Random issue - DHCP clients receive IP address but can't ping anything?

[themightymrp]

Thanks @psydii - what I don't understand is why the device can have a MAC address table that supports up to 16000 addresses and yet an ARP table that supports 1024?

 

What are the implications of settings the max learning number to zero? Will that cause all devices to lose the network? I've currently tweaked settings so that the MAC address table ages out after 3 minutes (default 5 mins) and the ARP ages out after 2 mins (default 20 mins). So far its been running since 11:05 and no further issues. The number of devices in the ARP list is hovering at around 1005 fairly constantly.

 

Obviously this isn't perfect but for now its like a god-send! There must be a setting I can use somewhere that cuts this list down?

 

The odd thing is this:

 

a) Core switch (HP 5310) has all the VLAN's on it for routing purposes and has all those addresses built up

 

b) Comms cabinet main link switches (HP 5130's too) only hold ARP entries for VLAN 1 - the switches VLAN. So about 40 to 50 addresses. They aren't retaining any ARP addresses for the end devices plugged into them. Is that correct? Is it down to the VLAN tagging that is sending all the ARP info to that core switch?

 

Does that make sense?

[psydii]

Yes that makes sense.

 

The arp cache is only used for IP communication over ethernet. The edge switches only talk IP when they are being managed. Everything else the edge switches do is Layer 2. Only the core switch needs to know the IP of devices, and hence only its ARP cache matters.

 

The size is limited deliberately because otherwise people would use these bargain basement edge switches as core network devices. :-(

 

I had an edit to my above post ready to go, (I've posted it now). You have taken option 4, and you should keep a close eye on things. The fact that you are hovering around 1000 arp entries with a maximum on 1024 is really too close for comfort. You will need to either get a switch more suitable for to be the core of a school lan, or re-jig your plan/routing design to make each switch room its own routing domain. You would probably loose the benefit or IRF at that point.

[themightymrp]

Right ok, thanks for that. I had hoped a £1000 switch would have been enough to cope but I guess not How much do you think I should be looking to pay for a core switch?

[psydii]

I'll defer to others on that. @njc235 has always been a great help when helping chose and support the right sort of switch for a job.

 

On 'bargin basement' my experience of the 51xx series was from the OfficeConnect variant (19xx series) which "didn't have a cli" but were otherwise identical. Those were bargins but also had the arp cache limitation. The switches branded 51xx are as far as I can tell identical to the 19xx but (and this i didn't realise) twice/three times the price, making them less bargain basement and more bog-standard. But the general point remains you need a switch that isn't limited in this way, and that will cost more (two/three times perhaps?).

Edited February 20, 2018 by psydii

[psydii]

Just been looking around at the 5130 series. It seems that for the 5130-HI varient the maximum number of dynamic ARP entries is 8192. Which ought to be enough for a few years.

https://support.hpe.com/hpsc/doc/public/display?docId=c04901863

[themightymrp]

Would this be a suitable model then? It has 4 SFP+ ports to support my existing 10Gig fibre modules and my current switch is using 23 of the 48 Ethernet ports, but I'm hopefully going to reduce that number by another 3 or 4 over the coming months as bits get virtualised and my NAS boxes are moved to a different network cabinet...

 

https://www.ebuyer.com/787744-hpe-5130-24g-4sfp-1-slot-hi-24port-managed-switch-jh323a?mkwid=s_dc&pcrid=51482425979&pkw=&pmt=&gclid=EAIaIQobChMI5Yus8ei02QIVzLftCh36jApjEAQYASABEgJma_D_BwE

[themightymrp]

Although, if I'm reading this correctly, I can get the 48 port model much cheaper :-/ ??

 

JH324A Hewlett Packard Enterprise 5130 48G 4SFP+ 1-slot HI Managed network switch L3 Gigabit Ethernet (10/100/1000) 1U Black JH324A

 

And it definitely has the correct part code of JH324A and isn't my current one of JG934A...

[themightymrp]

OK, the JH324A (5130 HI model, 48 port) is on order, should be hear Tuesday/Wednesday next week. Hopefully once that's configured I won't need to worry any more! Incidentally, I started touching the 1024 limit again so have limited the number of devices that can connect to one of our SSID's on Wi-Fi to 30. This particular SSID has bandwidth throttling on and is only used for sixth form and staff to add mobile phones to - so not major. This has kept the numbers at around 980 and no complaints as of yet.

 

Thanks to all for the advice - got there in the end!!