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New Member

CSM client side VLAN without a gateway?

Hi there,

We are running in bridge mode, and are having some weird arp table issues. I think I have it traced down to the fact that the CSM is arping for addresses, and the replies are getting to the CSM and getting cached, but the MSFC is never seeing them.

Would behavior like this happen if there is no gateway configured on the client side VLAN? Is a gateway on the client side VLAN a requirement?


Cisco Employee

Re: CSM client side VLAN without a gateway?

a gateway is not required.

If the CSM is sending the arp requests, there is no reason for any other device, including the MSFC, to see or do anything with the arp response.

If the MSFC is missing an arp entry, the MSFC should send an arp request.

You might be talking about the CAM/MAC-Address table.

This should get populated with any traffic. Not just arp.

So, could you please describe the issue you are having so we can tell you what information you need to capture to troubleshoot the issue.


Happy new Year.

New Member

Re: CSM client side VLAN without a gateway?

Let's see if I can explain this coherently, sorry if I don't...


What we're seeing is that a machine with multiple IP addresses tied to one NIC can only be reached via one of those IP addresses from a different VLAN. I look on the MSFC arp table, and I only see an entry with a MAC for that one IP address, none of the others. If I add a static ARP entry, I can then reach the other IP addresses from the other VLANs. So communication is possible, the ARP table is just not getting populated automatically.

-HOST A in VLAN A is pointing at the MSFC for it's gateway.

-HOST B in VLAN B is pointing at the MSFC for it's gateway.

-The CSM is in bridge mode. VLAN C is the client side VLAN. VLAN B is the server side VLAN.

-HOST A is trying ping HOST B. HOST A can ping HOST B on it's "main" IP address, but none of the others.

-The ARP table on the MSFC has an entry for the "main" IP address on HOST B, but no entries for any others.

-The ARP table on the CSM does have entries for the "extra" IP addresses on HOST B.

-A static ARP entry for an "extra" IP address on HOST B solves the problem. HOST A can then ping HOST B's "extra" IP address.

My thoughts:

The ARP table on the MSFC is not getting populated automatically from the CSM. As I see it, this is because HOST B is in VLAN B, which only has an interface on the CSM. The arp replies are going to the CSM successfully, but aren't getting to the MSFC because there is no gateway or route defined for VLAN B on the CSM.

The reason that anything at all works is that the hosts in VLAN B are initiating communication outbound to their gateway on the MSFC, so it's getting their MAC addresses that way. When a machine has multiple IP addresses, and it doesn't use them to communicate outbound, the MSFC doesn't learn the MAC for those addresses because the ARP replies are going to the CSM which isn't sharing.

Hopefully that makes sense, and it also makes sense why I'm thinking it's the lack of a gateway entry. Thanks for your help.

Cisco Employee

Re: CSM client side VLAN without a gateway?

if the MSFC does not have an arp entry for a particular ip address, it should broadcast an arp request on the vlan B. The CSM should get it and forward it to vlan C where your host should reside.

The host should then respond unicast to the MSFC going through the CSM.

There is no reason for the CSM to block any of these packets.

I would strongly suggest to capture a sniffer trace of the CSM etherchannel.

If you give my the CSM and MSFC config with the ip addresses being used on your hosts, I could maybe find something wrong.

Anyway this should work.


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