Anycast FRR

Anycast SID

The anycast SID is the same SID advertised by all routers within a group. On the network shown in Figure 2-75, Device D and Device E reside on the egress of an SR area. Traffic can reach the non-SR area through either Device D or Device E. The two devices can back up each other. In this situation, Device D and Device E can be configured in the same group and advertise the same prefix SID, the so called anycast SID.

An anycast SID's next hop directs to Device D that has the smallest IGP cost in the router group. Device D is called the optimal source that advertises the anycast SID, and the other device in the router group is the backup source. If the primary next-hop link or direct neighbor node of Device D fails, traffic can reach the anycast SID device through the other protection path. The anycast SID device can be the source that has the same primary next hop or another anycast source. When VPN traffic passes through an SR LSP, the same VPN private-network label must be configured for anycast.

Figure 2-75 Anycast SID

Anycast FRR

Anycast FRR allows multiple nodes to advertise the same prefix SID to form FRR. Common FRR algorithms use the SPT to compute a backup next hop. This applies to scenarios where a route is advertised by a single node instead of multiple nodes.

When a route is advertised by multiple nodes, these nodes must be converted to a single node before a backup next hop is computed for a prefix SID. Anycast FRR constructs a virtual node to represent the multiple nodes that advertise the same route and uses the TI-LFA algorithm to compute a backup next hop to the virtual node. The anycast prefix SID inherits the backup next hop from the created virtual node. This solution does not involve any modification of the algorithm for computing the backup next hop. The solution retains the loop-free trait so that no loop occurs between the computed backup next hop and the primary next hop of the peripheral node before convergence.

Figure 2-76 IGP FRR networking in a scenario where multiple nodes advertise the same route

As shown in Figure 2-76 (a), the cost of the Device A-to-Device B link is 5, and that of the Device A-to-Device C link is 10. Device B and Device C advertise the same route 10.1.1.0/24 simultaneously. TI-LFA FRR is enabled on Device A. Because the TI-LFA condition is not met, Device A cannot compute a backup next hop for the route 10.1.1.0/24. To address this problem, TI-LFA FRR can be used in scenarios where multiple nodes advertise the same route.

The implementation is as follows on the network shown in Figure 2-76 (b):

1. A virtual node is constructed between Device B and Device C. The virtual node is connected to both Device B and Device C. The costs of links from Device B and Device C to the virtual node are 0. The costs of links from the virtual node to Device B and Device C are infinite.
2. The virtual node advertises a prefix of 10.1.1.0/24. This means that the route is advertised by a single node.
3. Device A uses the TI-LFA algorithm to compute a backup next hop to the virtual node. The route 10.1.1.0/24 inherits the computation result. On the network shown in Figure 2-76 (b), Device A computes two links to the virtual node. The primary link is from Device A to Device B, and the backup link is from Device A to Device C.