Dell S4048 / N series / Core VLT questions

Hi Everyone,

We are having a few problems / issues with a large scale network put together with two dell S series 4048's at the core of the network (yes yes I know...)

So I've never come accross VLT before, and I was wondering if someone who knows more than me can help.

The whole network is not performing as it should, generating a whole TON of arp traffic and giving us slow response times to our switches and devices, causing WiFi disconnects and all sorts. This is not a small network either - think 18,000 users 4500 desktops and countless BYOD devices.

I'm just trying to diagnose the problem, but could do with a few answers to questions so I know where to look.

Just for clarity - I have done the obvious - disconnecting buildings and services, wireshark etc etc but this problem seems to be difficult to find.

The system was installed by an IT vendor who made a huge muck up of the project and sadly we can't get them in to clean up their mess, so its down to us!

Current Config:

Core - two Dell S4048's running latest firmware, configured as a VLT pair. This forms the core switch at the centre of the network. These switches are doing all layer 3 routing for the system.

Linked to

Aggrigation - two Dell N4000 series connected as VLT peer lags to both of the S4048's above. Most servers, and buildings plugged in here.

My questions:

1. In a Dell VLT scenario as above, we have configured a vlan interface ip address for each vlan. On both sides of the VLT this is configured as the same IP address, and for the corresponding vlan this is the IP we are giving to clients as the default gateway - is this an accepted config?
2. What should we have configured as our mac aging times etc, there are conflicting articles online (and dells documentation) about what this should be set too.
3. Ping times: Currently we are experiencing absolutely appalling ping times to the core of the network, and outlying switches (think 50-600ms) - some network devices do not priorities ping, but I think this is indicative of our problem. Even when the cores are isolated during down time, pinging them from just one device plugged in, gives a 2ms response time which does not seem right.
4. STP - Should we have per-vlan spanning tree turned on? Documentation suggests we use RSTP on the links to so that they are protected as they boot up. which leads too..
5. STP root. Should the S series above be the root bridge? some documentation seems to indicate that we use the aggregation layer below.
6. Dell to HP - anyone have any issues connecting Dell switches to HP 5400 and older HP switches?
7. Dell N series LACP trunks - do these normally have poor response times?

Happy to post config snippets etc if anyone things they can help...

Thanks,

Robin