Cisco NM-8B-U - HW ROUTERS L-M User Manual page 15

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
Figure 4
Figure 4
Boot-up
initialization
Blocking
state
Listening
state
Learning
state
Forwarding
state
When you enable spanning tree, every port in the switch, VLAN, or network goes through the blocking
state and the transitory states of listening and learning at power up. If properly configured, each Layer 2
interface stabilizes to the forwarding or blocking state.
When the spanning tree algorithm places a Layer 2 interface in the forwarding state, the following
process occurs:
1. The Layer 2 interface is put into the listening state while it waits for protocol information that
suggests that it should go to the blocking state.
2. The Layer 2 interface waits for the forward delay timer to expire, moves the Layer 2 interface to the
learning state, and resets the forward delay timer.
3. In the learning state, the Layer 2 interface continues to block frame forwarding as it learns end
station location information for the forwarding database.
4. The Layer 2 interface waits for the forward delay timer to expire and then moves the Layer 2
interface to the forwarding state, where both learning and frame forwarding are enabled.
illustrates how a port moves through the five stages.
STP Port States
Disabled
state
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
Feature Overview
15