IEEE 802.1w is a Rapid Spanning Tree Protocol (RSTP) that can be seen as an evolution of the
802.1 standard. What are the port roles described by 802.1w?
A.
Root port, designated port, alternate port, backup port, and disabled
B.
Standby port, alternate port, root port, and disabled
C.
Standby port, designated port, backup port, and disabled
D.
Root port, designated port, alternate port, and standby port
Explanation:
Port Roles
The role is now a variable assigned to a given port. The root port and designated port roles
remain, while the blocking port role is 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).
In order to simplify matters, the thing to remember about a BPDU is there is always a method to
compare any two of them and decide 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.
Root Port Roles
The port that receives 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. The STA elects a single root bridge in the wholebridged network (per-VLAN). The root bridge sends BPDUs that are more useful than the ones
any other bridge sends. The root bridge is the only bridge in the network that does not have a root
port. All other bridges receive BPDUs on at least one port.
Designated Port
A port is designated if it can send the best BPDU on the segment to which it is connected. 802.1D
bridges link together different segments, such as Ethernet segments, to create a bridged domain.
On a given segment, there can only be one path toward the root bridge. If there are two, there is a
bridging loop in the network. All bridges connected to a given segment listen to the BPDUs of each
and agree on the bridge that sends the best BPDU as the designated bridge for the segment. The
port on that bridge that corresponds is the designated port for that segment.
Alternate and Backup Port Roles
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.
A backup port receives more useful BPDUs from the same bridge it is on and is a port blocked.
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.http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml
#roles