Subject: Route Aggregation

From: wu min <[email-masked]>
Date: Tue, 23 Apr 2002 00:35:11 +0000

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?

2) Does OSPF perform route aggregation as BGP? If yes, then is it automatic
or have to be manually configured?

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

I would be very much appreciated if someone could clear my doubt.
Thanx.

-Tze Ven

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From: Acee Lindem <[email-masked]>
Date: Tue, 23 Apr 2002 08:09:36 -0400

> is  different from the longest match prefix? Is this safe?

This was changed. OSPF routers will always use the longest prefix
match (discounting other forwarding tricks). Refer to appendix G.5 in
RFC 2328.

> or have to be manually configured?

OSPF and BGP aggregation are quite different. OSPF will summarise the routes
within an area via configured area ranges. This will result in an aggregation
of sorts.

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

OSPF will install all 4 routes. In fact, I believe BGP will install
all 4 routes as well (although it may advertise an aggregated route to
its peers).

--
Acee