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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter11 Configuring Layer 2 Ethern et Interfaces
Layer 2 Interface Configuration Guidelines and Restrictions
Layer 2 Interface Configuration Guidelines and Restrictions
Keep the following guidelines and restrictions in mind when you configure Layer 2 interfaces:
In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance
of spanning tree for each VLAN allowed on the trunks. Non-Cisco 802.1Q switches mainta in only
one instance of spanning tree for all VLANs allowed on the tru nks.
When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switc h
combines the spanning tree instance of the native VLAN of the trunk with the spanning tree instance
of the non-Cisco 802.1Q switch. However, spanning tree information for each VLAN 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.
Make sure the native VLAN for an 802.1Q trunk is the same on bo th ends of the trunk link. If the
VLAN on one end of the trunk is different from the VLA N on the other end, spanning tree loops
might result.
Disabling spanning tree on any VLAN of an 802.1Q trunk can cause spanning tree loops.
Configuring Ethernet Interfaces for Layer 2 Switching
The following sections describe how to configure Layer 2 switching on a Catalyst4500 series switch:
Configuring an Ethernet Interface as a Layer 2 Trunk, page 11-6
Configuring an Interface as a Layer 2 Access Port, page 11-8
Clearing Layer 2 Configuration, page 11-9
Native VLAN (for 802.1Q only
trunks)
VLAN 1
STP1Enabled for all VLANs
STP port priority 128
STP port cost 100 for 10-Mbps Ethernet LAN ports
19 for 10/100-Mbps Fast Ethernet ports
19 for 100-Mbps Fast Ethernet ports
4 for 1000-Mbps Gigabit Ethernet ports
2 for 10,000-Mbps 10-Gigabit Ethernet LAN ports
1. STP = Spanning Tree Protocol
Table11-3 Layer 2 Ethernet Interface Default Configuration (continued)
Feature Default Value