DHCP Relay Wording + Diagram and L3 reachability for BMC access and PXE booting

[dshcherb]

This feature should not be called "DCHP relaying in MAAS". Rather "DHCP Relaying to a MAAS-managed DHCP server"
https://docs.ubuntu.com/maas/2.2/en/installconfig-network-dhcp#dhcp-relay
"DHCP relay
DHCP relaying in MAAS is an advanced feature"

Although expressed in the documentation, the key part here is that a DHCP server set up by MAAS is the one being relayed to:

MAAS rackd's DHCP <-- relayed requests <-- DHCP/BOOTP Relay Set up Manually <-- DHCP/BOOTP Client

http://www.tcpipguide.com/free/diagrams/bootprelay.png

http://www.tcpipguide.com/free/t_BOOTPRelayAgentsForwardingAgents-3.htm
http://www.tcpipguide.com/free/t_DHCPMessageRelayingandBOOTPRelayAgents.htm

I would like to illustrate that it is not mandatory to have a rackd on the same layer 2 network as the node that is trying to PXE boot.

L2 segment 1 <-- arbitrary layer 3 network <-- L2 segment 2

Those two diagrams would ideally need to be present in our docs (minus colors and style):
https://github.com/dshcherb/maas-concepts/blob/master/rackd_dhcp_pxe_relay.svg
http://www.tcpipguide.com/free/diagrams/bootprelay.png

The same applies for out-of-band management too - it is enough to have L3 reachability to manage a node via IPMI or other OOB protocols.

So, two cases:

• different L2 for a node trying to pxe boot and rackd (L3 reachability + DHCP/BOOTP relay);
• different L2 for BMC controller and rackd.

and

• a diagram that illustrates the above;
• a doc change "DHCP relay" -> "DHCP relay to MAAS-managed DHCP".

[degville]

Thank you for providing such a detailed summary of the issue. We'll updates the docs to clarify DHCP relaying.