BGP Well Known Community - no-advertise

dppagc

Hi I understand that the well known bgp community "no-advertise" command is used when not advertising routes to anyone.
However, why will anyone do that unless it is part of the LAN.
Can someone give a practical application of this?

kohr-ah

I have lets say a group of 5 routers.

A - B - C - D - E
A - E
/ \
B C
\ /
D

I want E to tell A that I am advertising this network into BGP. However please do not advertise it any further into B C or D you'd use it.

A good example of it.
Hacking Cisco: Lab 115 - BGP Communities - NO-ADVERTISE

Fitzi

No advertise means do not advertise to any neighbour at all ebgp or ibgp, there is an example here which might help to illustrate the point:

https://learningnetwork.cisco.com/thread/58299

Maybe you have a branch site with multiple subnet and one being a lab subnet that only the branch network wants to be able to access and you don't want the rest of your WAN to see this network. If you are using iBGP on your WAN you could mark this subnet as no advertise to keep it locally reachable only, this is opposed to the no-export command which will mark the route as no export to an upstream AS eg: outside your organisation.

There are probably some other corner cases where you want to keep some routes locally reachable as well.

dppagc

Okay I understand the scenario. But why isnt a default route used instead of no advertise?
Wont it be easier?

networker050184

Depends. What if there's is already another default route?

You aren't likely to use this one in the real world, but it's a nice trick to have to your sleeve.

EdTheLad

I typically use the "no-advertise" community when scale testing a device in a lab. I'll advertise 1mil+ eBGP routes from a tester to the router under test, i do not want that router to forward those routes to other peers.