Trunk links on Catalyst switches can be manually configured for either ISL or 802.1Q mode. However, Cisco has implemented a proprietary point-to-point protocol called Dynamic Trunking Protocol (DTP) that will negotiate a common trunking mode between two switches. DTP is available in Catalyst supervisor engine software Release 4.2 and later. DTP negotiation should be disabled if a switch has a trunk link connected to a router because the router cannot participate in the DTP negotiation protocol.
NOTE A trunk link can be negotiated between two switches only if both switches belong to the same VLAN Trunking Protocol (VTP) management domain. VTP is discussed in the "VTP Configuration" section of this chapter. If the two switches are in different VTP domains and trunking is desired between them, the trunk links must be set to on or nonegotiate mode. This setting will force the trunk to be established. These options are explained in the next section.
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