Trunk Agreement Compatibility
In best assembly networks, about-face trunks are configured application the aforementioned accepted throughout the
network. For instance, rather than acquiesce DTP to accommodate trunking,, abounding engineers configure
trunk interfaces to consistently block (switchport approach trunk) and attenuate DTP on ports that should
not trunk. IOS includes several commands that appulse whether a accurate articulation becomes a
trunk. Because abounding enterprises use a archetypal standard, it is accessible to balloon the nuances of how the
related commands work. This area covers those baby details.
Two IOS agreement commands appulse if and back two switches anatomy a trunk. The switchport
mode and switchport nonegotiate interface subcommands ascertain whether DTP alike attempts to
negotiate a trunk, and what rules it uses back the attack is made. Additionally, the settings on
the about-face ports on either ancillary of the articulation behest whether a block forms or not.
Table 2-9 summarizes the block agreement options. The aboriginal cavalcade suggests the configuration
on one switch, with the aftermost cavalcade advertisement the agreement options on the added about-face that would
result in a alive block amid the two switches.
Configuring Trunking on Routers
VLAN trunking can be acclimated on routers and hosts as able-bodied as on switches. However, routers do not
support DTP, so you charge manually configure them to abutment trunking. Additionally, you must
manually configure a about-face on the added end of the articulation to trunk, because the router does not
participate in DTP.
Table 2-9 Trunking Agreement Options That Lead to a Alive Trunk
Configuration Command
on One Side1
Short
Name Meaning
To Trunk,
Other Side
Must Be
switchport approach block Block Consistently trunks on this end; sends
DTP to advice added ancillary accept to
trunk
On, desirable,
auto
switchport approach trunk;
switchport nonegotiate
Nonegotiate Consistently trunks on this end; does not
send DTP letters (good when
other about-face is a non-Cisco switch)
On
switchport approach dynamic
desirable
Desirable Sends DTP messages, and trunks
if agreement succeeds
On, desirable,
auto
switchport approach dynamic
auto
Auto Replies to DTP messages, and
trunks if agreement succeeds
On, desirable
switchport approach admission Admission Never trunks; sends DTP to help
other ancillary ability aforementioned conclusion
(Never trunks)
switchport approach access;
switchport nonegotiate
Access (with
nonegotiate)
Never trunks; does not accelerate DTP
messages
(Never trunks)
1When the switchport nonegotiate command is not listed in the aboriginal column, the absence (DTP agreement is active)
is assumed.
NOTE If an interface trunks, again the blazon of trunking (ISL or 802.1Q) is controlled by the
setting on the switchport block encapsulation command. This command includes an option
for dynamically negotiating the blazon (using DTP) or configuring one of the two types. See
Example 2-5 for a sample of the syntax.