Server Mode

Server mode, which is configured by default, allows you to create, modify, and delete VLANs for the management domain. Configuration changes are then sent to all other participating members of the VTP domain. At least one VTP server should exist in the VTP management domain. Two or more switches can be configured as servers for redundancy. When a server’s power is cycled, the switch configured as a server maintains its global VLAN information.

Transparent Mode

Transparent mode allows VTP switches to be configured to not accept VTP information and to merely forward advertisements the switch receives to other switches participating in the VTP domain. Even though switches configured for transparent mode will still send VTP information and advertisements, they will not update their databases or send out information on VLAN or topology changes. In this mode, switches do not participate in any VTP functions, such as sending VTP advertisements or synchronizing VLAN information. VTP version 2 does allow a switch operating in transparent mode to forward the advertisements it receives out any configured trunk ports, thus allowing other switches participating in VTP to obtain the advertisements.

Methods for VLAN Identification

To logically identify which packets belong to which VLAN or VLAN group, Cisco Catalyst switches support many trunking protocols to put a header on frames. The headers identify the VLAN associated with the frame; each switch in the switch block removes the header before the frame exits the access link port or ports belonging to the identified VLAN.

Dynamic Trunking Protocol

The Dynamic Trunking Protocol (DTP) is another way of allowing trunk links to carry VLAN tagged frames across multiple switches or routers. DTP manages trunk negotiation in the Catalyst Supervisor Engine software releases 4.2 and later.

With previous versions of the Catalyst Supervisor Engine software, trunk negotiation was managed by the Dynamic Inter−Switch Link (DISL) protocol. Version 4.1 of the Catalyst Supervisor Engine software allows DISL auto−negotiation of ISL trunks only. In this version, you can manually configure DISL to auto−negotiate IEEE 802.1Q trunks.

In versions 4.2 and later, DTP supports and manages IEEE 802.1Q and ISL trunk links.

InterVLAN Routing

Access layer switches occupy the same place in the network as hubs. However, unlike hubs operating at Layer 1, switches examine the frames at Layer 2 and try to process the frames. A hub simply repeats the data to every single port, whereas a switch filters the data and forwards it directly to the destination port if the destination does not require routing. If data arrives on a port assigned to a VLAN and the destination is unknown, the switch repeats the signal only to the port or ports assigned to the VLAN, except for the port of arrival.

The switch also has the ability to regenerate packets, enabling the data to be forwarded even greater distances

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than a simple hub can achieve. This ability enables a switch to take the place of another device, called a repeater. A repeater is a device dedicated only to data regeneration. Repeaters allow data to be forwarded over greater distances, allowing the data to overcome regular data distance limitations for the type of physical media being used.

By filtering frames and regenerating forwarded frames and packets, the switch can split the network into many separate collision domains. This splitting allows for greater distances, dramatically lower collision rates, and higher numbers of nodes on the network. Each VLAN in the network is its own broadcast domain, and each port is its own collision domain. In a shared or flat network, every node is part of the same collision and broadcast domain.

Switches do not have the ability to forward frames based on Layer 3 addresses or the ability to forward data from one VLAN to another. A switch must forward these frames to a Layer 3 device for a routing decision. This device can be an external or internal route processor. An external route processor is your typical router. An internal router processor is considered a “router on a stick”; it is a module inside of the switch, but the switch must access it remotely to make forwarding decisions. Let’s take a look at an internal route processor.

Internal Route Processors

When a switch receives a packet from a port on one VLAN destined for the port of another VLAN, the switch must find a path on which to send the frame. Switches work at Layer 2 and are designed to isolate traffic to collision domains or subnets; they cannot by default forward data from one VLAN to another VLAN or network without some other Layer 3 devices. The Layer 2 device known as a “router on a stick” is used to route the data and create routing tables of other networks and devices.

Route processors can be used to route data between foreign VLANs and other logically segmented parts of the network, such as subnets. They also route data to remote WAN segments, networks, or the Internet.

Quite a few types of route processors are available for Catalyst switches. They include:

∙Route Switch Module (RSM)

∙Route Switch Feature Card (RSFC)

∙Multilayer Switch Module (MSM)

∙Multilayer Switch Feature Card (MSFC)

Note InterVLAN routing using RSM, RSFC, MSM, and MSFC will be covered in Chapter 6.

How InterVLAN Routing Works

Layer 3 routing takes place between VLANs. This can become a challenging problem for an administrator to overcome. As you already learned, there are two types of route processors: external and internal. An external route processor uses an external router (such as the Cisco devices you are familiar with) to route data from one VLAN to another VLAN. An internal route processor uses internal modules and cards located inside the switch route data between VLANs.

Each type of Layer 3 routable protocol that does not have to be IP can have its own mapping for a VLAN. In an IP network, each subnetwork is mapped to an individual VLAN. In an IPX network, each VLAN is mapped to the IPX network number. With AppleTalk, a cable range and AppleTalk zone name are associated with each VLAN.

By configuring VLANs, you control the size of your broadcast domains and keep local traffic local. However, when an end station in one VLAN needs to communicate with an end station in another VLAN, this communication is supported by interVLAN routing. You configure one or more routers to route traffic to the appropriate destination VLAN.

Figure 5.4 shows Switch 1 handling traffic for a PC in VLAN 1 and Switch 2 handing traffic for VLAN 2. The router has an ISL−configured interface connecting both switches.

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