network protocols handbook.pdf

270Protocols GuideCisco ProtocolsProtocol NameDTP: Cisco Dynamic TrunkingProtocolProtocol DescriptionDynamic Trunking Protocol (DTP), a Cisco proprietary protocolin the VLAN group, is for negotiating trunking on a link betweentwo devices and for negotiating the type of trunking encapsulation(802.1Q) to be used.There are different types of trunking protocols. If a port can becomea trunk, it may also have the ability to trunk automaticallyand, in some cases, even negotiate what type of trunking to useon the port. This ability to negotiate the trunking method with theother device is called dynamic trunking.The first issue is that both ends of a trunk cable had better agreethey’re trunking, or they’re going to be interpreting trunk framesas normal frames. End stations will be confused by the extra taginformation in the frame header, their driver stacks won’t understandit, and the end systems may lock up or fail in odd ways.To resolve this problem, Cisco created a protocol for switches tocommunicate intentions. The first version of it was VTP, VLANTrunking Protocol, which worked with ISL. The newer versionworks with 802.1q as well, and is called Dynamic Trunking Protocol(DTP).The second issue is creating VLAN’s. To configure VLAN’s individuallyswitch by switch, it is lot of work and easy to causeinconsistency, in that VLAN 100 could be Engineering on oneswitch, and Accounting on another. That would be a source ofconfusion in troubleshooting, and might also defeat your carefullycrafted VLAN security scheme. This issue is also addressedby VTP/DTP. You can create or delete a VLAN on one switch,and have the information propagate automatically to a group ofswitches under the same administrative control. This group ofswitches would be a VTP domain.Protocol StructureOn a Catalyst set-based switch, the syntax for setting up a linkas a trunk is:set trunk mod_num/port_num [on | desirable | auto | nonegotiate][isl | dot1q | negotiate] [vlan_range]Use this command to set the specified port or ports to trunking.The first set of keyword arguments govern the DTP modes:offdesirableautononegotiateForces the link to permanently not trunk, even if theneighbor doesn’t agreeCauses the port to actively attempt to become a trunk,subject to neighbor agreement (neighbor set to on,desirable, or auto )Causes the port to passively be willing to convert totrunking. The port will not trunk unless the neighbor isset to on or desirable. This is the default mode. Notethat auto-auto (both ends default) links will not becometrunks.Forces the port to permanently trunk but not sendDTP frames. For use when the DTP frames confusethe neighboring (non-Cisco) 802.1q switch. You mustmanually set the neighboring switch to trunking.The second set of keywords governs the type of VLAN taggingto use: ISL, 802.1q, or negotiate which to use.Related protocolsIEEE 802.1Q, VTP, ISL, DISLSponsor SourceDTP is a Cisco protocol.Referencehttp://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008019f048.html#1017196Understanding and Configuring VLAN Trunking Protocolhttp://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_5_2/cofigide/e_trunk.htmconfiguring VLAN TrunksonModeWhat the Mode DoesForces the link into permanent trunking, even if theneighbor doesn’t agree