LAN Switch Configuration 191
Dynamic Inter-Switch Link (DISL) protocol. DISL manages automatic ISL trunk negotiation.
The syntax for the trunk fast Ethernet interface configuration subcommand is as follows:
switch(config)# trunk [on
|
off
|
desirable
|
auto
|
nonnegotiate]
The options for the trunk command function are as follows:
·
on--Configures the port into permanent ISL trunk mode and negotiates with the
connected device to convert the link to trunk mode.
·
off--Disables port trunk mode and negotiates with the connected device to convert the
link to nontrunk.
·
desirable--Triggers the port to negotiate the link from nontrunking to trunk mode. The
port negotiates to a trunk port if the connected device is either in the on, desirable, or auto
state. Otherwise, the port becomes a nontrunk port.
·
auto--Enables a port to become a trunk only if the connected device has the state set to
on or desirable.
·
nonegotiate--Configures a port to permanent ISL trunk mode, and no negotiation takes
place with the partner.
As seen in the list, many options exist. Choices for these options are mostly personal
preference. Because trunks seldom change, my preference is to configure either on or off.
Figure 4-29 and Example 4-10 provide an expanded sample network, along with the additional
configuration required.
Notice that not only was trunking enabled on both Fast Ethernet ports, but each of the three
VLANs was statically configured on those ports. By also configuring the VLANs, the switch
treats the trunk ports as part of those VLANs. The router also must be configured to support
ISL, as seen earlier in this chapter.
To verify a trunk configuration, use the show trunk privileged EXEC command to display the
trunk parameters, as demonstrated in Example 4-10. The syntax is as follows:
Switch1(config)# show trunk [a
|
b]
The parameters a and b represent the Fast Ethernet ports:
·
Port a represents Fast Ethernet 0/26.
·
Port b represents Fast Ethernet 0/27.
ch04.fm Page 191 Monday, March 20, 2000 5:02 PM