Download Troubleshooting LAN Protocols

Dynamic Trunk Protocol (DTP)
ƒ What is DTP?
Automates ISL/802.1Q trunk configuration; operates between switches
Does not operate on routers; not supported on 2900XL or 3500XL
ƒ DTP synchronizes the trunking mode on link ends (i.e., native VLAN
mismatch, VLAN range mismatch, encapsulation, etc.)
ƒ DTP state on ISL/dot1Q trunking port can be set to “auto”, “on”, “off”,
“desirable”, or “non-negotiate”
ƒ Runs over link layer; assumes point-to-point link
ƒ DTP destination mac address is 01-00-0C-CC-CC-CC
ƒ Port should be able to operate as an access port—
to fall back to access mode
ƒ During negotiation do not participate in STP
ƒ VLAN1 should be added to trunk; in ISL DTP pkts send on VLAN1 and for
access or 802.1Q on native vlan
ƒ The HDLC protocol type for DTP is 0x2004 which is the SNAP format
BRKRST-3131
14513_04_2008_c2
© 2008 Cisco Systems, Inc. All rights reserved.
Cisco Public
29
DTP Packet Capture
BRKRST-3131
14513_04_2008_c2
© 2008 Cisco Systems, Inc. All rights reserved.
© 2006, Cisco Systems, Inc. All rights reserved.
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Cisco Public
30
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