Summary

In this chapter you have learned about one of the fundamentals of modern day switchingtrunking. Trunks are high-speed highways of the LAN that aggregate VLAN traffic onto a single Layer 2 link. You have also learned about bandwidth aggregation of LAN interfaces and how this technique can improve the performance and redundancy of your network without costly upgrades to next generation switching speeds.

Cisco Catalyst switches support the proprietary Cisco Inter-Switch Link (ISL) trunking protocol, as well as the standards based 802.1Q protocol. Trunk ports can use Dynamic Trunking Protocol (DTP) frames to negotiate whether a trunk is formed and the encapsulation (ISL of 802.1Q) used. Trunk ports can exist in several modes that determine how they negotiate trunking. The on or off mode explicitly forces a trunk on or off, with the on mode supporting the use of DTP frames. The nonegotiate mode explicitly forces a trunk on and disables the use of DTP frames. The auto and desirable modes determine whether a port passively (auto) uses DTP or actively (desirable) uses DTP to negotiate a trunk.

VLAN Trunking Protocol (VTP) is used to propagate VLAN databases over trunk interfaces. This protocol allows the centralized configuration of VLANs, with the resulting configurations automatically propagated out to the remaining switches in the network. VTP messages are propagated within a VTP domain, which is a collection of switches under common administrative control. For VTP to work, various parameters such as the domain name, version, password, and pruning must be configured identically throughout a VTP domain. VTP pruning allows for the automatic pruning of VLAN traffic from trunks that don't service a particular VLAN and, therefore, don't need to receive the traffic for that VLAN across a trunk. This pruning allows your network links to be optimized for the specific topology and distribution of users in your network.

EtherChannel is Cisco's proprietary bandwidth aggregation protocol, which all Cisco Catalyst switches support. EtherChannel forms bundles by using the Port Aggregation Protocol (PAgP), with each port operating in a specific PAgP mode. PAgP works by exchanging frames with remote ports to determine whether a bundle can be formed or not. The on or off mode explicitly forces an EtherChannel bundle on or off and disables the use of PAgP. The auto and desirable modes determine whether a port passively (auto) uses PAgP or actively (desirable) uses PAgP. The recommended PAgP mode for interswitch links between Cisco Switches is desirable because it handles link failures correctly.