Does a Router Stop Broadcast? – Spanning-Tree and Broadcast Guide

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Well I was at work.
And one of our customers connected a new device to that router. Additionally a switch was already connected to the router.
So there was only the switch connected to the router, then he connected another device which requested arp.
I got told no STP was configured.
Of course that would cause a loop if it was in between two switches. However doesn't the router stop the broadcast if the packet was reached on its interface.
Becasue i was checking on the router's CPU and it was quit high trying to process all broadcast arp that was going around in loops from the switch and the device.
I was so confused as to why the router didn't stop the broadcast packet?

Thanks

Best Answer

A router does stop broadcasts (unless configured otherwise).

However, it's not quite clear from your question how everything was connected. Multiple LAN ports on a router can often be configured to be a switch group - usually you'd connect multiple clients or switches to the router without connecting the switches. These switch group ports act like a switch (connecting parts of the same segment) and not like a router (connecting multiple subnets).

From the manual:

Interfaces ge-0/0/1 and fe-0/0/2 through fe-0/0/7 (port 0/1 through port 0/7) are configured as switched interfaces in a common VLAN on which the IP address 192.168.1.1/24 is configured.

Let this be a lesson:

  • Have STP active at all times unless only a single port (to a segment/VLAN) exists on a device.
  • Know how your equipment is set up and do not let anyone else fool with it.
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