Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 419)

3 10.40.0.0/24 10.1.0.2 0 100 0 BI
AS_PATH: 65001 4355 701 1 189
4 10.0.0.0/24 10.0.0.1 1 100 0 BI
AS_PATH: 65001 4355 3356 7170 1455
5 10.25.0.0/24 10.157.24.1 1 100 0 I
AS_PATH: 65001 4355 701
The first lookup results in an IBGP route, to network 10.0.0.0/24.
device# show ip route 10.0.0.1
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
Since the route to 10.0.0.1/24 is not an IGP route, the device cannot reach the next hop through IP, and
so cannot use the BGP4 route. In this case, since recursive next-hop lookups are enabled, the device
next performs a lookup for the next-hop gateway to 10.0.0.1’s next-hop gateway, 10.0.0.1.
device# show ip bgp route 10.0.0.0
Number of BGP Routes matching display condition : 1
Status A:AGGREGATE B:BEST b:NOT-INSTALLED-BEST C:CONFED_EBGP D:DAMPED
E:EBGP H:HISTORY I:IBGP L:LOCAL M:MULTIPATH m:NOT-INSTALLED-MULTIPATH
S:SUPPRESSED F:FILTERED s:STALE
Prefix Next Hop Metric LocPrf Weight Status
1 10.0.0.0/24 10.0.0.1 1 100 0 BI
AS_PATH: 65001 4355 1
The next-hop IP address for 10.0.0.1 is not an IGP route, which means the BGP4 route destination still
cannot be reached through IP. The recursive next-hop lookup feature performs a lookup on the next-
hop gateway for 10.0.0.1
device# show ip route 10.0.0.1
Total number of IP routes: 38
Network Address NetMask Gateway Port Cost Type
10.0.0.0 255.255.255.0 0.0.0.0 1/1 1 D
AS_PATH: 65001 4355 1 1
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.
Changing administrative distances
FastIron Ethernet Switch Layer 3 Routing Configuration Guide 419
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