Failover Clarification

Benjamin Wiechman benw at
Tue May 8 20:54:39 UTC 2007

This is the problem. Put three access points on the tower, and throw a /24
on it and run all three access points on that network. Realistically we can
put about 150-200 subscribers on that tower and still provide QOS. So we
have about 60% utilization (multiply this by 15-20 sites) - not going to get
more IPs. That is the management/ARIN problem. Say we use a /25 instead,
this will improve overall IP utilization, but we still are in the same boat
once we get to 120+ subs off that site.

We can run slightly larger networks, say a /26 per access point, but
multiply this by about 75 access points. This would lower out IP utilization
to less than 50%, and our upstream provider already drags their feet when we
come begging for more IPs. No way are they handing us almost 5000 IPs.

Or the bigger problem is that now if a subscriber on AP1 sends a broadcast
packet it is being retransmitted on AP1, and AP2, and AP3, possibly more
than once. There is no way to filter broadcast in the APs - they are simply
bridges (and not an 802.11 system). PPS limitation is about 2500... so there
isn't a lot of available overhead. Yes, if we could filter broadcast at the
access point this wouldn't be such a big deal.

So, to limit broadcast traffic we place each AP on its own VLAN and break up
the address space. This way if we do have a subscriber that is hammering our
network with broadcast traffic it is contained to one access point, not
every access point at the site.

Between a rock and hard place... IP management and ARIN vs QOS...

Back to the original question... how well is a failover configuration going
to handle this type of network? 

Ben Wiechman
ben at

-----Original Message-----
From: dhcp-users-bounce at [mailto:dhcp-users-bounce at] On Behalf
Of Simon Hobson
Sent: Tuesday, May 08, 2007 2:49 PM
To: dhcp-users at
Subject: Re: Failover Clarification

<benw at> wrote:

>We are a wireless ISP. To help control broadcast storms, which would
>us due to the relatively low pps capabilities of our wireless equipment we
>run rather small subnets on our access points, typically a /27 network per
>AP, adding additional subnets as required.

Wouldn't it be easier to filter packets in the access points - ie 
drop all broadcast packets from subscribers ?

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