STP Root Bridge Election – How is the Root Bridge Elected?

ethernetlayer2spanning tree

How does STP/RSTP determine which bridge is the root bridge, and how are paths to the root bridge selected?

Best Answer

Not sure how much you know about switching and spanning tree but basically when starting out all switches claim that they are the root. All switches send BPDUs (Bridge Protocol Data Unit) which contain a priority and the BID (Bridge ID).

The BID is 8 bytes long. 6 bytes is used for the MAC address of the bridge. 12 bits is used to indicate the VLAN, this is called extended system ID. 4 bits are used to set the priority. Lower priority means it is preferred compared to a higher. The priority is set in multiples of 4096.

If there is a tie in priority then the lowest MAC address will determine which bridge becomes the root.

To select the path to the root the cost to the root is calculated. As the BPDU travels from the root downstream the cost is increased INBOUND. 802.1D-1998 (legacy STP) had a cost of 19 for a FastEthernet interface. The newer standard 802.1D-2004 defines a cost of 200000 for FastEthernet.

If there is a tie in cost then choose the BPDU that came from the switch with lowest BID. If that is a tie as well (multiple links to same switch) port ID comes into play. The port ID is from the upstream switch as well and consists of a port priority and port ID which identifies the interface. The default port priority is 128.

There is a lot to spanning tree but these are the basic steps.

Related Solutions

How long does it take to Elect a Root Bridge in STP

I don't think it's possible to give a definitive answer on this because it will depend on the switch architecture, CPU power, the distance between the switches, the topology, the speed of the links and so on. I assume the delay would be something like this if we have three switches daisy chained:

+----------+           +----------+           +----------+
|          |           |          |           |          |
|   SW1    +-----------+  SW2     +-----------+  SW3     |
|          |           |          |           |          |
+----------+           +----------+           +----------+

Assume that SW1 has the best BID so that it should be elected root. Also assume that all switches have just been booted up, the way STP works all will claim to be root until they have heard a better BPDU. All links are Gigabit.

STP process runs and generates BPDU
BPDU must be serialized on to the link (serialization delay)
BPDU must be propagated between switches (propagation delay)
Act on BPDU received, change port roles
Send out BPDU on designated ports

All in all it should be in the millisecond range. It will be variable based on the speed and distance of links between switches and how fast they can process BPDUs. STP is fast to act on things where links go down, the issue is if you have unidirectional links or "passive" failures like a converter that won't give link down.

Cisco IOS – How to Determine the STP Designated Bridge

Found the answer:

show spanning-tree interface INT detail

This command goes deeper than the regular "show spanning-tree" and displays the designated bridge as well as root bridge.

Best Answer

Related Solutions

How long does it take to Elect a Root Bridge in STP

Cisco IOS – How to Determine the STP Designated Bridge

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