Design of switched layer 2 number of hosts

[ken.hollis]

Greetings and Salutations:

I have looked thru the Cisco (and Google) to no avail for answer to the question:

What is the suggested maximum number of hosts on a single ethernet switched LAN (VLAN). No hubs, all ports are 100Bt full duplex.

I saw the paper at:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd20e.htm

Under:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/

I also looked at (but didn't see anything relevent):

http://www.cisco.com/warp/customer/473/

The Broadcasts paper seems to suggest that under 1000 hosts on a single switched LAN you should be "OK". The CPU on the host machine only loses a fraction of it's CPU to overhead broadcasts. Other than that I don't really find any definite suggested maximum limits on a single switched LAN segment.

Any pointers / sugegstions would be helpful.

Thanks,

Ken

[ssymonds]

Hi Ken,

The number of hosts that you connect to a single LAN segment or VLAN (read broadcast-domain) would typically be dictated by the traffic characteristics that are present on the LAN, more specifically the projected or measured levels of broadcast traffic (L2 and L3). The links that you have cited give some indications of what you may expect from the point of view of broadcast levels and the expected impact on all of the segment hosts. Having 1000 hosts with minimal broadcast activity, or 20 hosts with high levels bc activity show that the 'numbers' of hosts are not the real issue, it's the traffic characteristics...

To summarise: from a purely operational standpoint, the suggestion is to scale the number of hosts per-LAN to the expected .. (research the application and protocol specifics), or measured levels of broadcast traffic present, versus the performance requirements.

rgds

steve

[j.chenevey]

Ken,

As the white paper suggested, the size of your switched network (broadcast domain) will depend largely on the types of applications & their broadcast characteristics. I've read similar Cisco guidance that says pure IP networks < 1000 hosts per VLAN, IP/IPX ~ 400-500 hosts per VLAN, AppleTalk ~ 200 hosts per VLAN. As faster processors come out as you've stated, CPUs are better able to handle broadcasts but they are still wasting CPU cycles with each broadcast thus reducing efficiency. But you are right, not definitive size. One suggestion I would make however, is that large broadcast domains indirectly means a larger population affected by network failures at L2 and L3. One thing to consider.

[cz96p6]

You should have 254 host per Vlan. You can connect workstation per your port density on a particular Catalyst Switch. 6509 backplain is 32 GBIC, 3524XL 8 GBIC. I won't worry about that.

[ken.hollis]

Greetings and Salutations:

O.K... Then the answer is < 1000 hosts per LAN / VLAN segment. This is a "normal" IP network, one gateway out and redundancy is not an issue (luckily). There should be minimal broadcasts at this time and when multicast is implemented we will have to look at those implications at that time.

Thank you for your help,

Ken