RFC 8227
MPLS-TP Shared-Ring Protection (MSRP) Mechanism for Ring Topology
Pages: 56
Proposed Standard
Proposed Standard
Part 2 of 4 – Pages 11 to 27
4.3. Ring Protection
This section specifies the ring protection mechanisms in detail. In general, the description uses the clockwise working ring tunnel and the corresponding anticlockwise protection ring tunnel as an example, but the mechanism is applicable in the same way to the anticlockwise working and clockwise protection ring tunnels. In a ring network, each working ring tunnel is associated with a protection ring tunnel in the opposite direction, and every node MUST obtain the ring topology either by configuration or via a topology discovery mechanism. The ring topology and the connectivity (Intact or Severed) between two adjacent ring nodes form the ring map. Each ring node maintains the ring map and uses it to perform ring protection switching. Taking the topology in Figure 4 as an example, LSP1 enters the ring at Node A and leaves the ring at Node D. In normal state, LSP1 is carried by the clockwise working ring tunnel (RcW_D) through the path A->B->C->D. The label operation is: [LSP1](Payload) -> [RCW_D(B)|LSP1](NodeA) -> [RCW_D(C)|LSP1](NodeB) -> [RCW_D(D)| LSP1](NodeC) -> [LSP1](Payload). Then at Node D, the packet will be forwarded based on the label stack of LSP1. Three typical ring protection mechanisms are described in this section: wrapping, short-wrapping, and steering. All nodes on the same ring MUST use the same protection mechanism. If the RPS protocol in any node detects an RPS message with a protection- switching mode that was not provisioned in that node, a failure of protocol will be reported, and the protection mechanism will not be activated. Wrapping ring protection: the node that detects a failure or accepts a switch request switches the traffic impacted by the failure or the switch request to the opposite direction (away from the failure). In
this way, the impacted traffic is switched to the protection ring tunnel by the switching node upstream of the failure, then it travels around the ring to the switching node downstream of the failure through the protection ring tunnel, where it is switched back onto the working ring tunnel to reach the egress node. Short-wrapping ring protection provides some optimization to wrapping protection, in which the impacted traffic is only switched once to the protection ring tunnel by the switching node upstream to the failure. At the egress node, the traffic leaves the ring from the protection ring tunnel. This can reduce the traffic detour of wrapping protection. Steering ring protection implies that the node that detects a failure sends a request along the ring to the other node adjacent to the failure, and all nodes in the ring process this information. For the impacted traffic, the ingress node (which adds traffic to the ring) performs switching of the traffic from working to the protection ring tunnel, and the egress node will drop the traffic received from the protection ring tunnel. The following sections describe these protection mechanisms in detail.4.3.1. Wrapping
With the wrapping mechanism, the protection ring tunnel is a closed ring identified by the egress node. As shown in Figure 4, the RaP_D is the anticlockwise protection ring tunnel for the clockwise working ring tunnel RcW_D. As specified in the following sections, the closed ring protection tunnel can protect both link failures and node failures. Wrapping can be applicable for the protection of Point-to-Multipoint (P2MP) LSPs on the ring; the details of which are outside the scope of this document.4.3.1.1. Wrapping for Link Failure
When a link failure between Nodes B and C occurs, if it is a bidirectional failure, both Nodes B and C can detect the failure via the OAM mechanism; if it is a unidirectional failure, one of the two nodes would detect the failure via the OAM mechanism. In both cases, the node at the other side of the detected failure will be determined by the ring map and informed using the RPS protocol, which is specified in Section 5. Then Node B switches the clockwise working ring tunnel (RcW_D) to the anticlockwise protection ring tunnel (RaP_D), and Node C switches the anticlockwise protection ring tunnel (RaP_D) back to the clockwise working ring tunnel (RcW_D). The
