Encapsulation Types - Cisco Catalyst 3550 series Software Configuration Manual

Configuring VLAN Trunks

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
The switch does not support Layer 3 trunks; you cannot configure subinterfaces or use the encapsulation
Note
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:
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Catalyst 3550 Multilayer Switch Software Configuration Guide
12-18
lists the Ethernet trunk encapsulation types and keywords.
Function
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.
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.
Make sure 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 potentially cause spanning-tree loops. We recommend that you
leave spanning tree enabled on the native VLAN of an 802.1Q trunk or disable spanning tree on
every VLAN in the network. Make sure your network is loop-free before disabling spanning tree.
Chapter 12 Configuring VLANs
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