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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 10 Configuring LAN Ports for Layer 2 Switching
Understanding How Layer 2 Switching Works
Understanding VLAN Trunks
These sections describe VLAN trunks on the Cisco 7600 series routers:
• Trunking Overview, page 10-3
• Encapsulation Types, page 10-3
Trunking Overview
Note For information about VLANs, see Chapter 14, “Configuring VLANs.”
A trunk is a point-to-point link between the router and another networking device. 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 ports:
• Inter-Switch Link (ISL)—ISL is a Cisco-proprietary trunking encapsulation.
Note The following switching modules do not support ISL encapsulation:
• WS-X6502-10GE
• WS-X6548-GE-TX, WS-X6548V-GE-TX, WS-X6548-GE-45AF
• WS-X6148-GE-TX, WS-X6148V-GE-TX, WS-X6148-GE-45AF
• 802.1Q—802.1Q is an industry-standard trunking encapsulation.
You can configure a trunk on a single Ethernet port or on an EtherChannel. For more information about
EtherChannel, see Chapter 12, “Configuring EtherChannels.”
Ethernet trunk ports support several trunking modes (see Table 10-2 on page 10-4). You can specify
whether the trunk uses ISL or 802.1Q encapsulation, and if the encapsulation type is autonegotiated.
Note You can configure LAN ports to negotiate the encapsulation type. You cannot configure WAN interfaces
to negotiate the encapsulation type.
The Dynamic Trunking Protocol (DTP) manages trunk autonegotiation on LAN ports. DTP supports
autonegotiation of both ISL and 802.1Q trunks within a VTP domain.
To autonegotiate trunking, the LAN ports must be in the same VTP domain. Use the trunk or
nonegotiate keywords to force LAN ports in different domains to trunk. For more information on VTP
domains, see Chapter 13, “Configuring VTP.”
Encapsulation Types
Table 10-1 lists the Ethernet trunk encapsulation types.
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