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3.3.1. Anycast-SID in SR-MPLS

                       +--------------+
                       |   Group A    |
                       |192.0.2.10/32 |
                       |    SID:100   |
                       |              |
                +-----------A1---A3----------+
                |      |    | \ / |   |      |
     SID:10     |      |    |  /  |   |      |     SID:30
203.0.113.1/32  |      |    | / \ |   |      |  203.0.113.3/32
       PE1------R1----------A2---A4---------R3------PE3
         \     /|      |              |      |\     /
          \   / |      +--------------+      | \   /
           \ /  |                            |  \ /
            /   |                            |   /
           / \  |                            |  / \
          /   \ |      +--------------+      | /   \
         /     \|      |              |      |/     \
       PE2------R2----------B1---B3---------R4------PE4
203.0.113.2/32  |      |    | \ / |   |      | 203.0.113.4/32
     SID:20     |      |    |  /  |   |      |     SID:40
                |      |    | / \ |   |      |
                +-----------B2---B4----------+
                       |              |
                       |   Group B    |
                       | 192.0.2.1/32 |
                       |    SID:200   |
                       +--------------+

            Figure 1: Transit Device Groups

The Figure 1 illustrates a network example with two groups of transit devices. Group A consists of devices {A1, A2, A3, and A4}. They are all provisioned with the anycast address 192.0.2.10/32 and the Anycast-SID 100.

Similarly, Group B consists of devices {B1, B2, B3, and B4}, and they are all provisioned with the anycast address 192.0.2.1/32 and the Anycast-SID 200. In the above network topology, each Provide Edge (PE) device has a path to each of the groups: A and B.

PE1 can choose a particular transit device group when sending traffic to PE3 or PE4. This will be done by pushing the Anycast-SID of the group in the stack.

Processing the anycast, and subsequent segments, requires special care.

                 +-------------------------+
                 |       Group A           |
                 |     192.0.2.10/32       |
                 |        SID:100          |
                 |-------------------------|
                 |                         |
                 |   SRGB:         SRGB:   |
  SID:10         |(1000-2000)   (3000-4000)|             SID:30
    PE1---+       +-------A1-------------A3-------+       +---PE3
           \     /   |    | \           / |    |   \     /
            \   /    |    |  +-----+   /  |    |    \   /
     SRGB:   \ /     |    |         \ /   |    |     \ /   SRGB:
  (7000-8000) R1     |    |          \    |    |      R3 (6000-7000)
             / \     |    |         / \   |    |     / \
            /   \    |    |  +-----+   \  |    |    /   \
           /     \   |    | /           \ |    |   /     \
    PE2---+       +-------A2-------------A4-------+       +---PE4
  SID:20         |   SRGB:         SRGB:   |             SID:40
                 |(2000-3000)   (4000-5000)|
                 |                         |
                 +-------------------------+

        Figure 2: Transit Paths via Anycast Group A

Considering an MPLS deployment, in the above topology, if device PE1 (or PE2) requires the sending of a packet to the device PE3 (or PE4), it needs to encapsulate the packet in an MPLS payload with the following stack of labels.

  • Label allocated by R1 for Anycast-SID 100 (outer label).

  • Label allocated by the nearest router in Group A for SID 30 (for destination PE3).

In this case, the first label is easy to compute. However, because there is more than one device that is topologically nearest (A1 and A2), determining the second label is impossible unless A1 and A2 allocated the same label value to the same prefix. Devices A1 and A2 may be devices from different hardware vendors. If both don't allocate the same label value for SID 30, it is impossible to use the anycast Group A as a transit anycast group towards PE3. Hence, PE1 (or PE2) cannot compute an appropriate label stack to steer the packet exclusively through the Group A devices. Same holds true for devices PE3 and PE4 when trying to send a packet to PE1 or PE2.

To ease the use of an anycast segment, it is recommended to configure identical SRGBs on all nodes of a particular anycast group. Using this method, as mentioned above, computation of the label following the anycast segment is straightforward.

Using an anycast segment without configuring identical SRGBs on all nodes belonging to the same anycast group may lead to misrouting (in an MPLS VPN deployment, some traffic may leak between VPNs).

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