Specifications

CHAPTER 3
Spanning Tree
n Link type: If you connect two switches through a point-to-point link and the local port becomes a designated port, it
exchanges a handshake with the other port to quickly transition to forwarding. Full-duplex links are assumed to be
point-to-point; half-duplex links are assumed to be shared.
n Backup and alternate ports: Ports that can transition to forwarding when no BPDUs are received from a neighbor
switch (similar to UplinkFast).
If an RSTP switch detects a topology change, it sets a TC timer to twice the hello time and sets the TC bit on all BPDUs
sent out its designated and root ports until the timer expires. It also clears the MAC addresses learned on these ports.
Only changes to the status of non-Edge ports cause a TC notification.
If an RSTP switch receives a TC BPDU, it clears the MAC addresses on that port and sets the TC bit on all BPDUs sent
out its designated and root ports until the TC timer expires. Enable and verify Rapid STP with the commands:
Switch(config)# spanning-tree mode rapid-pvst
Switch# show spanning-tree
A version of PVST+ is used with Rapid Spanning Tree, called Per-VLAN Rapid Spanning Tree (PVRST+). You should
still configure root and secondary root bridges for each VLAN when using RSTP.
Multiple Spanning Tree
With Multiple Spanning Tree (MST), you can group VLANs and run one instance of Spanning Tree for a group of
VLANs. This cuts down on the number of root bridges, root ports, designated ports, and BPDUs in your network.
Switches in the same MST Region share the same configuration and VLAN mappings. Configure and verify MST with
these commands:
(config)# spanning-tree mode mst
(config)# spanning-tree mst configuration
(config-mst)# name region_name
[ 40 ]
© 2010 Pearson Education, Inc. All rights reserved. This publication is protected by copyright. Please see page 112 for more details.
CCNP SWITCH 642-813 Quick Reference by Denise Donohue