Tag Archives: mobility

IP Local Area Mobility & IP network re-addressing

Cisco LAM enables a host to move to another subnet but keep its original IP address, and still be reachable across the network.

A router/layer 3 switch configured for LAM determines whether there are directly connected hosts that do not belong to the local IP subnet.

When this router sees traffic from a host that does not match the configured subnet, the router installs an ARP entry for the mobile host. The router then also installs a host route that points toward this interface.

The feature could be useful during a LAN refresh or DC relocation, when migrating from old IP ranges to new, if a device is overlooked but LAM is configured, that device will still be reachable even though it will be in the wrong IP subnet. Then the routers ARP cache can be inspected to see what ARP bindings exist, and the missed device can be re-configured.

to configure LAM.

int vlan 20
ip address 192.168.1.1 255.255.25.0
ip mobile arp

to add security (only permit allowed/known mobile host ranges)

int vlan 20
ip address 192.168.1.1 255.255.25.0
ip mobile arp access-group 1

access-list 1 permit 172.16.255.0 0.0.0.255

When using LAM, a router periodically checks to ensure that the mobile host is still there by querying it with ARP requests. This ensures that the redistributed route is still valid. The mobile host ARP keepalive times can be altered with the command:

ip mobile arp timers [keepalive minutes] [hold-time minutes]

The LAM default arp keepalive is 300 seconds with 900 seconds hold timer.