Dell PowerConnect B-RX BigIron RX Series Configuration Guide v02.7.02 - Page 811

Overview of BGP4 26 Figure 111 on page 739 shows a simple example of two BGP4 ASs. Each AS contains three BGP4 routers. All of the BGP4 routers within an AS communicate using IBGP. BGP4 routers communicate with other ASs using EBGP. Notice that each of the routers also is running an Interior Gateway Protocol (IGP). The routers in AS1 are running OSPF and the routers in AS2 are running RIP. The device can be configured to redistribute routes among BGP4, ISIS, RIP, and OSPF. They also can redistribute static routes. FIGURE 111 Example BGP4 ASs AS 1 IBGP OSPF IBGP AS 2 EBGP RIP IBGP IBGP OSPF IBGP OSPF IBGP RIP RIP Relationship between the BGP4 route table and the IP route table The BigIron RX's BGP4 route table can have multiple routes or paths to the same destination, which are learned from different BGP4 neighbors. A BGP4 neighbor is another router that also is running BGP4. BGP4 neighbors communicate using Transmission Control Protocol (TCP) port 179 for BGP communication. When you configure the device for BGP4, one of the configuration tasks you perform is to identify the BigIron RX's BGP4 neighbors. Although a router's BGP4 route table can have multiple routes to the same destination, the BGP4 protocol evaluates the routes and chooses only one of the routes to send to the IP route table. The route that BGP chooses and sends to the IP route table is the preferred route. This route is what the device advertises to other BGP neighbors. If the preferred route goes down, BGP4 updates the route information in the IP route table with a new BGP4 preferred route. NOTE If IP load sharing is enabled and you enable multiple equal-cost paths for BGP4, BGP4 can select more than one equal-cost path to a destination. A BGP4 route consists of the following information: • Network number (prefix) - A value comprised of the network mask bits and an IP address (/ ); for example, 192.215.129.0/18 indicates a network mask of 18 bits applied to the IP address 192.215.129.0. When a BGP4 BigIron RX advertises a route to one of its neighbors, the route is expressed in this format. • AS-path - A list of the other ASs through which a route passes. BGP4 routers can use the AS-path to detect and eliminate routing loops. For example, if a route received by a BGP4 router contains the AS that the router is in, the router does not add the route to its own BGP4 table. (The BGP4 RFCs refer to the AS-path as "AS_PATH".) • Additional path attributes - A list of additional parameters that describe the route. The route MED and next hop are examples of these additional path attributes. BigIron RX Series Configuration Guide 739 53-1001810-01