Brief overview was shared in earlier post of “Segment Routing – Topology Independent LFA”. In this post, I am showing some simulated results of per prefix Ti-LFA where in case of primary link failure, SR-MPLS with Ti-LFA enabled seamlessly re-route the destined traffic by excluding the direct link. If direct adjacent node need to be exclude from Ti-LFA calculation, it is called Ti-LFA node protection.

Below is SR-MPLS enabled topology used for simulation. XR-11 is sending traffic to loopback 1.1.1.12 (XR-12). There is no Ti-LFA configured on XR-11.

Verify that Ti-LFA is not configured

RP/0/0/CPU0:xrvr-11#show isis fast-reroute 1.1.1.12/32 detail
L2 1.1.1.12/32 [1020/115] medium priority
via 99.1.11.1, GigabitEthernet0/0/0/0, xrvr-1, SRGB Base: 16000, Weight: 0
No FRR backup (Check here)
src xrvr-12.00-00, 1.1.1.12, prefix-SID index 12, R:0 N:1 P:0 E:0 V:0 L:0
RP/0/0/CPU0:xrvr-11#

ISIS adjacency is configured for IPv4 and IPv6 address family.

RP/0/0/CPU0:xrvr-11#show isis adjacency systemid xrvr-1 detail
IS-IS 1 Level-2 adjacencies:
System Id Interface SNPA State Hold Changed NSF IPv4 IPv6
BFD BFD
xrvr-1 Gi0/0/0/0 *PtoP* Up 21 00:08:59 Yes None None
Area Address: 49.0001
Neighbor IPv4 Address: 99.1.11.1*
Adjacency SID: 24000 -> Adjacency SID for IPv4 address family. This is not protected, so no backup interface
Non-FRR Adjacency SID: 24001
Neighbor IPv6 Address: fe80::f816:3eff:fe8b:9b27*
Adjacency SID: 24002 -> Adjacency SID for IPv6 address family. This is not protected, so no backup interface
Non-FRR Adjacency SID: 24003
Topology: IPv4 Unicast
Topology: IPv6 Unicast
RP/0/0/CPU0:xrvr-11#

Verify MPLS forwarding. Most of the output is deleted for brevity

RP/0/0/CPU0:xrvr-11#show mpls forwarding
Local Outgoing Prefix Outgoing Next Hop Bytes
Label Label or ID Interface Switched
—— ———– —————— ———— ————— ————
16001 Pop SR Pfx (idx 1) Gi0/0/0/0 99.1.11.1 0
16002 16002 SR Pfx (idx 2) Gi0/0/0/0 99.1.11.1 0
16003 16003 SR Pfx (idx 3) Gi0/0/0/0 99.1.11.1 0
16004 16004 SR Pfx (idx 4) Gi0/0/0/0 99.1.11.1 2553
16005 Pop SR Pfx (idx 5) Gi0/0/0/1 99.5.11.5 0
16006 16006 SR Pfx (idx 6) Gi0/0/0/0 99.1.11.1 0
16007 16007 SR Pfx (idx 7) Gi0/0/0/0 99.1.11.1 0
16012 16012 SR Pfx (idx 12) Gi0/0/0/0 99.1.11.1 18048
16013 16013 SR Pfx (idx 13) Gi0/0/0/0 99.1.11.1 0
24000 Pop SR Adj (idx 1) Gi0/0/0/0 99.1.11.1 0
24001 Pop SR Adj (idx 3) Gi0/0/0/0 99.1.11.1 0
24002 Pop SR Adj (idx 1) Gi0/0/0/0 fe80::f816:3eff:fe8b:9b27
0
24003 Pop SR Adj (idx 3) Gi0/0/0/0 fe80::f816:3eff:fe8b:9b27
0
RP/0/0/CPU0:xrvr-11#

Verify end to end path for XR-12

RP/0/0/CPU0:xrvr-11#traceroute 1.1.1.12
Tracing the route to 1.1.1.12
1 99.1.11.1 [MPLS: Label 16012 Exp 0] 9 msec 9 msec 9 msec
2 99.1.2.2 [MPLS: Label 16012 Exp 0] 9 msec 9 msec 9 msec
3 99.2.3.3 [MPLS: Label 16012 Exp 0] 9 msec 9 msec 9 msec
4 99.3.12.12 9 msec 9 msec 9 msec

Configure Ti-LFA for IPv4 address family only on XR-11:

router isis 1
!
interface GigabitEthernet0/0/0/0
point-to-point
address-family ipv4 unicast
fast-reroute per-prefix
fast-reroute per-prefix ti-lfa
metric 500
!
!
interface GigabitEthernet0/0/0/1
point-to-point
address-family ipv4 unicast
fast-reroute per-prefix
fast-reroute per-prefix ti-lfa
metric 500
!

Once configured Ti-LFA, XR-11 will consider path via XR-5 in case failure between XR-11 and XR-1 as shown in the below topology.

Verify ISIS adjacency towards XR-1

RP/0/0/CPU0:xrvr-11#show isis adjacency systemid xrvr-1 detail
IS-IS 1 Level-2 adjacencies:
System Id Interface SNPA State Hold Changed NSF IPv4 IPv6
BFD BFD
xrvr-1 Gi0/0/0/0 *PtoP* Up 26 00:16:29 Yes None None
Area Address: 49.0001
Neighbor IPv4 Address: 99.1.11.1*
Adjacency SID: 24000 (protected) -> Adjacency SID is protected
Backup label stack: [16001] -> Adjacency SID 24000 is protected with 16001 backup
Backup stack size: 1
Backup interface: Gi0/0/0/1 -> Now backup interface is showing
Backup nexthop: 99.5.11.5 -> Next hop address is XR-5
Backup node address: 1.1.1.1 -> Backup node is XR-1
Non-FRR Adjacency SID: 24001
Neighbor IPv6 Address: fe80::f816:3eff:fe8b:9b27*
Adjacency SID: 24002 -> We have configured Ti-LFA for IPv4. Still IPv6 traffic is not protected
Non-FRR Adjacency SID: 24003
Topology: IPv4 Unicast
Topology: IPv6 Unicast
RP/0/0/CPU0:xrvr-11#