Filtering out default route from full Internet routing table

[marcel.schaaper]

Hi,

I have a question about filtering incoming bgp route updates from an internet provider. This provider sends the full internet routing table and default route and on an incoming prefix-filter on the customer switch (C6509-sup720) the default route is only accepted.

What happens on the 6509 switch when the BGP peer flaps?

Does it need to process all the internet routing updates, and if yes probably it cannot handle all these updates?

What happens with the CEF table, will the switch install the routes first?

Thanks in advance!

[Collin Clark]

If the BGP peer flaps, you'll lose all routes (assuming hard reset of BGP).

[marcel.schaaper]

Yes, but I'm wondering what happens on the customer switch, does it need to process all the internet routes first and then install the default route, because of te inbound filtering?

[Collin Clark]

The prefix filter will occur once the default route is learned. AFAIK there is no way for an ISP to tell a customer router to learn the default route first and then the full table after that.

[Richard Burts]

Marcel

Pretty clearly when the BGP peer goes down all the prefixes learned from that peer are flushed from the BGP table and from the IP forwarding table. Then when the BGP peer comes back up it will begin sending its advertisements. If the provider is sending advertisement of the full Internet routing table and a default route then the customer 6509 must process all of the advertisements. If the 6509 is using a prefix list that only permits the default route then the 6509 will only put a single entry into the BGP table and into the IP forwarding table. I have no reason to think that a 6509 would have problems in processing this.

In my experience most ISPs will offer customers a choice of what to advertise (frequently those choices are full Internet table; provider connected routes plus default; and default route only. If you are only going to use the default route then you should inquire to the provider about an option to only receive the default route.

HTH

Rick