your mail

Clayton Fiske clay at
Thu Nov 1 19:23:23 UTC 2001

On Thu, Nov 01, 2001 at 11:33:55AM -0600, Zhao, Xingguo wrote:
> I have a question regarding to redistribute IGR routes into BGP. Currently
> both Cisco 
> and Juniper support it.
> Here I don't understand what is the advantage of this redistribution?
> Because BGP is 
> potentially injecting information into the IGP and then sending such
> information back into BGP.
> Could anybody provide more description regarding the advantage and the
> disadvantage
> of redistribuing IGP routes into BGP?

How is BGP potentially injecting information into the IGP? The only way
that should be happening is if you are doing BGP -> IGP redistribution,
in which case I'd surely hope you aren't doing IGP -> BGP as well.

In my experience, most redistribution in either direction is used to
overcome design issues (such as two iBGP neighbors whose paths to each
other include a non-BGP-speaking router). I don't know that I would go
so far as to say there is absolutely no valid use for it, but I've
always seen it used strictly as a patch rather than as intended design.

The main advantage IMHO is getting something working immediately instead
of having to make design changes (when the boss wants it to work -now-),
though this certainly comes with a cost if the proper path is not
implemented shortly thereafter.

The primary disadvantages I see are:

1) Loss of information. Whether you go IGP -> BGP or vice versa, you lose
some of the specifics of routes. BGP and OSPF, for example, have different
attributes, many of which can't be mapped to a corresponding attribute in
either direction (AS-path is the most glaring example).

2) Risk of routing table pollution. More than one major ISP has had
network meltdowns of varying degrees due to redistribution mishaps.
Your policies and filters may seem to work fine, but you also may add
some routes in the future that will trip them up.

Again, I'm sure more than a few of us have used redistribution, but in
general I see it as being used in lieu of a proper solution rather than
being the proper solution itself.


More information about the NANOG mailing list