How auto summarization creates problem ?

Unanswered Question
Dec 19th, 2008

Can anybody knows how auto summarization creates problem ?

How router send packet to destination using

recursive method ?

I have this problem too.
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Giuseppe Larosa Fri, 12/19/2008 - 12:00

Hello Kaustubh,

1) auto-summarization can create problems if two routers announce to a third router that they can reach net because the backbone lan is in another major network for example the backbone lan is

R3 will see two paths to net and it will try to do load balancing.

But R1 can reach only some subnets within net 10/8 and so happens for R2.

Auto-summarization at major network boundaries cause problems if a major network is partitioned, from the point of view of routers that haven't an interface on the partitioned major network multiple paths to it exist as in the example above and so routing can fail.

Modern protocols like EIGRP, RIPv2 and BGP allows to disable auto-summary and they are usually configured in this way.

No auto-summary at major network boundary (class A,B,C) exists in link state protocols like OSPF and IS-IS

2) a router performs a recursion when the next hop for a route is part of another route:


B [200/0] via

and then

sh ip route

O [110/1500] via ...

in this case a network has been learned in iBGP (AD 200) and the packets for net have a BGP next hop of

This is a loopback address on a remote router.

Packets for the net follow the same path as packets for

This is recursion. On this a lot modern architectures and techologies have built their services: for example MPLS, MPLS VPN.

Hope to help


kaustubhchavan Fri, 12/19/2008 - 21:13

Hi Giuseppe,

answer given is really appricieble but u have any document link so that i can easily understand ?

Giuseppe Larosa Sat, 12/20/2008 - 04:14

Hello Kaustubh,

a little document that shows auto-summary with RIP

problems start when a major network (class A,B,C) is partitioned and some subnets are connected to R1, other to R2, ...

Then the core uses a different major network so some useful information is lost because each of R1 ... RN will say I can reach the whole network but it isn't true

Hope to help


Hope to help



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