Stp Protocol Timers; Creating The Spanning Tree Topology - Cisco 7604 Configuration Manual
Ios software configuration guide
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Chapter 20
Configuring STP and Prestandard IEEE 802.1s MST
STP Protocol Timers
Table 20-3
Table 20-3 STP Protocol Timers
Variable
Hello timer
Forward delay timer
Maximum age timer
Creating the Spanning Tree Topology
In
Figure
devices is set to the default (32768) and Switch A has the lowest MAC address. However, due to traffic
patterns, number of forwarding ports, or link types, Switch A might not be the ideal root bridge. By
increasing the priority (lowering the numerical value) of the ideal network device so that it becomes the
root bridge, you force an STP recalculation to form a new spanning tree topology with the ideal network
device as the root.
Figure 20-1 Spanning Tree Topology
DP
RP = Root Port
DP = Designated Port
When the spanning tree topology is calculated based on default parameters, the path between source and
destination end stations in a switched network might not be ideal. For instance, connecting higher-speed
links to a port that has a higher number than the current root port can cause a root-port change. The goal
is to make the fastest link the root port.
For example, assume that one port on Switch B is a fiber-optic link, and another port on Switch B (an
unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the
high-speed fiber-optic link. By changing the STP port priority on the fiber-optic port to a higher priority
(lower numerical value) than the root port, the fiber-optic port becomes the new root port.
OL-4266-08
describes the STP protocol timers that affect STP performance.
Description
Determines how often the network device broadcasts hello messages to other
network devices.
Determines how long each of the listening and learning states last before the
port begins forwarding.
Determines the amount of time protocol information received on an port is
stored by the network device.
20-1, Switch A is elected as the root bridge because the bridge priority of all the network
DP
DP
A
RP
DP
RP
DP
RP
B
C
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
D
DP
DP
Understanding How STP Works
20-5
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