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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 14 Co n figuring VLANs
Configur ing VLAN Trunks
To avoid t his, yo u s hould configu re int erfac es conn ect ed t o devices tha t do no t sup port DTP to n ot
forward D TP frame s, tha t i s, to t urn off DTP.
•If you do not int end to trunk ac ross thos e links, use th e switchport mode access interface
configurati on comm a nd t o d isable tr unk ing.
•To enabl e tr unk ing t o a d evice tha t do es no t sup por t DTP, use th e switchport mode trunk and
switchport nonegotiate inter face co nf igurati on comman ds to cause the inter face to be come a trun k
but to not generate DTP fra mes.
Note On GigaSt ack GB ICs, dyn am ic tr unkin g i s on ly supp ort ed w hen o nl y one p ort o f a G igaStac k GBI C i s
being used. If trunking is required on a GigaStack GBIC where both ports are in use, use the switchport
mode trunk and switchport nonegotiate interface configuration commands on both GBIC interfaces to
cause the interfaces to become trunks.

802.1Q Configuration Considerations

802.1 Q trunks impos e these lim itations on t he trunkin g strategy for a net work:
•In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance
of spann ing tree for e ach V LAN a ll owed on the trunk s. N on- Cisco devices mi ght su pport 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
comb ines the sp anni ng-tr ee in stan ce o f the V LAN o f the t runk wi th th e span nin g-tr ee in stanc e of
the no n-Cisc o 802 .1Q swi tch. Ho we v er , s panni ng-tre e inform ation f or each VL AN is ma intained by
Cisco swit ches se para ted by a c loud of non-C is co 80 2.1 Q sw itche s. T he no n- Cisco 80 2.1 Q c loud
separating the Cisco switches is treated as a single trunk link between the switches.
•Make sur e t he nat ive VLAN f or an 80 2. 1Q tru nk is t he s am e on bo th ends of the tr unk li nk. I f the
native VLA N on one end of t he trunk i s different from the na tive VLAN on the other end,
spanning-tree loops migh t result.
Table 14-4 Layer 2 Interface Modes
Mode Function
switchport mode access Puts th e i nte rface (ac cess por t) in to perm an en t no ntru nki ng mode and negot ia tes to
convert the link into a nontrunk link. The interface becomes a nontrunk interface even if
the neighboring interface is not a trunk interface.
switchport mode dynamic
desirable 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. The default switch-port mode for all Ethernet interfaces is dynamic desirable.
switchport mode dynamic auto Make s the interf ace able to convert th e link to a trunk li nk. The interf ace becom es a trunk
interface if the neighboring interface is set to trunk or desirable mode.
switchport mode trunk Puts the interf ac e int o perman ent tr unk ing mode an d negotiates to convert the link into a
trunk link. The interface becomes a trunk interface even if the neighboring interface is
not a trunk interface.
switchport nonegotiate Prevents the interface from generating DTP frames. You can use this command only when
the interface switchport mode is access or trunk. You must manua ll y con figure th e
neighboring interface as a trunk interface to establish a trunk link.