Cisco Voice-VLAN (VVLAN) inconsistencies

 | 12 Nov 2012 12:41

First off I’d like to say that this is just a minor issue, more relating for routers versus switch, I’m still a lot happier at how Cisco implements config and features as opposed to most if not all of their competitors…

At a customer I’ve recently had to commit a grave operational sin; to connect a small switch at the end of a floor patch. These things are normally operational nightmares as they have a tendency to quickly bring an entire LAN environment down to its knees when such a ‘switch’ is connected to the network twice. Always by accident but having management kick you for something someone else did is not anyone’s idea of fun. I won’t go into the underlying principles here as I’m assuming most who frequent my blog will know about broadcast storms, their causes and the tools and solutions available to mitigate the risks.

Our justification to operations was that we wanted a few more local LAN ports to test VoIP devices on than we had available through floor patches. As such I reasoned with Operations that this was a calculated choice to segregate our testing from the rest of the LAN but still make it as realistic as possible. Using the means available meant that I had to make do with a Cisco 1801. Single routed and 8 switched interfaces. Think of it as a router with one Ethernet interface and an 8 port HWIC-ESW nailed to it. Didn’t need the ATM or WiFi it has.

So I set out, disabling IP routing, admin down all non-Ethernet ports. set up the vlan database -old style, remember?-; I did not want this baby to participate in VTP, in fact I don’t think it even can! It’s limited to 8 vlans. Pulled two cables to it. One switched port as trunked with some data and voice vlans and configured the routed interface for management access.

All sweet and dandy, tested the BPDU-guard functionality prior to installation by connecting an access-port to the LAN. Clunk! it went down as desired, result I thought… Then when installing the LAN wouldn’t bring up the LAN port. Doh! I’d missed that the 1801 doesn’t send BPDU’s until a VLAN becomes active. I’d checked if spanning-tree was operational, and it wasn’t until I brought an interface up. So I disabled STP for all vlans in the VLAN database. Now my laptop received an IP address and the data VLANs all worked.

So, time to connect a Mitel phone. No dice, it received it’s first DHCP response with VLAn information, then it would just sit ennuncing it was waiting for a DHCP response. Dang, I’d configured the voice vlan so why did the switch not detect the phone, enable trunking so that the phone could send it’s DHCP request on the voice VLAN?

It was only when I started reading up on HWIC-ESW voice-VLAN config I noticed that Cisco hasn’t implemented the auto enable of dot1q trunking when a phone is detected… The solution is to add two lines of code; “switchport truck native vlan xyz” and “switchport mode trunk”. The crux is that this platform is at heart a router, not a native switch…

Cisco documentation

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