Between 16.30 and 17.00 this afternoon, the University's logical network will be reconfigured. This change will temporarily affect access to services running on the backbone. This includes almost all servers (mail, dns, web, proxy, novell, etc) as well as access to the Internet. It is expected that for most people the interuption will last no more than a few minutes.

Users who switch on their computers during the time the reconfiguration happens may find that they can't get an IP address from the DHCP servers. This can be solved by rebooting the computer once work has finished. In the same way, users who open their web browser during this time will find that they can't access the proxy server -- the solution in this case is to close the browser and re-open it once work has finished.

For those interested in the technicalities of this change, the idea is to move backbone services off VLAN 1 and onto a new VLAN (128). The reason for this change is that VLAN 1 exists by default on most VLAN (802.1q) aware devices and, as a result, appears all over campus. It is not usually possible to remove VLAN 1. This has two undesirable affects when VLAN 1 is used to host backbone services. The first is that, on occasion, machines on remote parts of the campus find their way onto this VLAN and could, in theory, disrupt or attack services offered on the backbone. The second is that broadcast traffic on the backbone is potentially trunked all over the network, sometimes over low bandwidth links. This is an inefficient use of resources.

During the course of the last couple of days we've tried to ensure that, so far as possible, traffic on the backbone is contained to switches where it is needed. We've done this by removing VLAN 1 from almost all inter-switch trunks. The VLAN ID, however, still exists on all switches. This has potentially confusing consequences. Once the change we're about to undertake is complete, VLAN 1 (where it exists) should no longer carry any form of traffic whatsoever. Any traffic on the backbone will be contained by VLAN 128, and any traffic still appearing on VLAN 1 will be in error.