[c-nsp] BGP/ISIS/administrative distance conundrum.

[Janez Novak]

Hi Jason,

You can change AD for default route only.

ip access-list standard ACL-DEFAULT-ROUTE
 permit 0.0.0.0

router isis
 distance 240 <R1> 0.0.0.0 ACL-DEFAULT-ROUTE
 distance 240 <R2> 0.0.0.0 ACL-DEFAULT-ROUTE

Kind regards,
Bostjan

On Wed, Sep 12, 2012 at 7:02 PM, Jason Lixfeld <jason at lixfeld.ca> wrote:
> I've been trying to sort out a routing paradox in my lab and I'm hoping someone might have some insight.
>
> Routers 1-4 are all speaking ISIS-L2.
> Routers 1,2 originate a default route into ISIS.
> Routers 1,2 speak MP-iBGP northbound and receive a default route from their respective northbound MP-iBGP sessions.
> Routers 1,2 redistribute ISIS-L2 routes into MP-iBGP so the north side has reachability.
>
> Problem - Because the AD of ISIS is lower than iBGP, R1 and R2 see the ISIS default route an ignore the BGP default route.  This causes a routing loop.
>
> From what I've read, it's not possible to apply some sort of filter to the the ISIS process on R1 or R2 to ignore a learned ISIS default route and just originate a default route.
>
> I can't see any way of adding a route-map to the MP-iBGP sessions on R1 or R2 or either of their northbound MP-iBGP routers to change the AD of the default to less than 115.
>
> I can modify the ISIS distance on R1 and R2 to be greater than iBGP, but that would ultimately lead to all ISIS routes on R1/R2 being ignored in favour of the MP-BGP routes for the same destinations being learned from northbound iBGP.  Likely more routing loops would ensue here.
>
> I could somehow tag the ISIS routes that are redistributed into MP-BGP on R1/R2 and write a route-map to filter those routes from being announced back down to R1 and R2 alleviating the loop.  I'm wondering though, is there's a more straightforward answer that I'm not seeing.
>
> Thanks in advance.
>
>
>
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/