Q Trunk Restrictions - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Catalyst 6000 series software configuration guide
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Understanding How VLAN Trunks Work
DTP is a point-to-point protocol. However, some internetworking devices might forward DTP frames
Note
improperly. To avoid this problem, ensure that trunking is turned off on ports connected to
non-switch devices if you do not intend to trunk across those links. When manually enabling trunking
on a link to a Cisco router, use the nonegotiate keyword to cause the port to become a trunk but not
generate DTP frames.

802.1Q Trunk Restrictions

The following configuration guidelines and restrictions apply when using 802.1Q trunks impose some
limitations on the trunking strategy for a network. Note these restrictions when using 802.1Q trunks:
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
5-4
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).
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).
When you connect a Cisco switch to a non-Cisco switch the CST is always on VLAN 1. The Cisco
switch sends an untagged IEEE BDPU (01-80-C2-00-00-00) on VLAN 1 for the CST and on the
native VLAN the Cisco switch sends an untagged Cisco BPDU (01-00-0C-CC-CC-CC) which the
non-Cisco switch forwards but does not act on (the IEEE BPDU is not forwarded on the native
VLAN).
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.
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 5
Configuring Ethernet VLAN Trunks
78-13315-02

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