Using the BGP ASN from multiple locations

Answered Question
Mar 12th, 2009

Hello,

I have a basic question about BGP. Can the same AS number be used by a organization at two different sites to peer with two different ISPs?

The scenario is as per what is shown below:

ASN 1111-a (100.100.100.0/24)---ASN XXX---ASN YYY---ASN 1111-b (200.200.200.0/24)

The AS number is 1111. The organization using it at location A to advertise network 100.100.100.0/24 to the ISP peer. At location B the organization is is using the same ASN to advertise 200.200.200.0/24 to another ISP. There is no peering between the two AS 1111 routers as they are physically separated.

Will this work?

Thanks,

AP

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 9 months ago

Hello Anand,

what you say look like possible

some notes:

you need to advertise separate public ip address blocks out of the two sites.

If you try to advertise something more specific that a /24 you may have problems with summarization of your routes at the ISP level.

So it can work in real world if you have at least two /24 public ip address blocks registered with your AS number

without knowing the details of the remote site routes it is difficult to say what is happening when something goes wrong.

>> The BGP rule that says an AS will not accept routes that have it's own ASN in the route

this can be overriden with the command neigh x.x.x.x allowas-in #times

where x.x.x.x is the ISP neighbor

you can still run an iBGP session between the two sites using the public ip addresses over the ISP links just to get the details of routes on the other site.

This can be useful for Disaster Recovery Strategies.

I would configure in this way in your case

Hope to help

Giuseppe

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Richard Burts Thu, 03/12/2009 - 11:56

AP

It is very common for a single organization to have multiple routers running BGP to route with several different iSPs. In this case the organization will use the same AS number on each of its routers running BGP.

Note that when the organization does this there should be a BGP session between the organization's own routers (running IBGP between its routers). It does not matter that they are physically separated - the organization's routers should run BGP between its own routers.

HTH

Rick

aprabhu12 Thu, 03/12/2009 - 12:24

Please bear with me. Just trying to understand BGP clearly.

Is it necessary for the organization to run BGP between it's own routers? What if the two locations do not have a high bandwidth enterprise WAN connection between them?

If each ISP is only feeding a default route to the organization at each location, what issues could arise if there was no peering between the organization's routers?

The BGP rule that says an AS will not accept routes that have it's own ASN in the route would not apply here as all each customer router is receiving is a default route.

The networks the customer is advertising should be able to reach each other. Don't you agree?

Thanks,

AP

Richard Burts Thu, 03/12/2009 - 12:38

AP

I can only tell you what the standard is in implementing BGP for Internet routing. And the standard is to run BGP between the organization's routers. Perhaps your situation is unique enough that not following the standard will not cause problems. We do not know enough about your situation to be able to know if there would be impact from not running BGP or not.

HTH

Rick

Correct Answer
Giuseppe Larosa Thu, 03/12/2009 - 12:59

Hello Anand,

what you say look like possible

some notes:

you need to advertise separate public ip address blocks out of the two sites.

If you try to advertise something more specific that a /24 you may have problems with summarization of your routes at the ISP level.

So it can work in real world if you have at least two /24 public ip address blocks registered with your AS number

without knowing the details of the remote site routes it is difficult to say what is happening when something goes wrong.

>> The BGP rule that says an AS will not accept routes that have it's own ASN in the route

this can be overriden with the command neigh x.x.x.x allowas-in #times

where x.x.x.x is the ISP neighbor

you can still run an iBGP session between the two sites using the public ip addresses over the ISP links just to get the details of routes on the other site.

This can be useful for Disaster Recovery Strategies.

I would configure in this way in your case

Hope to help

Giuseppe

dalal.sujal Tue, 11/12/2013 - 10:43

Hi Guiseppe, I had a question about the iBGP session establishment. So , how will the router in Site 1111-a get to know about the route to the IP address of the router at site 1111-b to which we want to peer without using allowas-in?

In my scenario, I dont want to use allowas-in because I dont want to break the loop detection in BGP. If I use allowas-in, I will have multiple routes for the same destination. The ones that came from the other site, and the one that came back from the ISP which were generated from the same site. My routers cannot maintain a routing table twice as big as the global routing table..I was thinking of a static route pointing to ISP..

Richard Burts Tue, 11/12/2013 - 12:25

In the original post site 1111-a has a publically routable address and site B has a publically routable address. So each site only needs a default route that sends their outbound traffic to their respective ISP and the packet can be routed to the peer.

HTH

Rick

NAGISWAREN2 Sun, 06/12/2011 - 07:50

Hi All,

If i would like to run BGP , where should i get the AS Number? from my ISP? If I have mutiple ISP? How about Public IP? Where should I get from? Any idea?

Giuseppe Larosa Mon, 06/13/2011 - 03:02

Hello Nagiswaren2,

note:opening a new thread would have been a better choice as your questions are close but not totally related to this thread.

you qualify for a public AS number if you are multihomed = you connect to multiple ISPs

public AS numbers and public IP subnets have to be requested to the regional  internet registry RIRs

europe:

www.ripe.net

america

www.arin.net

APNIC, ASIA

www.apnic.net

LACNIC south america

www.lacnic.net

the right one depends on the geographical region of your network

see

https://www.arin.net/knowledge/rirs/countries.html

Each RIR has its own policies and application forms to ask for ASNs ans IP networks.

Hope to help

Giuseppe

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