STP port states, RSTP and Rapid PVST+

Classic STP port states

A port cannot jump straight from blocking to forwarding, because the switches need time to agree on the new tree. If a port forwarded too early, a temporary loop could form. So classic 802.1D STP moves the port through waiting states.

The timers

• Hello time, 2 seconds: how often the root sends BPDUs.
• Forward delay, 15 seconds: time spent in listening, and again in learning.
• Max age, 20 seconds: how long a switch keeps the last BPDU it heard before deciding the path is gone.

So when a direct link fails, a blocked port takes 15 + 15 = 30 seconds to start forwarding. When the failure is indirect (the switch just stops hearing BPDUs), it first waits max age (20 s), so recovery takes 20 + 15 + 15 = 50 seconds. A PC that boots on a normal access port also waits 30 seconds before its traffic flows, which can make DHCP time out. Thirty to fifty seconds felt fine in 1990; today a 50-second outage drops every phone call and payment in progress.

RSTP: Rapid Spanning Tree (802.1w)

RSTP keeps the same election (root bridge, root ports, designated ports, costs) but changes how ports move to forwarding. Instead of waiting for timers, neighbouring switches negotiate directly with a proposal and agreement handshake on point-to-point links. Convergence typically takes about a second or less. RSTP is backward compatible: if it hears an old 802.1D BPDU on a port, it falls back to classic behaviour on that port.

RSTP also names the roles of non-forwarding ports more precisely:

• Root port and designated port: as in classic STP.
• Alternate port: a discarding port with another path to the root. It is a ready-made replacement for the root port and can take over almost instantly.
• Backup port: a discarding port that backs up a designated port on the same shared segment. You only see it with a hub, so it is rare today.
• Edge port: a port to an end device (like a PC). It goes to forwarding at once. On Cisco this is what PortFast configures.

Cisco flavours: PVST+ and Rapid PVST+

The IEEE standards originally ran one tree for all VLANs. Cisco runs a separate tree for every VLAN: PVST+ (Per-VLAN Spanning Tree Plus, classic timers) and Rapid PVST+ (per-VLAN RSTP). One tree per VLAN lets you load-share: make SW1 the root for VLAN 10 and SW2 the root for VLAN 20, so each uplink is busy for some VLANs instead of one uplink sitting idle. A third option, MST (802.1s), maps many VLANs onto a few trees; it is for larger networks.

DSW1(config)# spanning-tree mode rapid-pvst
DSW1(config)# spanning-tree vlan 10 root primary
DSW1(config)# spanning-tree vlan 20 root secondary

DSW2(config)# spanning-tree mode rapid-pvst
DSW2(config)# spanning-tree vlan 20 priority 24576
DSW2(config)# spanning-tree vlan 10 priority 28672

root primary is a macro, not a permanent setting: when you type it, the switch sets its priority to 24576, or, if the current root already has a priority lower than that, to 4096 less than the current root. root secondary sets 28672. Setting the priority number yourself is clearer and does not change later.

Reading show spanning-tree

DSW2# show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol rstp
  Root ID    Priority    24586
             Address     0011.1111.1111
             Cost        4
             Port        1 (GigabitEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    28682  (priority 28672 sys-id-ext 10)
             Address     0022.2222.2222

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- -----------
Gi0/1               Root FWD 4         128.1    P2p
Gi0/2               Desg FWD 4         128.2    P2p
Gi0/3               Altn BLK 4         128.3    P2p
Gi0/5               Desg FWD 4         128.5    P2p Edge

Read it top down: who is the root (Root ID), how far away it is (Cost), which port leads there (Port), then this switch's own ID, then one line per port with its role (Root, Desg, Altn, Back) and state (FWD, BLK, LRN). On the root bridge itself you see "This bridge is the root".