State Update packets
and Link State Acknowledgment packets to the multicast switch
ID AllDSwitches.
o Retransmissions of Link State Update packets are always
addressed directly to the nonresponding switch.
o Database Description packets and Link State Request are always
addressed directly to the other switch participating in the
database exchange process.
VLSP header
This 30-octet field contains the VLSP standard header. See
Section 10.4.
10.4 VLSP Packet Header
Every VLSP packet starts with a common 30-octet header. This header,
along with the data found in the network layer address information,
contains all the data necessary to determine whether the packet
should be accepted for further processing. (See Section 10.1.)
The format of the VLSP header is shown below. Note that the header
starts at offset 36 of the ISMP message body, following the network
layer address information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: frame header / ISMP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer address information :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
40 | (unused) | Type | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
44 | |
+ Source switch ID +
48 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
52 | | Area ID . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
56 | Area ID . . . | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
60 | Autype | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Authentication +
64 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
68 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
This 1-octet field contains the packet type. Possible values are
as follows:
1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment
Packet length
This 2-octet field contains the length of the protocol packet, in
bytes, calculated from the start of the VLSP header, at offset 20
of the ISMP message body. If the packet length is not an integral
number of 16-bit words, the packet is padded with an octet of zero
(see the description of the checksum field, below).
Switch ID
This 10-octet field contains the switch ID of the sending switch.
Area ID
This 4-octet field contains the area identifier. Since VLSP does
not support multiple areas, the value here is always zero.
Checksum
This 2-octet field contains the packet checksum value. The
checksum is calculated as the 16-bit one's complement of the one's
complement sum of all the 16-bit words in the packet, beginning
with the VLSP header, excluding the authentication field. If the
packet length is not an integral number of 16-bit words, the
packet is padded with an octet of zero before calculating the
checksum.
AuType
This 2-octet field identifies the authentication scheme to be used
for the packet. Since authentication is not supported by this
version of VLSP, this field contains zero.
Authentication
This 8-octet field is reserved for use by the authentication
scheme. Since authentication is not supported by this version of
VLSP, this field contains zeroes.
10.5 Options Field
Hello packets and Database Description packets, as well as link state
advertisements, contain a 1-octet options field. Using this field, a
switch can communicate its optional capabilities to other VLSP
switches. The receiving switch can then choose whether or not to
support those optional capabilities. Thus, switches of differing
capabilities potentially can be mixed within a single VLSP routing
domain.
Two optional capabilities are currently defined in the options field:
routing based on Type of Service (TOS) and support for external
routing beyond the local switch fabric. These two capabilities are
specified in the options field as shown below.
+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0|E|T|
+-+-+-+-+-+-+-+-+
The options field
T-bit
The T-bit specifies the switch's Type of Service (TOS) capability.
If the T-bit is set, the switch supports routing based on nonzero
types of service.
E-bit
The E-bit specifies the switch's external routing capability. If
the E-bit is set, the switch supports external routing.
Note: The current version of VLSP supports neither of these
capabilities. Therefore, both the T-bit and the E-bit are clear and
the options field contains a value of zero.
10.6 Packet Formats
This section contains detailed descriptions of the five VLS protocol
packets.
10.6.1 Hello Packets
Hello packets are sent periodically over multi-access switch
interfaces in order to discover and maintain neighbor relationships.
Note: Hello packets are not sent over point-to-point network links.
For point-to-point links, the VLS protocol relies on the VlanHello
protocol [IDhello] to notify it of neighboring switches.
Since all switches connected to a common network link must agree on
certain interface parameters, these parameters are included in each
Hello packet. A switch receiving a Hello packet that contains
parameters inconsistent with its own view of the interface will not
establish a neighbor relationship with the sending switch.
The format of a Hello packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | (unused -- must be 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
74 | HelloInt | Options | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
78 | DeadInt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
82 | |
+ Designated switch ID +
86 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
90 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
94 | |
+ Backup designated switch ID +
98 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
102 | |
+ +
: Neighbor list :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 70-octet field contains the network layer addressing
information and the standard VLS protocol packet header. The
packet header type field contains a value of 1.
HelloInt
This 2-octet field contains the interval, in seconds, at which
this switch sends Hello packets.
Options
This 1-octet field contains the optional capabilities supported by
the switch, as described in Section 10.5.
Priority
This 1-octet field contains the switch priority used in selecting
the designated switch and backup designated switch (see Section
6.3.1). If the value here is zero, the switch is ineligible to
become the designated switch or the backup designated switch.
DeadInt
This 4-octet field contains the length of time, in seconds, that
neighboring switches will wait before declaring the interface down
once they stop receiving Hello packets over the interface. The
value here is equal to the value of SwitchDeadInterval, as found
in the interface data structure.
Designated switch
This 10-octet field contains the switch ID of the designated
switch for this network link, as currently understood by the
sending switch. This value is set to zero if the designated
switch selection process has not yet begun.
Backup designated switch
This 10-octet field contains the switch ID of the backup
designated switch for the network link, as currently understood by
the sending switch. This value is set to zero if the backup
designated switch selection process has not yet begun.
