IS-IS LFA

[Iaroslav]

Is it possible to use LFA in IS-IS for broadcast links? Neighbours reachable via P2P links are used for repair-paths but broadcast links are not used at all.

CSR1#sho isis rib 

4.4.4.4/32  prefix attr X:0 R:0 N:1
  [115/L1/30] via 192.168.13.3(GigabitEthernet2), from 4.4.4.4, tag 0, LSP[4/9/68] 
     prefix attr: X:0 R:0 N:1
              (installed)
  [115/L2/30] via 192.168.13.3(GigabitEthernet2), from 192.168.13.3, tag 0, LSP[15/15/56] 
     prefix attr: X:0 R:1 N:1
  [115/L2/30] via 192.168.13.3(GigabitEthernet2), from 4.4.4.4, tag 0, LSP[15/14/62] 
     prefix attr: X:0 R:0 N:1
  [115/L2/31] via 192.168.12.2(GigabitEthernet1), from 192.168.24.2, tag 0, LSP[16/16/60] 
     prefix attr: X:0 R:1 N:1
  [115/L2/50] via 192.168.11.11(GigabitEthernet3), from 11.11.11.11, tag 0, LSP[11/11/67] 

CSR1#sho isis neighbors 

System Id       Type Interface     IP Address      State Holdtime Circuit Id
CSR2            L1   Gi1           192.168.12.2    UP    22       CSR1.01            
CSR3            L1   Gi2           192.168.13.3    UP    26       CSR1.02         

CSR1#sho ip ro repair-paths 
Gateway of last resort is not set
      4.0.0.0/32 is subnetted, 1 subnets
i L1     4.4.4.4 [115/30] via 192.168.13.3, 01:06:43, GigabitEthernet2

interface GigabitEthernet1
 ip address 192.168.12.1 255.255.255.0
 ip router isis 
end
!
interface GigabitEthernet2
 ip address 192.168.13.1 255.255.255.0
 ip router isis 
 
CSR1#sho run | s router isis
router isis
net 49.0000.0000.0000.0001.00
metric-style wide
log-adjacency-changes all
fast-reroute per-prefix level-1 all
fast-reroute per-prefix level-2 all

When I change interface type to P2P, repair-path are installed.

[Giuseppe Larosa]

Hello laroslav,

what you have seen is interesting.

the most current RFC about selection of LFA alternate next-hops should be

https://tools.ietf.org/html/rfc8518

in a point to point link a pre calculated next-hop is available if the p2p link is alive and cannot change.

In a broadcast network each IS node is adjacent with the pseudo node DIS, so a pre-defined IP next-hop for a prefix may not be available. In practice, the pseudonode might be seen as third party or not providing first hand information.

Also in broadcast link IS IS use LAN hellos different for level-1 and level-2 and actually level 1 DIS and level 2 DIS can be different nodes.

Hope to help

Giuseppe

[Iaroslav]

Hello, Giuseppe, 

This RFC describes the common behavior for IGP. OSPF works fine on bcast network type. IS-IS on the other hand exhibits inability to do so far. Could it be a bug? Or there is smth needed in config?