Cisco configuring: VLAN Trunking Protocol and Inter-VLAN Communication

Tip

The default mode is dependent on the platform. For the 2960/9200 series, the default mode is dynamic auto.

Tip

On a 2960/9200 series 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 and that other side is set to trunk or desirable. It is therefore recommended that you hard-code 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.

Tip

VLAN Trunking Protocol (VTP) domain names must match for a DTP to negotiate a trunk.

Caution

Cisco ISL has been deprecated. Depending on the age and model of your Cisco switch, you may be able to change the encapsulation type between dot1q and ISL.

Tip

With the switchport trunk encapsulation negotiate command set, the preferred trunking method is ISL.

Caution

The 2960, 2960-x, and 9200 series of switches support only dot1q trunking.

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:

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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

Caution

Due to the inherent risk in having VTP servers overwrite each other and cause VLANs to disappear, Cisco recommends as a best practice deploying VTP in transparent mode. If you are going to use a client/server model, use Version 3 and the use of a VTPv3 primary server to prevent accidental database overwrites.

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.

Note

The networks of the VLANs are directly connected to the router. Routing between these networks 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 networks associated with these VLANs appear in the routing table as directly connected networks.

Note

In production environments, VLAN 1 should not be used as the management VLAN because it poses a potential security risk; all ports are in VLAN 1 by default, and it is an easy mistake to add a nonmanagement user to the management VLAN.

Note

Rather than using an external router to provide inter-VLAN communication, a multilayer switch can perform the same task through the use of a switched virtual interface (SVI).