VCX, 2924PWR, And VLANS

[NathanB_1]

We have a fairly large network, consisting of a 3Com 5500 Core, with edge buildings of 4500's and 2924's.



The phones we have deployed at the buildings with 4500 series switches work fine, with option 184 passing out VLANs specific to each building.



However any phones deployed to buildings with 2924's won't register, and get an invalid DHCP lease. The option 184 is done identically as in the buildings that work, and if the phone is manually set to the voice VLAN for that building, it works just fine.



A sniffer trace shows that it is getting the option 184 correctly, and that the lease it is being offered, is on the DATA VLAN, NOT the voice.



It seems that even though the phone is being told its suppose to be on the VOICE VLAN, it still requests its lease on the data VLAN, and therefore gets a response with a data VLAN address.



Again, all of these to the 3Com 4500 series switches in other buildings are not a problem at all.



Thoughts?

[NathanB_1]

I finally ended up finding the issue myself.



On the 2900 switches, spanning tree's 'portfast'feature is disabled on all ports by default.



The opposite is true on the 4500 and 5500 switches.



Portfast basically enables the port to start forwarding packets right away, instead of waiting for STP to converge.



The phones were requesting DHCP before they were allowed to transmit, they would finally get an offer back at the exact moment they would hit the time-out, and choose to move on to layer 2 discovery.

[calama]

We have a multi site network (5 distant sites) with a centralise VCX in HQ, hosted on IBM system i. This machine acts as a dhcp server for Data and voice vlan.

In the HQ the backbone is built around 3Com 5500-G Core, and edge switch PoE 3Com 5500-E. In all distant sites, the distribution is assumed by 3Com 4200 switch. All switch port’s (HQ and distant sites) are configured in mode (hybrid), two vlans are known in ports.

Distant sites are connected via a WAN managed by the operator.

Regarding the HQ:

There is a dhcp scope to data Vlan (192.168.0.0/24)

There is a dhcp scope to voice Vlan (172.18.0.0/24)



Each distant site has a specific subnet

1st site :

To Data Vlan (192.168.1.0/24)

To voice Vlan (172.18.1.0/24)



2nd site :

To Data Vlan (192.168.2.0/24)

To voice Vlan (172.18.2.0/24)



And so on…



Phones in the HQ can get their IP address from dhcp server. I have attempted to do the same thing on distant sites, to allowed phones to get dynamic IP address, to perform this, I have done the following actions :

• Enable dhcp relay on WAN routers (One-Access)

• Create a dhcp scope (172.18.1.0/24) to first site

• In the distant site, change the mode port in mode “access vlan 2†which is the voice Vlan.

• Plug the phone in this port and start a frame capture via Wireshark



The phone send a dhcp discover, then the relay agent and the dhcp server offer an IP address but the phone don’t take care, and it resend a 2nd dhcp discover, the relay agent and the dhcp server reoffer the same IP address but the phone displays the message “DHCP lease invalid†.

The option 184 is done identically as the scope of HQ.



NathanB, It seems that I meet the same problem, your edge switch was a 3Com 2924 (Base-Line) and my edge swith is a 3Com 4200, Maybe STP it cause the same problem no ?



Thanks in advance.

[Ed Holt]

Let me throw my hat in the ring too.

I have 25 locations 24 are fine the main site gives me the same problem. The branch sites have a mix of 3com switches some 4400 some 4500 edge some 5500 at core some 4950 at core all pass the dhcp correctly through the 8800 core sw and the phones work like a champ. Main site has 4500 connected to the 8800 and I get the DHCP lease invalid message.When I change the PVID of those ports to the voice vlan the phones work correctly but that is not the point they need to work the way it is supposed to so I don't have to config ports specificaly for voice.

3Com Certified IP Telephony VCX/NBX Expert