payload that enters the ring at Node A and leaves the ring at Node D follows the path A->B->A->F->E->D->C->D. The label operation is: [LSP1](Payload) -> [RcW_D(B)|LSP1](Node A) -> [RaP_D(A)|LSP1](Node B) -> [RaP_D(F)|LSP1](Node A) -> [RaP_D(E)|LSP1] (Node F) -> [RaP_D(D)|LSP1] (Node E) -> [RaP_D(C)|LSP1] (Node D) -> [RcW_D(D)|LSP1](Node C) -> [LSP1](Payload). +---+#####[RaP_D(F)]######+---+ | F |---------------------| A | +-- LSP1 +---+*****[RcW_D(A)]******+---+ #/* *\# [RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#RaP_D(A) #/* *\# +---+ +---+ | E | | B | +---+ +---+ #\ *x# [RaP_D(D)]#\ [RcW_D(C)]*x#RaP_D(B) #\ *x# +---+*****[RcW_D(D)]****+---+ LSP1 +-- | D |-------------------| C | +---+#####[RaP_D(C)]####+---+ ----- Physical Links xxxxx Failure Links ***** RcW_D ##### RaP_D Figure 5: Wrapping for Link Failure4.3.1.2. Wrapping for Node Failure
As shown in Figure 6, when Node B fails, Node A detects the failure between A and B and switches the clockwise working ring tunnel (RcW_D) to the anticlockwise protection ring tunnel (RaP_D); Node C detects the failure between C and B and switches the anticlockwise protection ring tunnel (RaP_D) to the clockwise working ring tunnel (RcW_D). The node at the other side of the failed node will be determined by the ring map and informed using the RPS protocol specified in Section 5. The payload that enters the ring at Node A and exits at Node D follows the path A->F->E->D->C->D. The label operation is: [LSP1](Payload)-> [RaP_D(F)|LSP1](NodeA) -> [RaP_D(E)|LSP1](NodeF) -> [RaP_D(D)|LSP1](NodeE) -> [RaP_D(C)|LSP1] (NodeD) -> [RcW_D(D)|LSP1] (NodeC) -> [LSP1](Payload).
In one special case where Node D fails, all the ring tunnels with Node D as the egress will become unusable. The ingress node will update its ring map according to received RPS messages and determine that the egress node is not reachable; thus, it will not send traffic to either the working or the protection tunnel. However, before the failure location information is propagated to all the ring nodes, the wrapping protection mechanism may cause a temporary traffic loop: Node C detects the failure and switches the traffic from the clockwise working ring tunnel (RcW_D) to the anticlockwise protection ring tunnel (RaP_D); Node E also detects the failure and switches the traffic from the anticlockwise protection ring tunnel (RaP_D) back to the clockwise working ring tunnel (RcW_D). A possible mechanism to mitigate the temporary loop problem is: the TTL of the ring tunnel label is set to 2*N by the ingress ring node of the traffic, where N is the number of nodes on the ring. +---+#####[RaP_D(F)]######+---+ | F |---------------------| A | +-- LSP1 +---+*****[RcW_D(A)]******+---+ #/* *\# [RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#RaP_D(A) #/* *\# +---+ xxxxx | E | x B x +---+ xxxxx #\ */# [RaP_D(D)]#\ [RcW_D(C)]*/#RaP_D(B) #\ */# +---+*****[RcW_D(D)]****+---+ LSP1 +-- | D |-------------------| C | +---+#####[RaP_D(C)]####+---+ ----- Physical Links xxxxx Failure Nodes ***** RcW_D ##### RaP_D Figure 6: Wrapping for Node Failure4.3.2. Short-Wrapping
With the wrapping protection scheme, protection switching is executed at both nodes adjacent to the failure; consequently, the traffic will be wrapped twice. This mechanism will cause additional latency and bandwidth consumption when traffic is switched to the protection path.
With short-wrapping protection, protection switching is executed only at the node upstream to the failure, and the packet leaves the ring in the protection ring tunnel at the egress node. This scheme can reduce the additional latency and bandwidth consumption when traffic is switched to the protection path. However, the two directions of a protected bidirectional LSP are no longer co-routed under the protection-switching conditions. In the traditional wrapping solution, the protection ring tunnel is configured as a closed ring, while in the short-wrapping solution, the protection ring tunnel is configured as ended at the egress node, which is similar to the working ring tunnel. Short-wrapping is easy to implement in shared-ring protection because both the working and protection ring tunnels are terminated on the egress nodes. Figure 7 shows the clockwise working ring tunnel and the anticlockwise protection ring tunnel with Node D as the egress node.4.3.2.1. Short-Wrapping for Link Failure
As shown in Figure 7, in normal state, LSP1 is carried by the clockwise working ring tunnel (RcW_D) through the path A->B->C->D. When a link failure between Nodes B and C occurs, Node B switches the working ring tunnel RcW_D to the protection ring tunnel RaP_D in the opposite direction. The difference with wrapping occurs in the protection ring tunnel at the egress node. In short-wrapping protection, Rap_D ends in Node D, and then traffic will be forwarded based on the LSP labels. Thus, with the short-wrapping mechanism, LSP1 will follow the path A->B->A->F->E->D when a link failure between Node B and Node C happens. The protection switch at Node D is based on the information from its ring map and the information received via the RPS protocol.
