VLAN frequently asked questions


Are there any cases where a Distributed VLAN will not work?

Distributed VLAN domains will not span across routers, through switches in stand-alone mode or switches running another domain.

To satisfy Spanning Tree requirements, switches should not exceed the seven-hop limit. Since most applications are building a tree rather than a chain, you can still have a large number of switches in a VLAN domain (The firmware supports a maximum 200 switches in one domain).


What protocol is running?

The protocol is proprietary. The information carried by this protocol includes switch discovery, configuration updates and learning information.

This is the process that identifies other switches configured for distributed VLAN and matching domain. This process assigns the state SVL-port to any port connecting to another switch in the same domain. (All regular ports are named access ports.)

Configuration updates and the configuration control part of the protocol ensures that configuration takes place on only one switch at a time (since the configuration is shared among all the switches in the domain). When transferring the configuration changes to the other switches, a simple TFTP transfer is used.

Distributing the learned information synchronizes the filters in all the switches in a distributed VLAN domain. Each time a switch learns a new station, this information is forwarded to the MAC-VLAN server in the domain. This server ensures that all switches are informed of the changes in the learned information. The information exchanged here is MAC addresses and their VLAN relationship.


What does the use of this protocol cost?

There are some situations during start-up when the switches transmit packets every second. But for a stable network, the MAC-VLAN server transmits a hello packet (as a proprietary multicast) every two seconds to all switches in the domain, and the VLAN-PORT mapping entity transmits a packet every seven seconds between neigboring switches.

During configuration, live packets are transmitted between the VLAN server and the configuring client every second.


Is there a "master" to distribute the VLAN information?

Yes, a MAC-VLAN server is selected from the switches in a domain. The selection is based on a configurable priority (configurable because you might prefer a central switch to become the server) and the MAC addresses of the switches. The latter is used to ensure that only one switch becomes server even if priorities are all the same in the domain.

The MAC-VLAN server is also the server for configuration control. This ensures that only one switch in the domain is allowed to change the shared VLAN configuration at any given time. The mechanism is a simple token system, where a switch needs to obtain control of this token before making changes. The server also informs the switches in the domain about the current configuration version and state.


What are policy-based VLANS? How do they work?

The policy-based VLAN scheme is designed for network administrators who need to maintain control over constantly changing networks. It helps reduce TCO every time they add/drop/move users. A policy-based VLAN can be a combination of ports or MAC addresses or IP nets or IP hosts, making it flexible for a wide variety of applications. The following policies illustrate just a couple of applications:

Policy # 1: Separate AppleTalk users from rest of the network, except the printer, which they share with the rest of the network

Policy: Form a VLAN#1 of all the ports to which the AppleTalk users are attached, AND include the MAC address of the printer in the VLAN. Group the rest of the ports and the IP address of the printer to another VLAN#2.

Results: The AppleTalk traffic will be confined within VLAN#1. When an AppleTalk broadcast is forwarded to the switch, it will pass the traffic to the ports that are contained in VLAN#1 (and only those ports) as well as the printer. This allows the printer to respond to AppleTalk broadcasts as well as to the clients from the rest of the network.

Policy #2: Create a departmental VLAN that does not need re-configuration when a user moves.

Policy: Form a VLAN with MAC addresses of all the users in the department.

Result: This policy assumes that all of the users in the department being moved will retain the same computers after the move. Because the VLAN is based on their MAC addresses, rather than the switch ports which they are attached to, the switch will re-configure the VLAN automatically, every time a user in that department moves to another port on the switch. Hence, this policy helps reduce work for the network administrator.


Do Express 500 switches support overlapping VLANs?

Yes. Express 500 policy-based VLANs are very flexible. An administrator can assign each switch port, connected MAC address, IP net, or IP host to several different VLANs.


Does the number of VLANs scale with the number of switches in the stack?

No, the reason is that every switch in the stack belongs to a single logical switch and has to share the same VLAN table throughout the stack. The maximum number of VLANs that a switch or a stack supports is 128.


More info about

VLAN and routing concepts


Procedures

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