BigIron RX Series Configuration Guide 75753-1001986-01Changing administrative distances 26Changing administrative distancesThe BigIron RX can learn about networks from various protocols, including the EBGP portion ofBGP4 and IGPs such as OSPF, ISIS, and RIP. Consequently, the routes to a network may differdepending on the protocol from which the routes were learned.To select one route over another based on the source of the route information, the device can usethe administrative distances assigned to the sources. The administrative distance is aprotocol-independent metric that IP routers use to compare routes from different sources.The device re-advertises a learned best BGP4 route to the device’s neighbors even when the routetable manager does not also select that route for installation in the IP route table. The best BGP4route is the BGP4 path that BGP selects based on comparison of the paths’ BGP4 routeparameters. Refer to “How BGP4 selects a path for a route” on page 732.When selecting a route from among different sources (BGP4, OSPF, RIP, ISIS, static routes, and soon), the software compares the routes on the basis of each route’s administrative distance. If theadministrative distance of the paths is lower than the administrative distance of paths from othersources (such as static IP routes, RIP, or OSPF), the BGP4 paths are installed in the IP route table.Here are the default administrative distances on the BigIron RX:• Directly connected – 0 (this value is not configurable)• Static – 1 is the default and applies to all static routes, including default routes. This can beassigned a different value.• EBGP – 20• OSPF – 110• ISIS – 115• RIP – 120• IBGP – 200• Local BGP – 200• Unknown – 255 (the router will not use this route)Lower administrative distances are preferred over higher distances. For example, if the routerreceives routes for the same network from OSPF and from RIP, the router will prefer the OSPF routeby default. The administrative distances are configured in different places in the software. Thedevice re-advertises a learned best BGP4 route to neighbors by default, regardless of whether theroute’s administrative distance is lower than other routes from different route sources to the samedestination.• To change the EBGP, IBGP, and Local BGP default administrative distances, see theinstructions in this section.• To change the default administrative distance for OSPF, RIP, ISIS, refer to “Changingadministrative distances” on page 757.• To change the administrative distance for static routes, refer to “Configuring static routes” onpage 197To change the default administrative distances for EBGP, IBGP, and Local BGP, enter a commandsuch as the following.BigIron RX(config-bgp)# distance 200 200 200Syntax: distance
