Stp Port States - Cisco Catalyst 2000 Configuration Handbook

114 Cisco LAN Switching Configuration Handbook
Note The IEEE uses a nonlinear scale to relate the port bandwidth of a single link to its
port cost value. STP treats bundled links, such as Fast EtherChannel and Gigabit
EtherChannel, as a single link with an aggregate bandwidth of the individual links. As a
result, remember that the port or path cost used for a bundled EtherChannel will be based
on the bundled bandwidth. For example, a two-link Fast EtherChannel has 200 mbps band-
width and a path cost of 12. A four-link Gigabit EtherChannel has 4 gbps bandwidth and a
path cost of 2. Use Table 7-1 to see how these EtherChannel aggregate bandwidth and port
costs relate to the values of single or individual links.

STP Port States

Each switch port progresses through a sequence of states:
Disabled: Ports that are administratively shut down or shut down due to a fault con-
1.
dition. (MST calls this state discarding.)
Blocking: The state used after a port initializes. The port cannot receive or transmit
2.
data, cannot add MAC addresses to its address table, and can receive only BPDUs. If
a bridging loop is detected, or if the port loses its root or designated port status, it
will be returned to the blocking state. (MST calls this state discarding.)
Listening: If a port can become a root or designated port, it is moved into the listen-
3.
ing state. The port cannot receive or transmit data and cannot add MAC addresses to
its address table. BPDUs can be received and sent. (MST calls this state discarding.)
Learning: After the Forward Delay timer expires (default 15 seconds), the port
4.
enters the learning state. The port cannot transmit data but can send and receive
BPDUs. MAC addresses can now be learned and added into the address table.
Forwarding: After another Forward Delay timer expires (default 15 seconds), the
5.
port enters the forwarding state. The port can now send and receive data, learn MAC
addresses, and send and receive BPDUs.
STP Topology Changes
■ If a switch port is moved into the forwarding state (except when PortFast is enabled),
a topology change is signaled.
If a switch port is moved from the forwarding or learning state into the blocking
■
state, a topology change is signaled.
■ To signal a topology change, a switch sends TCN BPDUs on its root port every Hello
Time interval. This occurs until the TCN is acknowledged by the upstream designat-
ed bridge neighbor. Neighbors continue to relay the TCN BPDU on their root ports
until it is received by the root bridge.
■ The root bridge informs the entire spanning tree of the topology change by sending a
configuration BPDU with the topology change (TC) bit set. This causes all down-
stream switches to reduce their Address Table Aging timers from the default value
(300 seconds) down to the Forward Delay (default 15 seconds). This flushes inactive
MAC addresses out of the table faster than normal.