802.1Q Trunk Configuration Guidelines And Restrictions - Cisco WS-C6506 Software Manual

Understanding How VLAN Trunks Work
Table 5-3 Results of Possible Fast Gigabit Ethernet, and 10-Gigabit Ethernet Trunk Configurations (continued)
Local Port Trunk Mode and Trunk Encapsulation
Neighbor Port
Trunk Mode off
and Trunk isl or
Encapsulation dot1q
auto Local:
dot1q Nontrunk
Neighbor:
Nontrunk
desirable Local:
negotiate Nontrunk
Neighbor:
Nontrunk
auto Local:
negotiate Nontrunk
Neighbor:
Nontrunk
1. Using this configuration can result in spanning tree loops and is not recommended.
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 that are 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, enter the nonegotiate keyword to cause the port to become a trunk but not
generate DTP frames.

802.1Q Trunk Configuration Guidelines and Restrictions

The following configuration guidelines and restrictions apply when using 802.1Q trunks and impose some
limitations on the trunking strategy for a network:
•
•
•
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
5-4
on desirable
isl isl
Local: Local:
ISL trunk Nontrunk
Neighbor: Neighbor:
Nontrunk Nontrunk
Local: Local:
ISL trunk ISL trunk
Neighbor: Neighbor:
ISL trunk ISL trunk
Local: Local:
ISL trunk ISL trunk
Neighbor: Neighbor:
ISL trunk ISL trunk
When connecting Cisco switches through an 802.1Q trunk, make sure that 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 that 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 that is 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).
auto on desirable
isl dot1q dot1q
Local: Local: Local:
Nontrunk 1Q trunk 1Q trunk
Neighbor: Neighbor: Neighbor:
Nontrunk 1Q trunk 1Q trunk
Local: Local: Local:
ISL trunk 1Q trunk 1Q trunk
Neighbor: Neighbor: Neighbor:
ISL trunk 1Q trunk 1Q trunk
Local: Local: Local:
Nontrunk 1Q trunk 1Q trunk
Neighbor: Neighbor: Neighbor:
Nontrunk 1Q trunk 1Q trunk
Chapter 5 Configuring Ethernet VLAN Trunks
auto desirable
dot1q negotiate
Local: Local:
Nontrunk 1Q trunk
Neighbor: Neighbor:
Nontrunk 1Q trunk
Local: Local:
1Q trunk ISL trunk
Neighbor: Neighbor:
1Q trunk ISL trunk
Local: Local:
Nontrunk ISL trunk
Neighbor: Neighbor:
Nontrunk ISL trunk
auto
negotiate
Local:
Nontrunk
Neighbor:
Nontrunk
Local:
ISL trunk
Neighbor:
ISL trunk
Local:
Nontrunk
Neighbor:
Nontrunk
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