Wired Intelligent Edge

Hi all.

What kind of config are you using with regards to flow control and rate limit (broadcast limit) on your edge switches?

I have been recommended by various vendors to use flow control on all switch ports, whether it's connected to other switches, servers, PCs etc.

We are also using a limit of 20% on broadcast for all ports.

So a typical edge port is configured like this on our switches:

interface 1
   flow-control
   rate-limit bcast in percent 20
   untagged vlan xxx
   spanning-tree admin-edge-port
   spanning-tree bpdu-filter bpdu-protection
   loop-protect
   exit

20 percent seems to be the figure often mentioned in doco.

If you check a Layer2 segment that has about 1000 devices on it, you will find the background broadcast rate (mostly ARP) is about 5-7%.

So 20 is pretty generous.

If you use nice /24 subnets, then it should never get anywhere near 5%.

If you're talking VLANs where you're allowing server guys to patch their badly-configured servers in, you can easily get broadcasts of over 1Gb when they have crappy asymmetric routing/interface addressing mismatches. So the most important thing is to keep the servers segregated away from everything else.

Also, make sure you are dealing with multicast properly - design your multicast topology and turn off querier on all but the multicast cores. Enable IGMP. Otherwise multicast is going to be just like broadcast on your network, and you really don't know how much of it is going on, but it can include multimedia, so it's potentially very significant.

Thanks Vince. All our servers are in their own VLANs, and we use only /24 subnets. I will have a look at multicast config.

I just realised what I wrote was kind of meaningless - I said the background "noise" rate on a large broadcast segment was 5-7%....

5-7% of what???

What I meant was 5-7Mb/s.

The point is this isn't enough to really bother 100Mb interfaces. On ther other hand, it will completely knacker 10Mb interfaces, eg, scientific equipment, medical devices, and other instruments.

In any case, normal people try to use /24 subnets, so broadcast noise should never be much of a problem and you can safely use a much lower number than 20%, especially on Gb links, which are pretty much the norm nowadays.