3Com Switch on Cisco infrastructure??

Unanswered Question
Apr 22nd, 2008

Hello guys,

I am replacing a whole batch of 3Com switches with Cisco 3750s. However, I would like to continue using the existing 3Com phone system as my budget does not allow for a migration to Cisco IPT at this point.

The current telephony solution is a 3Com NBX 5000 IP PBX. The phone sets are 3Com 3102 Business Phones.

According to the limited documentation I have found, the 3Com phone allows PC connections directly to the phone much like the typical Cisco phone. However, I am not sure if it supports multiple VLANs (one voice, one data) and how to configure the connection even if it does?

The phone is a Class2 PoE device so I am confident there will be no issues with power.

Has anyone come across a deployment like this before?

Thanks in advance,

Ade

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legerity1_2 Wed, 04/23/2008 - 13:05

Ade,

3com phones do support multiple VLANs with some caveats - the NBX needs to be configured to use vlans and tags and *remote connectivity breaks*.

Here's an example "working" interface config:

interface fa1/0/5

switchport access vlan X

switchport mode access

switchport nonegotiate

switchport voice vlan Y

spanning-tree portfast

switchport mode access and switchport nonegotiate will keep DTP off the wire and the voice vlan lets the port recognize tagged packets from the 3com phones. Make sure your NBX is in the voice VLAN and you should be good for internal communication within a common subnet at layer 2.

That was the good news, now for the not so good news. The 3102 phones do some interesting things when it comes to which VLAN is used for what protocols. For example, the 3com phone is unable to ARP in the voice vlan - it always ARPs in the access vlan, causing issues with remote phone conversations (basically when it needs the first hop out of the voice VLAN to reach a remote subnet, it ARPs in the access VLAN, breaking communication). I spoke at length with 3com some time back, but there was no fix for this before. The only solution was to place the phone's access and voice vlan into the same vlan or not have remote phones - suboptimal to say the least. In this config, you're back to one port for the phone, another for the PC.

I would definitely do some testing preproduction - this is on the 3750 platform - not sure if there are any issues with any other Cisco chassis and 3com code revision - we're using 6.0.44

HTH

Geoff

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