[j-nsp] cogent bgp example?

[James Jun]

> Hi, I'm a small site and I'm just getting ready to go live with a
> cogent connection. But, I'm finding myself a bit confused by their
> Peer A / Peer B description. I can find some examples of multihop
> things, but none seem very clear.
> 
> Can someone send a basic cogent bgp setup?

[edit protocols bgp]
group cogent-in {
 type external;
 peer-as 174;
 local-address $peer-b-local;
 neighbor $peer-b {
   description Cogent Transit [CE<-PE] support at cogentco.com;
   multihop ttl 5;
 }
 import gen_transit_in;
}

group cogent-out {
 type external; 
 peer-as 174;
 neighbor $peer-a {
   description Cogent Transit [CE->PE] support at cogentco.com;
 }
 export transit_cogent_out;
 import no;
}

[edit routing-options]
static {
  $peer-b/32 next-hop $peer-a;
}


For Cisco:

neighbor $peer-a remote-as 174
neighbor $peer-a description Cogent Transit [CE->PE] support at cogentco.com
neighbor $peer-b remote-as 174
neighbor $peer-b description Cogent Transit [CE<-PE] support at cogentco.com
neighbor $peer-b ebgp-multihop 5
! set interface loopback1 with $peer-b-local address
neighbor $peer-b update-source loop1
!
address-family ipv4
 neighbor $peer-a activate
 neighbor $peer-a send-community both
 neighbor $peer-a remove-private-as
 neighbor $peer-a prefix-list To_Cogent out
 neighbor $peer-a route-map no in
 neighbor $peer-a route-map transit_export out
 neighbor $peer-b activate
 neighbor $peer-b prefix-list ISP-Ingress-Filter in
 neighbor $peer-b route-map transit_import in
 neighbor $peer-b route-map no out
 neighbor $peer-b filter-list 500 in
!
ip route $peer-b 255.255.255.255 $peer-a
!

Replace peer-a, peer-b with appropriate peer IP's and replace all
import/export/route-maps/prefix-lists/etc accordingly to your organization
routing policy.

Replace peer-b-local with the /32 ip address Cogent assigns you for
Loopback1 interface.

Peer A is a directly connected neighbor where you announce/advertise your
network routes to them.
Peer B is a multi-hop peer where you advertise NO routes to them, but
receive full internet routing table from them.

And one other thing:
Cogent sends you IP address of peer B over the bgp session with peer A, so
that your router will be able to establish multi-hop session with peer B.
I don't like relying on an ebgp peer to receive session address for another
ebgp peer (takes longer for peer B to come up) so I generally prefer using
manual static-route for peer B, pointed to peer A's directly connected
address.


Hope this helps.
james