Tuesday, February 2, 2010


Route Selection Using Policy Controls

Over the last ten years, the size of the Internet grew to an extremely large size. None of the interior protocols used by most companies (such as OSPF, RIP, and EIGRP) could successfully handle a network of this size. BGP was a new Exterior Gateway Protocol (EGP) created to handle routing tables of enormous size. This introductory chapter gives us the overview of BGP and the foundation configuration commands.



* …is a distance vector routing protocol
* …uses TCP as its layer four transport (TCP port 179)
* …does not use triggered updates
* …uses periodic keepalives to verify TCP connectivity
* …is extremely scalable, but is slow to converge

BGP is rarely necessary if your company has a single connection to the Internet. It is most useful when you have multiple or redundant Internet connections since it can then find the service provider with the fastest path to your destination.


Unlike most of the routing protocols you may have configured in the past, BGP does not dynamically discover other neighboring BGP routers. They must be statically configured. This is beneficial since the service provider keeps its BGP connections under tight security. Use the following syntax to configure a BGP neighbor relationship:

Router(config)# router bgp

Router(config-router)# neighbor remote-as

You can only configure a Cisco router for a single BGP autonomous system (AS) (you cannot enter multiple router bgp numbers). However, you can connect to a practically limitless number of neighboring autonomous systems.

Once you have formed your neighbor relationships (neighbors no longer show the idle or active states from the show ip bgp summary output), you are now able to specify which internal networks you would like to advertise into the BGP routing process. Remember, service provider will propagate the networks you advertise to the entire Internet.

There are two ways of advertising internal networks into the BGP routing process: the network or redistribute router configuration commands. The BGP network command operates differently than any other routing protocol. Typically, the network statement tells a routing process the networks on which it should operate. For example, if you typed network when using the RIP routing protocol, RIP would send advertisements out any interface that was using an address from the network. In BGP, the statement network causes BGP to advertise the network to all neighbor relationships it has formed (provided a network is installed on the interior routing table).

The redistribute command, however, works similarly to other routing protocols. The command redistribute eigrp 100 causes all EIGRP routes from autonomous system 100 to enter the BGP routing table.

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