Failover Clarification

Simon Hobson dhcp1 at
Tue May 8 22:19:31 UTC 2007

Benjamin Wiechman wrote:
>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.

I see the problem - oh for IPv6 eh ?

>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.

What's between the APs and the backhaul ? Could you insert a bridge 
or router (one port per AP) that can do filtering - or is this some 
'black box' of a system with APs on one side and a telco data line on 
the other ? I guess you've already thought about this.

>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?

 From my limited knowledge of failover, not at all. Firstly the 
balancing algorithms don't work well with very few available 
addresses. Secondly IIRC there are issues where the surviving server 
needs additional addresses when one server goes down - but I don't 
deal with failover personally so I don't know the details.

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