Encapsulation Types; Ieee 802.1Q Configuration Considerations - Cisco 3020 - Catalyst Blade Switch Configuration Manual

Configuring VLAN Trunks
Table 10-4 Layer 2 Interface Modes
Mode
switchport mode access
switchport mode dynamic auto
switchport mode dynamic
desirable
switchport mode trunk
switchport nonegotiate

Encapsulation Types

Table 10-5
Table 10-5 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
The trunking mode, the trunk encapsulation type, and the hardware capabilities of the two connected
interfaces decide whether a link becomes an ISL or IEEE 802.1Q trunk.

IEEE 802.1Q Configuration Considerations

The IEEE 802.1Q trunks impose these limitations on the trunking strategy for a network:
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Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
10-16
Function
Puts the interface (access port) into permanent nontrunking mode and negotiates to
convert the link into a nontrunk link. The interface becomes a nontrunk interface
regardless of whether or not the neighboring interface is a trunk interface.
Makes the interface able to convert the link to a trunk link. The interface becomes a trunk
interface if the neighboring interface is set to trunk or desirable mode. The default
switchport mode for all Ethernet interfaces is dynamic auto.
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 IEEE 802.1Q encapsulation on the trunk link.
an ISL (preferred) or IEEE 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 IEEE 802.1Q trunks, the switches maintain one
spanning-tree instance 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 IEEE 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 IEEE 802.1Q switch. However, spanning-tree information for each VLAN
is maintained by Cisco switches separated by a cloud of non-Cisco IEEE 802.1Q switches. The
non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated as a single trunk link between
the switches.
Chapter 10 Configuring VLANs
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