Trunking at Layer 2 basically comes in one of two flavors either dynamic or static. With dynamic trunking protocol or DTP developed by Cisco Systems switches can form Layer 2 trunks using either the industries standard IEEE 802.1Q or Cisco’s proprietary ISL protocol. Basically If a port can become a Layer 2 Trunk DTP provides this ability to negotiate the trunking method with the other device. This example demonstrates disabling the Layer 2 DTP negotiation, and manually configuring the trunking between switches.
Operational Trunking Status
- TOS = Trunk Operational Status
- TAS = Trunk Administrative Status
- TNS = Trunk Negotiation Status
- TOT = Trunk Operational Type
- TAT = Trunk Administrative Type
- TNT = Trunk Negotiation Type
The following example demonstrates how to disable the Layer 2 DTP messages between switches that are connected via a Trunk. I’m going to disable DTP on switch SW1, and then check the interface GigabitEthernet1/0/1 to verify that DTP frames have been disabled.
SW1#configure terminal
SW1(config)#interface gi1/0/1
SW1(config-if)#switchport nonegotiate
SW1(config-if)#switchport mode trunk
SW1(config-if)#speed 1000
SW1(config-if)#duplex full
SW1(config-if)#end
SW1#
Now lets take a look at the trunk negotiation on Switch SW1 after disabling DTP.
SW1#show dtp interface gi1/0/1
DTP information for GigabitEthernet1/0/1:
TOS/TAS/TNS: TRUNK/NONEGOTIATE/TRUNK
TOT/TAT/TNT: 802.1Q/802.1Q/802.1Q
Neighbor address 1: 000000000000
Neighbor address 2: 000000000000
Hello timer expiration (sec/state): never/STOPPED
Access timer expiration (sec/state): never/STOPPED
Negotiation timer expiration (sec/state): never/STOPPED
Multidrop timer expiration (sec/state): never/STOPPED
FSM state: S6:TRUNK
# times multi & trunk 0
Enabled: yes
In STP: no
Statistics
----------
0 packets received (0 good)
0 packets dropped
0 nonegotiate, 0 bad version, 0 domain mismatches,
0 bad TLVs, 0 bad TAS, 0 bad TAT, 0 bad TOT, 0 other
0 packets output (0 good)
0 native, 0 software encap isl, 0 isl hardware native
0 output errors
0 trunk timeouts
1 link ups, last link up on Mon Mar 01 1993, 00:10:13
0 link downs
SW1#
Now lets configure SW1’s GigiabitEthernet1/0/1 port with DTP enabled.
SW1#configure terminal
SW1(config)#interface gi1/0/1
SW1(config-if)#no switchport mode trunk
SW1(config-if)#switchport dynamic auto
SW1(config-if)#end
SW1#
Now lets take a look at the interface after enabling DTP negotiation again.
SW1#show dtp interface gi1/0/1
DTP information for GigabitEthernet1/0/1:
TOS/TAS/TNS: TRUNK/ON/TRUNK
TOT/TAT/TNT: 802.1Q/802.1Q/802.1Q
Neighbor address 1: 000000000000
Neighbor address 2: 000000000000
Hello timer expiration (sec/state): 13/RUNNING
Access timer expiration (sec/state): never/STOPPED
Negotiation timer expiration (sec/state): never/STOPPED
Multidrop timer expiration (sec/state): never/STOPPED
FSM state: S6:TRUNK
# times multi & trunk 0
Enabled: yes
In STP: no
Statistics
----------
0 packets received (0 good)
0 packets dropped
0 nonegotiate, 0 bad version, 0 domain mismatches,
0 bad TLVs, 0 bad TAS, 0 bad TAT, 0 bad TOT, 0 other
28957 packets output (28957 good)
28957 native, 0 software encap isl, 0 isl hardware native
0 output errors
0 trunk timeouts
1 link ups, last link up on Mon Mar 01 1993, 00:10:12
0 link downs
SW1#
My personal rule of thumb regarding the use of DTP is to turn it off on Trunks ports. I prefer to have total control of the Trunk ports between Switches, as well as the Trunk ports between Switches and Routers. I also disable DTP between Switches and Wireless LAN Controllers. Basically any port that needs to encapsulate Layer 2 VLAN information in the ethernet header.
I hope you found this post on Trunking helpful and informative. Be sure to let me know what you think by leaving suggestions, and feedback in the comments section below. You can find out more about these and other articles be checking out recent posts and archives. To learn more about me be sure to check out the About page. And as always thanks again for visiting The Packet.
