Reg: dhcp service unresponsive frequently

[Niall O'Reilly]

On 24 Jan 2020, at 7:14, Purva Rawan wrote:

> We have not configured dhcp relay agent in both dhcp server's 
> configuration .

That would be right; see below: the relay belongs to the network, not to 
the
server.

> In one of the server ,dhcp service listening on each interfaces.
> For another dhcp server ,dhcp relay agent is serving through Layer 3 
> switch.
>
> But we are facing this issue for both the servers.

It may help to think about this from the client back to the server,
rather than from the server(s) outwards.

I don't have a good picture of your network topology, or where the
relays and servers are sitting, or what experience you have, so I may
be explaining at the wrong level. If so, apologies.

Each client is connected to a layer-2 network, and needs a local DHCP
service on this network.

The local DHCP service must be provided either by a relay or by a 
server;
it's unusual to have a server connected to every client network.

The relay is typically part of the router configuration.

Each relay must be configured with the address of one or more servers;
one is enough; a second gives resilience; I wouldn't suggest more.

The server(s) can be on a network where no clients are connected; except
for very simple topologies, I prefer to keep the server on a non-client
network.

Each relay must be able to reach, and be reachable from, the server(s);
access lists and routing errors can obstruct reachability, perhaps
asymmetrically.

Each client network must be specified in the server configuration.

Log entries on the server will show you what the server thinks is going 
on.
You should see the DORA sequence: DISCOVER, OFFER, REQUEST, ACK; for
renewals, you'll normally just see REQUEST, ACK.

If you know or expect that the client is sending DISCOVER, but don't see
it in the server log, then you'll need to look at your network and work
out why the DISCOVER is not reaching the server either from the relay or
from a client on the same network as the server.

If you see OFFER in the server logs, but no REQUEST, you'll need to look
at the network in the opposite direction, and work out why the OFFER is
not reaching the client or relay.

I hope this helps.

Niall O'Reilly