DTP negotiation on an access switchport

Masoud Pourshabanian · ‎05-25-2015

Hello Everyone,

I surfed several website to find out whether DTP communicates over an access port or not, but as I read more, I got more confused. Finally, I came across a website which says DPT is sent just once from an access port. I just came up with this idea that access port says to its neighbor that I am an access port , so shut up.

Any other idea?

Please see the website below.

http://www.kimiushida.com/bitsandpieces/articles/packet_analysis_dtp/

Thanks

Reza Sharifi · ‎05-25-2015

Hi,

Have a look at this post by Peter.

https://supportforums.cisco.com/discussion/10812906/why-dtp-used

HTH

Masoud Pourshabanian · ‎05-25-2015

Thanks for the link,

I had seen that link before. It says an access link does not send DTP package, but why "switchport nonegotiate" command can be set on an access port?

Take a look at this link and also read its comments.

http://bradhedlund.com/2007/11/27/switchport-configurations-explained/

and also the link below from ciscopress. It say an access port ****negotiates**** to convert the link into a nontrunk link

http://www.ciscopress.com/articles/article.asp?p=2181837&seqNum=8

Thanks

Reza Sharifi · ‎05-25-2015

Hi,

The link I provided it says

"as the static access ports do not send DTP packets automatically).

"static access ports"

means dynamic access ports send DTP packets.

HTH

Reza Sharifi · ‎05-25-2015

Just to follow up and give you an example

port 1/0/1 on one switch is configured with mode dynamic desirable

interface GigabitEthernet1/0/1
switchport mode dynamic desirable
spanning-tree portfast
end

sw1(config-if)#

this swtich has an SVI for vlan 5

interface Vlan5
ip address 5.5.5.3 255.255.255.0
end

port 1/0/1 on the 2nd switch is configured statically with vlan 5.

interface GigabitEthernet1/0/1
switchport access vlan 5
spanning-tree portfast
end

Switch#

this swtich has an svi for vlan 5

interface Vlan5
ip address 5.5.5.2 255.255.255.0
end

Switch#

I can ping both IPs from both switches

sw1#ping 5.5.5.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
sw1#ping 5.5.5.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/5/9 ms
sw1#

as soon as I change the first switch mode from dynamic desirable to access no ping

sw1(config-if)#do sh run int gi1/0/1
Building configuration...

Current configuration : 86 bytes
!
interface GigabitEthernet1/0/1
switchport mode access
spanning-tree portfast
end

sw1(config-if)#^Z

sw1#ping 5.5.5.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
sw1#ping 5.5.5.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
sw1#

HTH

Masoud Pourshabanian · ‎05-25-2015

The second scenario makes sense for me because of Vlan mismatch on the link between two switches. If you put interface GigabitEthernet1/0/1 in SW2 in Vlan 5, it should work.

But your first scenario? I wonder how it works because when you have mode desirable and mode access on two links, the result will be access and again you will have Vlan mismatch.

Could you check "Show interface GigabitEthernet1/0/1 switchport" on both switches to check The interfaces are trunk or access?

Thanks for your time.

Reza Sharifi · ‎05-25-2015

this is the output from the side that is configured with desirable or statcic

static mode

sw1#sh interfaces gigabitEthernet 1/0/1 switchport
Name: Gi1/0/1
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: Off
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
sw1#sh run int g1/0/1
Building configuration..

desirable mode

sw1#sh interfaces gigabitEthernet 1/0/1 switchport
Name: Gi1/0/1
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none

The other switch:

Switch#sh interfaces gigabitEthernet 1/0/1 switchport
Name: Gi1/0/1
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 5 (test-vlan)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
Switch#

so the mode becomes trunk after adding desirable.

HTH

Masoud Pourshabanian · ‎05-25-2015

Thanks for your time and accurate answer.

Navanshu Sehgal · ‎05-25-2015

I tried this in the lab and found that using debug dtp packets and the output of "show dtp interface <>" the access port does not send DTP frames unless trunking is enabled.

Here is the sample output of show dtp interface:

LAN-SW-3750X_STACK#sh dtp int g2/0/2 (when trunking is enabled)
DTP information for GigabitEthernet2/0/2:
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):       2/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
3 packets output (3 good)
      3 native, 0 software encap isl, 0 isl hardware native
0 output errors
0 trunk timeouts
3 link ups, last link up on Sun Apr 10 2011, 12:37:48
3 link downs, last link down on Sun Apr 10 2011, 12:32:26