Chapter 10: Spanning Tree Algorithm
The Spanning Tree Algorithm (STA) can be used to detect and disable network
loops, and to provide backup links between switches, bridges or route rs. This a llow s
the switch to interact with other bridging devices (that is, an STA-compliant s witch,
bridge or router) in your network to ensure t hat on ly one route exists betw een any
two stations on the network, and provide backup links w hich automat ica lly tak e over
when a primary link goes down.
The spanning tree algorithms supported by this switch include these versions:
STP – Spanning Tree Protocol (IEEE 802.1D)
RSTP – Rapid Spanning Tree Protocol (IEEE 802.1 w)
MSTP – Multiple Spanning Tr ee Proto col (IEE E 802 .1s)
STP
– STP uses a distributed algorithm to select a bridging device (STP-compliant
switch, bridge or router) that serves as the root of the spann ing t ree netwo rk. It
selects a root port on each bridgin g device (ex cept for the root devi ce) wh ich incurs
the lowest path cost when forwarding a packet f rom th at devic e to the ro ot dev ice.
Then it selects a designated bridging device from each LAN which inc urs the low est
path cost when forwarding a packet from that LAN to the r oot devi ce. All por ts
connected to designated bridging devices are assigned as desig nated po rts. After
determining the lowest cost spanning tree, it enables all root ports and designated
ports, and disables all other ports. Network packets are therefore on ly forwa rded
between root ports and designated ports, eliminating any possible network loops.
Designated
Root
Root
Port
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x
Designated
Port
x
Designated
Bridge
Once a stable network topology has been established, all bridges listen for He llo
BPDUs (Bridge Protocol Data Units) transmitted from the Root Bridge. If a bridge
does not get a Hello BPDU after a predefined interval (Max imum Age ), the bridge
assumes that the link to the Root Bridge is d own. This bridg e will then initiate
negotiations with other bridges to reconfigure the network to reestabl ish a valid
network topology.
RSTP
– RSTP is designed as a general replacement for the slower, legacy STP.
RSTP is also incorporated into MSTP. RSTP achieves must faster reconfiguration
(i.e., around 1 to 3 sec onds, c ompared to 30 seco nds or mo re for STP ) by r educi ng
the number of state changes before active ports start learning, predefining an
alternate route that can be used when a node or port fails, and retain ing the
forwarding database for ports insensitive to changes in the tree structure when
reconfiguration occurs.

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