Layer 2 Lan Interface Configuration Guidelines And Restrictions; Restrictions; Guidelines - Cisco 6500 Series Software Configuration Manual

Layer 2 LAN Interface Configuration Guidelines and Restrictions

Layer 2 LAN Interface Configuration Guidelines and Restrictions
When configuring Layer 2 LAN ports, follow these guidelines and restrictions:

Restrictions

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Guidelines

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Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
7-6
10-Gigabit Ethernet ports do not support ISL encapsulation.
Non-Cisco 802.1Q switches maintain only a single instance of spanning tree (the Mono Spanning
Tree, or MST) that defines the spanning tree topology for all VLANs. When you connect a Cisco
switch to a non-Cisco switch through an 802.1Q trunk, the MST of the non-Cisco switch and the
native VLAN spanning tree of the Cisco switch combine to form a single spanning tree topology
known as the Common Spanning Tree (CST).
Because Cisco switches transmit BPDUs to the SSTP multicast MAC address on VLANs other than
the native VLAN of the trunk, non-Cisco switches do not recognize these frames as BPDUs and
flood them on all ports in the corresponding VLAN. Other Cisco switches connected to the
non-Cisco 802.1q cloud receive these flooded BPDUs. This allows Cisco switches to maintain a
per-VLAN spanning tree topology across a cloud of non-Cisco 802.1Q switches. The non-Cisco
802.1Q cloud separating the Cisco switches is treated as a single broadcast segment between all
switches connected to the non-Cisco 802.1q cloud through 802.1q trunks.
When connecting Cisco switches through an 802.1q trunk, make sure the native VLAN for an
802.1Q trunk is the same on both ends of the trunk link. If the native VLAN on one end of the trunk
is different from the native VLAN on the other end, spanning tree loops might result.
Disabling spanning tree on the native VLAN of an 802.1Q trunk without disabling spanning tree on
every VLAN in the network can cause spanning tree loops. We recommend that you leave spanning
tree enabled on the native VLAN of an 802.1Q trunk. If this is not possible, disable spanning tree
on every VLAN in the network. Make sure your network is free of physical loops before disabling
spanning tree.
When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning tree
BPDUs on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent
untagged to the reserved IEEE 802.1d spanning tree multicast MAC address (01-80-C2-00-00-00).
The BPDUs on all other VLANs on the trunk are sent tagged to the reserved Cisco Shared Spanning
Tree (SSTP) multicast MAC address (01-00-0c-cc-cc-cd).
Make certain that the native VLAN is the same on all of the 802.1Q trunks connecting the Cisco
switches to the non-Cisco 802.1Q cloud.
If you are connecting multiple Cisco switches to a non-Cisco 802.1Q cloud, all of the connections
must be through 802.1Q trunks. You cannot connect Cisco switches to a non-Cisco 802.1Q cloud
through ISL trunks or through access ports. Doing so will cause the switch to place the ISL trunk
port or access port into the spanning tree "port inconsistent" state and no traffic will pass through
the port.
Chapter 7 Configuring LAN Ports for Layer 2 Switching
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