Re: [isp-bgp] ibgp behavior with two links to same neighbor AS

[Jeffrey Belles <jaffa@...>]

Hi,

Colo Host schreef:
> Hi all, can someone tell me the normal behavior of iBGP
> between two routers who both peer with the same remote
> AS?  I have two routers peered with each other in my AS
> and they both have a link to the same remote AS and are
> receiving full routes.  Should my two routers be sharing
> their learned routes with each other even though they're
> basically learning the same thing?  
It is too easy to imply that they are basically learning the same thing. 
If you peer with one AS at two different locations, these two eBGP peers 
can hold different routing information. They just advertise _their_ best 
route. And they can be sending MEDs with their advertisements.

If that is the case they are also not sharing their full table.

> I'm seeing just one
> share its routes with the other but not vice versa, so if
> that one loses it's link to the remote AS, traffic trying
> to leave via that router gets stuck.  It does start flowing
> again about two minutes later because it seems the loss
> of that upstream link causes it to start receiving, or the
> other router to start advertising, the routes out through
> that other router.
In respect of my previous comment:
if you have router A & B peering with the same upstream AS, then in some 
cases router A and B might prefer the internal routes (learned via 
eachother) to get to certain prefixes.


|-----------------------------|
|            AS1              |
|---rtr1--------------rtr2----|
    |                 |
  A |               B |
    |                 |
|--rtrx--------------rtry-----|
|           AS2               |
|-----------------------------|


Say rtr1 advertises a prefix 10/8 with a MED of 5, and rtr2 is sending 
it with a MED of 10.

rtry will actually prefer to reach the 10/8 block over its internal 
neighbor rtrx and not advertise the rtr2 path.

If then link A would break, and the session expires, rtrx withdraws the 
routes learned from rtr1 and then rtry will advertise to rtrx the one he 
learned from eBGP.

HTH,
 -Jeffrey


> If that is normal behavior, are there common tricks that can
> be used to increase the propagation speed of routes via
> BGP, external and internal?  I'm finding that in a mixed-vendor
> network, some combos fully load the Internet table much
> quicker than others from their peers where bandwidth is
> not the limiting factor.
>
> Thanks,
>
> D



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