Mobile ARP

Well seeing as how I just missed a 4-point task on mobile ARP, I thought now was a good time to learn it. It's actually very simple and pretty cool once you get it working.

Topology is a little confusing so here it is in 2 parts:

PHYSICAL:

R1---CAT1===dot1q===CAT2---R8

LOGICAL:

VLAN 100---R1---R2---R5---R7---R8---VLAN 200

The task says that users on R8's LAN occasionally mover over to R1's LAN. They still need access to the network. What we do is configure R1 to listen for ARP packets from R8's subnet (VLAN 200).

We can test this by creating an SVI for VLAN 100 and giving it an IP in R8's subnet. When it tries to contact anyone, we will see a mobile route appear in R1's route table. This route then gets redistributed into the routing protocol (OSPF in this case). It appears as a /32 route so the longer match wins over any other route advertisement of VLAN 200.

All configuration is on R1:

1) CREATE THE ACL

R1(config)#access-list 8 permit 172.31.80.0 0.0.0.255


2) CONFIGURE MOBILE ARP ON INTERFACE

R1(config)#interface f0/0
R1(config-if)# ip mobile arp access-group 8


3) REDISTRIBUTE

R1(config)#router ospf 1
R1(config-router)# redistribute mobile subnets


4) VERIFY

Ceate an SVI on VLAN 100 with VLAN 200 IP address:

CAT1(config)#int vlan 100
CAT1(config-if)#ip address 172.31.80.100 255.255.255.0
CAT1(config-if)#^Z
CAT1#ping 172.31.80.8

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.31.80.8, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/219/1016 ms

Run debug and show commands on R1:

R1#debug ip mobile
IP mobility events debugging is on
Nov 29 21:17:14.138: Local MobileIP: route add 172.31.80.100

R1#show ip route mobile
172.31.0.0/16 is variably subnetted, 11 subnets, 6 masks
M 172.31.80.100/32 [3/1] via 172.31.80.100, 00:11:05, FastEthernet0/1
R1#

Traceroute shows how many hops we are actually going through:

CAT1#trace 172.31.80.8

Type escape sequence to abort.
Tracing the route to 172.31.80.8

1 172.31.10.1 4 msec 4 msec 0 msec
2 172.31.12.2 0 msec 4 msec 0 msec
3 172.31.100.5 12 msec 8 msec 12 msec
4 172.31.200.7 12 msec 8 msec 12 msec
5 172.31.78.8 12 msec * 8 msec
CAT1#