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Friday, October 29, 2010

Can we set up a BGP if there is only one connection leaving the network?

We are about to implement a new back up to our remote sites. We are using BT IP clear ADSL service and their request is for us to set up a BGP.Autonomous system so we can redistribute into their cloud.On reading up on this it states not to use BGP if you only have one connection leaving your network. We have over a hundred remote sites with 5 subnets in each. We are currently running OSPF.

Can some one guide us in the right direction?


You are correct in that if you only have one entry and exit point to each site there is no real need for BGP. However from my dealings with BT (although this was a couple of years back) they really only support BGP when you connect to their MPLS network. Sounds like this is still their position. You can use OSPF/static routing etc. but only if the SP supports it and i don't think BT do. It's basically for their ease of administration ie. BGP learned routes from you are automatically redistributed via MP-BGP to the other BT PE devices.

So you will need to run BGP between your CE device and the BT PE device. You will then have to redistribute the BGP routes you receive from their PEs into OSPF running on your CEs. To advertise your local networks you can either redistribute OSPF into BGP on your CE or use "network" statements on your CEs which is what we did.

It's important to understand that with this solution each OSPF process is isolated to each site so you only need one OSPF area per site. As for the BGP AS we used the same AS at all sites and used the "allowas-in 1" command under the router bgp config although if you want you can use separate BGP AS numbers at each site.

Edit - with a 150 sites it may well be pressing BT on whether they support anything else although i suspect that is when they will start offering their managed services ie. they manage the CE device in each site for you.

BGP Scenario- Design and Config

each site has 5 subnets so i would use network statements under BGP config rather than redistribute OSPF into BGP. If you can summarise the networks even better eg.

router bgp 65111

neighbor remote-as 65000 <-- where 65000 is BT AS number

neighbor allowas-in 1 <-- this means you can use the same AS number (65111) in all your sites

network mask

network mask

etc. for your subnets

note if you are peering with BT on loopbacks which they sometimes do then you would need to do the following in addition to the above -

1) add a route to your ADSL router telling it how to get to the BT loopback eg.

ip route x.x.x.x <-- where x.x.x.x is the IP of the physical interface on the BT PE router.

2) add this to your BGP config -

neighbor ebgp-multihop 2

Note also that for the network statements above eg etc. to be advertised there must be an exact match in the IGP routing table, so if you do a "sh ip route" you should see a route for with a /24 subnet mask. If you can summarise your networks then you can either -

1) use the "aggregate-address ...." command under your BGP config


2) create a static route to null for that summary address on your CE router eg.

ip address null0

and then under your bgp config simply do -

network mask

and remove the individual /24 network statements.

Under your OSPF config -

router ospf 10

redistribute bgp 65111 subnets

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