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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 9 Configuring the Switch Interfaces
Unders tanding Interface Types
These sections describes these types of interfaces:
•Access Por ts, page 9-2
•Trunk Po rts, page 9 -2
•Port-Ba sed VLA Ns , pa ge 9-3
•EtherCha nne l Po rt Gro ups, p age 9-3
•Connecting Interfaces, page 9-3

Access Ports

An access port belongs to and carries the traf fic of only one VLAN. T raff ic is receiv ed and sent in native
format s with no VLAN ta gging. Traffic arriving on an access port is assume d to belong to the VLAN
assigned to the port. If an access port receives a tagged packet (Inter-Switch Link [ISL] or 802.1Q
tagge d), the pac ket is dropp ed, th e source ad dress is no t learn ed, and th e fram e is counte d in the No
destination st atisti c. An access po rt can forwar d a tagged packet (802.1P an d 802.1Q ).
Two types of ac cess ports are sup por ted :
•Static access ports are manually assigned to a VLAN.
•VLAN m embe rsh ip of d ynami c a cce ss por ts is l ear ned th rough i ncomi ng p ac kets. By de fau lt, a
dynamic access por t is a me mb er of n o VL AN, an d f or w ar d in g to an d from th e p ort is en ab led o nl y
when the VLAN membership of the port is discovered. Dynamic access ports on the switch are
assigned to a VLAN by a VLAN Membership Policy Server (VMPS). The VMPS can be a
Catalyst 6000 series swit ch; the Catal yst 2950 switch does not suppor t the function of a V MPS.

Trunk Ports

A trunk port carries the traff ic of multiple VLANs and by default is a member of all VLANs in the VLAN
database . O nly IE EE 802. 1Q tru nk por t s are su pport ed. A n IEE E 80 2.1Q tru nk por t sup port s
simulta neous tag ged a nd un tag ged t raffic. An 8 02.1Q tr unk po rt is assi g ned a d efault Port VLA N ID
(PVID), a nd a ll untag g ed tr affic trav el s o n the p o rt def a ult PVI D. All untagged tra ffic and tag ged tr affic
with a NULL VLAN ID are assumed to belong to the port default PVID. A packet with a VLAN ID equal
to th e out go ing por t de fault PVI D i s s ent unta gg ed. A ll o t her t r affic i s sen t wi th a VL AN t ag.
Although by defau lt, a tr unk por t is a me mber of every VLA N k nown to th e VT P, you can li mit VLA N
member ship by configurin g an allowed list of VLA Ns for eac h trunk port . The list of allowed VLANs
does not affect any other port but the associated trunk port. By default, all possible VLANs (VLAN ID 1
to 1005 when t he standard sof tware ima ge [SI] is insta lled or VLAN ID 1 to 4094 wh en the enh anced
software im age [E I] is inst alled) ar e in the al lowed list. A tr unk port ca n only beco me a mem ber of a
VLAN if VT P kno ws of the VL AN and the VLAN is in the enabled s tate. If VTP le arns of a ne w , enabled
VLAN and the VLAN is in the allowed list for a trunk port, the trunk port automatically becomes a
member of that VLAN and traffic is forwarded to and from the trunk port for that VLAN. If VTP learns
of a new, enabled VLAN that is not in the allowed list for a trunk port, the port does not become a
member of the VLAN, and no traffic for the VLAN is forwarded to or from the port.
Note VLAN 1 cannot be excluded from the allowed list.
For more information about trunk ports, see Chapter 14, “Configuring VLANs.”