TELE33324 Lecture Notes - Lecture 11: Packet Switching, Bridge Protocol Data Unit, Mac Address
11 Dec 2016
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Spanning Tree Terms
Before I get into describing the details of how STP works in the network, you
need to understand some basic ideas and terms and how they relate within the
layer 2 switched network:
• Root bridge The root bridge is the bridge with the best bridge ID. With
STP, the key is for all the switches in the network to elect a root bridge
that becomes the focal point in the network. All other decisions in
the network—such as which port is to be blocked and which port is to be
put in forwarding mode—are made from the perspective of this root
bridge. Once a root bridge is elected on the network, all other bridges
must make a single path to this root bridge. The port with the best path to
the root bridge is called the root port.
• BPDU All the switches exchange information to use in the selection of the
root switch as well as in subsequent configuration of the network. Each
switch compares the parameters in the Bridge Protocol Data Unit
(BPDU) that it sends to one neighbor with the ones that it receives from
other neighbors.
• Bridge ID The bridge ID is how STP keeps track of all the switches in
the network. It is determined by a combination of the bridge priority
(32,768 by default on all Cisco switches) and the base MAC address. The
bridge with the lowest bridge ID becomes the root bridge in the network.
• Nonroot bridges These are all bridges that are not the root bridge.
Nonroot bridges exchange BPDUs with all bridges and update the STP
topology database on all switches, preventing loops and providing a
measure of defense against link failures.
• Port cost Port cost determines the best path when multiple links are used
between two switches. The cost of a link is determined by the bandwidth
of a link.
• Root port The root port is always the link directly connected to the root
bridge, or the lowest path cost to the root bridge. If more than one link
connects to the root bridge, then a port cost is determined by checking the
bandwidth of each link. The lowest-cost port becomes the root port. If
multiple upstream switches have the same cost, the bridge with the lower
advertising bridge ID is used. When multiple links connect to the same
device, the port connected to the lowest port number on the upstream
switch will be used.
• Designated port A designated port is one that has been determined as
having the best (lowest) cost to the root bridge via its root port. A
designated port will be marked as a forwarding port.
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