Cisco ME 3400 User Manual

14-11
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-07
Chapter 14 Configuring STP
Configuring Spanning-Tree Features
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 802.1Q switch.
However, all PVST+ or rapid-PVST+ information is maintained by Cisco switches separated by a cloud
of non-Cisco
802.1Q switches. The non-Cisco 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+.
For more information on IEEE 802.1Q trunks, see Chapter 11, “Configuring VLANs.”
Configuring Spanning-Tree Features
• Default Spanning-Tree Configuration, page 14-11
• Spanning-Tree Configuration Guidelines, page 14-12
• Enabling Spanning Tree on an ENI, page 14-13 (required)
• Disabling Spanning Tree, page 14-15 (optional)
• Configuring the Root Switch, page 14-15 (optional)
• Configuring a Secondary Root Switch, page 14-17 (optional)
• Configuring Port Priority, page 14-17 (optional)
• Configuring Path Cost, page 14-19 (optional)
• Configuring the Switch Priority of a VLAN, page 14-20 (optional)
• Configuring Spanning-Tree Timers, page 14-21 (optional)
Default Spanning-Tree Configuration
Table 14-3 shows the default spanning-tree configuration.

Cisco ME 3400 Specifications

General

Category	Switch
Rack Mountable	Yes
Jumbo Frame Support	Yes
Authentication Method	RADIUS, TACACS+
RAM	128 MB
Flash Memory	32 MB
Power Device	Internal power supply
Model	ME 3400
Layer	Layer 2
MAC Address Table Size	8000 entries
Remote Management Protocol	SNMP, Telnet, SSH, HTTP, HTTPS
Features	VLAN support, IGMP snooping, Quality of Service (QoS)
Compliant Standards	IEEE 802.3, IEEE 802.3u, IEEE 802.1D, IEEE 802.1Q, IEEE 802.3ab, IEEE 802.3x
Memory	128 MB
Power Supply	AC 120/230 V (50/60 Hz)
Dimensions (H x W x D)	4.4 cm x 44.5 cm x 24.2 cm
Routing Protocol	Static routing