This lookup results in an IGP route that is a directly-connected route. As a result, the BGP4 route destination is now reachable through
IGP, which means the BGP4 route can be added to the IP route table. The IP route table with the BGP4 route is shown here.
device# show ip route 10.0.0.0/24
Total number of IP routes: 38
Network Address NetMask Gateway Port Cost Type
10.0.0.0 255.255.255.0 10.0.0.1 1/1 1 B
AS_PATH: 65001 4355 1
The device can use this route because it has an IP route to the next-hop gateway. Without recursive next-hop lookups, this route would
not be in the IP route table.
Changing administrative distances
BGP4 devices can learn about networks from various protocols, including the EBGP portion of BGP4, and IGPs such as OSPF and RIP,
the routes to a network may differ depending 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 use the administrative distances assigned
to the sources. The administrative distance is a protocol-independent metric that IP devices use to compare routes from different
sources.
The device re-advertises a learned best BGP4 route to neighbors even when the route table manager does not also select that route for
installation in the IP route table. The best BGP4 route is the BGP4 path that BGP4 selects based on comparison of the paths’ BGP4
route parameters.
When selecting a route from among different sources (BGP4, OSPF, RIP, static routes, and so on), the software compares the routes on
the basis of the administrative distance for each route. If the administrative distance of the paths is lower than the administrative distance
of paths from other sources (such as static IP routes, RIP, or OSPF), the BGP4 paths are installed in the IP route table.
The default administrative distances on the device are:
•
Directly connected - 0 (this value is not configurable)
•
Static - 1 is the default and applies to all static routes, including default routes. This can be assigned a different value.
•
EBGP - 20
•
OSPF - 110
•
RIP - 120
•
IBGP - 200
•
Local BGP4 - 200
•
Unknown - 255 (the device will not use this route)
Lower administrative distances are preferred over higher distances. For example, if the device receives routes for the same network from
OSPF and from RIP, the device will prefer the OSPF route by default. The administrative distances are configured in different places in
the software. The device re-advertises a learned best BGP4 route to neighbors by default, regardless of whether the administrative
distance for the route is lower than the administrative distances of other routes from different route sources to the same destination:
•
To change the EBGP, IBGP, and Local BGP4 default administrative distances, refer to the instructions in this section.
•
To change the default administrative distance for OSPF, RIP, refer to
Configuring a static BGP4 network
•
To change the administrative distance for static routes, refer to the instructions in this section.
To change the default administrative distances for EBGP, IBGP, and Local BGP4, enter a command such as the following.
device(config-bgp-router)# distance 200 200 200
Syntax:
[no] distance
external-distance
internal-distance
local-distance
Optional BGP4 configuration tasks
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