BGP AS Numbers

[gavin.mckee]

Hi,

I have very little experience with BGP, and I am about to get a baptism of fire! Anyway we are implementing WAN links with a new provider. The network engineering team suggest we use 7 differnt AS numbers to form the WAN. If we use 7 different AS numbers do we have to use EBGP between the AS's and IGP between routers within an AS?

Any help will be rated.

Thanks

Gavin

[pkhatri]

Gavin,

If you are being asked to form a network using 7 different AS numbers, then one would presume that it is a very large network. Networks are usually split up into multiple ASs in order to scale better. The configuration that results is called a BGP confederations. Under such a setup, you would run a single IGP over the whole network, IBGP within each AS and EBGP between the ASs.

However, there is a lot of complexity involved. I would have to question the use of so many ASs - there are other mechanisms such as route reflector clusters that can be used to overcome such issues.

I suggest you read the following to get up to speed on BGP:

http://www.cisco.com/warp/public/459/bgp-toc.html

Paresh

[globalnettech]

Hello Gavin,

in addition to Paresh's post, it might be interesting to know why the network engineering team suggests the use of that many ASs, they might very well have a valid reason. Can you give more details about the setup of your network ? Basically, you are right: iBGP is used for connections between BGP routers within the same AS, and eBGP is used for connections between routers in different ASs.

Regards,

GNT

[mheusinger]

Hi,

One question: is your solution an MPLS VPN offered by the SP?

Then each location should have a separate private AS number for simpler configuration and troubleshooting. Within each location you would use iBGP between CE routers and an IGP (OSPF, ...) for all other routers.

If you setup your own WAN solution based on leased lines, then you can for sure use one AS and the comments of Paresh apply.

I have seen the "one AS per organizational unit" approach as well. In the environment I have seen each country of an international company got its own private AS also because each country had a separate (more or less independant) IT department. This scenario reflects more or less the definition of an AS:

An AS is a collection of networks under a single technical administration.

In case you provide more details about your planned setup and involved technologies, we can be more specific in our answers.

Hope this helps! PLease rate all posts.

Regards, Martin