+---+#####[RaP_D(F)]######+---+ | F |---------------------| A | +-- LSP1 +---+*****[RcW_D(A)]******+---+ #/* *\# [RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#RaP_D(A) #/* *\# +---+ +---+ | E | | B | +---+ +---+ #\ *x# [RaP_D(D)]#\ [RcW_D(C)]*x#RaP_D(B) #\ *x# +---+*****[RcW_D(D)]****+---+ LSP1 +-- | D |-------------------| C | +---+ +---+ ----- Physical Links xxxxx Failure Links ***** RcW_D ##### RaP_D Figure 7: Short-Wrapping for Link Failure4.3.2.2. Short-Wrapping for Node Failure
For the node failure that happens on a non-egress node, the short- wrapping protection switching is similar to the link failure case as described in the previous section. This section specifies the scenario of an egress node failure. As shown in Figure 8, LSP1 enters the ring on Node A and leaves the ring on Node D. In normal state, LSP1 is carried by the clockwise working ring tunnel (RcW_D) through the path A->B->C->D. When Node D fails, the traffic of LSP1 cannot be protected by any ring tunnels that use Node D as the egress node. The ingress node will update its ring map according to received RPS messages and determine that the egress node is not reachable; thus, it will not send traffic to either the working or the protection tunnel. However, before the failure location information is propagated to all the ring nodes using the RPS protocol, Node C switches all the traffic on the working ring tunnel RcW_D to the protection ring tunnel RaP_D in the opposite direction based on the information in the ring map. When the traffic arrives at Node E, which also detects the failure of Node D, the protection ring tunnel RaP_D cannot be used to forward traffic to Node D. With the short-wrapping mechanism, protection switching can only be performed once from the working ring tunnel to the protection ring tunnel; thus, Node E MUST NOT switch the traffic that is already carried on the protection ring tunnel back to the working
ring tunnel in the opposite direction. Instead, Node E will discard the traffic received on RaP_D locally. This can avoid the temporary traffic loop when the failure happens on the egress node of the ring tunnel. This also illustrates one of the benefits of having separate working and protection ring tunnels in each ring direction. +---+#####[RaP_D(F)]######+---+ | F |---------------------| A | +-- LSP1 +---+*****[RcW_D(A)]******+---+ #/* *\# [RaP_D(E)]#/*[RcW_D(F)] [RcW_D(B)]*\#RaP_D(A) #/* *\# +---+ +---+ | E | | B | +---+ +---+ #\ */# [RaP_D(D)]#\ [RcW_D(C)]*/#RaP_D(B) #\ */# xxxxx*****[RcW_D(D)]****+---+ LSP1 +-- x D x-------------------| C | xxxxx +---+ ----- Physical Links xxxxx Failure Nodes ***** RcW_D ##### RaP_D Figure 8: Short-Wrapping for Egress Node Failure4.3.3. Steering
With the steering protection mechanism, the ingress node (which adds traffic to the ring) performs switching from the working to the protection ring tunnel, and at the egress node, the traffic leaves the ring from the protection ring tunnel. When a failure occurs in the ring, the node that detects the failure with an OAM mechanism sends the failure information in the opposite direction of the failure hop by hop along the ring using an RPS request message and the ring-map information. When a ring node receives the RPS message that identifies a failure, it can determine the location of the fault by using the topology information of the ring map and updating the ring map accordingly; then, it can determine whether the LSPs entering the ring locally need to switch over or not. For LSPs that need to switch over, it will switch the LSPs from the working ring tunnels to their corresponding protection ring tunnels.
