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Trunk and Access Link between Switches

Posted on 2014-10-04
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Last Modified: 2014-10-04
As shown on the topology below, I have 2 Cisco Switches linked together.
I have R3 and R4 configured as hosts with No IP Routing command.
I can ping from R3 to R4  and back.
Initially I thought a link between 2 switches is always a Trunk, but I do not see a Trunk formed between Switch1 and Switch2:

SW
Switch1#sh interfaces fastEthernet 1/1 trunk

Port      Mode         Encapsulation  Status        Native vlan
Fa1/1     off          802.1q         not-trunking  1

Port      Vlans allowed on trunk
Fa1/1     1

Port      Vlans allowed and active in management domain
Fa1/1     1

Port      Vlans in spanning tree forwarding state and not pruned
Fa1/1     1
Switch1#

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Switch2#show interfaces fastEthernet 1/0 trunk

Port      Mode         Encapsulation  Status        Native vlan
Fa1/0     off          802.1q         not-trunking  1

Port      Vlans allowed on trunk
Fa1/0     1

Port      Vlans allowed and active in management domain
Fa1/0     1

Port      Vlans in spanning tree forwarding state and not pruned
Fa1/0     1
Switch2#

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Question by:jskfan
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Predrag Jovic earned 250 total points
ID: 40361085
You have flat LAN, only VLAN is VLAN1, which is there by default. So, all hosts are in the same VLAN.

Link between switches don't have to be trunk.
Even auto-negotiation of links as trunks  is not recommended (as it is by default) by Cisco.

But if you want switching in GNS3 it is not so easy.

You need to use router with EtherSwitch  module.
http://www.youtube.com/watch?v=f868S9NnmHg

And even then you will not have full switch functionality. It is recommended for learning switching to have real Cisco switches.
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by:Don Johnston
Don Johnston earned 250 total points
ID: 40361116
Initially I thought a link between 2 switches is always a Trunk, but I do not see a Trunk formed between Switch1 and Switch2:
For just about all IOS switches, this is true as DTP is enabled by default.  But I suspect that you're running GNS. In which case all bets are off since switches in GNS are simulated.
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Author Comment

by:jskfan
ID: 40361319
Correct I am using Switch 3700 with slot NM-16ESW on GNS3
So when I linked the switches , will their ports be Access ports but in the same Vlan 1 ?
I can ping between R3 and R4 and back, so thy got to be in the same Vlan:

Switch1#sh interface fastEthernet 1/1 switchport
Name: Fa1/1
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: Disabled
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1
Priority for untagged frames: 0
Override vlan tag priority: FALSE
Voice VLAN: none
Appliance trust: none
Switch1#

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Switch2#sh interfaces fastEthernet 1/0 switchport
Name: Fa1/0
Switchport: Enabled
Administrative Mode: static access
Operational Mode: static access
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: native
Negotiation of Trunking: Disabled
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Trunking VLANs Active: 1
Priority for untagged frames: 0
Override vlan tag priority: FALSE
Voice VLAN: none
Appliance trust: none
Switch2#

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by:Don Johnston
Don Johnston earned 250 total points
ID: 40361324
Administrative Mode: static access
Operational Mode: static access

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This means that the ports are statically configured as access ports and not using DTP to negotiate trunking.  I'm not sure if switch modules can do DTP.  You could always try "switchport mode dynamic desirable" and see if it accepts it.
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LVL 27

Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 250 total points
ID: 40361328
For switching (at least some parts) Cisco packet tracer is better switch simulator for learning switching.
Just in case you didn't know.
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Author Comment

by:jskfan
ID: 40361335
Switch1(config-if)#switchport mode ?
  access  Set trunking mode to ACCESS unconditionally
  trunk   Set trunking mode to TRUNK unconditionally

I believe the Switch simulator on GNS3 does not have the mode dynamic desirable for an interface..

Regarding Packet Tracer 6.1.1, it has this command:
Switch(config-if)#switchport mode dynamic ?
  auto       Set trunking mode dynamic negotiation parameter to AUTO
  desirable  Set trunking mode dynamic negotiation parameter to DESIRABLE

 but Packet Tracer  lacks other commands that the simulated Switch on GNS3 has.
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LVL 27

Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 250 total points
ID: 40361342
You have some commands in packet tracer, and you have some commands in GNS3. Combine both while learning (preparing for exam). Some command are missing on both. That's way I would recommended  you to buy some inexpensive switches from eBuy if you can.
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Assisted Solution

by:Don Johnston
Don Johnston earned 250 total points
ID: 40361347
I believe the Switch simulator on GNS3 does not have the mode dynamic desirable for an interface..
That's what I suspected.  But it could be that the module doesn't support DTP.
but Packet Tracer  lacks other commands that the simulated Switch on GNS3 has.
It's really impossible to find a single emulator/simulator platform that will duplicate every environment and configuration.  GNS is about the best for routers.  Switches are the real challenge.
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Author Closing Comment

by:jskfan
ID: 40361358
Thank you Guys!!
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