Question on LFA

[chintan-shah]

Hi,

I have been doing various test for LFA on PRP2 IOS-XR 4.0.3.

It seems that Cisco do not consider Node protection as a option while computing LFA and finding alternate backup path.

Consider below topology :

All routers are in ISIS L1 area and primary route from Node A to Node C loopback is via Node B . During testt, I am seeing LFA from Node A is again via Node B via second link between A and B (based on IGP metric).I would have expected the backup path would be from A to C Direct link ( i.e. avoid Node B in backup path in case it fails , hence node protection) but It looks cisco don’t care about node protection.

I have configured per-prefix feature.

Can some one clarify if it is expected and if yes, Is there any way to consider node protection during LFA to install backup path.

Regards,

Chintan

[Nagendra Kumar Nainar]

Hi Chintan,

To my knowledge, it is working as expected. As you can see in RFC 5286, Section 3.2, for node protection, alternate link should not go via the node to be protected.

"

For an alternate next-hop N to protect against node failure of a
   primary neighbor E for destination D, N must be loop-free with
   respect to both E and D.  In other words, N's path to D must not go
   through E. 

"

In your case, by increasing the metric from 20 to 41 or more should provide you node protection.

HTH,

Nagendra

[chintan-shah]

Hi Nagendra,

Thanks for pointing RFC section. it looks like it doesn't meet inequality 3 hence can't have node protection.

So if i go with below topology , based on this section 3.2, here also i should not expect node protection.

Right ?

Regards,

Chintan

[Nagendra Kumar Nainar]

Hi Chintan,

Yes. The alternate path from NodeA to NodeC is again via Node B (A->D->B->C). So Node protection is not possible.

HTH,

Nagendra