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BT Global Services: BGP FAQSuppose you want to reduce the proportion of traffic from you to the Internet via your BT Global Services connection. In order to achieve this, you will be altering how your own router makes the routing decision. If you have connections to two or more ISP's, you should be able to reach any destination on the Internet via any of those connections. Left alone, your router will use the as-path length of the routes you receive in order to make its routing decision.One of the parameters you can use to influence these routing decisions is local preference. Because the local preference is looked at before the as-path length by your router, just setting a local preference on one of the connections would shift all your traffic to one link. For this reason, you have to select certain routes which will be routed by BT Global Services, and some by others. Note: on Cisco routes, the default local preference is 100. The higher the local preference, the more preferred the route is. Also although you are selecting a path for your traffic, your router will still be able to use the routes seen from your other provider(s) in case of failure of your BT Global Services or other link. ExampleSuppose 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 3499If you wanted to route all the traffic to Sprint (AS 1239) and AT&T (AS 7018) destinations via your other provider NewNet, you would do so by selectively applying a local preference lower than 100 to those routes. Step 1: Create an as-path access-list detailing the as-path patterns you want NOT preferred via BT Global Servicesconf t ip as-path access-list 90 permit ^5400_7018 ip as-path access-list 90 permit ^5400_1239 exit Step 2: Create a route-map to match those as-path patterns and set the local preference lower than 100conf t route-map btgs-in match as-path 90 set local-preference 80 exit exit Step 3: Apply the route-map to your BT Global Services connectionconf t router bgp 8888 neighbor 166.49.0.1 route-map btgs-in in exit exit Step 4: Clear your session with BT Global Servicesclear ip bgp 166.49.0.1Note: your peering will be reset and traffic to BT Global Services will be interrupted by this command. Possible variationsIf you wish to decrease even more the amount of traffic through BT Global Services, you can experiment by changing the as-path access-list to include more ASes. Below are some lines you can include in your as-path access-list to further decrease the amount of traffic using your BT Global Services link. You can decide to include all of them or only some of them.Note: if you want to change an existing access-list, you need to first delete it (no ip as-path access-list 90) and the re-create it. ip as-path access-list 90 permit ^5400_1_ ip as-path access-list 90 permit ^5400_286 ip as-path access-list 90 permit ^5400_3549 ip as-path access-list 90 permit ^5400_6461 ip as-path access-list 90 permit ^5400_2914 ip as-path access-list 90 permit ^5400_702 ip as-path access-list 90 permit ^5400_1755 ip as-path access-list 90 permit ^5400_5511 ip as-path access-list 90 permit ^5400_8297 ip as-path access-list 90 permit ^5400_1299 ip as-path access-list 90 permit ^5400_174 ip as-path access-list 90 permit ^5400_8709 ip as-path access-list 90 permit ^5400_2548 ip as-path access-list 90 permit ^5400_16631 ip as-path access-list 90 permit ^5400_3300Back to main page |