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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
OL-8978-04
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