EGP, BGP (Exterior Gateway Routing, Border Gateway Protocol)
See IGP, EGP (Routing Protocols). This will talk more in-depth info on Exterior Gateway Routing Protocols between primarily Autonomous Systems (AS, Networking).
Here BGP: Border Gateway Protocol is used to route between Autonomous Systems (AS, Networking). They are used to route between ASes. Used to route between Ases :
• Exchanges network reachability information. Says you can get to network X via this path of ASes
• Needs to be concerned with policies (business and political)
• See RFC 1771, BGP-4
• Runs over a reliable transport protocol (TCP)
• Uses a modified Distance Vector Protocol, but really it's Path-Vector Routing
It's important to note that it only exchanges routing information at the AS level. So there is not routing info about the internals of the ASes. It exchanges the full AS path info the the destination IP.
IP 5.0.0.0/8 has path <AS4, AS 9, AS 7, AS 3> from your local machine.
An Example
Here essentially the BGP speakers share with their neighbors, similar to the Shortest Path Algorithm (Networking), RIP, the paths of ASes for each subnet. For choosing a path:
- If there is no policy, then just choose the shortest path
- Business Reasons (ex: one may only want to use AS 50 above as a backup to preserve expensive traffic for more $$).
- All of this is decided within the BGP speaker.
You're Routing Table will not have info on ASes, but on the BGP speaker it routes certain IPs (ex: Cal Polys) to whatever direction its policy says to do.
Interior Boarder Gateway Protocol
How does the information on the BGP speaker get to information within the cloud/AS? Here comes interior Boarder Gateway Protocol (iBGP):
- Is a modified use of BGP but for interior route distrbution
- Used in large ASes
- Redistributes the BGP routes to internal routers
- Allows internal routers to learn the best gateway (from routes learned by BGP)
- If iBGP is not used, some other interior route distribution software is needed (so you essentially always need iBGP).