Accepting Only Default Route with Cisco N9K

[CiscoUser098234]

Cisco Community,

I'm planning on using an N9K as an L3 switch\Router at my smaller datacenter locations and plugging my transit provider directly into it.  My transit provider provides a full table and this obviously will not work with an N9K.  Do you see any problems with creating a prefix list that only accepts default (0.0.0.0/0)?  Are these prefix lists implicit deny in that if I only accept 0.0.0.0/0 it will reject everything else?

In locations with two providers I plan on getting just the default from both of my transit providers and using a combo of local preference + AS prepending prefer one over the other to make a true "active\passive" setup.

There may be some cases where I want to prefer one transit provider over another.  Is it possible to just accept default in the manner specified above but also specify specific ASNs that I want to accept from the provider to let BGP "do it's thing".

I've seen these Nexus crank several tens of Gbps.  See any issues if I'm accepting a few thousand routes from BGP (default + peering at the an IX location)?

[paul driver]

Hello

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@CiscoUser098234 wrote:

Do you see any problems with creating a prefix list that only accepts default (0.0.0.0/0)?  Are these prefix lists implicit deny in that if I only accept 0.0.0.0/0 it will reject everything else?

In locations with two providers I plan on getting just the default from both of my transit providers and using a combo of local preference + AS prepending prefer one over the other to make a true "active\passive" setup.

There may be some cases where I want to prefer one transit provider over another.  Is it possible to just accept default in the manner specified above but also specify specific ASNs that I want to accept from the provider to let 

Using an access-list to accept just defaults sounds like it would be applicable as you wouldn’t want to rely on your ISP to do this.
As this is just the one L3 switch/router connecting to two ISP rtrs you could use the bgp weight path attribute for the preferred egress path on those static routes and preferably As-Path pre-pending for ingress traffic from the ISP.

I would say you also need to make sure you don’t become a transit path between the two ISP’s which can be accomplished by just advertising only locally originated routes to each service provider

Lastly to save on the N9Ks cpu/memory resources having to process the filtering of the whole internet table down just to a static default you could even implement ( if its supported ) Outbound router filtering(ORF), which will tell the ISP rtrs to filter its advertised routes to you before they send them which will be based a specified outbound route filter you send to them.

[Giuseppe Larosa]

Hello @CiscoUser098234 ,

the prefix-list to accept only the default route is correct.

However, you need to verify that your ISPs are sending a default route, because you might be receiving a full BGP table without a default route. (actually a full BGP table does not need a default route as it describes all the public IPv4 prefixes).

In case you are not receiving the prefix 0.0.0.0/0 you need to ask to your ISP to generate a default route in eBGP for you on their routers connecting to your Nexus 9k.

Hope to help

Giuseppe