1 Reply Latest reply: Oct 31, 2010 11:56 AM by Keith Barker - CCIE RS/Security, CISSP RSS

    RSTP Topology Changes on Link Speed changes...

    James Elstone

      Hi All,

       

      Long time lurker, first time poster.  I have a quick question and wanted to ask the audience!

       

      I have attached a random network diagram of which I have been exploring RSTP with.

       

      Background:

      I created the diagram randomly and am manually calculating RSTP port states based on a priority change, and then a link failure, and then a link speed change to 100Mbps.  The link lines are annotated with "speed / short path cost" for my reference. I am assuming that there is a single vlan (1).  All links are full duplex point to point.

       

      The priorty of Switch ..a3 has been made lower than all other Switches.  The link between Switch ..a1 and ..a3 is down due to a network error (Layer 1 say..) and the link between Switch ..a2 and ..a1 has been changed to 100Mbps.  With Switch ..a8 as the historic root switch before the link speed change, is the port towards ..a1 from ..a8 (on a8), a designated port, or should this be blocking?  If so, would ..a1 become a root switch and the network be split in half?

       

      I am reasonable sure that the root port on ..a1 is correct, but am open to alternative answers!

       

      Thoughts on a postcard please!

       

      James.