BGP Session with multiple ASN and multiple network over single link with ISP

[psc18@yahoo.com]

Hi,

I want to configure BGP With my ISP but I have 2 ASN And 2 different networks associated with each ASN.

Existing BGP is there using one ASN and one network and I want to add one more ASN and network.

[nazimkha]

You can have only one ASN number configured per router exception would be for using stub AS which doesnt seem to be your use case.

If you still need to use 2 ASN then you need to have an additional router for that BGP session

[psc18@yahoo.com]

Hi,

I read somewhere that we can either use local-AS command but Iam not sure of how to do that.
Also, VLAN can be used and then have 2 BGP Sessions but I would like to avoid using 2 BGP sessions if possible.

[Harold Ritter]

You can certainly use local-as to do that. One session could be established with the ASN configured on the BGP process and the other one could use "neighbor local-as ≤second ASN≥ no-prepend replace-as", which would make it appear as if you used the second ASN. But you certainly need to session to that.

The other way would be transfer both prefixes under the same ASN and use only that one ASN to peer with your service provider. You need to ask ARIN, RIPE or whatever internet registry organization you got your prefixes from and they will be able to help.

Regards,

[psc18@yahoo.com]

Thanks Harold,

Due to some business restriction I cannot move both the prefixes to the same ASN.

Can you share the full sample configurations for using local AS and if I use local AS, will it create multiple BGP Sessions ?

I am concerned as I don't want to put much load on the router and if I get the full routing table twice (one for each BGP session) then I will also have to increase the memory in the router.

Since I have an old 7606 router, so I am trying to find a way which will not put much load on the router.

Regards.

[Harold Ritter]

It would look something like this.

router bgp <your asn1>

nei <peer1> remote-as <sp asn>

nei <peer2> remote-as <sp asn>

nei <peer2> local-as <your asn2> no-prepend replace-as

If you do not want to overload the 7600 with both full Internet feeds, you could simply filter all of the routes inbound on one of the two feeds. One session would only serve to advertise one of your prefixes, while the other would serve to advertise the other prefix and receive the full internet feed.

Regards,

[psc18@yahoo.com]

Hi Harold,

But how will the system bifurcate among the networks associated with each ASN.

Like  I have AS1 with 2 Class C Network 204.X.X.X and 198.X.X.X

now the new AS2 has another Class C Network 200.X.X.X

[Harold Ritter]

I am not sure I understand your question. Your ISP will have 2 sessions with the 2 different ASNs. The 3 prefixes will be learnt by your ISP and routing will happen as usual. Does that answer your question? If not, can you clarify the question.

Regards,

[psc18@yahoo.com]

Hi  Harold,

I wanted to understand that since my ISP will have 2 sessions with my 2 different ASN's, so in that case,

how the ISP will determine which prefix is being advertised by which ASN ? 

and how the incoming routing will happen ?

My current config is as below

router bgp AS1
no synchronization
bgp log-neighbor-changes
network 204.X.X.0
neighbor 138.X.X.X remote-as 11111
neighbor 138.X.X.X route-map LOCALONLY out
neighbor 180.X.X.X remote-as 22222
neighbor 180.X.X.X route-map LOCALONLY out
auto-summary

and the new AS  I have purchased is AS2 and it has the new class c network 200.X.X.0, so as per your recommendations I should do the below config

router bgp AS1
no synchronization
bgp log-neighbor-changes
network 204.X.X.0

network 200.X.X.0  (New network of the new AS2)
neighbor 138.X.X.X remote-as 11111
neighbor 138.X.X.X route-map LOCALONLY out
neighbor 180.X.X.X remote-as 22222
neighbor 180.X.X.X route-map LOCALONLY out

neighbor 138.X.X.X local-as AS2 no prepend replace-as  (but will I have to do this with both the ISP's ?)
auto-summary

Regards

[Harold Ritter]

> how the ISP will determine which prefix is being advertised by which ASN ? 

They ISPs receive one prefix on the session with one ASN and the other two prefixes on the session with the other ASN. You just need to make sure that you only advertise the proper prefixe(s) on the respective session using your outbound route-map.

>neighbor 138.X.X.X remote-as 11111
>neighbor 138.X.X.X route-map LOCALONLY out
>neighbor 180.X.X.X remote-as 22222
>neighbor 180.X.X.X route-map LOCALONLY out

Why do use a different remote-as f you only peer with one ISP?

Regards,

[psc18@yahoo.com]

Hi Harold,

We actually have 2 ISP's, so thats why there are 2 different remote-as.

You said the ISP's receive one prefix on the session with one ASN and other two prefix on the other ASN but how to achieve that ? How to advertise the prefix based on the ASN they are actually associated with.