[c-nsp] IOS XR PIC (+ multipath)

[Adam Vitkovsky]

You're the best Oli
Thank you very much for clarifying the multipath-protect feature. 
Right for multipath there are alternate paths installed in FIB already so in
case one fails others serve as backup. 

I've just realized I forgot the "advertise best-external" cmd in configs
below
-which is necessary to get the alternate paths propagated across the network

Now to my question
I was strongly advised by the SE team NOT to use the combo of multipath +
backup-path:
The reasoning behind it was basically that it was not tested by big ISPs -so
I guess no one really knows how it behaves in production networks. 
I also heard an argument that best-external could cause loops in some cases
-though I can't really think of such a scenario -and would love to be
schooled on that. 
 
router bgp 123
 vrf test
  address-family ipv4 unicast
   additional-paths selection route-policy RP-PIC
   advertise best-external
   maximum-paths eibgp 8

So please are there any issues/corner cases where it can cause issues -or
it's just a lack of production testing? 

adam

-----Original Message-----
From: Oliver Boehmer (oboehmer) [mailto:oboehmer at cisco.com] 
Sent: Tuesday, January 08, 2013 5:55 PM
To: Adam Vitkovsky; cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] IOS XR PIC (+ multipath)

Adam,


>
>More specifically with the new CLI for the feature in 4.2 :
>
>route-policy RP-PIC
>  set path-selection backup 1 install
>end-policy
>!
>router bgp 123
> address-family vpnv4 unicast
>  additional-paths selection route-policy RP-PIC
>
>
>I'm more interested in the advanced multipath/primary-backup
>implementation:
>
>-so for equal cost paths I'll have the balancing in place -and for 
>unequal cost paths I'll have the PIC in place

well, no. For equal-cost, you also have PIC in place, but as the router
already has two (or more) path installed and active in the FIB, it doesn't
need to install a backup path. so that's PIC-like behaviour, without any
config. The config ...

>
>route-policy RP-PIC
>  set path-selection backup 1 install multipath-protect end-policy

... also causes the PE to install a backup path even for those cases. I
can't think of a scenario where this is needed, but I might be missing
something.

	oli