Tech-invite3GPPspecsGlossariesIETFRFCsGroupsSIPABNFsWorld Map

RFC 8227

 
 
 

MPLS-TP Shared-Ring Protection (MSRP) Mechanism for Ring Topology

Part 2 of 4, p. 11 to 27
Prev Section       Next Section

 


prevText      Top      ToC       Page 11 
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

Top      Up      ToC       Page 12 
   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

Top      Up      ToC       Page 13 
   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 Failure

4.3.1.2.  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).

Top      Up      ToC       Page 14 
   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 Failure

4.3.2.  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.

Top      Up      ToC       Page 15 
   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.

Top      Up      ToC       Page 16 
                         +---+#####[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 Failure

4.3.2.2.  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

Top      Up      ToC       Page 17 
   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 Failure

4.3.3.  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.

Top      Up      ToC       Page 18 
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).

Top      Up      ToC       Page 19 
   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.

Top      Up      ToC       Page 20 
                                                   /+-- 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 Fails

4.3.3.2.  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.

Top      Up      ToC       Page 21 
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.

Top      Up      ToC       Page 22 
             +---+      +---+      +---+      +---+      +---+
             | A |------| B |------| C |------| G |------| H |
             +---+      +---+      +---+      +---+      +---+
               |                     |                     |
               |                     |                     |
               |        Ring1        |        Ring2        |
               |                     |                     |
               |                     |                     |
             +---+      +---+      +---+      +---+      +---+
             | F |------| E |------| D |------| J |------| I |
             +---+      +---+      +---+      +---+      +---+

                 Figure 12: Dual-Node Interconnected Rings

4.4.2.  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

Top      Up      ToC       Page 23 
   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

Top      Up      ToC       Page 24 
   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

Top      Up      ToC       Page 25 
                          +---+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 Rings

4.4.4.  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.

Top      Up      ToC       Page 26 
   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

Top      Up      ToC       Page 27 
   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).



(page 27 continued on part 3)

Next Section