Redistribution / Summarisation Query

Unanswered Question
Jun 24th, 2009


The background:

Have peered (BGP) two of my internal routers through my firewalls to our main extranet provider (at two different sites within my network). The aim is to move to a more dynamic routing setup to incorporate a DR/BC approach (availability - tick in the box).

Peering is all working but I have quite a lot of additional routes (well over 700) however these can be summarised to 5 networks. I was hoping the provider would do this for me but having no joy with them at the moment.

What I would like to do is to summarise these routes and redistribute them into my internal routing protocol (EIGRP).

I was thinking of redistributing with a route-map which points to an ip prefix-list with these 5 subnets. Is this a good way of doing it? I have sequenced the list so I can add to it if I ever need too in the future.

The internal routers are on a stick so only have one interface to work with.

It seems messy, is there a better way to do it?



I have this problem too.
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Giuseppe Larosa Wed, 06/24/2009 - 06:05

Hello Stuart,

if your objective is to send summary EIGRP routes you can do it on a per interface basis using

ip summary-address eigrp asn prefix mask ADm

multiple commands are allowed

be aware that EIGRP automatically generates a route to null0 for loop avoidance you need to specify an ADm > EBGP distance (for safety I would use 201 > iBGP)

if each router has multiple interfaces to the core you need apply the same set of summary address commands on all of them.

OR you can use

router bgp xx

aggregate-address x.x.x.x mask summary-only

the aggregate is created if at least a component route is in the BGP table.

then in the EIGRP process as you say you can use the filter list invoked by the route-map to allow only the 5 summaries

Depending on the way the route-map is written you may need some refinement for example a second clause where you specify other prefixes (if present and needed) to be advertised.

Without specifying an empty final clause all what doesn't match is silently filtered.

Hope to help


s-sheffield Wed, 06/24/2009 - 06:34

Hi Giuseppe

firstly thankyou for your time on this matter.

I will try the summary-address first (never thought of that one). I take it this will stop the 700+ routes flying around my network and be restricted to my initial redistribution router.

I did contemplate the aggregate route but unsure what impact this would have on the bgp side, I'm assuming that nothing will go back to my provider (poison reverse etc kicking in). Would I need a standard redistribution statement - will this take into account the aggregate route?

you have pointed me in a more productive direction (thought mine solution was too messy)




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