03-12-2009 11:40 AM - edited 03-04-2019 03:54 AM
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
Solved! Go to Solution.
03-12-2009 12:59 PM
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
03-12-2009 11:56 AM
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
03-12-2009 12:24 PM
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
03-12-2009 12:38 PM
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
03-12-2009 12:59 PM
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
11-12-2013 10:43 AM
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..
11-12-2013 12:25 PM
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
11-12-2013 12:38 PM
Got it ! Thanks !
06-12-2011 07:50 AM
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?
06-13-2011 03:02 AM
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
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide