Solved: MPLS and AS numbers for my sites

Joshua Engels · ‎08-11-2013

Hey guys,

I am new to MPLS but some experience with BGP in a multihoming enviornment so have a little bit of understanding on BGP configuration.

That said, I am working with a provider on setting up multiple MPLS sites. Some sites I will manage the router and some of the more remote sites we will have the ISP manage the router. I am in the process of setting up the first site and one question is evading me. Site A (Baton Rouge) we are managing the router. The AS number is "65003". The ISP's AS is "1". I have setup Baton Rouge and it is now peered successfully to the ISP. My question is regarding the AS number on the other end at Site B (Atlanta). Will it have the same AS number as my BR site or will it be different? Again, as we start to bring on other sites for my company through this ISP, will they assign different AS numbers for each site or will it be the same for each site?

The reason I ask is I am building a mock lab to try and understand how routing is going to work between sites.

Vignesh Rajendran Praveen · ‎08-12-2013

Hello Joshua,

The AS number for Site B (Atlanta) can either be 65003 or another number. But usually the ISP takes the customer's input while doing the design. Just in case the ISP uses the same AS number 65003 for site B CE then they need to use AS-Overide command on the PEs of Site A & Site B at their end towards your BGP sessions respectively. If different AS numbers are used at customer Site A & B then the ISP does not need to use the AS-Overide BGP command on their PEs for these two sites.

The purpose of the AS-Overide BGP command is to avoid routing loops.

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Thanks & Regards,

Vignesh R P

View solution in original post

Vivek Ruhil · ‎08-12-2013

Hi

Some points for your consideration

1. With all CE routers having the same AS Number:

a. Biggest advantage is you don't need to think or even bother about maintaining a list of BGP AS numbers per site and ensure that the SP uses "as-override" on their PE routers. Which they do.

b. This is the most widely deployed option.

2. With all CE routers having the different AS Number:

a. You will have to maintain a list and manage AS numbers across all sites, if the number of sites are huge its a hectic and you will have to stay within the private range, though, the private range is huge so no worries there.

b. The advantage this offers is that supposing you have a routing loop or some routing problem, you will be able to look at your bgp routes and figure out which site is misbehaving since you have site dependent AS numbers in the AS Path.So, this option basically allows you to keep a track of all the sites and easily identify any problems in then network.

c. Many Customers do use this option also, since it is giving any added advantage of tracking.