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Community Member

3750 arp problem

We have issues with a 3750 WS-C3750-24PS-and connectivity to a Siemens Optipoint 420 IP phone. Switch is not seeing/recognising ARP packets from the IP phone. We connected the IP phone and a PC to a hub and then to switchport, both PC and IP phone can ping each other and PC can ping through the switch though IP phone cant. We have an ethereal capture from PC on the hub and can see ARP requests from the IP phone but we do not see these when running debug arp on the switch.

The IP phone works fine when connected to a 2950 or 3Com switch (all switchports involved are in the same Vlan).

Any suggestions to why the 3750 is not recognising the IP phone ARP packets.

3 REPLIES
Bronze

Re: 3750 arp problem

Hello,

Firstly, can you see the IP phone's mac address in the 3750's table, and the PC's?

Secondly, I would do a "show interfaces gigx/y trunk" and see what type of encapsulation is being negotiated.

I would be starting to look at trunking as the problem first-off.

Hope this helps,

Michael.

Community Member

Re: 3750 arp problem

Thanks for the reply Michael,

1. We can see the IP Phone's in the PC's arp table but not in the 3750's arp table.

2. We tried it with trunking disabled and set to auto (neither worked). Also the set-up works fine on an access port on a 2950 switch.

3750#sh int fa 1/0/4 trunk

Port Mode Encapsulation Status Native vlan

Fa1/0/4 off negotiate not-trunking 1

Port Vlans allowed on trunk

Fa1/0/4 1

Port Vlans allowed and active in management domain

Fa1/0/4 1

Port Vlans in spanning tree forwarding state and not pruned

Fa1/0/4 1

3750#sh int fa1/0/4 trunk

Port Mode Encapsulation Status Native vlan

Fa1/0/4 auto negotiate not-trunking 1

Port Vlans allowed on trunk

Fa1/0/4 1

Port Vlans allowed and active in management domain

Fa1/0/4 1

Port Vlans in spanning tree forwarding state and not pruned

Fa1/0/4 1

Regards

Phil.

Community Member

Re: 3750 arp problem

Update - Now working

Michael,

We now have it working root cause was QOS settings on the Siemens Optipoint 420 IP Phone.

In summary the IP Phone has QOS settings for Layer2 and Layer3 (which were defaulted to both enabled). However any time L2 is enabled it wouldn't work (there is some reference to L2 in that when it is disabled it forces Vlan discovery mode to Manual with default ID of 0). Looks like when L2 QOS is enabled it is trying to discover the Vlan dynamically and having problems.

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