advertise prefix which isnt in rib

[Amirmahdi.M]

Hello everyone

I need to advertise a prefix to a customer which we have an ebgp connection with but the problem is that the prefix isnt in my routers rib.

i used to use a network command in bgp and static route the prefix to my upstream router but im looking for another way.

is it possible to somehow advertise the prefix to the customer despite it not being in the routing table?

thanks

Amir

[MHM Cisco World]

Static route to null0 is solve this issue 

[David Ruess]

Hello,

I don't believe there is. You need to have it in the routing table for BGP to advertise it. That usually means a learned prefix from an IGP or a static entry. This is to prevent the BGP speakers from advertising routes it doesn't know how to route to itself. 

Secondly if its not in your own RIB but your are trying to advertise it, when return traffic comes from the customer looking for that route how will the router know what to do with it if that route isn't even in its own routing table?

If you need to you I believe you can advertise a default route to the customer without having one yourself to get return traffic from them.

I would just create a static NULL 0 route and advertise it into BGP.

-David

[ESam]

In our current scenario, our PE routers are facing a similar challenge. They are receiving a default route from our ISP, but we are unable to propagate this route downstream to our CE routers through eBGP peering. The reason for this limitation is that our CE routers are already receiving a default route from a different Internet gateway within their LAN.

Instead of advertising the default route, we need to advertise specific Microsoft public prefixes that correspond to Office 365 services. The challenge is in the fact that our PE routers lack these Microsoft subnets in their RIB, making it impossible to advertise them even with outbound prefix configurations in place.

[MHM Cisco World]

What meaning of leak to RIB in PE?

[Flavio Miranda]

Hi @Amirmahdi.M 

Take a look in the command

neighbor x.x.x.x advertise-map Advertise non-exist-map Non-Exist

[David Ruess]

@Flavio Miranda I thought about that too but I believe conditional advertisements also will not work as they cannot create routes. In order to advertise a route it has to be in the RIB. A conditional advertisements withholds routes until the criteria is met. It cannot create networks to advertise it doesn't already know about.

[paul driver]

Hello

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@ESam wrote:I need to advertise a prefix to a customer which we have an ebgp connection with but the problem is that the prefix isnt in my routers rib.

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You shouldnt advertise routes to anyone that you do not own or have no rib entry for,
However If you have a client that needs to route towards a specific destination via your rtr then one possible way could be to apply some policy based routing (PBR) on the client itself pointing towards your router but this would still require you to have a valid route of your own towards that destination prefix and If you do then, I would say applying some conditional IPSLA static routing with a redistribution of that static into bgp should also work.

Editied:- the prefix-list noroute is for negating the advertsiment of the redistributed static towards any upstream ebgp peers.

Example:
ip sla 1
 icmp-echo 1.1.1.1 source-ip 10.1.34.9
ip sla schedule 1 life forever start-time now
track 10 ip sla 1 reachability

ip route 15.15.15.0 255.255.255.0 gig1 10.1.34.8 tag 10 track 10
ip prefix-list noroute seq 5 deny 15.15.15.0/24
ip prefix-list noroute seq 10 permit 0.0.0.0/0 le 32

route-map static permit 10
match tag 10

router bgp 100
redistribute static route-map static
neighbor 10.1.34.8 prefix-list noroute out