Prior to 802.1w, Cisco implemented a number of proprietary enhancements to 802.1D to improve
convergence in a Layer 2 network. Which statement is correct?
A.
Only UplinkFast and BackboneFast are specified in 802.1w; PortFast must be manually
configured.
B.
Only PortFast is specified in 802.1w; UplinkFast and BackboneFast must be manually
configured.
C.
None of the proprietary Cisco enhancements are specified in 802.1w.
D.
PortFast, UplinkFast, and BackboneFast are specified in 802.1w.
Explanation:
Spanning-tree PortFast causes a spanning-tree port to enter the forwarding state immediately,
bypassing the listening and learning states. You can use PortFast on switch ports connected to a
single workstation or server to allow those devices to connect to the network immediately, rather
than waiting for spanning tree to converge.
UplinkFast provides fast convergence after a spanning-tree topology change and achieves load
balancing between redundant links using uplink groups. An uplink group is a set of ports (per
VLAN), only one of which is forwarding at any given time. Specifically, an uplink group consists of
the root port (which is forwarding) and a set of blocked ports, except for self-looping ports. The
uplink group provides an alternate path in case the currently forwarding link fails.
BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs
from its designated bridge. An inferior BPDU identifies one switch as both the root bridge and the
designated bridge. When a switch receives an inferior BPDU, it indicates that a link to which the
switch is not directly connected (an indirect link) has failed (that is, the designated bridge has lost
its connection to the root bridge). Under normal spanning-tree rules, the switch ignores inferior
BPDUs for the configured maximum aging time, as specified by the aging time variable of the “set
spantree MaxAge” command. The switch tries to determine if it has an alternate path to the root
bridge. If the inferior BPDU arrives on a blocked port, the root port and other blocked ports on the
switch become alternate paths to the root bridge. (Self-looped ports are not considered alternate
paths to the root bridge.) If the inferior BPDU arrives on the root port, all blocked ports become
alternate paths to the root bridge. If the inferior BPDU arrives on the root port and there are no
blocked ports, the switch assumes that it has lost connectivity to the root bridge, causes the
maximum aging time on the root to expire, and becomes the root switch according to normal
spanning-tree rules.
If the switch has alternate paths to the root bridge, it uses these alternate paths to transmit a new
kind of PDU called the Root Link Query PDU. The switch sends the Root Link Query PDU out all
alternate paths to the root bridge. If the switch determines that it still has an alternate path to the
root, it causes the maximum aging time on the ports on which it received the inferior BPDU to
expire. If all the alternate paths to the root bridge indicate that the switch has lost connectivity to
the root bridge, the switch causes the maximum aging times on the ports on which it received an
inferior BPDU to expire. If one or more alternate paths can still connect to the root bridge, the
switch makes all ports on which it received an inferior BPDU its designated ports and moves them
out of the blocking state (if they were in blocking state), through the listening and learning states,
and into the forwarding state.