Understanding Vlan Trunks - Cisco 4500M Software Manual

Chapter 12 Configuring Layer 2 Ethernet Interfaces

Understanding VLAN Trunks

A trunk is a point-to-point link between one or more Ethernet switch interfaces and another networking
device such as a router or a switch. Trunks carry the traffic of multiple VLANs over a single link and
allow you to extend VLANs across an entire network.
Two trunking encapsulations are available on all Ethernet interfaces:
•
•
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundle. For more
information about EtherChannel, see
Ethernet trunk interfaces support different trunking modes (see
trunk uses ISL encapsulation, 802.1Q encapsulation, or if the encapsulation type is autonegotiated.
To autonegotiate trunking, make sure your interfaces are in the same VTP domain. Use the trunk or
nonegotiate keywords to force interfaces in different domains to trunk. For more information on VTP
domains, see
Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP). DTP supports autonegotiation
of both ISL and 802.1Q trunks.
Encapsulation Types
Table 12-1
Table 12-1 Ethernet Trunk Encapsulation Types
Encapsulation Type Encapsulation Command
ISL switchport trunk encapsulation isl
802.1Q switchport trunk encapsulation dot1q
Negotiate switchport trunk encapsulation negotiate
The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected
interfaces determine whether a link becomes an ISL or 802.1Q trunk.
OL-6696-01
Inter-Switch Link (ISL) Protocol—ISL is a Cisco-proprietary trunking encapsulation.
Note The blocking Gigabit ports on the WS-X4418-GB and WS-X4412-2GB-T modules do not
support ISL. Ports 3 to 18 are blocking Gigabit ports on the WS-X4418-GB module. Ports
1to 12 are blocking Gigabit ports on the WS-X4412-2GB-T module.
802.1Q—802.1Q is an industry-standard trunking encapsulation.
Chapter 27, "Understanding and Configuring VTP."
lists the Ethernet trunk encapsulation types.
Chapter 17, "Understanding and Configuring EtherChannel."
Table
Purpose
Specifies ISL encapsulation on the trunk link.
Specifies 802.1Q encapsulation on the trunk link.
Specifies that the interface negotiate with the
neighboring interface to become an ISL (preferred)
or 802.1Q trunk, depending on the configuration
and capabilities of the neighboring interface.
Software Configuration Guide—Release 12.2(25)EW
Overview of Layer 2 Ethernet Switching
12-2). You can specify whether the
12-3