A Tek-Tips forum member recently reported that one of his technicians improperly cabled a Nortel i2002/i2004 Internet Telephone (plugging both the ports on the back of the Internet Telephone into the network switch) causing a loop which took down their entire network. The member was curious about how to configure Spanning Tree to help prevent this problem. I went digging and found the following information in the current phone firmware release notes;
Network Loop (Applies to IP Phone 2002, 2004, 2007, 1120E, 1140E)
These firmware releases include a fix to help prevent network loop scenarios from being introduced into the network, and the resultant network outages that can occur. The network loop avoidance fix was first introduced in 0604D9H, 0621C2B, 0624C1E and 0625C1E. One important note when upgrading to 0604DBN, 0621C4T, 0624C4L or 0625C4L from any load previous to 0604D9H, 0621C2B, 0624C1E or 0625C1E respectively, is that IP Phones that were inadvertently mis-wired during initial installation will not be allowed to work until the cabling problem is corrected. This fix is only an issue if the installer, when installing the Nortel IP Phone 2002, 2004, 2007, 1120E or 1140E, inadvertently connected the network Ethernet cable to the PC Ethernet port on the back of the phone, instead of connecting it to the network Ethernet port on the back of the phone. Phase II IP Phones (2002 and 2004) running firmware previous to 0604D9H, IP Phones 2007 running firmware previous to 0621C2B and IP Phone 1120E and 1140E running firmware previous to 0624C1C and 0625C1C respectively will work when incorrectly connected, but this does introduce the potential for network degradation. These new firmware loads will try and safe guard the network by trying to prevent phones that are mis-cabled to function. This means that the IP Phones that are working on a previous release of firmware may stop working if they are not correctly wired.
But realize that a mis-cabled phone may still work, even with the new firmware, if the network infrastructure supports Auto MDIX. If the network infrastructure supports Auto MDIX, network loop can still occur if the network is not running the Spanning Tree Protocol (STP) or a similar loop avoidance protocol.
As a preventative measure to reduce the potential for network degradation, and to prevent mis-cabled phones from ceasing to work when their firmware is upgraded, please consider taking the necessary steps to ensure your Nortel IP phones network cables are plugged into the correct ports on the back of the phone – network cable into the network Ethernet port, and the PC Ethernet cable (if connecting a PC) to the PC Ethernet port (little computer icon) on the back of the phone.
I’ve highlight a very important caveat above in RED. While this was and is a great feature of the new phone firmware the important piece to realize here is that if the network switch supports Auto MDIX, which the Nortel Ethernet Routing Switch 5520 and Ethernet Switch 470 PWR do you can’t rely on this feature alone to protect your network.
Ever since the release of the Nortel Ethernet Switch 470 we now configure Spanning Tree on every port with the exception of the core MLT/SMLT uplinks. Prior to the availability of the “Auto MDIX” feature a technician would need a crossover cable to physically put a loop between two switch ports. We made sure there were never any crossover cables left lying around. With the arrival of the “Auto MDIX” feature technicians could now put a loop in the switch with a standard straight-thru cable, which happened on a number of occasions. In order to prevent this problem we reconfigured every closet to run Spanning Tree locally on that switch. We would not run it on the uplinks but we would run it on all other ports in the switch/stack.
Here are some of the commands to enable Spanning Tree with Fast Learning on ports 1-46 of an ERS 5520 switch;