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Setting the Administrative Distance for a Route

The administrative distance is an integer in the range 0–255 that is associated with each route known to a router. The distance represents how reliable the source of the route is considered to be. A lower value is preferred over a higher value. An administrative distance of 255 indicates no confidence in the source; routes with this distance are not installed in the routing table. As shown in Table 22, default distances are provided for each type of source from which a route can be learned.

Table 22: Default Administrative Distances for Route Sources

Route Source	Default Distance
Connected interface	0
Static route	1
External BGP	20
OSPF	110
IS-IS	115
RIP	120
Internal BGP	200
Unknown	255

If the IP routing table contains several routes to the same prefix—for example, an OSPF route and an IBGP route—the route with the lowest administrative distance is used for forwarding.

By default, BGP propagates received BGP routes to EBGP routes only if the BGP route is used for forwarding traffic—that is, if it is the route with the lowest administrative distance in the IP forwarding table. However, you can modify this behavior by using the bgp advertise-inactive command. See Advertising Inactive Routes for more information.

You can use the distance bgp command to configure the administrative distance associated with routes. If you choose to set an administrative distance, you must specify a value for all three of the following types of routes:

external—Administrative distance for BGP external routes. External routes are routes for which the best path is learned from a BGP peer external to the AS. Acceptable values are from 1 to 255. The default value is 20.
internal—Administrative distance for BGP internal routes. Internal routes are those routes that are learned from a BGP peer within the same AS. Acceptable values are from 1 to 255. The default value is 200.

local—Administrative distance for BGP local routes. Local routes are those routes locally originated by BGP. BGP can locally originate routes if you issue the network command, if you configure redistribution into BGP, or by means of a non-AS-set aggregate route. Acceptable values are from 1 to 255. The default value is 200.

Caution: Changing the administrative distance of BGP internal routes is considered dangerous and is not recommended. One problem that can arise is the accumulation of routing table inconsistencies, which can break routing.

You can use the distance bgp command to configure these preferences. The following commands leave the internal distance at 200, set the external distance to 150, and set the local distance to 80:



host1(config)#router bgp 100 

host1(config-router)#network 172.28.0.0 

host1(config-router)#neighbor 156.128.5.5
remote-as 310 

host1(config-router)#neighbor 142.132.1.1
remote-as 50 

host1(config-router)#distance bgp 150 200
80

distance bgp

Use to set the administrative distance for all BGP routes.
You must specify the following:
- external-distance—Administrative distance for routes external to the AS in the range 1–255. The default is 20.
- internal-distance—Administrative distance for routes internal to the AS in the range 1–255. The default is 200.
- local-distance—Administrative distance for local routes in the range 1–255. The default is 200.
The default value is 20 for external routes, 200 for internal route, and 200 for local routes.
The new distance is applied to all routes that are subsequently placed in the IP routing table. To apply the new distance to routes that are already present in the IP routing table, you must use the clear ip routes * command to reinstall BGP routes in the IP routing table.
Use the no version to return the distances to their default values, 20, 200, and 200.
See distance bgp.

Example 1

Routes learned from other sources can be preferred to routes learned by means of BGP. Consider the network structure shown in Figure 38.

Figure 38: Administrative Distances

Image g013193.gif

Suppose router KC originates 172.17.24.0/21 and advertises the route to router Chicago by means of EBGP. Both router KC and router Chicago are directly connected to the network represented by 172.17.24.0/21. If you issue the show ip route command on router Chicago, the BGP route does not appear. Instead, only the connected route is displayed.

Both routes are in the IP routing table, but the show ip route command displays only the best route. (Use the show ip route all command to display all best routes; in this case the BGP route and the connected route.) Connected routes have a default distance of 0. Routes learned by means of EBGP have a default value of 20. The connected route is a better route than the EBGP route and appears in the command display.

In practice, if two BGP peers are connected to the same network, both peers should originate the route.

Example 2

Consider the network structure shown in Figure 39. Router Chicago originates prefix 192.168.11.0/24 and advertises it by means of EBGP to router Albany. Router Albany advertises the route to router Boston by means of IBGP.

Router Albany also redistributes the route into the interior gateway protocol RIP, which informs router NY of the route. Router NY propagates the route to router Boston by means of RIP, from which it is injected into BGP.

In this example, both router Albany and router Boston have synchronization turned on. When synchronization is on, BGP propagates a received route to EBGP peers, even if the IP forwarding table contains a non-BGP route with a better administrative distance than the BGP route. This example demonstrates why synchronization is needed.

Figure 39: Administrative Distance and Synchronization

Image g013194.gif

Router Boston does not advertise the route externally to router Philly. At first, this is because router Boston has not yet heard about the prefix from router NY, and therefore the IGP route does not appear in router Boston’s IP routing table.

BGP routes are not propagated until a route to the prefix by means of any IGP appears in the IP routing table. In other words, routers connected by means of an IGP must have a route to the prefix before a BGP speaker can advertise the route it learned from a peer.

When the RIP route appears on router Boston, the router has both an IBGP route and a RIP route to the same prefix. Even though the RIP route has a better administrative distance, the IBGP route is propagated to router Philly because synchronization is turned on.

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