Encapsulation Types; 802.1Q Configuration Considerations - Cisco WS-C3560-48PS-S Software Configuration Manual

Configuring VLAN Trunks
Table 12-4 Layer 2 Interface Modes (continued)
Mode
switchport mode dynamic
desirable
switchport mode trunk
switchport nonegotiate

Encapsulation Types

Table 12-5
Table 12-5 Ethernet Trunk Encapsulation Types
Encapsulation
switchport trunk encapsulation isl
switchport trunk encapsulation dot1q
switchport trunk encapsulation negotiate Specifies that the interface negotiate with the neighboring interface to become
Note The switch does not support Layer 3 trunks; you cannot configure subinterfaces or use the encapsulation
keyword on Layer 3 interfaces. The switch does support Layer 2 trunks and Layer 3 VLAN interfaces,
which provide equivalent capabilities.
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.

802.1Q Configuration Considerations

802.1Q trunks impose these limitations on the trunking strategy for a network:
•
Catalyst 3560 Switch Software Configuration Guide
12-18
Function
Makes the interface actively attempt to convert the link to a trunk link. The interface
becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto
mode.
Puts the interface into permanent trunking mode and negotiates to convert the
neighboring link into a trunk link. The interface becomes a trunk interface even if the
neighboring interface is not a trunk interface.
Prevents the interface from generating DTP frames. You can use this command only
when the interface switchport mode is access or trunk. You must manually configure the
neighboring interface as a trunk interface to establish a trunk link.
lists the Ethernet trunk encapsulation types and keywords.
Function
Specifies ISL encapsulation on the trunk link.
Specifies 802.1Q encapsulation on the trunk link.
an ISL (preferred) or 802.1Q trunk, depending on the configuration and
capabilities of the neighboring interface. This is the default for the switch.
In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance
of spanning tree for each VLAN allowed on the trunks. Non-Cisco devices might support one
spanning-tree instance for all VLANs.
When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch
combines the spanning-tree instance of the VLAN of the trunk with the spanning-tree instance of
the non-Cisco 802.1Q switch. However, spanning-tree information for each VLAN is maintained by
Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud
separating the Cisco switches is treated as a single trunk link between the switches.
Chapter 12 Configuring VLANs
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