CCNP BCMSN Notes – Traditional Spanning Tree Protocol


is defined in IEEE 802.1D.


STP messages are carried by Bridge Protocol Data Unit (BPDU) frames; BPDUs are multicast to 01:80:c2:00:00:00.

BPDU types:

       Configuration – Used for spanning-tree computation

       Topology Change Notification (TCN) – Used to announce changes in the network

Configuration BPDUs are sent out every port every two seconds by default.

Root Bridge Election

A root bridge is elected to serve as a common reference point for the topology.

A switch’s bridge ID is composed of two parts:

       Bridge priority (2 bytes) – Administratively set; defaults to 32,768 (0×8000)

       MAC address (6 bytes) – One of the switch’s MAC addresses

All switches assume they are the root bridge at boot. The actual root bridge is the switch with the lowest bridge ID.

Configuration BPDUs are only generated by the root bridge; all other bridges insert their own sender ID and relay them.

Root Port Election

All non-root switches must designate a single interface as the root port (the port with the best path to the root bridge).

All interfaces are assigned an 8-bit cost derived from their speed.

Port costs:

      Bandwidth Cost

      4 Mbps 250

      10 Mbps 100

      16 Mbps 62

      45 Mbps 39

      100 Mbps 19

      155 Mbps 14

      622 Mbps 6

      1 Gbps 4

     10 Gbps 2

The port with the lowest path cost to the root bridge is designated as the root port.

The root path cost noted in a BPDU is incremented by the cost assigned to the port on which it was received.

Designated Port Selection

If multiple switches reside on a segment, the one with the lowest root path cost has the designated port; the other ports will be set to blocking.

Designated port selection process:

1. Lowest root bridge ID

2. Lowest root path cost

3. Lowest sender bridge ID

4. Lowest sender port ID

STP States

       Disabled – Shutdown

       Blocking – The first state when an interface comes up; only receives BPDUs; indefinite Duration

       Listening – Can send and receive BPDUs; able to participate in STP; duration specified by forward delay timer

       Learning – Can send and receive BPDUs and learn MAC addresses; duration specified by forward delay timer

       Forwarding – Normal operation; indefinite duration

STP Timers

       Hello Time – The rate at which configuration BPDUs are advertised by the root bridge (default is 2 seconds)

       Forward Delay – Length of time a port spends in both the listening and learning states(default is 15 seconds)

       Max Age – Life of the most recent BPDU advertised from the root bridge (default is 20 seconds)

Timers can be individually adjusted manually on the root bridge, or automatically adjusted by modifying the network diameter (number of switch hops which extend from the root).

STP Types

       Common Spanning Tree (CST) – Defined in 802.1Q; one tree for all VLANs

       Per-VLAN Spanning Tree (PVST) – Cisco proprietary; one tree per VLAN

       Per-VLAN Spanning Tree Plus (PVST+) – PVST featuring compatibility with CST BPDUs

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