732 BigIron RX Series Configuration Guide53-1001986-01Overview of BGP426Relationship between the BGP4 route table and the IP routetableThe device’s BGP4 route table can have multiple routes or paths to the same destination, which arelearned from different BGP4 neighbors. A BGP4 neighbor is another router that also is runningBGP4. BGP4 neighbors communicate using Transmission Control Protocol (TCP) port 179 for BGPcommunication. When you configure the device for BGP4, one of the configuration tasks youperform is to identify the device’s BGP4 neighbors.Although a router’s BGP4 route table can have multiple routes to the same destination, the BGP4protocol evaluates the routes and chooses only one of the routes to send to the IP route table. Theroute that BGP chooses and sends to the IP route table is the preferred route. This route is what thedevice advertises to other BGP neighbors. If the preferred route goes down, BGP4 updates theroute information in the IP route table with a new BGP4 preferred route.NOTEIf IP load sharing is enabled and you enable multiple equal-cost paths for BGP4, BGP4 can selectmore 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 (address>/ ); for example, 192.215.129.0/18 indicates a network mask of 18 bitsapplied to the IP address 192.215.129.0. When a BGP4 device advertises a route to one of itsneighbors, the route is expressed in this format.• AS-path – A list of the other ASs through which a route passes. BGP4 routers can use theAS-path to detect and eliminate routing loops. For example, if a route received by a BGP4 routercontains 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 routeMED and next hop are examples of these additional path attributes.NOTEThe device re-advertises a learned best BGP4 route to the device’s neighbors even when the routetable manager does not select that route for installation in the IP route table. This can happen if aroute from another protocol, for example, OSPF, is preferred. The best BGP4 route is the route thatBGP selects based on comparison of the BGP4 route path’s attributes.After a device successfully negotiates a BGP4 session with a neighbor (a BGP4 peer), the deviceexchanges complete BGP4 route tables with the neighbor. After this initial exchange, the deviceand all other RFC 1771-compliant BGP4 routers send UPDATE messages to inform neighbors ofnew, changed, or no longer feasible routes. BGP4 routers do not send regular updates. However, ifconfigured to do so, a BGP4 router does regularly send KEEPALIVE messages to its peers tomaintain BGP4 sessions with them if the router does not have any route information to send in anUPDATE message. Refer to “BGP4 message types” on page 734 for information about BGP4messages.How BGP4 selects a path for a routeWhen multiple paths for the same route prefix are known to a BGP4 router, the router uses thefollowing algorithm to weigh the paths and determine the optimal path for the route. The optimalpath depends on various parameters, which can be modified.
