01-08-2009 08:59 PM - edited 03-06-2019 03:20 AM
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!
01-08-2009 09:24 PM
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
01-08-2009 09:37 PM
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.
01-08-2009 09:54 PM
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
01-08-2009 10:05 PM
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)#
01-08-2009 11:11 PM
Hi Roberto,
Thanks for that. Did the same configuration, however the 2950 never complains about an encapsulation mismatch.
Lejoe
01-08-2009 11:15 PM
yes, I suspect that. Bottom line is this is a mis-configuration and unpredictable things will happen.
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