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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

  • What is the router model and configuration? – Ron Maupin Jan 24 '18 at 22:41
  • It was a juniper SRX210 – Ahmed Jan 24 '18 at 22:46
  • Sorry.. can't remember of the configs, but assume no STP was configured. It was still stop the ARP packet as it is a broadcast right – Ahmed Jan 24 '18 at 22:47
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    This is a hard lesson to not disable STP, even if you are not using it. It can act as a last-resort failsafe. – Ron Maupin Jan 24 '18 at 23:05
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    It's possible that the router/firewall has some bridging enabled. Please post the configuration so we can see what's going on. – Ron Trunk Jan 25 '18 at 15:42
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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.
  • ahhhhh. Thank you very much. Make's sense now, because i didn't know some ports on router can be configured as a switch group. It all makes sense. I am just a junior network engineer, just tryna learn. We live and learn by the day. Thanks :) – Ahmed Jan 26 '18 at 20:25
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Yes, a router stops all broadcast packets which are identified with the destination address: FF:FF:FF:FF:FF:FF

Another answer to a similar question answers this in great detail here: How does a router prevent broadcast radiation.

Quote:


When a router receives a packet, it gets inspected, then forwarded out the appropriate interface or it gets dropped. When a router receives a broadcast packet, it drops it (excluding directed-broadcasts, dhcp, etc).

When a switch receives a frame, it either forwards it on to a known interface or floods it out all of its ports if it doesn't know where to go. When a broadcast frame comes along, it get's flooded out all interfaces. Every machine in your segment sees it. Excessive amounts of these constitute a storm.

The most common way for a broadcast storm to happen is from a switching loop. If you somehow get a switching loop on your network, these broadcasts will perpetually send this data back and forth forever, or until you remove the loop. This will cause data to hit every machine on your segment. This can cause your network to stop.

When you have a router in between multiple layer 2 segments, each is inherently protected from the other. Remember, a router won't forward on broadcasts. For instance: simple illustration of two switches connected to the same router
LAN 1 can be all sorts of messed up, and LAN 2 will be none the wiser because ROUTER won't forward LAN 1 broadcast packets on to anyone.

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Just for clarity, it is a bit misleading to say a router stops broadcasts. The rule is that a router will only forward broadcast packets onto the interfaces that lead back into the same network that the packet came from. For example, if a router has two interfaces on the same network, it will forward a broadcast coming from one of those devices to the other.

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    Not usually. Two routed ports connected to the same network don't forward broadcasts (by default). Two bridged ports do forward broadcasts. – Zac67 Feb 2 '18 at 6:28

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