Subject: Re: Route Aggregation

From: "Balaji R (Networking) - CTD, Chennai." <[email-masked]>
Date: Tue, 23 Apr 2002 11:46:55 +0530

-----Original Message-----
From: wu min [mailto:[email-masked]]
Sent: Tuesday, April 23, 2002 6:05 AM
To: [email-masked]
Subject: Route Aggregation

Hi,

I have some questions and I hope someone out there can help.

1) I understand that when a router tries to route a packet it must use the
packet's destination address to search for the longest match  prefix entry
in it routing table to determine how and where the packet is supposed to be
forwarded. Supposely the router break this rule and use a prefix which is
not the longest match regardless of the existent of a longer one, what would
be the consequences assuming that the next hop router for the chosen prefix
is  different from the longest match prefix? Is this safe?
        Not all the time. If the other router thinks that you are the next
hop for the longer prefix, this will result in a routing loop.

2) Does OSPF perform route aggregation as BGP? If yes, then is it automatic
or have to be manually configured?
Yes, OSPF performs route aggregation at area boundaries and during
redistribution of routes from other routing protocols. The address ranges
that are to be aggregated are manually configured as per RFC 2328. I don't
know the implications of doing it dynamically. Can someone throw more light
on this?
3) A router running certain distributed routing protocol receives link state
advertisements about other routers. When it tries to build a  routing table
it found out that several prefixes in which one is a subset of the other. In
addition it found that all the prefixes have similar next hop router. Is it
allowable for these prefixes to be aggregated and assigned with a prefix
with the most general one? To clarify my question, let me use one example.
Given that a router  has the following fictious routing information:

i) prefix = 10.2.153.178/32; next-hop = router2
ii) prefix = 10.2.153/23; next-hop = router2
iii) prefix = 10.2.154/23; next-hop = router2
iv) prefix = 10.2/16; next-hop = router2

The question is, is it possible to aggregate them such that we have only the
following prefix in the routing table?

iv) prefix = 10.2/16; next-hop = router2

Are you talking about External-LSA's here? If so, it is better not to
aggregate as the intent of the sender might to update just the LSA that
changes. (Otherwise, the sender would have probably aggregated the routes).
If you aggregated, you wouldn't be able to do incremental routing table
calculation.
I would be very much appreciated if someone could clear my doubt.
Thanx.

-Tze Ven

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From: Eric Gray <[email-masked]>
Date: Tue, 23 Apr 2002 10:32:56 -0400

>hop for the longer prefix, this will result in a routing loop.
I doubt that this is ever 'safe' - though you may get away with it.

In order to get away with it, other routers must not have a preferred route
such that the packet will subsequently arrive at the same router.  The case
where the peer router thinks the local router is the next hop will not
often
result in a routing loop - it would be a black hole instead - because
'split-
horizon' behavior will prevent it from returning the packet to the router
from which it has received it.  But there are a ton of other ways in which
the packet may end up arriving at the local router again, resulting in a
loop.

>...

--
--
Eric Gray (mailto:[email-masked])
http://www.mindspring.com/~ewgray