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

[Osama I Dosary]

Thank you Oliver. But is there a good reliable  alternative to 
redistribution of the default route?
Is this what most ISPs are doing?
Or do they just have BGP everywhere?

/Osama

Oliver Boehmer (oboehmer) wrote:

>>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
>  
>