How STP Works 39
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.
How RSTP Differs to
STP
RSTP works in a similar way to STP, but it includes additional information
in the BPDUs. This information allows each bridge to confirm that it has
taken action to prevent loops from forming when it wants to enable a
link to a neighbouring bridge. This allows adjacent bridges connected via
point-to-point links to enable a link without having to wait to ensure all
other bridges in the network have had time to react to the change.
So the main benefit of RSTP is that the configuration decision is made
locally rather than network-wide which is why RSTP can carry out
automatic configuration and restore a link faster than STP.
STP Example Figure 11
shows a LAN that has STP enabled. The LAN has three
segments, and each segment is connected using two possible links.
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