Configuring Lan Interfaces For Layer 2 Switching - Cisco 7604 Configuration Manual
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Configuring LAN Interfaces for Layer 2 Switching
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Configuring LAN Interfaces for Layer 2 Switching
These sections describe how to configure Layer 2 switching on the Cisco 7600 series routers:
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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
10-6
The following configuration guidelines and restrictions apply when using 802.1Q trunks and impose
some limitations on the trunking strategy for a network. Note these restrictions when using 802.1Q
trunks:
When connecting Cisco switches through an 802.1q trunk, make sure the native VLAN for an
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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
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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
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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).
Non-Cisco 802.1Q switches maintain only a single instance of spanning tree that defines the
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spanning tree topology for all VLANs. When you connect a Cisco router to a non-Cisco router
through an 802.1Q trunk, the MST of the non-Cisco router and the native VLAN spanning tree
of the Cisco router 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
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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.
Make certain that the native VLAN is the same on all of the 802.1q trunks connecting the Cisco
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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
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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 causes the router to place
the ISL trunk port or access port into the spanning tree "port inconsistent" state and no traffic
will pass through the port.
Configuring a LAN Port for Layer 2 Switching, page 10-7
Configuring a Layer 2 Switching Port as a Trunk, page 10-8
Configuring a LAN Interface as a Layer 2 Access Port, page 10-14
Configuring a Custom IEEE 802.1Q EtherType Field Value, page 10-15
Chapter 10
Configuring LAN Ports for Layer 2 Switching
OL-4266-08
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