CCIE R&S Written 1.30 Implement Trunk and Trunk Protocols, EthernChannel and load-balance

DTP – Dynamic Trunking Protocol

The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, AND for negotiating the type of trunking encapsulation to be used. It works on the Layer 2 of the OSI model. VLAN trunks formed using DTP may utilize either IEEE 802.1Q or Cisco ISL trunking protocols.
DTP should not be confused with VTP, as they serve different purposes. VTP communicates VLAN existence information between switches. DTP aids with trunk port establishment. Neither protocol transmits the data frames that trunks carry.

DTP negotiation can be disabled two ways, with the switchport mode access command, or with the switchport nonegotiate command. This design is most commonly used when a switch is trunking to a device that does not support DTP, such as an IOS router’s routed interface (not an Ethernet Switch interface), or a server’s NIC card.

Types of switchport operational modes are as follows:-

Dynamic desirable (default mode on Catalyst 2950 and 3550)
Dynamic auto (default mode on Catalyst 3560)
dotq-tunnel (Not an option on the Catalyst 2950.)
Using these different trunking modes, an interface can be set to trunking or nontrunking or using DTP to negotiate trunking with the neighboring interface.

Commands of interest are :-

Operational Modes

switchport nonegotiate
switchport mode access
switchport mode dot1q-tunnel
switchport mode dynamic auto
switchport mode dynamic desirable
switchport mode trunk

Encapsulation Negotiation

switchport trunk encapsulation negotiate
switchport trunk encapsulation dot1q
switchport trunk encapsulation isl

Viewing the settings of a port would be achieved using :-

show interface fastethernet 0/1 switchport

Viewing the trunks on the switch and their respective settings can be achieved using

show interface trunks

DTP Grid:

Switchport Mode Access Dynamic Desirable Dynamic Auto Trunk
Access No Trunk No Trunk No Trunk No Trunk
Dynamic Auto No Trunk Trunk No Trunk Trunk
Dynamic Desirable No Trunk Trunk Trunk Trunk
Trunk No Trunk Trunk Trunk Trunk


Two negotiation protocols exist for EtherChannel, PAgP and LACP. This is analogous to the trunk protocols of ISL and 802.1q. PAgP is proprietary as is ISL, LACP is an open standard and is used by 802.3ad and 802.1q is also an open standard.

Negotiation can also be turned off and ports forced into EtherChannel.

channel-group 1 mode active                                                            LACP
channel-group 1 mode auto | auto non-silent                            PAgP
channel-group 1 mode desirable | desirable non-silent         PAgP
channel-group 1 mode on
channel-group 1 mode passive                                                         LACP

Verification of the mode used on your EtherChannel configuration can be viewed by :-

show etherchannel protocol

A more comprehensive overview of EtherChannel configuration would be to use:-

show etherchannel summary

Configuration of the EtherChannel group is then completed using the command:-

interface port-channel 1

Where you can find normal interface configuration commands which get propagated to all interfaces in the channel group should they be executed.

Configuring Load-Balancing for the EtherChannel can be configured using the global command:-

port-channel load-balance dst-ip
port-channel load-balance dst-mac
port-channel load-balance src-dst-ip
port-channel load-balance src-dst-mac
port-channel load-balance src-ip
port-channel load-balance src-mac

Number of Ports Load Balancing
8 EQUAL 1:1:1:1:1:1:1:1
7 2:1:1:1:1:1:1
6 2:2:1:1:1:1
5 2:2:2:1:1
4 EQUAL 2:2:2:2
3 3:3:2
2 EQUAL 4:4

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