38 CHAPTER 4: USING RESILIENCE FEATURES
STP Configuration After all the bridges on the network have agreed on the identity of the
Root Bridge, and have established the othe r relevant parameters, each
bridge is configured to forward traffic only betwee n its Root Port and the
Designated Bridge Ports for the respective network se gments. 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 brid ges 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 bet ween itself and the
Root Bridge, has gone down. The bridge the n reconfigures the network
to cater for the change. If you have configured an SNMP trap destination,
when the topology of your network chang es, the first bridge to detect
the change sends out an SNMP trap.
CAUTION: Netw ork loops can occur if aggregated links are ma nually
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 bridg es connected via
point-to-point links to enable a link without h aving 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 i s why RSTP can carry out
automatic configuration and restore a link faster than STP.
STP Example Figure 8 shows a LAN that has STP enabled. The LAN has three segments,
and each segment is connected using two possible links.