ipv6 default - ::/3 ?

[Sander Steffann]

Hi Garry,

> I’ve been seeing an odd v6 situation and wondered if any may be seeing the same.
>  
> I have two different v6 providers originating a default v6 (expected to be ::/0) into two different routers of mine.
> I’m not seeing it from either ;  however my routers tell me I am RX’ing a ::/3 from each which I believe is a bogon.
> They claim they are default-originating a ::/0  , are not advertising a ::/3   ,  nor do they have this prefix in their tables.
>  
> As it looks oddly similar to 2000::/3, I’m curious if it’s being mucked with within IOS when received.
> I’m seeing this on Cisco76xx under SRE6,7 & 7a.
> I’ve tried removing filtering/routemaps with no change in behavior.  I’ve not had an opportunity to sniff it.
>  
> Outside of the aspect of accepting/using a default, anyone else seeing this or know why it may be?

This is a bug that I ran into earlier this year. It happens when you set the BGP timers to a low value on the receiving router. Here is an example. This is before setting the timers:

> router1#sh bgp ipv6 unicast neighbors 2001:DB8:810:1::1 received-routes 
> BGP table version is 1, local router ID is 10.0.56.5
> Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
>              r RIB-failure, S Stale, m multipath, b backup-path, x best-external, f RT-Filter
> Origin codes: i - IGP, e - EGP, ? - incomplete
> 
>   Network          Next Hop            Metric LocPrf Weight Path
> *  ::/0             2001:DB8:810:1::1
>                                             0             0 65531 i
> 
> Total number of prefixes 1

Then I change the timers:

> router1#conf t
> Enter configuration commands, one per line.  End with CNTL/Z.
> router1(config)#router bgp 65530
> router1(config-router)#neighbor 2001:DB8:810:1::1 timers 10 30 

Now clear the session and look again:

> router1(config-router)#do clear bgp ipv6 unicast 2001:DB8:810:1::1
> router1(config-router)#do sh bgp ipv6 unicast neighbors 2001:DB8:810:1::1 received-routes
> BGP table version is 54339, local router ID is 10.0.56.5
> Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
>              r RIB-failure, S Stale, m multipath, b backup-path, x best-external, f RT-Filter
> Origin codes: i - IGP, e - EGP, ? - incomplete
> 
>   Network          Next Hop            Metric LocPrf Weight Path
> *  ::/3             2001:DB8:810:1::1
>                                             0             0 65531 i
> 
> Total number of prefixes 1 
> router1(config-router)#

And suddenly we have the ::/3 route. The very annoying thing is that it doesn't disappear when removing the timers:

> router1(config-router)#no neighbor 2001:DB8:810:1::1 timers 10 30
> router1(config-router)#do clear bgp ipv6 unicast 2001:DB8:810:1::1
> router1(config-router)#do sh bgp ipv6 unicast neighbors 2001:DB8:810:1::1 received-routes
> BGP table version is 54339, local router ID is 10.0.56.5
> Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
>             r RIB-failure, S Stale, m multipath, b backup-path, x best-external, f RT-Filter
> Origin codes: i - IGP, e - EGP, ? - incomplete
> 
>  Network          Next Hop            Metric LocPrf Weight Path
> *  ::/3             2001:DB8:810:1::1
>                                            0             0 65531 i
> 
> Total number of prefixes 1 
> router1(config-router)#

Only reloading the box after removing the timers fixes this...

This was tested on a test-box without a active support contact so I couldn't create a TAC case for it, but I did report it to people inside Cisco. The firmware was an 15.1XB* release from lisp.cisco.com from around April. I don't know the exact XB release anymore though :-(

- Sander