BGP next-hop

Fri Oct 1 00:17:45 UTC 2010

On Sep 30, 2010, at 5:37 PM, Randy Bush <randy at psg.com> wrote:

> i was recently bitten by a cousin of this
> 
> research router getting an ebgp multi-hop full feed from 147.28.0.1
> (address is relevant)
> 
> it is on a lan with a default gateway 42.666.77.11 (address not
> relevant), so it has
> 
>    ip route 0.0.0.0  0.0.0.0  42.666.77.11
> 
> massive flapping results.  
> 
> it seems it gets the bgp route for 147.28.0.0/16 and then can not
> resolve the next hop.  it would not recurse to the default exit.
> 
> of course it was solved by
> 
>    ip route 147.28.0.0  255.255.0.0  42.666.77.11
> 
> but i do not really understand in my heart why i needed to do this.
> 

Looks like a classic race condition, in that 147.28/16, upon arrival, becomes a better route for the recursed next-hop (which really is a recursed lookup on your default)  So you get

147.28/16 -> 147.28.0.1, and then 147.28.0.1 looks best through the learned route.

Of course, this would appear to be a matter of how it is implemented.  Because in fact, the 147 route isn't yet in the routing table, so your default should apply.  The static seems to force a recursion to the 666 nh.  

I'll wait for your friend to send the implementation details, but from a glance, it looks like a defensive (lazy?) attempt to avoid a recursion loop during the update receive process.

Btw, this will happen on a Juniper (or at least it used to).  I'll have to check to confirm.

Chris

> randy
> 