4.3.3.1. Steering for Link Failure
Ring Map of F +--LSP1 +-+-+-+-+-+-+-+ +---+ ###[RaP_D(F)]### +---/ +-+-+-+-+-+-+-+ |F|A|B|C|D|E|F| | F | ---------------- | A | |A|B|C|D|E|F|A| +-+-+-+-+-+-+-+ +---+ ***[RcW_D(A)]*** +---+ +-+-+-+-+-+-+-+ |I|I|I|S|I|I| #/* *\# |I|I|S|I|I|I| +-+-+-+-+-+-+ #/* *\# +-+-+-+-+-+-+ [RaP_D(E)] #/* [RcW_D(B)] *\# [RaP_D(A)] #/* [RcW_D(F)] *\# +-+-+-+-+-+-+-+ #/* *\# |E|F|A|B|C|D|E| +---+ +---+ +-- LSP2 +-+-+-+-+-+-+-+ | E | | B | +-+-+-+-+-+-+-+ |I|I|I|I|S|I| +---+ +---+ |B|C|D|E|F|A|B| +-+-+-+-+-+-+ #\* */# +-+-+-+-+-+-+-+ #\* [RcW_D(E)] [RcW_D(C)] */# |I|S|I|I|I|I| [RaP_D(D)] #\* */# +-+-+-+-+-+-+ #\* */# [RaP_D(B)] +-+-+-+-+-+-+-+ +---+ [RcW_D(D)] +---+ +-+-+-+-+-+-+-+ |D|E|F|A|B|C|D| +-- | D | xxxxxxxxxxxxxxxxx | C | |C|D|E|F|A|B|C| +-+-+-+-+-+-+-+ LSP1 +---+ [RaP_D(C)] +---+ +-+-+-+-+-+-+-+ |I|I|I|I|I|S| LSP2 |S|I|I|I|I|I| +-+-+-+-+-+-+ +-+-+-+-+-+-+ ----- Physical Links ***** RcW_D ##### RaP_D I: Intact S: Severed Figure 9: Steering Operation and Protection Switching When Link C-D Fails As shown in Figure 9, LSP1 enters the ring from Node A while LSP2 enters the ring from Node B, and both of them have the same destination, which is Node D. In normal state, LSP1 is carried by the clockwise working ring tunnel (RcW_D) through the path A->B->C->D, and the label operation is: [LSP1](Payload) -> [RcW_D(B)|LSP1](NodeA) -> [RcW_D(C)| LSP1](NodeB) -> [RcW_D(D)|LSP1](NodeC) -> [LSP1](Payload). LSP2 is carried by the clockwise working ring tunnel (RcW_D) through the path B->C->D, and the label operation is: [LSP2](Payload) -> [RcW_D(C)|LSP2](NodeB) -> [RcW_D(D)|LSP2](NodeC) -> [LSP2](Payload).
If the link between Nodes C and D fails, according to the fault detection and distribution mechanisms, Node D will find out that there is a failure in the link between C and D, and it will update the link state of its ring topology, changing the link between C and D from normal to fault. In the direction that is opposite to the failure position, Node D will send the state report message to Node E, informing Node E of the fault between C and D, and E will update the link state of its ring topology accordingly, changing the link between C and D from normal to fault. In this way, the state report message is sent hop by hop in the clockwise direction. Similar to Node D, Node C will send the failure information in the anticlockwise direction. When Node A receives the failure report message and updates the link state of its ring map, it is aware that there is a fault on the clockwise working ring tunnel to Node D (RcW_D), and LSP1 enters the ring locally and is carried by this ring tunnel; thus, Node A will decide to switch the LSP1 onto the anticlockwise protection ring tunnel to Node D (RaP_D). After the switchover, LSP1 will follow the path A->F->E->D, and the label operation is: [LSP1](Payload) -> [RaP_D(F)| LSP1](NodeA) -> [RaP_D(E)|LSP1](NodeF) -> [RaP_D(D)|LSP1](NodeE) -> [LSP1](Payload). The same procedure also applies to the operation of LSP2. When Node B updates the link state of its ring topology, and finds out that the working ring tunnel RcW_D has failed, it will switch the LSP2 to the anticlockwise protection tunnel RaP_D. After the switchover, LSP2 goes through the path B->A->F->E->D, and the label operation is: [LSP2](Payload) -> [RaP_D(A)|LSP2](NodeB) -> [RaP_D(F)|LSP2](NodeA) -> [RaP_D(E)|LSP2](NodeF) -> [RaP_D(D)|LSP2](NodeE) -> [LSP2](Payload). Assume the link between Nodes A and B breaks down, as shown in Figure 10. Similar to the above failure case, Node B will detect a fault in the link between A and B, and it will update its ring map, changing the link state between A and B from normal to fault. The state report message is sent hop by hop in the clockwise direction, notifying every node that there is a fault between Nodes A and B, and every node updates the link state of its ring topology. As a result, Node A will detect a fault in the working ring tunnel to Node D, and switch LSP1 to the protection ring tunnel, while Node B determines that the working ring tunnel for LSP2 still works fine, and it will not perform the switchover.
