ARP table is full - what can I do?

[MrBrownPr]

I'll put my hands up straight away and admit to being something of a novice at networking and this has me stumped.

 

We recently discovered that when our Aerohive WiFi system was installed the company that provided it included in the cost a 5 year management licence. That expired and we suddenly found ourselves unable to change settings on the APs. When we enquired with Aerohive they wanted us to pay £2,800 per annum, money we definitely did not have. Our only option was to switch over to Aerohive Connect but this meant losing the DHCP facility that we were using on the APs for some of our VLANs.

 

I setup new DHCP ranges on our server over Easter and all seemed fine, but once we had everyone back in the school it was clear the network was being absolutely hammered, laptops would be allocated IP addresses by the DHCP but would never receive them, people wouldn't be able to access the internet or network resources, etc. Connecting to the core switch I could see constant "ARP table is full" messages.

 

I adjusted the configuration and had one of the VLANs pick up addresses from the DHCP on our Protex box, sacrificing the guest WiFi network in the process. ARP table was no longer showing as full and people were able to access the network without issue again.

 

The core switch is a 3COM 4200G. It seems the ARP table limit is 256 entries. Aging timer is set to 20 minutes.

 

I'm looking for suggestions about what I can do here. Preferably suggestions with zero or at least very little cost. I'm thinking that I somehow need to get a separate DHCP to provide addresses to clients on one of our VLANs so they are not passing through the core switch and being added to the ARP table.

 

I've considered whether we need to replace the core switch but I am struggling to find details of switches that have a higher ARP table limit. I don't want to spend money to find myself in the same boat.

 

And I also have a nagging question at the back of my mind about whether the ARP table is really the problem or just a symptom of a different issue that needs fixing?

 

Open to any and all suggestions, please!

[PlantHead]

I think the 3COM is only a layer 2 switch. If you have vlans and need to route between Vlans you are going to need a layer 3 switch which can do high speed routing at your core.

I'm no expert so hopefully someone else can give you a bit more of definitive answer.

[MrBrownPr]

I think the 3COM is only a layer 2 switch.

I did suspect that, but my Google Fu couldn't provide a definitive answer as to whether it was a layer 2 or layer 3 switch. I even found references to it being a "layer 2+", i.e. a layer 2 with some features of layer 3 which further muddies the waters!

[FN-GM]

In Cisco you have something called an SDM template. It allows you to allocate switch resources and increases and de-crease the size of tables. Might be worth seeing if there is one for 3Com.

[pantscat]

It is a Layer-3 switch, but it's an edge switch, not really suitable for use as a core unit.

 

If money is tight have a look for a refurbished unit - something like a HP Procurve 5406zl is available pretty cheaply.

[MrBrownPr]

It is a Layer-3 switch, but it's an edge switch, not really suitable for use as a core unit.

 

If money is tight have a look for a refurbished unit - something like a HP Procurve 5406zl is available pretty cheaply.

 

Is there any option that doesn't involve spending money? A quick look on eBay (if you know of other/better suppliers for refurbs please let me know!) suggest I am still looking at around £600+.

 

(It's also huge and won't fit in my current cabinet without a lot of work...)

[keithu]

https://community.hpe.com/t5/Comware-Based/4200G-Trap-ARP-table-full/td-p/2315386#.XNWL9ejdthG

[Bruce123]

https://community.hpe.com/t5/Comware-Based/4200G-Trap-ARP-table-full/td-p/2315386#.XNWL9ejdthG

 

Interesting solution, the default of 20 mins sounds unnecessarily long anyway. Dropping to 5 mins should probably resolve the immediate problem (for a while - until more clients are added). And it would increase the frequency of ARP broadcasts (unlikely to have any noticeable effect on network performance).

 

If you’re looking for a more long-term solution and don’t have much cash (you should speak to someone about that) you could do worse than purchase this switch (a refurbished with warranty):

 

Cisco 3750X

 

It comes in many variations. Get the 24/48 port 1Gbit version with POE (as per your existing switch spec). It’s a full later 3 switch. I don’t know what the max ARP table size is but will probably be at least 4,000.

 

If you’re not familiar with Cisco CLI, you’d need someone to setup the initial config but it doesn’t sound like there is very much to translate over.

 

Thanks,

 

Bruce.

Edited May 12, 2019 by Bruce123

[Bruce123]

Interesting solution, the default of 20 mins sounds unnecessarily long anyway. Dropping to 5 mins should probably resolve the immediate problem (for a while - until more clients are added). And it would increase the frequency of ARP broadcasts (unlikely to have any noticeable effect on network performance).

 

If you’re looking for a more long-term solution and don’t have much cash (you should speak to someone about that) you could do worse than purchase this switch (a refurbished with warranty):

 

Cisco 3750

 

It comes in many variations. Get the 24/48 port 1Gbit version with POE (as per your existing switch spec). It’s a full later 3 switch. I don’t know what the max ARP table size is but will probably be at least 4,000.

 

If you’re not familiar with Cisco CLI, you’d need someone to setup the initial config but it doesn’t sound like there is very much to translate over.

 

Thanks,

 

Bruce.

 

Also, I’m not 100% what happened to cause the issue in the first place...

Edited May 12, 2019 by Bruce123