TIP: The default mode is dependent on the platform. For the 2960, the default mode is dynamic auto.
TIP: On a 2960 switch, the default for all ports is to be an access port. However, with the default DTP mode being dynamic auto, an access port can be converted into a trunk port if that port receives DTP information from the other side of the link if that side is set to trunk or desirable. It is therefore recommended to hardcode all access ports as access ports with the switchport mode access command. This way, DTP information will not inadvertently change an access port to a trunk port. Any port set with the switchport mode access command ignores any DTP requests to convert the link.
Setting the Encapsulation Type
Depending on the series of switch that you are using, you might have a choice as to what type of VLAN encapsulation you want to use: the Cisco proprietary Inter-Switch Link (ISL) or the IEEE Standard 802.1q (dot1q). The 2960 switch supports only dot1q trunking.
TIP: With the switchport trunk encapsulation negotiate command set, the preferred trunking method is ISL.
CAUTION: The 2960 series switch supports only dot1q trunking.
VLAN Trunking Protocol (VTP)
VTP is a Cisco proprietary protocol that allows for VLAN configuration (addition, deletion,
or renaming of VLANs) to be consistently maintained across a common administrative domain.
Using Global Configuration Mode
NOTE: Only VLANs included in the pruning-eligible list can be pruned. VLANs 2 through 1001 are pruning eligible by default on trunk ports. Reserved VLANs and extended-range VLANs cannot be pruned. To change which eligible VLANs can be pruned, use the interface-specific switchport trunk pruning vlan command:
Switch(config-if)#switchport trunk pruning vlan remove 4, 20-30
! Removes VLANs 4 and 20-30
Switch(config-if)#switchport trunk pruning vlan except 40-50
! All VLANs are added to the pruning list except for 40-50
Verifying VTP
NOTE: If trunking has been established before VTP is set up, VTP information is propagated throughout the switch fabric almost immediately. However, because VTP information is advertised only every 300 seconds (5 minutes), unless a change has been made to force an update, it can take several minutes for VTP information to be propagated.
Inter-VLAN Communication Using an External Router: Router-on-a-Stick
NOTE: The subnets of the VLANs are directly connected to the router. Routing between these subnets does not require a dynamic routing protocol. In a more complex topology, these routes need to either be advertised with whatever dynamic routing protocol is being used or be redistributed into whatever dynamic routing protocol is being used.
NOTE: Routes to the subnets associated with these VLANs appear in the routing table as directly connected networks.
Inter-VLAN Communication Tips
- Although most routers support both ISL and dot1q encapsulation, some switch models only support dot1q (the 2950 and 2960 series, for example).
- If you need to use ISL as your trunking protocol, use the command encapsulation isl x, where x is the number of the VLAN to be assigned to that subinterface.
- Recommended best practice is to use the same number of the VLAN number for the subinterface number. It is easier to troubleshoot VLAN 10 on subinterface fa0/0.10 than on fa0/0.2.
- The native VLAN (usually VLAN 1) cannot be configured on a subinterface for Cisco IOS Software releases that are earlier than 12.1(3)T. Native VLAN IP addresses therefore need to be configured on the physical interface. Other VLAN traffic is configured on subinterfaces:
Router(config-if)#encapsulation dot1q 1 native
Router(config-if)#ip address 192.168.1.1 255.255.255.0
Router(config-if)#interface fastethernet 0/0.10
Router(config-subif)#encapsulation dot1q 10
Router(config-subif)#ip address 192.168.10.1 255.255.255.0
Configuration Example: Inter-VLAN Communication
Figure 13-1 illustrates the network topology for the configuration that follows, which shows
how to configure inter-VLAN communication using commands.
ISP Router
CORP Router
L2Switch1 (Catalyst 2960)
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