[c-nsp] Drawbacks of Redistributing Default Route from BGP intoIGP(ISIS)

[Oliver Boehmer (oboehmer)]

> We are a service provider, and have two upstream service providers,
> connecting to two different border routers (at different locations).
> Each upstream SP is sending us a default route back to them. Not all
> our routers speak BGP, so we need a way to send them the default
> route, where if one upstream fails they get the other dynamically.
> We are currently using default-originate in ISIS, but it is not
> dynamic. Then we thought of using route-map condition with the
> default-originate, but it didn't seem as straight forward as
> redistribution. 
> 
> What are the drawbacks of redistributing the default route into ISIS
> on the border routers?
> Is there an architecturally better way to do this?

Redistribution from BGP into an IGP is usually not being done, reasoning
is that simple config mistakes can cause a network meltdown if you end
up injecting several thousands of prefixes into an IGP. 

If you do it carefully and secure it with multiple safety nets (i.e. a)
filtering and/or limiting the number of prefixes you accept from your
eBGP peer, b) use route-maps to only redistribute 0/0 into ISIS, and
possibly c) limit the number of redistributed routes via the "IS-IS
Limit on Number of Redistributed Routes" feature), it is a valid
approach IMHO.

	oli