Cisco Systems OL-8241-02 Manual De Usuario

The CSS supports configuration of Spanning-Tree Protocol (STP) bridging for an 
Ethernet interface in a VLAN or for a trunked Ethernet interface. Spanning-tree 
bridging is used to detect, and then prevent, loops in the network. You can define 
the bridge spanning-tree path cost, priority, and state for an Ethernet interface or 
for a trunked Ethernet interface. Ensure you configure the spanning-tree bridging 
parameters the same on all switches running STP in the network. 

When connecting a Cisco Catalyst switch to a CSS using an 802.1Q trunk and the 
Spanning-Tree Protocol, the Catalyst runs a spanning-tree instance for each 
VLAN. When you configure an 802.1Q trunk on an Ethernet interface for the 
Catalyst switch, the bridge protocol data units (BPDUs) are tagged with the 
corresponding VLAN ID and the destination MAC address changes from the 
standard 01-80-C2-00-00-00 to the proprietary 01-00-0c-cc-cc-cd. This 
modification allows Cisco switches operating in a non-Cisco (a mix of other 
vendors) 802.1Q trunk environment to maintain spanning-tree states for all 
VLANs. Although the CSS maintains a spanning-tree instance for each VLAN as 
well, the CSS uses the standard 01-80-C2-00-00-00 destination MAC address for 
all BPDUs (tagged or untagged). When you connect a Cisco Catalyst switch to a 
CSS over an 802.1Q trunk, the result is that neither switch recognizes the other's 
BPDUs, and both assume root status. If a spanning-tree loop is detected, the 
Catalyst switch goes in to blocking mode on one of its looped ports.

This section includes the following topics:

For details about globally configuring spanning-tree bridging parameters for the 
CSS (such as bridge aging time, forward delay time, hello time interval, and 
maximum age), refer to 

.