Key Point

Do not confuse the STP learning state with the learning process that the switch goes through to populate its MAC address table. The STP learning state is a transitory state. While a switch can learn MAC addresses from data frames received on its ports that are in the STP learning state, it does not forward those frames. In a stable network, switch ports are in either the forwarding or blocking state. Ports in the blocking state do not listen to data frames and therefore do not contribute to the switch's MAC address table. Ports in the forwarding state do, of course, listen to (and forward) data frames, and those frames populate the switch's MAC address table.

STP Options

Figure 2-5 illustrates that it could take up to 50 seconds for a blocked port to transition to the forwarding state after a failure has occurred in the forwarding path. This lengthy time is one of the drawbacks of STP.

Several features and enhancements to STP can help to reduce the convergence time, that is, the time it takes for all the switches in a network to agree on the network's topology after that topology has changed. The following are some of these features that are implemented in Cisco switches:

• PortFast This feature should be used for ports that have only end-user stations or servers attached to them, in other words, for ports that are not attached to other switches (so that no BPDUs are received on the port). Because no other switches are attached, the port cannot be part of a loop, so the switch immediately puts the port in the forwarding state. Thus, the port transitions to the forwarding state much faster than it otherwise would.

• UplinkFast This feature is intended to be used on redundant ports on access layer switches.111 If the root port (pointing to the root bridge) on a switch goes down, the nondesignated port (the redundant blocking port) on the switch is quickly put in the forwarding state, rather than going through all the other states.

• BackboneFast This feature helps to reduce the convergence time when links other than those directly connected to a switch fail. This feature must be deployed on all switches in the network if it is to be used.

Rapid STP (RSTP)

RSTP is defined by IEEE 802.1w. RSTP incorporates many of the Cisco enhancements to STP, resulting in faster convergence. Switches in an RSTP environment converge quickly by communicating with each other and determining which links can be forwarding, rather than just waiting for the timers to transition the ports among the various states. RSTP ports take on different roles than STP ports. The RSTP roles are root, designated, alternate, backup, and disabled. RSTP port states are also different than STP port states. The RSTP states are discarding, learning, and forwarding. RSTP is compatible with STP.



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