CCIE Pursuit Blog

August 8, 2007

Configuring ISL Trunks Between 3560s

Filed under: Cisco,Cisco Certification,IOS,Switching — cciepursuit @ 9:33 am

I’m making my final run through the Internetwork Expert Advanced Tech labs and this week I’m working on the switching labs.  My goal is to get a deep level understanding of the technologies before I go on to doing practice labs.  I am going through the labs very slowly and looking into the different ways to perform a task.  This means trying to explore all aspects of even the simplest tasks, such as this one:

“Configure an ISL trunk link between sw1 and sw2[on port fa0/13]”.  (Both switches are 3560s)

sw1 (3560) fa0/13—————–fa0/13 sw2 (3560)

I can think of 5 different ways to accomplish this task:

1) Hardset both switchports to trunk
2) Hardset one switchport to trunk and set the other to use DTP  dynamic auto(default)
3) Hardset one switchport to trunk and set the other to use DTP  dynamic desirable
4) Configure both switchports as DTP dynamic desirable
5) Configure one switchports as DTP dynamic desirable and the other as DTP  dynamic auto(default)

Note: Trivial solutions can be achieved with “switch nonegotiate” and “switch trunk encap isl”

By knowing the different methods to accomplish a simple task such as setting up an ISL trunk you can quickly zero in on a correct configuration when the lab gives you specific parameters.  For instance, if the task requires you to create an ISL trunk between two 3560s without using the default DTP settings and not to specify a trunking encapsulation, you would use option 4.


1) Hardset both switchports to trunk(my preferred method):

sw1:
sw1#sh run int fa0/13
Building configuration…

Current configuration : 93 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk
end

sw1#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1
—–output truncated—–

Fa0/13      1
sw1#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
—–output truncated—–

sw2:
sw2#sh run int fa0/13
Building configuration…

Current configuration : 93 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk
end

sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1
—–output truncated—–

sw2#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
—–output truncated—–

2) Hardset one switchport to trunk and set the other to use DTP  dynamic auto(default):

sw1:
sw1#sh run int fa0/13
Building configuration…

Current configuration : 93 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk
end

sw1#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1
—–output truncated—–

sw1#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
—–output truncated—–

sw2:
sw2#sh run int fa0/13
Building configuration…

Current configuration : 34 bytes
!
interface FastEthernet0/13
end

sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      auto         n-isl          trunking      1
—–output truncated—–

sw2#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: dynamic auto
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)
—–output truncated—–

3) Hardset one switchport to trunk and set the other to use DTP  dynamic desirable:

sw1:
sw1#sh run int fa0/13
Building configuration…

Current configuration : 93 bytes
!
interface FastEthernet0/13
 switchport trunk encapsulation isl
 switchport mode trunk

end

sw1#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      on           isl            trunking      1
—–output truncated—–

sw1#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
—–output truncated—–

sw2:
sw2#sh run int fa0/13
Building configuration…

Current configuration : 69 bytes
!
interface FastEthernet0/13
 switchport mode dynamic desirable
end

sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      desirable    n-isl          trunking      1
—–output truncated—–

sw2#sh int fa0/13 switch
Name: Fa0/13
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)
—–output truncated—-

4) Configure both switchports as DTP dynamic desirable:

sw1:

sw1#sh run int fa0/13
Building configuration…

Current configuration : 69 bytes
!
interface FastEthernet0/13
 switchport mode dynamic desirable
end

sw1#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      desirable    n-isl          trunking      1
—–output truncated—–

sw1#sh int fa0/13 switch
Name: Fa0/13
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)
—–output truncated—–

sw2:
sw2#sh run int fa0/13
Building configuration…

Current configuration : 69 bytes
!
interface FastEthernet0/13
 switchport mode dynamic desirable
end

sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      desirable    n-isl          trunking      1
—–output truncated—–

sw2#sh int fa0/13 switch
Name: Fa0/13
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)
—–output truncated—–

5) Configure one switchports as DTP dynamic desirable and the other as DTP  dynamic auto(default):

sw1:
Building configuration…

Current configuration : 69 bytes
!
interface FastEthernet0/13
 switchport mode dynamic desirable
end

sw1#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      desirable    n-isl          trunking      1
—–output truncated—–

sw1#sh int fa0/13 switch
Name: Fa0/13
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)
—–output truncated—–

sw2:
sw2#sh run int fa0/13
Building configuration…

Current configuration : 34 bytes
!
interface FastEthernet0/13
end

sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      auto         n-isl          trunking      1
—–output truncated—–

sw2#sh int fa0/13 switch
Name: Fa0/13
Switchport: Enabled
Administrative Mode: dynamic auto
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)
—–output truncated—–

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1 Comment »

  1. […] Configuration For ISL Trunk Building on this post, let’s explore a favorite type of lab task –  the dreaded ”minimum […]

    Pingback by Minimum Configuration For ISL Trunk « CCIE Pursuit — August 8, 2007 @ 9:48 am | Reply


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