Cisco Systems RJ-45-to-AUX manual Vlan Trunking, Trunk Types

Cisco also recommends a one−to−one ratio between VLANs and subnets. This means that you must understand how users are broken up by subnets. If you have 1,000 users in a building and 100 users are in each subnet, then you should have 10 VLANs.

VLAN Trunking

There are two types of VLAN links: a trunk link and an access link. An access link is part of only one VLAN, referred to as the native VLAN of the port. All the devices are attached to an access link, which connects your physical workstation to the network. Access link devices are totally unaware of a VLAN membership, or that a switched network exists at all. The devices only know that they are part of a broadcast domain. They have no understanding of the network they are attached to and don’t need to know this information.

Tip Remember, an access link device cannot communicate with devices outside of its VLAN or subnet without the use of a router or internal route processor.

Trunk links, on the other hand, can carry multiple VLANs. A trunk link is a link that carries all the VLANs in a network and tags each frame as it enters the trunk link and spans the network. You probably have heard this term used in telephone systems. The trunk link of a telephone system carries multiple telephone conversation and lines on a single cable. Trunk links that connect switches and carry VLANs to other switches, routers, or servers use the same theory.

When an administrator assigns a port to a VLAN, that port can be a member of only one VLAN. In order for VLANs to span multiple connected switches, a trunk link must be used. This link cannot be used to connect to the average Network Interface Card (NIC) found on the back of the PC.

Frame tagging is used when a frame travels between two devices that support a trunked link. Each switch that the frame reaches must be able to identify the VLAN the frame is a member of based on the tagging information, in order to determine what to do with the frame and how to apply it to the filtering table.

Because the trunk link uses frame tagging to identify which VLAN a frame belongs to, each device connecting to the trunk link must be able to interpret and read this VLAN tag. Intel has created some NICs for servers that understand the frame tagging involved with a trunk link. However, in most situations, this trunk link tagging is removed at the Access layer switch, and the destination address never knows that the frame it received was tagged with information to allow it to span the switch fabric.

What happens if the frame reaches a switch or router that has another trunk link? The device will simply forward the frame out of the proper trunk link port. Once the frame reaches a switch at the Access layer, the switch will remove the frame tagging. It does this because the end device needs to receive the frames without having to understand the VLAN tagging. Remember, the end device (such as a workstation) does not understand this frame tagging identification.

If you are using NetFlow switching hardware (discussed in Chapter 6) on your Cisco switches, it will allow devices on different VLANs to communicate after taking just the first packet through the router. The router will then send the correct routing information back to the NetFlow device. This process allows the router to be contacted only once to let VLAN frames be routed from port to port on a switch, rather than from port to router and back to the port for each frame.

Trunk Types

Trunk links are point−to−point, high−speed links from 100 to 1000Mbps. These trunked links between two switches, a switch and a router, or a switch and a server carry the traffic of up to 1,005 VLANs at any given time.

Four different methods or protocols allow you to track VLAN frames as they traverse the switch fabric:

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