Chapter 18 Configuring STP

Understanding Spanning-Tree Features

The switch supports the IEEE 802.1t spanning-tree extensions, and some of the bits previously used for the switch priority are now used as the VLAN identifier. The result is that fewer MAC addresses are reserved for the switch, and a larger range of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID. As shown in Table 18-1, the 2 bytes previously used for the switch priority are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID.

Table 18-1

Switch Priority Value and Extended System ID

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch Priority Value

 

Extended System ID (Set Equal to the VLAN ID)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bit 16

Bit 15

 

Bit 14

Bit 13

Bit 12

Bit 11

Bit 10

Bit 9

Bit 8

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

32768

16384

 

8192

4096

2048

1024

512

256

128

64

32

16

8

4

2

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Spanning tree uses the extended system ID, the switch priority, and the allocated spanning-tree MAC address to make the bridge ID unique for each VLAN. Because the switch stack appears as a single switch to the rest of the network, all switches in the stack use the same bridge ID for a given spanning tree. If the stack master fails, the stack members recalculate their bridge IDs of all running spanning trees based on the new MAC address of the new stack master.

Support for the extended system ID affects how you manually configure the root switch, the secondary root switch, and the switch priority of a VLAN. For example, when you change the switch priority value, you change the probability that the switch will be elected as the root switch. Configuring a higher value decreases the probability; a lower value increases the probability. For more information, see the “Configuring the Root Switch” section on page 18-16, the “Configuring a Secondary Root Switch” section on page 18-18, and the “Configuring the Switch Priority of a VLAN” section on page 18-21.

Spanning-Tree Interface States

Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When an interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding state, it can create temporary data loops. Interfaces must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for forwarded frames that have used the old topology.

Each Layer 2 interface on a switch using spanning tree exists in one of these states:

Blocking—The interface does not participate in frame forwarding.

Listening—The first transitional state after the blocking state when the spanning tree decides that the interface should participate in frame forwarding.

Learning—The interface prepares to participate in frame forwarding.

Forwarding—The interface forwards frames.

Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on the port, or no spanning-tree instance running on the port.

An interface moves through these states:

From initialization to blocking

From blocking to listening or to disabled

From listening to learning or to disabled

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

 

OL-9775-02

18-5

 

 

 

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Cisco Systems 3750E manual Spanning-Tree Interface States, Switch Priority Value, Bit, 18-5