& Switching v5 Workbook -
Advanced Technology Labs - IP
CCIE Routing
CCIE R&S v5
Routing
Longest Match Routing
A Note On Section Initial Configuration Files: You must load the
initial configuration files for the section, named Basic IP Addressing,
which can be found in
. Reference the Advanced Technology Labs With
Addressing Diagram to complete this task.
Task
Configure R4 and R5 with IPv4 static routes to each other’s Loopback0 prefixes via
the Ethernet segment between them.
Configure R4 and R5 with IPv4 static routes for 150.1.0.0/16 prefix via the DMVPN
cloud.
Ensure that traffic between R4’s and R5’s Loopback0 prefixes is primarily routed over
the Ethernet segment, and DMVPN cloud is used only if Ethernet link is DOWN.
Configuration
R4:
ip route 150.1.5.5 255.255.255.255 GigabitEthernet1.45
ip route 150.1.0.0 255.255.0.0 155.1.0.5
R5:
ip route 150.1.4.4 255.255.255.255 GigabitEthernet1.45
ip route 150.1.0.0 255.255.0.0 155.1.0.4
Verification
IPv4 routing logic uses longest match routing to determine which entry to use from
the routing table for forwarding. This principle can be used to achieve both
redundancy and traffic engineering, as is the case for traffic routed between
Loopback0 prefixes of R4 and R5. When R5 performs a lookup on the final
destination 150.1.4.4, the longest match is 150.1.4.4/32 out GigabitEthernet1.45.
Although technically there are multiple routes to this destination, 150.1.4.4/32 and
150.1.0.0/16, the /32 route wins. For any other destination from 150.1.0.0/16 range,
such as 150.1.3.3, the longest match will be 150.1.0.0/16 via 155.1.0.4. This allows
traffic for a portion of the IP address space to be routed one way, and traffic for a
different portion of the IP address space to be routed another way:
R5#show ip route 150.1.4.4
Routing entry for150.1.4.4/32
Known via "static", distance 1, metric 0 (connected)
Routing Descriptor Blocks: * directly connected,via GigabitEthernet1.45
Route metric is 0, traffic share count is 1
!R5#traceroute 150.1.4.4
Type escape sequence to abort.
Tracing the route to 150.1.4.4
VRF info: (vrf in name/id, vrf out name/id)1 155.1.45.4 3 msec * 2 msec
Redundancy is accomplished in this example when the Ethernet link between R4
and R5 fails. As long as this link is UP and associated IPv4 static routes can be
installed in the routing table, the 150.1.4.4/32 route will be installed in the routing
table. However, if the GigabitEthernet1.45 link is in the UP/DOWN or DOWN/DOWN
states, it cannot be installed in the routing table; the same goes for any routes that
recurse to GigabitEthernet1.45. The result is that when the link is down, traffic
between Loopbacks of R4 and R5 is rerouted out the DMVPN cloud using the
150.1.0.0/16 prefix as the longest match.
R5#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.R5(config)#interface GigabitEthernet1.45
R5(config-if)#shutdown
!R5#show ip route 150.1.4.4
Routing entry for150.1.0.0/16
Known via "static", distance 1, metric 0
Routing Descriptor Blocks: *155.1.0.4
Route metric is 0, traffic share count is 1
!R5#traceroute 150.1.4.4
Type escape sequence to abort.
Tracing the route to 150.1.4.4
VRF info: (vrf in name/id, vrf out name/id)1 155.1.0.4 5 msec * 5 msec