Uplinkfast and RSTP

I have a question on uplinkfast's migration into RSTP.  In RSTP, when a switch's root port fails and the ALTN port gets used, does the switch still generate a dummy multicast frame that is sent upstream to the other switches so that they can learn where the MAC addresses reside after flushing their CAM tables out?  I'm hoping it does, otherwise it would seem like a step backwoulds from using uplinkfast in terms of reducing unknown flooding.

Comments

  • Per CCIE R&S v5 SG...

    “A switch that detects a topology change on a port (that is, one of its own Non-Edge ports transitions into the Forwarding state) or learns about a topology change on a port (a BPDU with the TC flag set is received on its Root or Designated Port) will do the following:

    - Set a so-called tcWhile timer to the value of the Hello time plus one second (older revisions of RSTP set this value to twice the Hello time) on all remaining Non-Edge Designated ports and Root Port if any, except the port on which the topology change was detected or learned.

    - Immediately flush all MAC addresses learned on these ports.

    - Send BPDUs with the TC flag set on these ports every Hello seconds until the tcWhile timer expires.

     

    This way, information about a topology change is rapidly flooded along the spanning tree in the form of BPDUs with the TC flag set, and causes switches to immediately flush their CAM tables for all ports except those ports on which the topology change was detected or learned, as they point in the direction of the topology change where a set of MAC addresses might have become reachable “through a new or improved path.Edge ports never cause a topology change event, and MAC addresses learned on them are not flushed during topology change event handling.”

     

    HTH

  • You've missed the point of my question entirely.  I know the things you mentioned. I'm trying to see if there is a way the new mac's are learned without unknown flooding?

  • I have a question on uplinkfast's migration into RSTP.  In RSTP, when a switch's root port fails and the ALTN port gets used, does the switch still generate a dummy multicast frame that is sent upstream to the other switches so that they can learn where the MAC addresses reside after flushing their CAM tables out?  I'm hoping it does, otherwise it would seem like a step backwoulds from using uplinkfast in terms of reducing unknown flooding.

    In RSTP i don't think it does - remember RSTP is standard based, whereas uplink fast was a Cisco proprietary optimisation.

    in a RSTP a environment the CAM table is flushed for affected VLANs which will cause unknown unicast flooding.

  • Per Cisco's documentation,

    "The 802.1w topology change mechanism clears the appropriate entries in the Content Addressable Memory (CAM) tables of the upstream bridge. This removes the need for the dummy multicast generation process of UplinkFast." 

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