Neighbor list
This variable-length field contains a list of switch IDs of each
switch from which the sending switch has received a valid Hello
packet within the last SwitchDeadInterval seconds.
10.6.2 Database Description Packets
Database Description packets are exchanged while an adjacency is
being formed between two neighboring switches and are used to
describe the contents of the topological database. For a complete
description of the database exchange process, see Section 7.2.
The format of a Database Description packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | (unused -- must be 0) | Options | Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
74 | Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
78 | |
+ +
: Link state advertisement headers :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 70-octet field contains the network layer addressing
information and the standard VLS protocol packet header. The
packet header type field contains a value of 2.
Options
This 1-octet field contains the optional capabilities supported by
the switch, as described in Section 10.5.
Flags
This 1-octet field contains a set of bit flags that are used to
coordinate the database exchange process. The format of this
octet is as follows:
+-+-+-+-+-+-+-+-+
|0|0|0|0|0|I|M|MS
+-+-+-+-+-+-+-+-+
I-bit (Init)
The I-bit is used to signal the start of the exchange. It is set
while the two switches negotiate the master/slave relationship and
the starting sequence number.
M-bit (More)
The M-bit is set to indicate that more Database Description
packets to follow.
MS-bit (Master/Slave)
The MS-bit is used to indicate which switch is the master of the
exchange. If the bit is set, the sending switch is the master
during the database exchange process. If the bit is clear, the
switch is the slave.
Sequence number
This 4-octet field is used to sequence the Database Description
packets during the database exchange process. The two switches
involved in the exchange process agree on the initial value of the
sequence number during the master/slave negotiation. The number
is then incremented for each Database Description packet in the
exchange.
To acknowledge each Database Description packet sent by the
master, the slave sends a Database Description packet that echoes
the sequence number of the packet being acknowledged.
Link state advertisement headers
This variable-length field contains a list of link state headers
that describe a portion of the master's topological database.
Each header uniquely identifies a link state advertisement and its
current instance. (See Section 11.1 for a detailed description of
a link state advertisement header.) The number of headers
included in the list is calculated implicitly from the length of
the packet, as stored in the VLSP packet header (see Section
10.4).
10.6.3 Link State Request Packets
Link State Request packets are used to request those pieces of the
neighbor's database that the sending switch has discovered (during
the database exchange process) are more up-to-date than instances in
its own database. Link State Request packets are sent as the last
step in bringing up an adjacency. (See Section 7.3.)
The format of a Link State Request packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | Link state type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
74 | |
+ Link state ID +
88 | |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
82 | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
86 | |
+ Advertising switch ID +
90 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
94 | |
: . . . :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 70-octet field contains the network layer addressing
information and the standard VLS protocol packet header. The
packet header type field contains a value of 3.
Link state type
This 4-octet field contains the link state type of the requested
link state advertisement, as stored in the advertisement header.
Link state ID
This 10-octet field contains the link state ID of the requested
link state advertisement, as stored in the advertisement header.
Advertising switch
This 10-octet field contains the switch ID of advertising switch
for the requested link state advertisement, as stored in the
advertisement header.
Note that the last three fields uniquely identify the
advertisement, but not its instance. The receiving switch will
respond with its most recent instance of the specified
advertisement.
Multiple link state advertisements can be requested in a single
Link State Request packet by repeating the link state type, ID,
and advertising switch for each requested advertisement. The
number of advertisements requested is calculated implicitly from
the length of the packet, as stored in the VLSP packet header.
10.6.4 Link State Update Packets
Link State Update packets are used to respond to a Link State Request
packet or to advertise a new instance of one or more link state
advertisements. Link State Update packets are acknowledged with Link
State Acknowledgment packets. For more information on the use of
Link State Update packets, see Section 7 and Section 8.
The format of a Link State Update packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | # advertisements |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
74 | |
+ +
: Link state advertisements :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 70-octet field contains the network layer addressing
information and the standard VLS protocol packet header. The
packet header type field contains a value of 4.
# advertisements
This 4-octet field contains the number of link state
advertisements included in the packet.
Link state advertisements
This variable-length field contains a list of link state
advertisements. For a detailed description of the different types
of link state advertisements, see Section 11.
10.6.5 Link State Acknowledgment Packets
Link State Acknowledgment Packets are used to explicitly acknowledge
one or more Link State Update packets, thereby making the
distribution of link state advertisements reliable. (See Section
8.2.6.)
The format of a Link State Acknowledgment packet is shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 | |
: Network layer addressing / VLSP header :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
70 | |
+ +
: Link state advertisement headers :
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network layer addressing / VLSP header
This 70-octet field contains the network layer addressing
information and the standard VLS protocol packet header. The
packet header type field contains a value of 5.
Link state advertisement headers
This variable-length field contains a list of link state headers
that are being acknowledged by this packet. Each header uniquely
identifies a link state advertisement and its current instance.
(See Section 11.1 for a detailed description of a link state
advertisement header.) The number of headers included in the list
is calculated implicitly from the length of the packet, as stored
in the VLSP packet header (see Section 10.4).
(next page on part 4)