/+-- LSP1 +-+-+-+-+-+-+-+ +---+ ###[RaP_D(F)]#### +---/ +-+-+-+-+-+-+-+ |F|A|B|C|D|E|F| | F | ----------------- | A | |A|B|C|D|E|F|A| +-+-+-+-+-+-+-+ +---+ ***[RcW_D(A)]**** +---+ +-+-+-+-+-+-+-+ |I|S|I|I|I|I| #/* x |S|I|I|I|I|I| +-+-+-+-+-+-+ #/* x +-+-+-+-+-+-+ [RaP_D(E)] #/*[RcW_D(F)] [RcW_D(B)]x [RaP_D(A)] #/* x /+-- LSP2 +-+-+-+-+-+-+-+ +---+ +---/ +-+-+-+-+-+-+-+ |E|F|A|B|C|D|E| | E | | B | |B|C|D|E|F|A|B| +-+-+-+-+-+-+-+ +---+ +---+ +-+-+-+-+-+-+-+ |I|I|S|I|I|I| #\* */# |I|I|I|I|I|S| +-+-+-+-+-+-+ #\*[RcW_D(E)] [RcW_D(C)] */# +-+-+-+-+-+-+ [RaP_D(D)] #\* */# [RaP_D(B)] +-+-+-+-+-+-+-+ #\* */# +-+-+-+-+-+-+-+ |D|E|F|A|B|C|D| +---+ ***[RcW_D(D)]*** +---+ |C|D|E|F|A|B|C| +-+-+-+-+-+-+-+ +-- | D | ---------------- | C | +-+-+-+-+-+-+-+ |I|I|I|S|I|I| LSP1 +---+ ###[RaP_D(C)]### +---+ |I|I|I|I|S|I| +-+-+-+-+-+-+ LSP2 +-+-+-+-+-+-+ ----- Physical Links ***** RcW_D ##### RaP_D Figure 10: Steering Operation and Protection Switching When Link A-B Fails4.3.3.2. Steering for Node Failure
For a node failure that happens on a non-egress node, steering protection switching is similar to the link failure case as described in the previous section. If the failure occurs at the egress node of the LSP, the ingress node will update its ring map according to the received RPS messages; it will also determine that the egress node is not reachable after the failure, thus it will not send traffic to either the working or the protection tunnel, and a traffic loop can be avoided.
4.4. Interconnected Ring Protection
4.4.1. Interconnected Ring Topology
Interconnected ring topology is widely used in MPLS-TP networks. For a given ring, the interconnection node acts as the egress node for that ring, meaning that all LSPs using the interconnection node as an egress from one specific ring to another will use the same group of ring tunnels within the ring. This document will discuss two typical interconnected ring topologies: 1. Single-node interconnected rings In single-node interconnected rings, the connection between the two rings is through a single node. Because the interconnection node is in fact a single point of failure, this topology should be avoided in real transport networks. Figure 11 shows the topology of single-node interconnected rings. Node C is the interconnection node between Ring1 and Ring2. +---+ +---+ +---+ +---+ | A |------| B |----- -----| G |------| H | +---+ +---+ \ / +---+ +---+ | \ / | | \ +---+ / | | Ring1 | C | Ring2 | | / +---+ \ | | / \ | +---+ +---+ / \ +---+ +---+ | F |------| E |----- -----| J |------| I | +---+ +---+ +---+ +---+ Figure 11: Single-Node Interconnected Rings 2. Dual-node interconnected rings In dual-node interconnected rings, the connection between the two rings is through two nodes. The two interconnection nodes belong to both interconnected rings. This topology can recover from one interconnection node failure. Figure 12 shows the topology of dual-node interconnected rings. Nodes C and D are the interconnection nodes between Ring1 and Ring2.
