[c-nsp] ARP-cache Timeouts for ASA5520

Wed Jun 11 04:00:26 EDT 2008

Hi,

Where are you doing the load balancing?

I assume that because you are using the ASA's in Active/Standby then 
it's not the ASA's, therefore it would be some other device/s.

In that case how is the load balancing working in the other devices?  do 
the load balancers have individual mac addresses or have a floating mac 
address? or the secondary load balancer take the mac of the first when 
it fails (similar to how the ASA's can do failover), or even load 
balance by rewriting packets?

I think 4 hours is unrealistic, one would expect the device with the mac 
address that could fail would be ping monitored and at which time debug 
would be carried and the arp cache could be cleared manually. (I know 
that's not the point)

The arp timeout is easily changeable, documentation from the ASA's on 
device documentation says:

"ARP Timeout—Sets the amount of time before the security appliance 
rebuilds the ARP table, between 60 to 4294967 seconds. The default is 
14400 seconds. Rebuilding the ARP table automatically updates new host 
information and removes old host information. You might want to reduce 
the timeout because the host information changes frequently. Although 
this parameter appears on the ARP Static Table panel, the timeout 
applies to the /dynamic/ ARP table."

I suspect this value is the most convenient for the majority of users 
but not yourself.

Are you able to lab your setup? perhaps you can test the impact on 
changing the arp cache, or simulate a load balancer failure to see how 
the ASA's react.

Kind regards,
Alasdair

Casey, J Bart wrote:
> We are looking at the possibility of purchasing load-balancers for our
> web servers(2) and mail gateways(2).  Unfortunately, we don't have a lot
> of money to throw at this solution and are therefore looking at the most
> economic solutions available.  As a result, one of our options is what I
> consider to be for a small office and not necessarily for an enterprise
> environment.  As a result, there are a few features that I consider to
> be lacking.  One in particular is the ability to make a pair of
> load-balancers highly-available.  
>
>  
>
> The documentation from the manufacturer states that failover in an
> HA-pair takes 5 seconds plus the amount of time it takes for the
> ARP-cache to time out.  In our case, the default ARP-cache timeout for
> our ASA5520s is 14400 seconds or 4 hours.  That would mean if one of the
> load-balancers failed, it could take up to 4 hours before the ASA begins
> to forward packets to the backup device.  In my mind, if we are trying
> to be "highly-available", this is unacceptable.  However, I understand
> that this value was most likely arrived at as a result of testing and is
> really more of a best-practice.  
>
>  
>
> I called my SE to get recommendations/suggestions.  He was very helpful
> in answering my questions and confirming my thoughts that lowering that
> timeout would most likely increase CPU load and if the CPU load
> increased enough would potentially affect the stability of the ASA and
> thereby the stability of any network which depends on that device.  I
> asked for his advice based on experience for lowering that timeout and
> he mentioned 5000 seconds or approximately 1 hour 23 minutes (from a
> previous implementation).  This is better but still not in an acceptable
> range for something that's supposed to be "highly-available".  
>
>  
>
> I have been following the thread titled "Gratuitous ARP and PIX" and it
> seems like David is wrestling with some of the same type issues that I
> am.  The only difference is that he mentions that his ARP-cache is set
> to time out at 5 minutes which seems very low but maybe appropriate for
> his environment.  Like David, I am hesitant to lower the value.
> However, I'm just curious what the thoughts are from the others on this
> list about how far I can push that value down.
>
>  
>
> Here are the facts:
>
>  
>
> 1.       I am running a pair of 5520s in active/standby with stateful
> failover.
>
> 2.       Devices are running with a single context for now.  I am
> considering multiple contexts in the future.
>
> 3.       The average bandwidth through the device is about 34Mbps with a
> slated increase of 12Mbps/year over the next 4 years (When the devices
> will be replaced).  Total approaching approximately 84Mbps.
>
> 4.       This particular pair of devices does not have any service
> modules
>
> 5.       There's no VPN taking place on this pair except for one group
> configured for emergency purposes.  Obviously, that will go away if we
> go to multiple contexts.
>
> 6.       There are currently 4 VLANs trunked to one interface(4
> sub-interfaces), 1 outside interface, 1 inside interface, 1 LAN failover
> interface and 1 State failover interface.
>
> 7.       There are about 80 ACL lines, however, almost every single line
> references a Network Object Group and a Service Group.  So, in truth,
> there are probably a few hundred ACL lines.
>
> 8.       There are 130+ NAT Statements
>
> 9.       These devices also run OSPF on the outside and inside
> interfaces (2 OSPF areas) with about 6 peers.
>
> 10.   The current CPU utilization is 9% (approximately 15Mbps and 70
> connections/second).  It's the summer and the majority of students
> aren't on campus and therefore, bandwidth utilization is down.  I also
> don't have any history on the CPU utilization for high-traffic time.
>
> 11.   The ARP-cache timeout is set to the default of 14400
>
> 12.   There are currently 34 entries in the ARP table
>
>  
>
> So, all of that being said, I welcome the thoughts of the members of
> this list with regard to adjusting the ARP-cache timeout.
>
>  
>
> Thank you in advance for your help.
>
>  
>
> J. Bart Casey
>
> Network Engineer
>
> Wofford College
>
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Lumison
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