Specifications

CHAPTER 2

VLAN Implementation

in either the ISL encapsulation or the 802.1Q tag. The switch on the other end of the trunk removes the ISL or 802.1Q

information, checks the VLAN of the frame, and adds the internal tag. If the exit port is a user port, the original frame is

sent out unchanged, making the use of VLANs transparent to the user.

If a nontrunking port receives an ISL-encapsulated packet, the port cannot remove the ISL header. By default, the system

installs ISL system CAM entries and drops ISL packets. In special, rare circumstances, these CAM entries are installed

for every active VLAN in the switch. To prevent such collisions, enter the no-isl-entries enable command on

switches connected to other switches. If the ISL header and footer cause the MTU size to be exceeded, it might be

counted as an error.

If a nontrunking port receives an 802.1Q frame, the source and destination MAC addresses are read, the tag field is

ignored, and the frame is switched normally at Layer 2.

Configuring a Trunk Link

Ports can become trunk ports either by static configuration or dynamic negotiation using Dynamic Trunking Protocol

(DTP). A switch port can be in one of five DTP modes:

n Access: The port is a user port in a single VLAN.

n Trunk: The port negotiates trunking with the port on the other end of the link.

n Non-negotiate: The port is a trunk and does not do DTP negotiation with the other side of the link.

n Dynamic Desirable: Actively negotiates trunking with the other side of the link. It becomes a trunk if the port on

the other switch is set to trunk, dynamic desirable, or dynamic auto mode.

n Dynamic Auto: Passively waits to be contacted by the other switch. It becomes a trunk if the other end is set to

trunk or dynamic desirable mode.

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CCNP SWITCH 642-813 Quick Reference by Denise Donohue

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