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Community Member

Best practice subnet size for gigabit network access layer

Hi,

Is there a recommended subnet size with respect to layer 2 broadcast domains in a well behaving access layer gigabit network?

I remember reading something about and interpacket delay but am having a hard time finding info on it.

Ta

David

10 REPLIES

Re: Best practice subnet size for gigabit network access layer

/24 per broadcast domain is the normal recommendation.

HTH>

Hall of Fame Super Silver

Re: Best practice subnet size for gigabit network access layer

Community Member

Re: Best practice subnet size for gigabit network access layer

Awesome responses!

I am assuming though there are some exceptions, such as the Cisco Wireless LWAPP architecture where the access points create tunnels back from access point to WLC so the WLC becomes the determining point, which then allows for the mobility requirements of wireless?

Re: Best practice subnet size for gigabit network access layer

Well not in all cases, if you have under 254 mobile devices, spread over a campus network - you still only need 1 x /24. Using local switching - all client traffic is tunnled via the LWAPP AP back to the WLC and switched from the WLC onto the LAN.

HTH>

Community Member

Re: Best practice subnet size for gigabit network access layer

True - I was looking at the cases where there are more then 254 device, which is our case due to iphones, desktops, pdas etc, etc and how we can accommodate them safely (and transparently) without moving to mobility groups.

http://oreilly.com/catalog/802dot112/chapter/ch21.pdf seems to suggest up to 900 hosts per vlan attached to a wlc may be acceptable.

Re: Best practice subnet size for gigabit network access layer

Well in that kind of a case - you should consider:-

1) WLAN for Phones - which I would strongly recommend for it's own QoS policy, WMM, CAC and AP Load Sharing.

2) not having ALL AP's have ALL WLANS - select which WLANS need to be sent to specific areas for specific AP's. i.e you dont want the Cafe WLAN available from the classrooms (campus examples)

3) Create IP Subnets based on equipment types, WLANS.

Bad practise is to bundle all wireless in one WLAN - bad planning and bad security.

HTH>

Hall of Fame Super Silver

Re: Best practice subnet size for gigabit network access layer

Hello,

I strongly agree with Andrew using a single ip subnet for voip wireless phones and PCs is not recommended.

We have made a deployment using WISMs and several client vlans and it is working well.

Hope to help

Giuseppe

Community Member

Best practice subnet size for gigabit network access layer

With the WLC version 7.0 you can create interface groups and have multiple vlans per WLAN.

http://www.cisco.com/en/US/products/ps10315/products_tech_note09186a0080b78900.shtml

Hall of Fame Super Gold

Re: Best practice subnet size for gigabit network access layer

Have you browsed the Wireless LAN Controller (WLC) Configuration Best Practices (http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a0080810880.shtml)?

Super Bronze

Best practice subnet size for gigabit network access layer

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Posting

As the other posters have noted, a /24 often is a good starting point for a maximum subnet size.  Occasionally it's too large, and conversely sometimes it can be pushed larger.  The determinate is broadcast volume.

On something like Cisco's WLC and/or WiSM, you can often safely push client subnets larger than a /24 because there's not the same broadcast issue between wireless clients on the same subnet.  If your have "temporary" wireless clients, you might also want to use a much larger subnet to allow for a larger DHCP pool, as IPs can be tied up, with transitiory clients until their lease expires.  (Of course, you can also decrease the lease time to recover inactive IPs faster.)

Recently, I've been involved in a project where we've been migrating multiple internal Wireless (WLC or WiSM) subnets (all same SSID and security) to a single /22 or /21.  So far, we haven't seen any problems, although rarely more than a couple of hundred active wireless client at anyone time.  Definitely, a little easier to manage.

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