Advanced Configuration and Management Guide

Static Ports

Static ports are permanent members of the protocol VLAN. The ports remain active members of the VLAN regardless of whether the ports receive traffic for the VLAN’s protocol. You must explicitly identify the port as a static port when you add it to the VLAN. Otherwise, the port is dynamic and is subject to aging out.

In addition, static ports never “leak” broadcast packets of other protocol types. (See “Broadcast Leaks” on page 16-10.)

Excluded Ports

If you want to prevent a port in a port-based VLAN from ever becoming a member of a protocol, IP sub-net, IPX network, or AppleTalk cable VLAN configured in the port-based VLAN, you can explicitly exclude the port. You exclude the port when you configure the protocol, IP sub-net, IPX network, or AppleTalk cable VLAN.

Broadcast Leaks

Dynamic ports differ from static ports in an important way. Static ports never allow broadcasts for protocols other than the protocol of the VLAN to be forwarded on the port. Thus, an IP protocol VLAN forwards only IP broadcast packets and never broadcasts any Layer 3 broadcasts of other protocol types. If you want to ensure that no broadcasts other than those of the VLAN’s protocol get through, use static ports.

Dynamic ports “leak” every eighth broadcast packet of another protocol type through the port. Thus, if an IP protocol VLAN receives eight AppleTalk broadcast packets, the VLAN port drops the first seven packets but sends the eighth packet. This behavior enables a PC, Macintosh computer, or workstation that joins the network to find its servers, even if the LAN segment the device is on is configured as part of a protocol VLAN for a different protocol. For example, if a few of your network users have Macintosh computers, they can still find their printers or other servers even if the network segment they are on is part of an IP protocol VLAN.

The VLAN ports maintain separate counters for each protocol. Thus, if a port in an IP protocol VLAN receives four AppleTalk broadcast packets and four DECnet broadcast packets, the port still does not forward any of the packets. Only when the port receives eight AppleTalk broadcast packets or eight DECnet broadcast packets does the port send the eighth packet of that protocol type.

Figure 16.8 shows an example of a Layer 3 IP protocol VLAN with dynamic ports. Since the ports have dynamic membership, they are “leaky”. They forward every eighth broadcast packet of non-IP protocols. For example, when the Macintosh computer sends its eighth broadcast packet, the VLAN forwards the packet. In a VLAN with static ports, the VLAN never forwards broadcast packets of other protocol types.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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User-configured port-based VLAN

Active Dynamic Ports

Candidate Ports

Figure 16.8 Protocol VLAN with “leaky” (dynamic) ports

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