BIRD - iBGP between both RS - invalid NEXT-HOP notification.

[Ondrej Zajicek]

On Thu, Mar 20, 2014 at 11:20:20AM +0200, Javor Kliachev wrote:
> Hello,
>
> Unfortunately, I still have no response from anybody. The messages  
> continue to come into our logs.
> The iBGP session is UP since more than 1 week and till this moment we  
> don't have complaints by some member.

>> Here is part of my bird.log:
>> 07-03-2014 14:19:41 R0_29: Invalid NEXT_HOP attribute in route  

This message means that BIRD tries to send a route with NEXT_HOP attribute
X to a neighbor with IP address X.

That happens because both iBGP and RS eBGP do not change NEXT_HOP,
so if you have neighbor N connected to both:

RS1  ---  RS2
  \       /
   \     /
    \   /
      N

And it propagates the same route to both RS1 and RS2, then you have the
same route both times in RS1 and RS2. Usually the directly received is
prefered, but if for some reason the one received through IBGP is
preferred (perhaps N sends a withdraw, which is propagated first to RS1
so directly received route disappears but the one received from RS2 by
iBGP is still in RS1 for a moment), then it is scheduled for propagation
back to N, but the check for NEXT_HOP denies it.

I am not sure now whether this is some non-standard situation or the
check should be silent. Generally i would advise against connecting
route servers through IBGP. You get most routes two times, therefore
double memory requirements. Another reason is that IBGP (unless with
ADD-PATH extension) will silently eliminate non-preferred routes,
which would cause hidden unexpected problems.

-- 
Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."
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