ISL and 802.1Q Configuration
Cisco switches use the Dynamic Trunk Protocol (DTP) to dynamically learn whether the device
on the other end of the cable wants to perform trunking and, if so, which trunking protocol to use.
DTP learns whether to trunk based on the DTP mode defined for an interface. Cisco switches
default to use the DTP desirable mode, which means that the switch initiates sending DTP
messages, hoping that the device on the other end of the segment replies with another DTP message.
If a reply is received, DTP can detect whether both switches can trunk and, if so, which type of
trunking to use. If both switches support both types of trunking, they choose to use ISL. (An
upcoming section, “Trunk Configuration Compatibility,” covers the different DTP modes and how
they work.)
With the DTP mode set to desirable, switches can simply be connected, and they should
dynamically form a trunk. You can, however, configure trunking details and verify the results with
show commands. Table 2-8 lists some of the key Catalyst IOS commands related to trunking.
46 Chapter 2: Virtual LANs and VLAN Trunking
Figure 2-4 lists several details regarding Switch1’s trunking configuration and status, as shown in
Example 2-5. R1 is not configured to trunk, so Switch1 will fail to negotiate trunking. Switch2 is
a Catalyst 3550, which supports both ISL and 802.1Q, so they will negotiate trunking and use ISL.
Switch3 and Switch4 are Catalyst 2950s, which support only 802.1Q; as a result, Switch1
negotiates trunking, but picks 802.1Q as the trunking protocol.
Figure 2-4 Trunking Configuration Reference for Example 2-5
Table 2-8 VLAN Trunking–Related Commands
Command Function
switchport | no switchport Toggle defining whether to treat the interface as a switch
interface (switchport) or as a router interface (no switchport)
switchport mode Sets DTP negotiation parameters
switchport trunk Sets trunking parameters if the interface is trunking
switchport access Sets nontrunking-related parameters if the interface is not
trunking
show interface trunk Summary of trunk-related information
show interface type number trunk Lists trunking details for a particular interface
show interface type number
switchport
Lists nontrunking details for a particular interface
Example 2-5 Trunking Configuration and show Command Example–Switch1
! The administrative mode of dynamic desirable (trunking) and negotiate (trunking
! encapsulation) means that Switch1 attempted to negotiate to trunk, but the
! operational mode of static access means that trunking negotiation failed.
! The reference to “operational trunking encapsulation” of native means that
! no tagging occurs.
SW1
Gi0/1
R1
Not Configured
to Trunk
Defaults to DTP Desirable
Supports ISL or .1Q
Defaults to DTP Desirable
Does Not Support ISL (2950)
Defaults to DTP Desirable
Does Not Support ISL (2950)
Fa0/12
Fa0/24
Fa0/1
SW2
SW3 SW4
VLAN Trunking: ISL and 802.1Q 47
Switch1# show int fa 0/1 switchport
Name: Fa0/1
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Voice VLAN: none (Inactive)
Appliance trust: none
! Next, the show int gig 0/1 trunk command shows the configured mode
! (desirable), and the current status (N-ISL), meaning negotiated ISL. Note
! that the trunk supports the extended VLAN range as well.
Switch1# show int gig 0/1 trunk
Port Mode Encapsulation Status Native vlan
Gi0/1 desirable n-isl trunking 1
Port Vlans allowed on trunk
Gi0/1 1-4094
Port Vlans allowed and active in management domain
Gi0/1 1,21-22
Port Vlans in spanning tree forwarding state and not pruned
Gi0/1 1,21-22
! Next, Switch1 lists all three trunks – the segments connecting to the other
! three switches – along with the type of encapsulation.
Switch1# show int trunk
Port Mode Encapsulation Status Native vlan
Fa0/12 desirable n-802.1q trunking 1
Fa0/24 desirable n-802.1q trunking 1
Gi0/1 desirable n-isl trunking 1
Port Vlans allowed on trunk
Fa0/12 1-4094
continues
Example 2-5 Trunking Configuration and show Command Example–Switch1 (Continued)
Fa0/24 1-4094
Gi0/1 1-4094
Port Vlans allowed and active in management domain
Fa0/12 1,21-22
Fa0/24 1,21-22
Gi0/1 1,21-22
Port Vlans in spanning tree forwarding state and not pruned
Fa0/12 1,21-22
Fa0/24 1,21-22
Gi0/1 1,21-22