Remote LFA or TILFA?

Load rlfa.or.tilfa.init.cfg

#R3-R6, R10 only
configure
load bootflash:rlfa.or.tilfa.init.cfg
commit replace
y

The R4-R10 link metric is now 20. Is RLFA possible, or is TILFA required? Test this out by configuring the protection you believe is required.

Answer

The answer is that RLFA is possible.

#All routers
router isis 1
 add ipv4 uni
  mpls ldp auto-config
!
mpls ldp

#R10
mpls ldp
 add ipv4 
  discovery targeted-hello accept

#R3
router isis 1
 int gi0/0/0/4
  add ipv4 uni
   fast-reroute per-prefix
   fast-reroute per-prefix remote-lfa tunnel mpls-ldp

Explanation

You might think there is currently no PQ node, which would require TILFA. R3 can only reach R5 and R6 without traversing the protected link. R3 cannot reach R10 without traversing the link, as it has ECMP paths to R10.

However, R3 can use an “Extended P” space. Because R3 simply switches a packet out a given interface, R3 can use the P space of its own neighbors and extend its P-space. So R3 can use R5’s P space. This is because when R5 receives a packet destined for R10, R5 does not use the protected link. (This is called the extended P space because you are now looking at R5’s P space, not R3’s P space).

The diagram now looks like this:

R10 is available as a PQ node. The label stack is as follows: <R5’s label for R10>/<R10’s label for 4.4.4.1>.

We can verify this using the following show commands:

24005 is R5’s LDP label for 10.10.10.1/32. 24002 is R10’s label for 4.4.4.1/32.

What is the max metric of the R4-R10 link that still allows for RLFA?

No matter what the metric of the R4-R10 link is, R5 can always reach it without crossing the R3-R4 link (as long as no other link metrics change). So the metric at which R10 no longer becomes a Q node is when RLFA is no longer possible. (Put another way, an RLFA is no longer possible when R10 does not use the R10-R4 link as the only path to get to 4.4.4.1/32).

The metric via the path R10-R6-R5-R3-R4 is 40. If the link metric becomes at least 40, we no longer have a PQ node.

#R4
router isis 1
 interface GigabitEthernet0/0/0/10
  address-family ipv4 unicast
   metric 40

#R10
router isis 1
 interface GigabitEthernet0/0/0/4
  address-family ipv4 unicast
   metric 40

R3 no longer has a backup path for 4.4.4.1/32:

R10 now has ECMP paths to 4.4.4.1, so it is no longer a Q node. It uses the protected link to reach 4.4.4.1/32.