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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP
Unders tan din g RSTP

Understandin g RSTP

The RSTP takes adv antage of point-to-point wiring and provides rapid conver gence of the spanning tree.
Reconfigur ation of the spanning tree can occur in less tha n 1 second (in contrast to 50 se conds with the
default setti ngs in the 802.1D spanning tree), whic h is critical for networks carryi ng delay-se nsitive
traffic such as voice and video.
These sec tion descr ibes how the RSTP works:
•Port Roles and the Active Topology, page 12-2
•Rapid Convergence, pag e 12-3
•Synchroniz ation o f Po rt R oles , p age 12 -4
•Bridge Pro toco l D ata U n it Format an d Pr ocessi n g, pa ge 12 -5
For configuration infor ma tio n, see the “Configuring RSTP and MSTP Features” section on page 12-11 .

Port Roles and the Active Topology

The RST P pr ovides r ap id c onvergence of the s panni ng t ree by assi gning po rt r ole s and by de ter mining
the active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch with the highest
switch priorit y (lo west numer ical prio rity v alue ) as the root swit ch as described in “Election of the Root
Switch” section on pag e 11-3. The n the RSTP assigns one of these por t roles to individual port s:
•Root port—provides the best pa th (lo west cos t) when the switch forw ards pack ets to the r oot switch.
•Designat ed port—connects to the designated switch, which incurs the lowest path cost when
forwarding packets from that LAN to the root switch. The port through which the designated switch
is attached to the LAN is called the designated port.
•Altern at e p ort—offers an alternate path toward the root switch to that provided by the current root
port.
•Backup por t—acts as a backup for the path provided by a designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected together in a loopback by
a poin t-t o-p oint link o r w hen a swi tch has two o r m or e co nne ctio ns t o a sh ar ed LAN segme nt.
•Disabled por t—has no role within the operation of the spanning tree.
A port with the root or a designated port role is included in the active topology. A port with the alternate or
backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the network, the RSTP ensures that e very root
port a nd de signa ted p ort imme dia tel y t ransi tion t o the for war ding stat e w hile a ll a ltern at e a nd bac kup
ports a re a lways in t he d i scard ing sta te (equ ivalent to bloc kin g i n 80 2.1D) . The por t st at e cont ro ls t he
operati on of t he f orward ing an d le arni ng p roc esses. Table 12-1 pr ovides a co mpari so n o f 802.1D an d
RSTP port sta tes.
Table 12-1 Port State Comparison
Operatio nal Status STP Port Sta t e RSTP Port State Is Port Included in th e
Active Topology?
Enabled Blocking Discarding No
Enabled Listening Discarding No
Enabled Learning Learning Yes