Multiple BGP Instances

[mibritzman]

I have a need for multiple BGP AS instances on the same 2611 router. Does Cisco support this type of BGP deployment? OR Is there another way to handle multiple BGP AS connections on the same router? Thanks.

[rwiesmann]

Hi,

a router allows you only to define one AS on it. But you still could peer to different ASs. so you basically can handle multiple bgp as connections but the information is kept in the same routing table. You would need Multiprotokoll BGP with which you could create mulitiple address families within bgp and keep the the routing information seperate in different vrf's

Hope that helps

Roger

[mibritzman]

The situation is we are migrating from one MPLS provider to another. Both use BGP and there will be a time when both MPLS circuits,i.e. eBGP ASs will be active. It provider of course has their own unique AS number. So I was trying minimize the circuit downtime and keep them both active for a short time. Thanks for your input. I will look into it.

[Harold Ritter]

As Roger says, it is not possible to define more than one AS per router.

What are you trying to achieve. If all you need to do is to appear as multiple different ASes to your BGP peers, the local-as feature could be a solution.

Please refer to the following document for more details on the local-as and replace-as features:

local-as feature:

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800949cd.shtml

replace-as/dual-as features:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1829/products_feature_guide09186a00802c0816.html

Hope this helps,

[mibritzman]

The situation is we are migrating from one MPLS provider to another. Both use BGP and there will be a time when both MPLS circuits,i.e. eBGP ASs will be active. It provider of course has their own unique AS number. So I was trying minimize the circuit downtime and keep them both active for a short time. Thanks for your input. I will look into it.

[Harold Ritter]

You can certainly use the local-as feature to do that. Here's an example of how it can be achieved:

router bgp 65001

neighbor remote-as

neighbor local-as 100 no-prepend replace-as

neighbor remote-as

neighbor local-as 200 no-prepend replace-as

In this example, you use AS100 for the session to provider one and AS200 to provider 2.

Hope this helps,

[jain.manish94]

Hello please can you tell me why we use it.

One router mutilple ASN.

why we use Local AS in BGP.

I know it when you want to run multiple ASN in the same router. but why we do it. ?

Please reply.

[mjoaquin23]

I think your problem is not about having 2 different ASs defined on a single router. You don't have to change your AS number even if you are connected to both providers. I think your problem is more of protecting your AS from becoming a transit AS to both providers. I don't see any sense in advertising or receiving the same advertisement from both providers, so you can bring up both BGP sessions to both providers, but block all incoming and outgoing advertisements to your NEW one, by using filters(attribute manipulation, route-maps, prefix-list, filter-list, etc). Once you need to migrate your traffic from old to the new provider, you can just release all filters on the new link and shut or block traffic on the OLD one.

Advertisements from your new provider can be seen even if your BLOCK filters are on(sh ip bgp neigh receive).

If you need to see all bgp routes from the same provider at the same time, you can still accomplish this by releasing your BLOCK filters on the inbound of the NEW one(keep outbound filters), BUT assign a higher weight for routes coming in from your OLD provider, to make sure that your traffic still goes through teh OLD direction.

[Danilo Dy]

You can't have multiple BGP instances, but you can make some workaround to advertise your multiple AS.

Example:

1. You have the following

AS: 1

Network: 10.0.0.0

AS: 2

Network: 20.0.0.0

2. Your upstream ISP

AS: 3

Link IP: 30.0.0.1

3. Configuration

!

router bgp 1

network 10.0.0.0

network 20.0.0.0

neighbor 30.0.0.1 remote-as 3

neighbor 30.0.0.1 route-map BGP-OUT out

!

access-list 1 permit 10.0.0.0 0.255.255.255

access-list 1 deny any

access-list 2 permit 20.0.0.0 0.255.255.255

access-list 2 deny any

!

route-map BGP-OUT permit 1

match ip address 1

!

route-map BGP-OUT permit 2

match ip address 2

set as-path prepend 2

-------------------------

AS 2 will be seen behind AS 1.

I have used it, it works. I have two routers running BGP/iBGP connected to two ISP, I have two AS. It's been in production for 3 years :)

NOTE: The above is just a minimum configuration. I post it this way so that you will understand.

[shujath.syed]

Hi - Why are all the cisco support threads so old like 5years ago,12yrs, where can I see something recently discussed?

[pgasparovic]

Even in 2020 man, even in 2020! Sometimes you win a pot of gold pot in them, like me today. This one and other from 2012