Sunday, November 7, 2010


We'll now go into the final and biggest protocol in the world: The Border Gateway Protocol (BGP). Interior Gateway Protocols are the protocols we use to do routing internally. Exterior Gateway Protocols are protocols used to route the Internet. EGP used to be a protocol, but now it's become a category of protocols, and BGP is the only EGP protocol employed now.
BGP is used when you have multiple ISPs. When you run BGP, you're not so much concerned which direction you're going OUT. You're more concerned about how people get INTO your organization! Organizations want to make sure that they have a redundant connection for people to access their sites if ISPs go down. To do this, they'll need to advertise their public address block to the BGP domain. Such usage of more than 1 ISP is known as multi-homing.

BGP runs on top of TCP, unlike other protocols which have their own transport mechanism. TCP is used for reliability, and updates are incremental and triggered. The metric of BGP is the biggest metric in all routing protocols. The metric for BGP is the BGP Attributes. The protocol goes through 7 different attributes looking for an attribute to break the tie. It is the slowest routing protocol to converge, which is good for the Internet.

There are many styles of BGP implementation. CCNP focuses on using BGP from the perspective of the enterprise. The simplest outgoing access style is simply to have one ISP be the primary, and the other ISP be the backup, or do a simple per-conversation load-balance.

The second style of implementation is to have Partial Updates. You can selectively receive updates for blocks of addresses that are important to you. For example, you can selectively opt to receive addresses belonging to your offices in other countries.

The final style of implementation is the Full Updates. You will need to have a very powerful router to support this as you will be receiving all routes in the Internet.

BGP is technically a Distance Vector protocol, but most people call it a "Path Vector" protocol. Without tuning, BGP behaves just like RIP! The path chosen is based on the lowest number of AS they see in the AS_PATH.

BGP communicates with these types of packets:

Open packets start the session with another router. It is like the initial Hello packet.

Keepalive messages make sure that the neighbors are still around. It is like the subsequent Hello packets.

Update packets gives network reachability information. This is the bulk of messages. The messages contain "prefixes", which is another term for "routes".

Notification packets tells the neighbors of a bad event. There is no good notification. It is also used to close a session.

Below is the list of tables used by BGP:
-Neighbor Table
-BGP Table
-Routing Table

Neighbor Table keeps a list of configured neighbors. Neighbors in BGP are statically configured.

BGP Table is the list of ALL BGP routes. This is a huge table.

Routing Table keeps a list of the best routes.

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