Bridge Id, Switch Priority, And Extended System Id; Spanning-Tree Interface States - Cisco NCS 4200 Series Configuration Manual

Bridge ID, Switch Priority, and Extended System ID

Bridge ID, Switch Priority, and Extended System ID

The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which controls
the selection of the root switch. Because each VLAN is considered as a different logical bridge with PVST+
and rapid PVST+, the same switch must have as many different bridge IDs as VLANs configured on it. Each
VLAN on the switch has a unique 8-byte bridge ID. The two most-significant bytes are used for the switch
priority, and the remaining six bytes are derived from the switch MAC address.
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 Switch Priority Value and Extended System ID, the two 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 1: Switch Priority Value and Extended System ID
Switch Priority Value
Bit
16
32768
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.
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
Switch
a VLAN

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 STP port
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 determines that
LAN Switching Configuration Guide, Cisco IOS XE Everest 16.6.1 (Cisco NCS 4200 Series)
40
Extended System ID (Set Equal to the VLAN ID)
Bit Bit Bit Bit
15 14 13 12
16384 8192 4096 2048
section, the Configuring a Secondary Root Switch
section.
the interface should participate in frame forwarding.
Bit Bit Bit 9 Bit 8 Bit 7
11 10
1024 512 256 128 64
section, and the
Configuring PVST+ and RPVST+
Bit 6 Bit 5 Bit 4 Bit 3 Bit 2
32 16 8 4 2
Configuring the Root
Configuring the Switch Priority of
Bit 1
1