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This passage is from :http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#roles
Alternate and Backup Port Roles
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These two port roles correspond to the blocking state of 802.1D. A blocked port is defined as not being the designated or root port. A blocked port receives a more useful BPDU than the one it sends out on its segment. Remember that a port absolutely needs to receive BPDUs in order to stay blocked. RSTP introduces these two roles for this purpose. -
An alternate port receives more useful BPDUs from another bridge and is a port blocked. This is shown in this diagram:
This distinction is already made internally within 802.1D. This is essentially how Cisco UplinkFast functions. The rationale is that an alternate port provides an alternate path to the root bridge and therefore can replace the root port if it fails. Of course, a backup port provides redundant connectivity to the same segment and cannot guarantee an alternate connectivity to the root bridge. Therefore, it is excluded from the uplink group.
As a result, RSTP calculates the final topology for the spanning tree that uses the same criteria as 802.1D. There is absolutely no change in the way the different bridge and port priorities are used. The name blocking is used for the discarding state in Cisco implementation. CatOS releases 7.1 and later still display the listening and learning states. This gives even more information about a port than the IEEE standard requires. However, the new feature is now there is a difference between the role the protocol determines for a port and its current state. For example, it is now perfectly valid for a port to be designated and blocking at the same time. While this typically occurs for very short periods of time, it simply means that this port is in a transitory state towards the designated forwarding state.
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