3Com SuperStack 3C17300, Implementation Manual

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44 C
HAPTER
4: U
SING
R
ESILIENCE
F
EATURES
■
The identity of the bridge that is to be the Root Bridge. The Root
Bridge is the central reference point from which the network is
configured.
■
The Root Path Costs for each bridge — that is, the cost of the paths
from each bridge to the Root Bridge.
■
The identity of the port on each bridge that is to be the Root Port.
The Root Port is the one that is connected to the Root Bridge using
the most efficient path, that is, the one that has the lowest Root
Path Cost. Note that the Root Bridge does not have a Root Port.
■
The identity of the bridge that is to be the Designated Bridge of
each LAN segment. The Designated Bridge is the one that has the
lowest Root Path Cost from that segment. Note that if several
bridges have the same Root Path Cost, the one with the lowest
Bridge Identifier becomes the Designated Bridge.
All traffic destined to pass in the direction of the Root Bridge flows
through the Designated Bridge. The port on this bridge that connects
to the segment is called the Designated Bridge Port.
STP Configuration After all the bridges on the network have agreed on the identity of the
Root Bridge, and have established the other relevant parameters, each
bridge is configured to forward traffic only between its Root Port and the
Designated Bridge Ports for the respective network segments. All other
ports are blocked, which means that they are prevented from receiving or
forwarding traffic.
STP Reconfiguration Once the network topology is stable, all the bridges listen for Hello BPDUs
transmitted from the Root Bridge at regular intervals. If a bridge does not
receive a Hello BPDU after a certain interval (the Max Age time), the
bridge assumes that the Root Bridge, or a link between itself and the
Root Bridge, has gone down. The bridge then reconfigures the network
to cater for the change. If you have configured an SNMP trap destination,
when the topology of your network changes, the first bridge to detect
the change
sends out an SNMP trap.
CAUTION: Network loops can occur if aggregated links are manually
configured incorrectly, that is, the physical connections do not match the
assignment of ports to an aggregated link. RSTP and STP may not detect
these loops. So that RSTP and STP can detect all network loops you must
ensure that all aggregated links are configured correctly.