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Cisco Bug: CSCuq78959 - "ISL" trunking encapsulation causing layer 2 loop in 151-2.SY3

Last Modified

Sep 14, 2019

Products (1)

  • Cisco Catalyst 6000 Series Switches

Known Affected Releases

15.1(1)SY2.9 15.1(2)SY3

Description (partial)

Symptom:
If we connect two ports (default switchport mode dynamic desirable) in a module (that supports both "isl" and dot1Q") with a hard loop then they negotiate a trunk link as expected but they agree on encapsulation type "isl". At this stage both these ports move to Designated/forwarding state where we expect one of them  to move to Back/BLK state. BPDU statistics on both these ports indicate that they only send BPDUs but they do not see a single BPDU from the other, which might makes the port believe that they are connected to a host, hence they move to STP forwarding state. 

As soon as we hardcode the trunking encapsulation type to dot1Q (switcport mode is still dynamic desirable) OR downgrade the software to SXI releases, they immediately start seeing BPDUs from each other and block one of these two ports.. 

The STP behavior with "ISL" type trunking encapsulation need to be corrected  in  15.1 train (as it is there in SXI releases). Release note of 15.1(1)SY indicates that it does not support "ISL". If "ISL" is really not supported by design then probably we need to change the default trunk negotiation type to "dot1Q" and need to disable any option of ?isl? encapsulation type.

Conditions:
SW release 15.1 train

Connect two ports (default switchport mode dynamic desirable) in a module (that supports both "isl" and dot1Q") with a hard loop .
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