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| 3.4 |  | 802.1w Rapid Spanning-Tree Protocol |
| 3.4.3 |  | RSTP port roles |

The RSTP role is now a variable assigned to a given port. The root port and designated port roles remain, while the blocking port role is now split into the backup and alternate port roles. The spanning-tree algorithm (STA) determines the role of a port based on Bridge Protocol Data Units (BPDUs). To keep things simple, the thing to remember about a BPDU is that there is always a way of comparing any two of them and deciding whether one is more useful than the other. This is based on the value stored in the BPDU and occasionally on the port on which they are received. This considered, the following paragraphs explain very practical approaches to port roles.

As in standard Spanning-Tree Protocol 802.1D, the port receiving the best BPDU on a bridge is the root port. This is the port that is the closest to the root bridge in terms of path cost.

A port is designated if it can send the best BPDU on the segment to which it is connected. As with 802.1D, bridges create a bridged domain by linking together different segments (Ethernet segments for example). On a given segment, there can be only one path to the root bridge, multiple paths would create a bridging loop in the network.

Alternate and backup 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 would send 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 is a port blocked by receiving more useful BPDUs from another bridge. A backup port is a port blocked by receiving more useful BPDUs from the same bridge it is on.

This backup port distinction was already made internally within 802.1D and is essentially how Cisco uplink fast functions. The rationale behind 802.1w is that an alternate port provides an alternate path to the root bridge and could therefore replace the root port should it fail. A backup port provides redundant connectivity to the same segment and cannot guarantee an alternate connectivity to the root bridge. It was therefore excluded from the uplink group.

RSTP calculates the final topology for the spanning tree using exactly 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. There is now a difference between the role the protocol has determined 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 will typically happen for very short periods of time, it simply means that this port is in a transitory state towards designated forwarding.