isl and dot1q

Unanswered Question
Jan 8th, 2009

Hi every body!

My understanding is in order for trunk to form, both ends must be configured with same trunk encapsulation i.e dotq or isl.

However, i encountered something in my book which says if one end is configured with isl and the other with dot1q, then dot1q end will regard isl encapsulated frames as giant.

My reasoning is there would not be any trunk because encapsulation mismatch .Am i correct?

thanks a lot!

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lejoe.thomas Thu, 01/08/2009 - 21:24

Hi Sarah,


The only way trunk will stay up is if its manually configured to trunk. But because of encapsulation mismatch, frames will not be correctly delivered. It's as good as ports being in the blocked state.


HTH


Lejoe

Roberto Salazar Thu, 01/08/2009 - 21:37

If one end is dot1Q and the other is ISL and you force it to turnk with "switch mode trunk" both of the interfaces will flap between trunk and non-trunk, and if you have udld configured on both interfaces they will eventually err-disable and remain down. It would not even come up. If you let the ports negotiate trunk by removing the "switch mode trunk" the interfaces will come up but it will not trunk hence the ports are access ports.

lejoe.thomas Thu, 01/08/2009 - 21:54

Hi Roberto,


When ports are manually configured to trunk and encap isl and dotl1 on first port and second port respectively, I couldnt find any flapping, when did the sample configuration between 2950 (dotlq by default) and 3550 (isl). Both ports stated to be in trunk mode with status up. Could you explain more on that?


Lejoe

Roberto Salazar Thu, 01/08/2009 - 22:05

Here see for yourself:


c6509-E#sh run int gig 4/47

Building configuration...


Current configuration : 373 bytes

!

interface GigabitEthernet4/47

switchport

switchport trunk encapsulation dot1q

logging event link-status

logging event trunk-status

logging event bundle-status

logging event spanning-tree

load-interval 30

speed 1000

duplex full

udld port aggressive

flowcontrol receive on

flowcontrol send off

end


c6509-E#sh cdp neigh

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone


Device ID Local Intrfce Holdtme Capability Platform Port ID

c7609 Gig 4/47 156 R S I CISCO7609 Gig 4/23

c6509-E#conf t

Enter configuration commands, one per line. End with CNTL/Z.

c6509-E(config)#int gig 4/47

c6509-E(config-if)#switchport mode trunk

c6509-E(config-if)#

*Jan 9 05:27:00.513: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet4/47, changed state to down

*Jan 9 05:27:03.529: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet4/47, changed state to up

*Jan 9 05:27:04.010: %DTP-SP-5-TRUNKPORTON: Port Gi4/47 has become dot1q trunk

*Jan 9 05:27:20.369: %DTP-SP-5-ILGLCFG: Illegal config(on,isl--on,dot1q) on Gi4/47

*Jan 9 05:27:21.369: %DTP-SP-5-ILGLCFG: Illegal config(on,isl--on,dot1q) on Gi4/47

*Jan 9 05:27:22.437: %DTP-SP-5-ILGLCFG: Illegal config(on,isl--on,dot1q) on Gi4/47

c6509-E(config-if)#

c6509-E(config-if)#do show int trunk


Port Mode Encapsulation Status Native vlan

Gi4/47 on 802.1q trunking 1


Port Vlans allowed on trunk

Gi4/47 1-4094


Port Vlans allowed and active in management domain

Gi4/47 1,5,10,30,100,200


Port Vlans in spanning tree forwarding state and not pruned

Gi4/47 1,5,10,30,100,200

c6509-E(config-if)#

*Jan 9 05:28:02.401: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet4/47, changed state to down

*Jan 9 05:28:02.421: %LINK-3-UPDOWN: Interface GigabitEthernet4/47, changed state to down

*Jan 9 05:28:02.913: %DTP-SP-5-NONTRUNKPORTON: Port Gi4/47 has become non-trunk



ON the other end it's the same thing, it actually went errdisable:

c7609#sh run int gig 4/23

Building configuration...


Current configuration : 355 bytes

!

interface GigabitEthernet4/23

switchport

switchport trunk encapsulation isl

logging event link-status

logging event trunk-status

logging event bundle-status

logging event spanning-tree

load-interval 30

speed 1000

duplex full

udld port aggressive

flowcontrol receive on

flowcontrol send off

end


c7609#sh cdp neigh

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone


Device ID Local Intrfce Holdtme Capability Platform Port ID

c6509-E Gig 4/23 153 R S I WS-C6509- Gig 4/47



c7609(config-if)#switch mode trunk

c7609(config-if)#

1d02h: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet4/23, changed state to down

1d02h: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet4/23, changed state to up

c7609(config-if)#

1d02h: %DTP-SP-5-TRUNKPORTON: Port Gi4/23 has become isl trunk

c7609(config-if)#

1d02h: %DTP-SP-5-ILGLCFG: Illegal config(on,isl--on,dot1q) on Gi4/23

c7609(config-if)#


c7609(config-if)#

1d02h: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet4/23, changed state to down

1d02h: %LINK-3-UPDOWN: Interface GigabitEthernet4/23, changed state to down

1d02h: %UDLD-SP-4-UDLD_PORT_DISABLED: UDLD disabled interface Gi4/23, unidirectional link detected

1d02h: %PM-SP-4-ERR_DISABLE: udld error detected on Gi4/23, putting Gi4/23 in err-disable state

1d02h: %DTP-SP-5-NONTRUNKPORTON: Port Gi4/23 has become non-trunk

c7609(config-if)#


lejoe.thomas Thu, 01/08/2009 - 23:11

Hi Roberto,


Thanks for that. Did the same configuration, however the 2950 never complains about an encapsulation mismatch.


Lejoe

Roberto Salazar Thu, 01/08/2009 - 23:15

yes, I suspect that. Bottom line is this is a mis-configuration and unpredictable things will happen.

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