Changes in the core

On Tuesday 2nd March we made two significant changes in the Core. Both are inline with current Cisco best practice are have been implemented as part of our Backbone ‘feature update’ project.


We have moved to using VLAN Trunking Protocol (VTP) In the core. This allows us to maintain a central VLAN database and simplifies VLAN provisioning. Further, we are now able to name each VLAN which will aid troubleshooting and design. The two ‘B’ core switches, BOUCS and BMUS are now VTP servers, with the remaining ‘C’ switch/routers acting as clients.

At the time of writing the FroDo network is in VTP transparent mode (effectively the feature is switched off) and in a different domain. Over the coming weeks we will move the FroDos into the same VTP domain as the core.

Oxford Core VTP Logical

Oxford Core VTP Logical


We have migrated the Core from IEEE 802.1D STP protocol to Rapid Spanning Tree Protocol plus (RSTP+). The main advantage is that convergence will take approximately 5 seconds, as opposed to the circa 50 seconds taken by 802.1D.

RSTP+ is fully backwards compatible with 802.1D; for the time being we are still running 802.1D on the FroDo network.

There has been an unexpected side effect which is that some sites not running any form of STP are experiencing intermittent packet loss. Please note that although you may not have any redundant links, the Core does, so it is important to enable the feature.

Jeremy Stretch has written an excellent blog on the subject: here. I highly recommend subscribing to this blog if you use Cisco equipment.

Over the coming weeks we will migrate the FroDo network to RSTP+. Each migration may cause an outage of circa 5 seconds to the connected Units. Therefore the migrations will take place during the JANET at risk period of 07:00 – 09:00 on a Tuesday morning.

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