Can a router be configured to forward ARP requests?

[s.debenito]

Hello all,

I have the following problem:

I am administering a campus network composed of just one building. Now we are using Layer 2 configured ports for all the interconnections between the Catalysts (in the distribution and core layers). Of course the topology has redundancy. This involves loops and hence a constant worry about Spanning-Tree protocol and its 40-50 seconds of downtime every time a topology change happens.

So, I want to stop using Layer 2 connections and begin using Layer 3 VLANs interfaces instead. The problem is that we have some VLANs expanded along 2 different Catalysts, so it is impossible that a PC connected to one of these Catalysts be able to communicate with another machine in the same VLAN but on the other Catalyst, since the ARP Requests will not pass through the router's VLANs interfaces.

The question is: Can I configure the Catalysts for them to allow forwarding ARP requests through their interfaceS?

Thanks in advance !!

[dmcdonald]

Not precisely, but you can accomplish something very similar.

The feature is called ip mobile arp, and is documented at http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/ipras_r/ip1_i2g.htm#wp1078546

In older versions of IOS, the interface with mobile arp applied needed to be in a bridge-group. I haven't tried it recently.

Once the mobile-arp entries are formed, you will need to redistribute the mobile routes in your routing protocol, so, for example, if you wanted machines with a 192.168.50.x address to be able to be found on vlan 70:

ip access-list standard roaming-hosts

permit 192.168.50.0 0.0.0.255

!

interface vlan 70

ip address 192.168.70.1 255.255.255.0

ip mobile arp access-group roaming-hosts

!

router eigrp 1

network 192.168.40.1 255.255.255.252

redistribute connected

redistribute mobile metric 100000 2 1 255 1500

---

A couple of other notes:

1. Using recent versions of IOS, you could run rapid per-vlan spanning tree and decrease your failover time from 45 seconds to about 4 seconds. The command, which must be done on every switch, is

spanning-tree mode rapid-pvst

2. For fastest convergence use "routed" interfaces rather than switched interfaces. i.e., instead of:

interface vlan 401

ip address 192.168.40.1 255.255.255.252

!

interface gigabitethernet 0/1

switch

switch trunk allow vlan 401

switch trunk encapsulation dot1q

do:

interface Gigabitethernet 0/1

ip address 192.168.40.1 255.255.255.252

ip route-cache flow

The advantage is that the failure detect time will go from about 10 seconds to sub-second, providing much faster convergence times.

3. ip mobile arp is a great tool for transistioning devices, but it is preferable to use separate routed address space for your long term solution. You should plan to migrate devices to different IP addresses as soon as possible.