+---+ +---+ +---+ +---+ +---+ | A |------| B |------| C |------| G |------| H | +---+ +---+ +---+ +---+ +---+ | | | | | | | Ring1 | Ring2 | | | | | | | +---+ +---+ +---+ +---+ +---+ | F |------| E |------| D |------| J |------| I | +---+ +---+ +---+ +---+ +---+ Figure 12: Dual-Node Interconnected Rings4.4.2. Interconnected Ring Protection Mechanisms
Interconnected rings can be treated as two independent rings. The RPS protocol operates on each ring independently. A failure that happens in one ring only triggers protection switching in the ring itself and does not affect the other ring, unless the failure is on the interconnection node. In this way, protection switching on each ring is the same as the mechanisms described in Section 4.3. The service LSPs that traverse the interconnected rings use the ring tunnels in each ring; within a given ring, the tunnel is selected using normal ring-selection procedures. The traversing LSPs are stitched on the interconnection node. On the interconnection node, the ring tunnel label of the source ring is popped, then LSP label is swapped; after that, the ring tunnel label of the destination ring is pushed. In the dual-node interconnected ring scenario, the two interconnection nodes can be managed as a virtual node group. In addition to the ring tunnels to each physical ring node, each ring SHOULD assign the working and protection ring tunnels to the virtual interconnection node group. In addition, on both nodes in the virtual interconnection node group, the same LSP label is assigned for each traversed LSP. This way, any interconnection node in the virtual node group can terminate the working or protection ring tunnels targeted to the virtual node group and stitch the service LSP from the source ring tunnel to the destination ring tunnel. When the service LSP passes through the interconnected rings, the direction of the working ring tunnels used on both rings SHOULD be the same. In dual-node interconnected rings, this ensures that in normal state the traffic passes only one of the two interconnection nodes and does not pass the link between the two interconnection
nodes. The traffic will then only be switched to the protection path if the interconnection node that is in working path fails. For example, if the service LSP uses the clockwise working ring tunnel on Ring1, when the service LSP leaves Ring1 and enters Ring2, the working ring tunnel used on Ring2 should also follow the clockwise direction.4.4.3. Ring Tunnels in Interconnected Rings
The same ring tunnels as described in Section 4.1 are used in each ring of the interconnected rings. In addition, ring tunnels to the virtual interconnection node group are established on each ring of the interconnected rings, that is: o one clockwise working ring tunnel to the virtual interconnection node group o one anticlockwise protection ring tunnel to the virtual interconnection node group o one anticlockwise working ring tunnel to the virtual interconnection node group o one clockwise protection ring tunnel to the virtual interconnection node group The ring tunnels to the virtual interconnection node group are shared by all LSPs that need to be forwarded to other rings. These ring tunnels can terminate at any node in the virtual interconnection node group. For example, all the ring tunnels on Ring1 in Figure 13 are provisioned as follows: o To Node A: R1cW_A, R1aW_A, R1cP_A, R1aP_A o To Node B: R1cW_B, R1aW_B, R1cP_B, R1aP_B o To Node C: R1cW_C, R1aW_C, R1cP_C, R1aP_C o To Node D: R1cW_D, R1aW_D, R1cP_D, R1aP_D o To Node E: R1cW_E, R1aW_E, R1cP_E, R1aP_E o To Node F: R1cW_F, R1aW_F, R1cP_F, R1aP_F o To the virtual interconnection node group (including Nodes F and A): R1cW_F&A, R1aW_F&A, R1cP_F&A, R1aP_F&A
All the ring tunnels on Ring2 in Figure 13 are provisioned as follows: o To Node A: R2cW_A, R2aW_A, R2cP_A, R2aP_A o To Node F: R2cW_F, R2aW_F, R2cP_F, R2aP_F o To Node G: R2cW_G, R2aW_G, R2cP_G, R2aP_G o To Node H: R2cW_H, R2aW_H, R2cP_H, R2aP_H o To Node I: R2cW_I, R2aW_I, R2cP_I, R2aP_I o To Node J: R2cW_J, R2aW_J, R2cP_J, R2aP_J o To the virtual interconnection node group (including Nodes F and A): R2cW_F&A, R2aW_F&A, R2cP_F&A, R2aP_F&A
+---+ccccccccccccc+---+ | H |-------------| I |--->LSP1 +---+ +---+ c/a a\ c/a a\ c/a a\ +---+ +---+ | G | Ring2 | J | +---+ +---+ c\a a/c c\a a/c c\a aaaaaaaaaaaaa a/c +---+ccccccccccccc+---+ | F |-------------| A | +---+ccccccccccccc+---+ c/aaaaaaaaaaaaaaaaaaa a\ c/ a\ c/ a\ +---+ +---+ | E | Ring1 | B | +---+ +---+ c\a a/c c\a a/c c\a a/c +---+aaaaaaaaaaaaa+---+ LSP1--->| D |-------------| C | +---+ccccccccccccc+---+ Ring1: ccccccccccc R1cW_F&A aaaaaaaaaaa R1aP_F&A Ring2: ccccccccccc R2cW_I aaaaaaaaaaa R2aP_I Figure 13: Ring Tunnels for the Interconnected Rings4.4.4. Interconnected Ring-Switching Procedure
As shown in Figure 13, for the service LSP1 that enters Ring1 at Node D and leaves Ring1 at Node F and continues to enter Ring2 at Node F and leaves Ring2 at Node I, the short-wrapping protection scheme is described as below.
