SR Anycast SID

Load basic.isis.sr.enabled.cfg

configure
load bootflash:basic.isis.sr.enabled.cfg
commit replace
y

SR is already enabled using ISIS.

Configure an anycast prefix of 46.46.46.1/32 on Lo46 of both R4 and R6. Using an Anycast SID, ensure that TI-LFA will work properly for this prefix.

Answer

#R4, R6
int Lo46
 ip add 46.46.46.1/32
!
router isis 1
 int Lo46
  add ipv4 uni
   prefix-sid index 46 n-flag-clear

Explanation

By default, when a prefix-SID is configured, the N (node) flag is set. However, an Anycast SID cannot have the N flag set, as it is not specific to one single node.

Interestingly, traffic will still work fine if you forget to clear the N flag for any Anycast SID. You’ll just see a logging message like below. But ECMP traffic to both R4 and R6 works just fine.

RP/0/0/CPU0:Oct 24 12:07:58.733 UTC: isis[1011]: %ROUTING-ISIS-6-ANYCAST_N_FLAG_SET : 46.46.46.1/32 is an anycast route with the segment-routing Node flag set on R4

A problem will appear when you try to use TI-LFA. TI-LFA assumes that the N flag set means the prefix is actually specific to only one node, so TI-LFA will break if you forget to clear the N flag. The problem is that TI-LFA will think that the SID belongs to one single node, and the LSP might use the prefix SID to get to a specific node, and then use that node’s Adj SID. If the LSP actually goes through the other node in the anycast set, the Adj SID will be meaningless to that node, and the LSP will be broken. If TI-LFA knows that the prefix SID does not belong to a single node (because the N flag is unset), this mistake will not happen.

We can see that the N flag is cleared on the LSPs of R4 and R6: