IP Proxy=arp

[cmelbourne]

Hi,

if i have a vlan (say vlan 2) that has the following setup.

interface vlan 2

ip address 10.1.1.254 255.255.255.0

ip address 10.2.1.254 255.255.255.0 secondary

ip address 10.3.1.254 255.255.255.0 secondary

ip proxy-arp

!

now, i have a number of devices in vlan 2 with the following

hosta IP ADD 10.1.1.1 mask 255.0.0.0

hostb IP ADD 10.2.1.1 mask 255.0.0.0

hostC IP ADD 10.3.1.1 mask 255.0.0.0

i have been told the masks CANNOT change for some reason.

they all need to communicate with each other which is fine and this works no problem, they now need to communicate with devices in vlan 3

so see below vlan 30 config

interface vlan 30

ip address 10.30.1.254 255.255.255.0

host in vlan 30 is say 10.30.1.1 mask 255.255.255.0 and g/w of 10.3.1.254

so i enabled proxy-arp  on vlan 2 so that the router (msfc) will arp this and this works so host A,b,c in vlan 2 can ping host A in vlan 30. this all works perfect. EXCEPT, for no reason i know have every so often loss of communication between HOSTA in vlan 2 and HOSTB in vlan2, it comes and goes.

any ideas?

[Richard Burts]

From your reference to msfc would I be correct in understanding that this is a layer 3 switch like a 6500? Are host A and host B in vlan 2 connected on ports of this switch, or are they connected to some other switch which is connected to this switch?

If the subnet mask on both hosts is 255.0.0.0 then both hosts consider themselves in the same network/subnet. This means that they will arp for each other and should communicate directly with each other. They really do not need the switch vlan interface. And this problem is not related to proxy arp.

At the times when host A can not communicate with host B, can A and B both communicate with host C, and can host C communicate with both A and B? This would be a very good test to perform if you have not done so yet.

I can think of 2 situations which could produce the symptoms that you describe:

- host A or host B has temporarily become un-available. This might be particularly possible if one or both hosts are connected to a different switch which connects to the main switch. The test of communicating with host C is a good way to determine if this is the case.

- host A or host B might temporarily not have the MAC address of the other host in their arp table. A good test for this would be to examine the output of arp -a on both hosts and verify whether the MAC of the other host is in the arp table.

HTH

Rick