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Software Configuration Guide—Release 12.2(25)SG
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Chapter11 Configuring Layer 2 Ethern et Interfaces
Overview of Layer 2 Ethernet Switching
Understanding VLAN Trunks
A trunk is a point-to-point link between one or more Ethe rnet 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:
•Inter-Switch Link (ISL) Protocol—ISL is a Cisco-proprietary trunking en capsulation.
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 encapsulati on.
You can configure a trunk on a single Ethernet interface or on an EtherChannel bundl e. For more
information about EtherChannel, see Chapter 16, “Understanding and Configuring E therChannel.”
Ethernet trunk interfaces support different trunking modes (see Table 11-2). You can specify whether the
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 Chapter27, “Unde rstanding and Configuring VTP.”
Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP). DTP supports autonegotiation
of both ISL and 802.1Q trunks.

Encapsulation Types

Table11-1 lists the Ethernet trunk encapsulation types.
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 tr unk.
Table11-1 Ethernet Trunk Encapsulation Types
Encapsulation Type Encapsulation Command Purpose
ISL switchport trunk encapsulation isl Specifies ISL encapsulation on the trunk link.
802.1Q switchport trunk encapsulation dot1q Specifies 802.1Q encapsulation on the trunk link.
Negotiate switchport trunk encapsulation negotiate 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.