BT Global Services: BGP FAQ

Suppose that you want to reduce the proportion of traffic from the Internet to you via your BT Global Services connection. This can be achieved by making the routes advertised through your BT Global Services connection less 'attractive', by artificially lengthening the as-path. This is known as 'prepending' or as-path stuffing.

Let us assume that you have a BGP connection to BT Global Services AS5400, and that the peering address is 166.49.0.1. You have a second BGP connection to NewNet AS3499, for which the peering address is 6.0.0.1. Your own AS is 8888.

Your current BGP configuration should look like this:

router bgp 8888
neighbor 166.49.0.1 remote-as 5400
neighbor 6.0.0.1 remote-as 3499

Step 1: Create a route-map to prepend your own AS on your connection to BT Global Services

conf t
route-map Global Services-out permit 10
set as-path prepend 8888
exit

Step 2: Apply the route-map to your NewNet peering

conf t
router bgp 8888
neighbor 166.49.0.1 route-map btgs-out out
exit
exit

Step 3: Clear your BGP session with BT Global Services

clear ip bgp 166.49.0.1

Note: your peering will be reset and traffic to BT Global Services will be interrupted by this command.

Possible variations

1. If this does not reduce the amount of traffic through BT Global Services and increase the traffic on your other connection, you may want to prepend twice, by using a route-map like:

route-map bgts-out permit 10
set as-path prepend 8888 8888

This would add your own AS, 8888, twice to the as-path when you advertise to BT Global Services.

2. If it reduces the amount of traffic too much, you may want to only prepend on some of the networks you advertise, instead of all of them. For example, if you have three networks, 195.2.1.0/24, 195.2.2.0/24 and 195.2.3.0/24, and you wanted to as-prepend on the first two networks, you would go about it like this:

access-list 88 permit 195.2.1.0 0.0.0.255
access-list 88 permit 195.2.2.0 0.0.0.255

route-map btgs-out permit 10
match ip address 88
set as-path prepend 8888

route-map btgs-out permit 20

router bgp 8888
neighbor 166.49.0.1 route-map btgs-out out

This configuration would only prepend your AS, 8888, to those two networks.

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