General Networking Concepts:-Section 5

                                                       Spanning Tree Protocol

Switching Loops:

A Layer-2 switch belongs to only one broadcast domain, and will forward both broadcasts and multicasts out every port but the originating port.

When a switching loop is introduced into the network, a destructive broadcast storm will develop within seconds. A storm occurs when broadcasts are endlessly forwarded through the loop. Eventually, the storm will choke off all other network traffic.

Consider the following example:

If HostA sends out a broadcast, SwitchD will forward the broadcast out all ports in the same VLAN, including the trunk ports connecting to SwitchB and SwitchE. In turn, those two switches will forward that broadcast out all ports, including the trunks to the neighboring SwitchA and SwitchC.

The broadcast will loop around the switches infinitely. In fact, there will be two separate broadcast storms cycling in opposite directions through the switching loop. Only powering off the switches or physically removing the loop will stop the storm.

Spanning Tree Protocol (STP)

Spanning Tree Protocol (STP) was developed to prevent the broadcast storms caused by switching loops. STP was originally defined in IEEE 802.1D.

Switches running STP will build a map or topology of the entire switching network. STP will identify if there are any loops, and then disable or block as many ports as necessary to eliminate all loops in the topology.

A blocked port can be reactivated if another port goes down. This allows STP to maintain redundancy and fault-tolerance.

However, because ports are blocked to eliminate loops, STP does not support load balancing unless an EtherChannel is used. EtherChannel is covered in great detail in another guide.

STP switches exchange Bridge Protocol Data Units (BPDU’s) to build the topology database. BPDU’s are forwarded out all ports every two seconds, to a dedicated MAC multicast address of 0180.c200.0000.

Building the STP topology is a multistep convergence process:

·        A Root Bridge is elected

·        Root ports are identified

·        Designated ports are identified

·        Ports are placed in a blocking state as required, to eliminate loops

The Root Bridge serves as the central reference point for the STP topology. STP was originally developed when Layer-2 bridges were still prevalent, and thus the term Root Bridge is still used for nostalgic reasons. It is also acceptable to use the term Root Switch, though this is less common.

Once the full topology is determined, and loops are eliminated, the switches are considered converged.

STP is enabled by default on all Cisco switches, for all VLANs.

Electing an STP Root Bridge

The first step in the STP convergence process is electing a Root Bridge, which is the central reference point for the STP topology. As a best practice, the Root Bridge should be the most centralized switch in the STP topology.

A Root Bridge is elected based on its Bridge ID, comprised of two components in the original 802.1D standard:

·        16-bit Bridge priority

·        48-bit MAC address

The default priority is 32,768, and the lowest priority wins. If there is a tie in priority, the lowest MAC address is used as the tie-breaker.