Switching - Introducing Spanning Tree Protocol (STP)

A loop avoidance mechanism called STP has been developed to alleviate the 3 issues above.

There are two varieties of Spanning Tree Protcol, STP (802.1d) and RSTP ('Rapid' 802.1w) both work in a similar way.

A Spanning Tree Algorithm examines the switched network, and, through a series of decisions (discussed below) places each port in the loop into either Forwarding or Blocking state, therefore breaking the loop.

Types of port in STP:

1. Designated Port (DP) Closest to Root i.e. All Ports on the Root Switch.
2. Route Port (RP) One Per Switch.
3. Blocking Port.

The process the switches go through to decide which port they place in blocking or forwarding state is governed by the position of a 'Root Switch' (also known as 'Root Bridge'):

• To decide which switch is the root switch, each switch begins by claiming to be the Root Switch they all send out an STP messages called BPDU's (Bridge Protocol Data Unit). A fight-off happens to determine the final root based on a unique BID (Bridge Identifier) contained within the BPDU, the BID is made up of two components; a priority value and the switch MAC address. By default each switch begins off with a priority of 32,768. The switch with the lowest BID wins the honor of being the Root Bridge.
• All ports on the root switch are placed into forwarding state. These ports are known as 'Designated Ports' (DP). You can NOT have a Root Port or Blocking Port on the root switch.

• All the remaining non-root switches determine which port is closest to the root switch and places the port with the least cost (based on hops and/or bandwidth) into forwarding state. These are known as 'Root Ports' (RP).
• There can only ever be one RP per segment, and so if there are more than one paths with the same cost the decision is made based on the BID of the neighboring switches, the port connecting to the switch with the lowest BID becomes a RP.
• All ports are placed in blocking state and the loops stopped.

Once the switches have all assigned ports then the switched network is said to be "Converged".

As discussed above the BID is made up of:

• A Priority Value
• MAC Address

The Priority Value of the BID can be manually changed by an administrator, this is useful of you want to force a switch in the middle of a network to be a Root Switch.

note: If a newer switch is plugged into a working network of a different manufacturer it may disrupt the network if manual BID priorities are not set. A Cisco Switch (MAC Vendor ID 00000C) will win over a 3COM switch (MAC Vendor ID 001AFC) because Cisco equipment has a lower MAC address.

STP is switched on in all Cisco switches by default, if you have no loops for example:

You could switch STP off on all the switches, but this is not advisable, as all it would take is someone to plug a cable in the wrong port to cause a loop.

note: If two links between two switches, the decision of which port to block can no longer be based on the BID because they would be the same for both links, the selection is therefore made on the lowest port number.

During the STP selection process ports can be in one of the following statuses:

• Disabled
• Blocked
• Listening *
• Learning *
• Forwarding

* Ports in Listening & Learning state can take upto 30 seconds, during which no traffic will pass through the port. This may disrupt services such as DHCP and Network Neighborhood. Switching on 'portfast' by using the "spanning-tree portfast" command will skip the Listening and Learning stage. Do not set portfast on inter switch ports or branch ports, however you could set portfast on a leaf port (ports connected to hosts).