Root bridge election-RSTP

closetgeek

Can anyone explain "Election of the Root Bridge" and the RP Root port.

luke_bibby

Each switch in the network compares the sender Bridge ID (BID) of all received Bridge PDUs (BPDU) to its own BID and determines who has the numerically lowest value, starting from the Priority (default 32768 + VLAN ID) then the MAC address is used when a tiebreaker occurs (that is, all swithces have the same Priority value configured). Remember that the BID = (Priority + VLAN ID), (MAC Address of the sending switch). This is the process of 'electing the root bridge',

Note that before any BPDUs are received, each switch thinks that they are the root bridge, and will continue to do so until they receive a BPDU with a lower advertised sender BID.

The root port (RP) is the switch port on a non-root switch which has the lowest cost to reach the root bridge.

miller811

Also remember you can influence the election by changing the priority on a switch.

you can use the
spanning-tree vlan XX root primary command, which tells the switch to allows adjust its priority lower than any other switch that enters the network.

spanning-tree vlan XX root secondary command, which tells the switch to become the backup if the root fails

spanning-tree vlan xx priority <0-61440> bridge priority in increments of 4096

if all switches are left as the default 32768, they are then elected by MAC address, which would probably mean that an old 1900 switch could be elected root, instead of a new high powered switch

Hope that helps.