In normal state, LSP1 follows R1cW_F&A in Ring1 and R2cW_I in Ring2. At the interconnection Node F, the label used for the working ring tunnel R1cW_F&A in Ring1 is popped, the LSP label is swapped, and the label used for the working ring tunnel R2cW_I in Ring2 will be pushed based on the inner LSP label lookup. The working path that the service LSP1 follows is: LSP1->R1cW_F&A (D->E->F)->R2cW_I(F->G->H->I)->LSP1. In case of link failure, for example, when a failure occurs on the link between Nodes F and E, Node E will detect the failure and execute protection switching as described in Section 4.3.2. The path that the service LSP1 follows after switching change to: LSP1->R1cW_F &A(D->E)->R1aP_F&A(E->D->C->B->A)->R2cW_I(A->F->G->H->I)->LSP1. In case of a non-interconnection node failure, for example, when the failure occurs at Node E in Ring1, Node D will detect the failure and execute protection switching as described in Section 4.3.2. The path that the service LSP1 follows after switching becomes: LSP1->R1aP_F&A(D->C->B->A)->R2cW_I(A->F->G->H->I)->LSP1. In case of an interconnection node failure, for example, when the failure occurs at the interconnection Node F, Node E in Ring1 will detect the failure and execute protection switching as described in Section 4.3.2. Node A in Ring2 will also detect the failure and execute protection switching as described in Section 4.3.2. The path that the service traffic LSP1 follows after switching is: LSP1->R1cW_F&A(D->E)->R1aP_F&A(E->D->C->B->A)->R2aP_I(A->J->I)->LSP1.4.4.5. Interconnected Ring Detection Mechanism
As shown in Figure 13, in normal state, the service traffic LSP1 traverses D->E->F in Ring1 and F->G->H->I in Ring2. Nodes A and F are the interconnection nodes. When both links between Nodes F and G and between Nodes F and A fail, the ring tunnel from Node F to Node I in Ring2 becomes unreachable. However, the other interconnection Node A is still available, and LSP1 can still reach Node I via Node A. In order to achieve this, the interconnection nodes need to know the ring topology of each ring so that they can judge whether a node is reachable. This judgment is based on the knowledge of the ring map and the fault location. The ring map can be obtained from the Network Management System (NMS) or topology discovery mechanisms. The fault location can be obtained by transmitting the fault information around the ring. The nodes that detect the failure will transmit the fault information in the opposite direction hop by hop using the RPS protocol message. When the interconnection node receives the message that informs the failure, it will calculate the
location of the fault according to the topology information that is maintained by itself and determines whether the LSPs entering the ring at itself can reach the destination. If the destination node is reachable, the LSP will leave the source ring and enter the destination ring. If the destination node is not reachable, the LSP will switch to the anticlockwise protection ring tunnel. In Figure 13, Node F determines that the ring tunnel to Node I is unreachable; the service LSP1 for which the destination node on Ring2 is Node I MUST switch to the protection ring tunnel (R1aP_F&A), and consequently, the service traffic LSP1 traverses the interconnected rings at Node A. Node A will pop the ring tunnel label of Ring1 and push the ring tunnel label of Ring2 and send the traffic to Node I via the ring tunnel (R2aW_I).
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