[c-nsp] Default Route recalculated every 60 seconds.

[James Bensley]

On Sat, 18 Apr 2020 at 07:11, Bradley Ordner <bradinusa at hotmail.com> wrote:
>
> I am about to leave an Enterprise environment due to Pre Covid-19 redundancy and I just need to find the root cause of this issue before I leave.
>
> We recently built a Layer 2 Circuit over a Providers SDN Backbone so we could get a 2Gb Internet link. We peered with this neighbor and filtered a partial table, so we get about 30000 routes. For some reason, every 30 seconds the default route uptime resets to 00:00 in the routing table. I spoke with Carrier, they made a few changes and one was the BGP advertisement timer. It is now set to 60 seconds and now the default route resets every 60 seconds.
>
> The carrier, keeps blaming my side so I opened a Cisco TAC case and they haven't got around to looking at it yet, probably because it really sounds like it is the carrier side. I took some packet captures and indeed every 60 seconds an update with the default is sent. Our router constantly accepts this, recalculates and enters it into the routing table.
>
> I can't seem to figure out if this is some type of bug or not. The router has been rebooted and is due for IOS upgrade shortly, but wanted to see if anyone has seen this or point me in the right direction.
>
> Thanks
>
> Brad

Hi Brad,

Do you know if your provider uses IOS-XR on their PE? There was a bug
a couple of years ago in XR (I've searched on cisco.com but can't find
the BugID right now) in which XR was re-advertising the default route
every 30 or 60 seconds. We had it, and if I recall correctly it wasn't
being withdrawn, just a new BGP UPDATE was sent to supersede the
existing route, so as other posters have said, packet capture the BGP
TCP packets with your provider or use some "debug bgp" commands to see
whats really going on.

Also, maybe reconsider if need them to advertise a default route if
you're getting a partial table from them.

Cheers,
James.