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ISIS LFA

Hi I am trying to find a decent set of docs regarding the ISIS Loop Free Alternates on IOS-XR 4.3.1 for the ASR9K platforms.

There seem to be lots of parameters to the CLI that dont appear in the documentation.

I have for example a node with two uplinks, both running ISIS L2.

but when I do the following:

router isis isp

     address-family ipv4 unicast

          fast-reroute per-prefix priority-limit high level 2

          spf prefix-priority high PE-Loopbacks

!

I dont see any backups when I run the command

show isis fast-reroute

I see that you can specify the command under the interface ( but nowhere does it state this is a nessesary step ?? )

router isis isp

     interface te 0/2/0/0

          address-family ipv4 unicast

               fast-reroute per-prefix

!

Has someone got a working config they can share?

many thanks

1 REPLY
Silver

Re: ISIS LFA

Hi, I tried this in pre-stage whilst building a new SP network earlier this year.

The configuration I used was as follows:

router isis AS65000

!

interface GigabitEthernet0/1/1/0

  point-to-point

  address-family ipv4 unicast

   fast-reroute per-prefix level 2

   fast-reroute per-prefix exclude interface TenGigE0/1/0/1 level 2

   fast-reroute per-prefix lfa-candidate interface TenGigE0/0/0/0 level 2

   fast-reroute per-prefix lfa-candidate interface TenGigE0/0/0/1 level 2

   fast-reroute per-prefix lfa-candidate interface TenGigE0/1/0/0 level 2

   mpls ldp sync level 2

All i had to do was define the backup paths for each interface under the ISIS process. Note this is L2 ISIS only.

I removed it before putting this into production as my customer couldn't decide which paths they wanted as backup!

I did some convergence time measurements whilst the kit was still in the lab, so its not real world as the routers were all directly cabled together, but without LFA it took typically 50ms for ISIS to reconverge following a p2p cable disconnect, with LFA on this time reduced to less than 1ms!

The only show command output I recorded was a show route, a snippit of which is here:

i L2 10.250.250.96/30 [115/20000] via 10.250.250.2, 00:10:23, TenGigE0/0/0/0

                      [115/0] via 10.250.250.6, 00:10:23, TenGigE0/0/0/3

i L2 10.250.250.100/30 [115/0] via 10.250.250.2, 00:10:23, TenGigE0/0/0/0

                       [115/20000] via 10.250.250.6, 00:10:23, TenGigE0/0/0/3

You can see the backup paths are already in the route table, but with a higher metric.

This was on 4.3.0

HTH

Andy

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