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SITUATION: I have two switches SW1 and SW2. These two switches are connected with two trunk ports. While SW1 is connected to PC1, SW2 is connected to PC2. 

If PC1 sends a broadcast frame to PC2 via SW1. SW1 transmits the frame to SW2. However, recall i have two connecting trunk ports. So when the frame from SW1 gets to SW2 through one of the trunking ports. The frame is flooded out of all of SW2 ports except that it was received on. Hence, the frame loops back to SW1 and this cycle continues causing Broadcast storm.

QUESTION: ASIDE STP, HOW CAN I AVERT LOOP ON THIS TOPOLOGY.

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There are three basic approaches here:

  1. Block the redundant link(s) using a spanning-tree protocol - MSTP, RSTP, STP or RPVST+.
  2. Aggregate the physical links into a single, logical link (link aggregation group/LAG by LACP, static, EtherChannel, ...). That way, forwarding a broadcast to all ports but the one it was received on applies to the entire LAG and no broadcast returns to SW1.
  3. Use a forwarding scheme that is based on link-state routing (Dijkstra's algorithm), like Shortest Path Bridging or TRILL.
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QUESTION: ASIDE STP, HOW CAN I AVERT LOOP ON THIS TOPOLOGY.

Today one of the two ports between sw1 and sw2 should be blocked by STP.

If you don’t want one of the two trunk ports blocked by STP, put both trunk ports on an LACP bundle on both sides of the links.

The frame is flooded out of all of SW2 ports except that it was received on. Hence, the frame loops back to SW1 and this cycle continues causing Broadcast storm.

If you get a broadcast storm, you have done something wrong on the trunk ports connecting the two switches. Spanning tree is like brakes in a car… if you refuse to use your brakes in traffic, you crash.

Likewise you must use a protocol like STP to break layer2 switched loops (or bundle your multiple interfaces as an LACP interface to break loops).

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