Meraki MS - L2 Broadcast Behaviour

[Pavan_Pawar]

Hello Experts,

I'm doing POC for switches before using them in production.

We are doing broadcast testing but got weird results.

I have configured the LAN interfaces 1,2 and 3 and passed access VLAN 100 on them.

When I'm sending a broadcast packet from interface 1, Ideally it should be received at interfaces 2 and 3 but broadcast randomly being sent to interface 2 or sometimes to 3 not to both at the same time.

Packet - 

mac_src aa:aa:aa:aa:aa:10 mac_dst ff:ff:ff:ff:ff:ff 

ipv4_src 101.10.10.10 ipv4_dst 101.10.10.100

Even when I send an ARP broadcast from interface 1 it also gives random results, the packet only received at interface 2, not at 3

packet - mac_src aa:aa:aa:aa:aa:10 mac_dst ff:ff:ff:ff:ff:ff eth_type 0x806

arp_tpa 101.10.10.1 ipv4_src 101.10.10.10

Is the above behaviour expected?

Is there any document that describes the BUM(Broadcast/Unicast/Multicast) traffic behaviour on Meraki MS switches?

SW :  MS 15.21.1

HW : MS250-24P

-Pavan Pawar

[DarrenOC]

hi @Pavan_Pawar - do you have anything configured under Storm-Control which could be limiting the Broadcast traffic?

[Pavan_Pawar]

Storm-Control is not configured, the behaviour I stated earlier is the same so you can also test in your lab

[DarrenOC]

hi @GreenMan , @Ryan_Miles , @GiacomoS - any input on this one from the Meraki collective?

[GiacomoS]

Hey folks!

Ooohhh, this is a fun one. Pavan, do you happen to have taken a packet capture on the receiving port upstream from port 2 and 3? I'd just like to confirm that this is not a capture problem ("cosmetic"), rather than the actual traffic not being forwarded. 

This is basic switching behaviour here, if what you are observing is in fact happening, I'd expect a lot more problems.

Re the documentation, we would not be deviating from what the RFC demands in terms of handling any sort of BUM.

If you are still seeing the same behaviour with the upstream captures, I'd suggest to have a case open as well, so we can do a proper analysis. 

Keep me posted!

Giac

[Pavan_Pawar]

I have packet capture setup at all three interfaces so when I send a broadcast packet from interface 1, either I see it received at interface 2 or sometimes at 3

Can you test in the lab?

This is also the same for multicast.

[JacekJ]

How do you capture the packets? By interfaces you mean capturing via Meraki dashboard or and the end device?

This is crucial, I've seen weird things in the dashboard captures sometimes, so you should capture data on the end devices to be on the safe side and I think its what @GiacomoS is suggesting.

[ww]

Ms switch dashboard captures are really bad. Missing packets. 1 way traffic etc.  You should take the captures on each client connected to the ports

[DarrenOC]

Worth configuring port mirroring to capture the traffic across all configured ports?

[GiacomoS]

Hey folks,

So, especially in a situation of congestion we prioritise getting traffic out of the door, so packet captures are one of the first things that suffer the consequences of prioritisation. 

If we are trying to unravel whether there is a problem, we definitely need a capture on the other side of the port (from a client, or another switch, doesn't matter too much as long as it can do a capture) , and then we can compare it with what you are observing right now, as it could be just a cosmetic problem.

Pavan, we don't yet have a clear problem statement here, we would need the data to corroborate what you are observing before we can proceed to lab and investigate further in a Support case. 

I'd recommend following the tips you got throughout the thread so far before we move on to the next steps!

Let us know how you get on! 😁

Giac

[Pavan_Pawar]

Problem Description:  Wierd L2 BUM traffic Behaviour

SW - MS 15.21.1

HW - MS250-24P

Topology :

                                 Interface 3

                                         |

                                         | L2 Vlan 100

Interface 1 -----------Switch MS 250----------- Interface 2

                                        |

                                        |

                                   Interface 4

All ports are part of VLAN 100(access port), Nothing else is configured on the switch like storm control.

On all ports, the Servers are connected which can capture the packets.

This is my lab device hence there is no production traffic or any other traffic load on the switch.

Summary :

• When we send broadcast traffic from Interface 1, It is received on some of the interfaces not all(like sometimes I see the broadcast packet received on Interface 2 or 3 or 4 sometime 3 and 4).
• I have a packet capture setup to see on what interface I receive the broadcast.

Packet Test:

Injected packet from interface 1 - 

mac_src aa:aa:aa:aa:aa:10 mac_dst ff:ff:ff:ff:ff:ff

eth_type 0x800 ip_version 4

vlan_vid untagged

ipv4_src 101.10.10.10 ipv4_dst 101.10.10.100 

ip_proto 6

I received the above packet on interface 3 only(validated through captures setup on all connected servers.). 

At the same time, I took the combined captures on the Dashboard for interfaces 2,3,4. it also shows one packet.
Ideally, it should show three broadcast packets as the captures are for combined interfaces 2,3,4.

I just want to confirm if you see this behaviour in your LAB for the MS switch.

Regards,

Pavan Pawar

[GIdenJoe]

I think you need non Meraki switches at each port participating in the test and have them to the port mirroring so you have an honest setup so you can verify the actual behavior.