Friday, February 20, 2015

RSTP vs STP port States

The goal of RSTP is to improve STP convergence. The first step in achieving this was to deprecate some of the 802.1D port states.

Below is a table showing the three port states for RSTP compared to the five ports states of 802.1D STP.

Administrative State STP State (802.1D) RSTP State (802.1w)
Disabled Disabled Discarding
Enabled Blocking Discarding
Enabled Listening Discarding
Enabled Learning Learning
Enabled Forwarding Forwarding

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