about concept "group", "shared-network", and "subnet",

Bruce Hudson Bruce.Hudson at Dal.Ca
Wed Apr 20 20:54:06 UTC 2011

> If this drawing show up un-mangled, I'll be amazed...

    No need to be amazed. :)
>>The key test is this. Can you unplug a device in the
>>network, and plug into the same socket a device in the
>>network and have it work correctly WITH NO OTHER CHANGES ? If not
>>then they are not the same network and the subnets should not be in a
>>shared network.

> It's a little different than that, we are also using QinQ and all of our 
> dhcp is relayed, so depending on where your encapsulation dot1q and second 
> dot1q are defined (which particular router performing L3 relay) terminates
> the QinQ is which giaddr your discover will be forwarded with to dhcpd.

    I do not think Simon's test was meant to involve DHCP specifically. If
the two networks share a broadcast domain, you should be able to take a server
with a address, change its address to something on the
network and have it work. Actually, that is the inverse of Simon's test;
changing the address and leaving the port the same versus changing the port
and leaving the address intact.
   QinQ is a complication that should make no difference. I think you are
looking at the wrong level. I have two easier tests for you.

    1)	If you run a sniffer on a host on your "TEST ISG-10K" network, does
	it see broadcast traffic from "PRODUCTION ISG-10K" hosts, including
	DHCP traffic but other traffic such as ARP requests may be easier
	to see. If yes, they are part of the same broadcast domain.

    2)	Do the DHCP requests logged on the DHCP server come from the same
	relay address or different addresses? A properly functioning DHCP
	relay with interfaces in multiple broadcast domains should use its
	address facing that domain. If requests come from the same address
	you probably have a shared network. That or a broken relay. 

	If there is a different relay address for each subnet, appropriate
	to that subnet, definitely get rid of your shared subnets.
Bruce A. Hudson				| Bruce.Hudson at Dal.CA
ITS, Networks and Systems		|
Dalhousie University			|
Halifax, Nova Scotia, Canada		| (902) 494-3405

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