- Each AS contain indipenent routing policies.
- Border Gateway Protocol (BGP) is
- an interdomain routing protocol
- designed to provide loop-free routing between separate routing domains
- that contain independent routing policies (autonomous systems)
- BGP is mainly used to connect a local network to an external network to gain access to the Internet or to connect to other organizations.
- BGP uses a path-vector routing algorithm to exchange network reachability information with other BGP speaking networking devices.
- Network reachability information is exchanged between BGP peers in routing updates.
- Network reachability information contains
- the network number,
- path specific attributes,
- and the list of autonomous system numbers that a route must transit through to reach a destination network.
- BGP prevents routing loops
- by rejecting any routing update that contains the local autonomous system number
- because this indicates that the route has already traveled through that autonomous system and a loop would therefore be created.
- The BGP path-vector routing algorithm is a combination of the
- distance-vector routing algorithm and the AS-path loop detection.
- NULL vs LOOPBACK interfaces
Correct Answer by Paul Stewart - CCIE Security, CCSI on Mar 14, 2011 3:56 PM
Paul Stewart - CCIE Security, CCSI
There is a software interface called the loopback interface. However, there isn't really a null interface, but you can create null routes. A null route just indicates to the router to throw packets containing a certain destination away.
Do they offer the same function?
A loopback is used for the router ID for OSPF and EIGRP, no similar function exists with a null route. We can bring a loopback directly into the routing process by using the "network x.x.x.x" command. No similar command exists for null routes and a null route is not used for any type of router ID. Either can be used for redistribution. To redistribute a loopback, you would use the "redistribute connected". To redistribute a null route, you would use "redistribute static".
- Asymmetric Routing: Incoming and outgoing routes are different between source & destination.
