OSPF LSA filtering

Tom Thomas tthomas at torrentnet.com
Tue Dec 11 03:43:29 UTC 2001

Hi Simon,

I have some experience in this area that might help. By default, OSPF floods
new LSAs out all interfaces in the same area, except the interface on which
the LSA arrives. OSPF will flood based on the characteristics we discussed
earlier in this chapter, I am mentioning this because OSPFs specific
behavior is continue flooding until an acknowledgement on the Link State
Update packet is received. Some redundancy is desirable, because it ensures
robust flooding and accurate routing. However, too much redundancy can waste
bandwidth and might destabilize the network due to excessive link and CPU
usage in certain topologies. For example the bandwidth consumed by OSPF in a
fully meshed topology could be considerable and it might them be desirable
to block flooding. You can block OSPF flooding of LSAs two ways, depending
on the type of networks:

1. On broadcast, nonbroadcast, and point-to-point networks, you can block
flooding over specified OSPF interfaces.

2. On point-to-multipoint networks, you can block flooding to a specified
On broadcast, nonbroadcast, and point-to-point networks, to prevent flooding
of OSPF LSAs out of a specific interface, use the following command in
interface configuration mode:

ospf database-filter all out

On point-to-multipoint networks, to prevent flooding of OSPF LSAs to a
specific neighbor, use the following command in router configuration mode:

neighbor ip-address database-filter all out

Hopefully this maybe of assistance to you, good luck!

Tom Thomas

-----Original Message-----
From: owner-nanog at merit.edu [mailto:owner-nanog at merit.edu]On Behalf Of
Rizkalla, Simon
Sent: Monday, December 10, 2001 7:30 PM
To: nanog at merit.edu
Subject: OSPF LSA filtering

Does anyone have any idea how to filter out LSA updates between areas?  I am
using the Cisco 7204 VXR routers as ABRs and Enterasys routers as ASBRs.
However, I do not want routes to be propagated from the backbone area to any
other area.  The limitation is that the Enterasys gear we are using does not
support NSSAs.


Simon Rizkalla

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