VLAN and Routing Concepts

A VLAN is a group of stations connected using their MAC addresses, which you connect using rules (called policies).

Stations belonging to the same VLAN have unconstrained MAC connectivity with each other. If the station is not a member, no traffic is allowed. To ensure connectivity, each station must be a member of at least one VLAN. A station can also be a member of multiple VLANs. If a station is not a member of any VLAN, you can use a router to connect the station.


Example

Station A is the only station in VLAN 1 and thus cannot connect to any other station, B is connected to both C and D and E has no connectivity at all because it is not in a VLAN.

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Layer 3 and IP Links

VLANs (not physical LANs) make it easier to move stations to other physical locations because they do not require any reconfiguration. If two stations that do not share a VLAN want to communicate, a router is still required. The router is either an external device (for example, an Intel® Express Router ER8100) or the switch's internal routing.

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The implementation of the Layer 3 is in layers to support multiple protocols. The lowest layer is the VLAN, and the VLAN maps directly to a number of ports on the switch. On top of the VLAN you build Layer 3 links which lay the foundation for the next layer.

Where you can route IP from one VLAN to another (via two of these links), we have an IP link.

Once you have a Layer 3 link, you can build an IP link or IPX link on top of this (and more if required). You now have access from the ports through VLANs and through Layer 3 links to the protocol links (IP, IPX, etc).

The Express 510T Switch has one Layer 3 link and an IP link. The Express 550 Routing Switches can have up to 40 Layer 3 links and just as many IP and IPX links.

Domains

You can configure a switch to run VLAN policies locally (policies for stacks can be configured regardless of VLAN mode) so it will not look for other members of its domain. This is called a stand-alone VLAN (as opposed to the distributed VLAN).

You can run a switch with either stand-alone or distributed VLANs.

Note:

You cannot run both stand-alone and distributed VLANs on the same switch, nor can the same active VLAN-knowledge database exist in separate VLAN domains.

The VLAN policy database and auto-learning of stations physical locations enables the switch to automatically setup and control all the logical links between the relevant users.

When a user changes location, the network automatically learns the new location (based on the packets received).

Both unicast and broadcast packets are forwarded to the relevant station in the VLAN (without disturbing users in other VLANs); multicast packets are forwarded to all stations. As the number of VLANs increase, it is important to 'prune' the multicast because the router domain does not have a physical constraint. Since these VLANs do not map directly into router domains, it is possible to include the same MAC address in more than one VLAN. This is called overlapping VLANs (see below).

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Overlapping VLANs

Unfortunately, overlapping VLANs might cause problems. For example, if each VLAN represents a router-domain (see below). There is no restriction on the number of overlapping VLANs a station can be a member of.

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VLANs ensure that packets which should be flooded and multicast packets are only flooded to switches with access to specific stations; these stations belong to the VLAN to which the packet should be sent.

Globally significant information

You can configure globally significant information on all the switches. Globally significant information such as the MAC address or IP address is not specific to a particular switch.The port on a switch is not globally significant information; it is called locally significant information. To avoid configuration conflicts when switches are linked, each configuration has a version number and only the highest can edit the configuration (online) at a given time.

A switch can either run stand-alone or distributed VLANs. For distributed VLANs, the global configuration (IP and MAC address) is automatically distributed. For VLANs, the local configuration (ports) must be configured on each switch. Runtime relationships (about stations and VLANs) is distributed in the VLAN's domain.

Note (stack only):

When you have a stack local policies can be configured for the whole stack, no matter of the VLAN mode.

The switch automatically ensures that the globally significant configuration is distributed to all switches in the domain. An IP address learned by the switch is not distributed, but the MAC address is.

Note:

VLANs do NOT give the switch the ability to route between VLANs in different network address domains. In other words, a router is still needed between those domains.


More info about

VLAN FAQ


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