Re: BGP next-hop

[Christian Martin <christian.martin () teliris com>]

On Sep 30, 2010, at 5:37 PM, Randy Bush <randy () 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