STP/RSTP/MST Comparison

STP State Comparison.

IEEE 802.1D STP states:

  • Disabled > Blocking > Listening > Learning > Forwarding.

IEEE 802.1W RSTP states:

  • Discarding > Learning > Forwarding.

With RSTP, the 802.1D states disabled, blocking and listening are combined into the RSTP initial state of Discarding.

General RSTP notes:

Root Ports as per normal STP and Alternate ports.  Backup (BCK) ports exist as backups to Alternate ports but only on shared media!

  • RSTP point-to-point port:  any port running in full-duplex mode.
  • RSTP edge port: a port on the edge of the network connected to a single host e.g. PC. RSTP does not consider a topology change to have taken place when an edge port is involved (doesn’t generate TCN BPDU).

When a non-edge port moves to forwarding mode, RSTP sents  TCN BPDU’s out of all non-edge designated ports. Switches receiving TCN BPDU’s clear all entries from their MAC address tables except the port the TCN arrived on.  The TCN is then flooded out of the designated ports until the change reaches all switches in the RSTP domain.

BPDU Generation:

  • 802.1D – the root bridge generates BPDU’s every two seconds that downstream switches read and relay on.
  • 802.1W – all switches generate BDPU’s every two seconds.  This allows all switches in the network to contribute to link failure detection.  If a switch doesn’t see three BPDU’s from its neighbor, the link is considered down and all information concerning the port that should have received the BPDU is aged out immediately.

SW4#show spanning-tree vlan 1

Spanning tree enabled protocol rstp
Root ID    Priority    24577
Address     5000.0001.0000
Cost        8
Port        2 (GigabitEthernet0/1)
Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
Address     5000.0004.0000
Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
——————- —- — ——— ——– ——————————–
Gi0/0               Desg FWD 4         128.1    P2p Peer(STP)  <— 802.1D neighbor
Gi0/1               Root FWD 4         128.2    P2p
Gi0/2               Desg FWD 4         128.3    P2p
Gi0/3               Desg FWD 4         128.4    P2p

Multiple Spanning Tree (MST)

  • IEEE 802.1S standard.
  • Maps multiple VLAN’s to a single instance of STP.
  • Reduced overhead compared to PVST.
  • Multiple instances of MST permitted to bind “groups” of  VLAN’s to different spanning tree instances which in turn allows a form of VLAN load balancing across switches where different root bridges can be specified for the different “groups”.
  • Switches in an MST region must agress on the MST configuration name, the instance/VLAN mapping able and the MST revision number. Switches that do not match are considered to be in different regions.
  • MST BPDU’s contain the rev number, the config name and a digest value of the mapping table.
  • Upto 16 MST instances can exist within a region.
  • IST (Internal Spanning Tree) keeps communications inside MST regions loop-free.










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