Will Connecting MX84 to 2 Cisco switch cause loop?

[Ebrithil]

I have an MX84 and thinking of connecting 2 Cisco switch to its LAN port. On the 2 switches I will set the uplink to MX84 as trunk mode. I was thinking if this can cause a switching loop?

I have looked into how the BPDU packets pass through MX. The MX receive the BPDU packet and just re-forwards it to the same broadcast domain as to where it was received, so this will send BPDU packet to the other switch until it gets back from the sender of the BPDU packets and will cause a loop.

I have thought of enabling BPDU filter on the interface pointing to the MX but this will still not stop the the loop from occurring. Please see below for the topology that I have. (Assuming that a port is blocked by STP)

[KarstenI]

What you describe is exactly how STP behaves. The BPDU have to get back to the switches so that a port can go into blocking on one of the switches. It can not block on the MX as the MX does not use STP.

Just make sure that BPDUs pass through, for example if you configure a Trunk on the MX without allowing VLAN1 that can also cause a loop that doesn't get blocked.

[Ebrithil]

Thank you for your response. Sorry if this is confusing. On the current topology, what I wanted to know is that if I configured trunk mode on the uplinks to the MX will this cause a switching loop.

[KarstenI · Accepted solution]

No, it doesn't have to as long you don't break STP. I would expect that most deployments have a trunk to the MX. Either because the core-routing is done on the MX for smaller deployments, or because multiple VLANs (Transfer to Core, Guests, internal DMZs) terminate on the MX.

[Ebrithil]

Thank you very much for your help 🙂

[PhilipDAth]

I would avoid dual connecting the MX84 for reliability.  It will come back to bite you 1 or 2 times a year with outages.