ARP cache size increase: how to find if this is necessary?

[Rahul]

I have seen a few reccomendations which reccomend increasing the ARP cache
size for networks where more than 512 hosts are on the same IP subnet. e.g.
here:
https://wiki.fysik.dtu.dk/niflheim/System_administration#kernel-arp-cache

But is there a way of knowing from logs etc. if this fix is indeed needed
for my network? I have two different subnets 192.168 and 10.0 each with
approximately 300 servers on them (each server has a low and high speed eth
card and hence the twin subnets)

[Raja Kommula]

ARP entry usually expires in 30 mins. You can use simple ping to
findout if the entry is expiring sooner than the default timeout. When
ARP entry is missing, ping response takes longer than usual time. So
if you ping the server, say every 10 mins, and if you get the response
quickly then you are same. Otherwise you probably have to increase the
cache size.
--Raja

Post by Rahul
I have seen a few reccomendations which reccomend increasing the ARP cache
size for networks where more than 512 hosts are on the same IP subnet. e.g.
here:https://wiki.fysik.dtu.dk/niflheim/System_administration#kernel-arp-c...
But is there a way of knowing from logs etc. if this fix is indeed needed
for my network? I have two different subnets 192.168 and 10.0 each with
approximately 300 servers on them (each server has a low and high speed eth
card and hence the twin subnets)
--
Rahul

[Rahul]

Post by Raja Kommula
ARP entry usually expires in 30 mins. You can use simple ping to
findout if the entry is expiring sooner than the default timeout. When
ARP entry is missing, ping response takes longer than usual time. So
if you ping the server, say every 10 mins, and if you get the response
quickly then you are same. Otherwise you probably have to increase the
cache size.
--Raja

Thanks! I tried that but it is hard to say. All my responses are around 11-
18 microsecs roundtrip. But the varaiability in switch latencies etc. make
it hard to know if this is an ARP cache issue or not. Are there any other
ways of finding out?

[Pascal Hambourg]

Hello,

Post by Rahul

Post by Raja Kommula
ARP entry usually expires in 30 mins. You can use simple ping to
findout if the entry is expiring sooner than the default timeout. When
ARP entry is missing, ping response takes longer than usual time. So
if you ping the server, say every 10 mins, and if you get the response
quickly then you are same. Otherwise you probably have to increase the
cache size.

Thanks! I tried that but it is hard to say. All my responses are around 11-
18 microsecs roundtrip. But the varaiability in switch latencies etc. make
it hard to know if this is an ARP cache issue or not. Are there any other
ways of finding out?

Of course : just check directly the contents of the ARP cache.

[Rahul]

Post by Pascal Hambourg
Of course : just check directly the contents of the ARP cache.

Thanks Pascal! I did dump it with the arp command. But how do I correlate
this with if or not I need to increase the size of the cache? I find the
lenght of the output variable depending on which machine I execute this
command on.

[Pascal Hambourg]

Post by Rahul

Post by Pascal Hambourg
Of course : just check directly the contents of the ARP cache.

Thanks Pascal! I did dump it with the arp command.

You can do it with the command "ip neigh" too, or by reading directly in
/proc/net/arp.

Post by Rahul
But how do I correlate
this with if or not I need to increase the size of the cache? I find the
lenght of the output variable depending on which machine I execute this
command on.

Compare the output length with the cache size.