Monday, February 16, 2015

Default STP(Spanning-Tree Protocol) port costs

Rehashing STP at the moment for CCIE studies and thought I'd make a quick note about the default STP port costs and how they've evolved with link speed increases over time.

Things to note with the table below:

  • The default costs used by Catalyst switches correspond to the 802.1D-1998 version of the standard if PVST(Per-Vlan Spanning-Tree) or Rapid-PVST(Rapid Per-Vlan Spanning-Tree) are used.
  • If MSTP (802.1s) is used then Catalyst switches use the default costs that correspond to the 802.1D-2004 version.
  • Just to make things a little more confusing PVST and Rapid-PVST can be forced into using 802.1D-2004 default port costs via the 'spanning-tree pathcost method long' global configuration command. By default PVST and Rapid-PVST use 'spanning-tree pathcost method short' which causes the switch to us the older revision of the costs(802.1D-1998).

    Juniper by default uses the 802.1D-2004 default port costs when either RSTP or MSTP is enabled on its switching equipment.

    Port Speed Pre-802.1D-1998 Cost 802.1D-1998 Cost* 802.1D-2004 Cost**
    10Mbps 100 100 2000000
    100Mbps 10 19 200000
    1000Mbps(1Gbps) 1 4 20000
    10000Mbps(10Gbps) 1 2 2000
    * Cisco Catalyst default for PVST and Rapid-PVST
    ** Juniper default for RSTP\MSTP & Cisco Catalyst default for MSTP

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