[j-nsp] cogent bgp example?

[Amos Rosenboim]

Hello James,

Can you explain the logic behind Cogent BGP topology?
Are they going through all these configuration efforts just so their  
PE routers will not have to carry full internet feed?

Regards

Amos Rosenboim
amos at oasis-tech.net



On Aug 20, 2008, at 11:21 PM, James Jun wrote:

>> 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
>
>
>
>
>
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