broadcasts

carl_townshend · ‎06-17-2008

Hi all, when having broadcasts on a lan, what is the normal exeptable limit on a lan ?

grichardson661 · ‎06-17-2008

Hi Carl,

I think Cisco advise to have a maximum of 500 users per broadcast domain (subnet).

Somebody please correct me if i'm wrong.

Cheers,

Garry

grichardson661 · ‎06-17-2008

In fact Carl, i'm pretty sure its 200 users not 500! let me try and look it up.

Joseph W. Doherty · ‎06-17-2008

"Normal" is whatever is required for correct functioning on a particular LAN, and as such, can vary greatly. For instance, Windows hosts, depending on their configuration where they are doing more host name resolution through the NetBIOS calls (i.e. pre-WINS), will issue many more broadcasts then if relying on IP host name resolution.

As a very rough rule, if there's more than 1% of the bandwidth being consumed by broadcasts, you might want to investigate whether it's valid and whether it could be reduced. As another rough rule, if activating some type of broadcast suppression, again without a baseline, you might try 50%.

carl_townshend · ‎06-17-2008

hi there, so if I have more than 1 percent broadcasts on my lan, then i need to look into it?

when rate limiting broadcasts say to 10% on the interface, does that mean that 10% of the interface speed is only allowed for broadcasts?

Joseph W. Doherty · ‎06-17-2008

"hi there, so if I have more than 1 percent broadcasts on my lan, then i need to look into it? "

Probably be a good case to do so. In normal operations, especially IP, there are few valid reasons to send to ALL hosts. Even when there are valid reasons, a well behaved application will minimize the need to do do. For instance, ARP uses a broadcast to find a destination's IP to MAC but then caches the result.

"when rate limiting broadcasts say to 10% on the interface, does that mean that 10% of the interface speed is only allowed for broadcasts?"

You would need to verify that on the platform in question, but I would expect that to be the case in most instances.