323
Configuring STP
Information About Configuring STP
In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST backbone, and a
PVST+ switch cannot connect to multiple MST regions.
When a network contains switches running rapid PVST+ and switches running PVST+, we recommend that the
rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instances. In the rapid-PVST+
spanning-tree instances, the root switch must be a rapid-PVST+ switch. In the PVST+ instances, the root switch must be
a PVST+ switch. The PVST+ switches should be at the edge of the network.
STP and IEEE 802.1Q Trunks
The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for a network. The
standard requires only one spanning-tree instance for
all
VLANs allowed on the trunks. However, in a network of Cisco
switches connected through IEEE 802.1Q trunks, the switches maintain one spanning-tree instance for
each
VLAN
allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco switch uses PVST+ to
provide spanning-tree interoperability. If rapid PVST+ is enabled, the switch uses it instead of PVST+. The switch
combines the spanning-tree instance of the IEEE 802.1Q VLAN of the trunk with the spanning-tree instance of the
non-Cisco IEEE 802.1Q switch.
However, all PVST+ or rapid-PVST+ information is maintained by Cisco switches separated by a cloud of non-Cisco IEEE
802.1Q switches. The non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated as a single trunk link
between the switches.
PVST+ is automatically enabled on IEEE 802.1Q trunks, and no user configuration is required. The external spanning-tree
behavior on access ports is not affected by PVST+.
VLAN-Bridge Spanning Tree
Cisco VLAN-bridge spanning tree is used with the fallback bridging feature (bridge groups), which forwards non-IP
protocols such as DECnet between two or more VLAN bridge domains or routed ports. The VLAN-bridge spanning tree
allows the bridge groups to form a spanning tree on top of the individual VLAN spanning trees to prevent loops from
forming if there are multiple connections among VLANs. It also prevents the individual spanning trees from the VLANs
being bridged from collapsing into a single spanning tree.
To support VLAN-bridge spanning tree, some of the spanning-tree timers are increased.
Table 38
Spanning-Tree Interoperability and Backward Compatibility
PVST+
MSTP
Rapid PVST+
PVST+
Yes
Yes (with restrictions)
Yes (reverts to PVST+)
MSTP
Yes (with restrictions)
Yes
Yes (reverts to PVST+)
Rapid PVST+
Yes (reverts to PVST+)
Yes (reverts to PVST+)
Yes
Содержание IE 4000
Страница 12: ...8 Configuration Overview Default Settings After Initial Switch Configuration ...
Страница 52: ...48 Configuring Interfaces Monitoring and Maintaining the Interfaces ...
Страница 108: ...104 Configuring Switch Clusters Additional References ...
Страница 128: ...124 Performing Switch Administration Additional References ...
Страница 130: ...126 Configuring PTP ...
Страница 140: ...136 Configuring CIP Additional References ...
Страница 146: ...142 Configuring SDM Templates Configuration Examples for Configuring SDM Templates ...
Страница 192: ...188 Configuring Switch Based Authentication Additional References ...
Страница 244: ...240 Configuring IEEE 802 1x Port Based Authentication Additional References ...
Страница 274: ...270 Configuring SGT Exchange Protocol over TCP SXP and Layer 3 Transport Configuring Cisco TrustSec Caching ...
Страница 298: ...294 Configuring VLANs Additional References ...
Страница 336: ...332 Configuring STP Additional References ...
Страница 408: ...404 Configuring DHCP Additional References ...
Страница 450: ...446 Configuring IGMP Snooping and MVR Additional References ...
Страница 490: ...486 Configuring SPAN and RSPAN Additional References ...
Страница 502: ...498 Configuring Layer 2 NAT ...
Страница 559: ...555 Configuring Network Security with ACLs How to Configure Network Security with ACLs Creating a Numbered Extended ACL ...
Страница 770: ...766 Configuring IPv6 MLD Snooping Related Documents ...
Страница 930: ...926 Configuring IP Unicast Routing Related Documents ...
Страница 956: ...952 Configuring IPv6 Unicast Routing Configuring IPv6 network 2010 AB8 2 48 network 2010 AB8 3 48 exit address family ...
Страница 976: ...972 Configuring Cisco IOS IP SLAs Operations Additional References ...
Страница 978: ...974 Dying Gasp ...
Страница 990: ...986 Configuring Enhanced Object Tracking Monitoring Enhanced Object Tracking ...
Страница 994: ...990 Configuring MODBUS TCP Displaying MODBUS TCP Information ...
Страница 996: ...992 Ethernet CFM ...
Страница 1030: ...1026 Working with the Cisco IOS File System Configuration Files and Software Images Working with Software Images ...
Страница 1066: ...1062 Using an SD Card SD Card Alarms ...