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A new location I've been visiting has 3 fiber connected buildings using Cisco switches. They are each on a separate subnet and separate vlan.

192.168.0.0/24 (VLAN 10) Building 1
192.168.1.0/24 (VLAN 20) Building 2
192.168.2.0/24 (VLAN 30) Building 3

Each building also has its own Domain forest, ie: Domain1, Domain2, Domain3 for each building.

The routes are setup on the L3 switches and I can ping 192.168.1.100 fom 192.168.0.100 without a problem.

I can't figure out though why Building 2 is able to see broadcast traffic from the other VLANs. Well, maybe I worded that wrong. Basically Building 2 computers can successfully join Domain1, Domain2, or Domain3. Which I assume means there are some Windows Server broadcast messages available so they know where to look.

Members of VLAN 10 (building 1) can only join Domain1, when attempting to join Domain2 or Domain3 they cannot find them which is what I would expect.

I checked the CLI of the switch in building 2 and there is no IP helper or Directed Broadcast settings enabled.

Anyone have any advice?

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  • Are there any ACLs on the switches?
    – ewwhite
    May 18, 2015 at 21:50

1 Answer 1

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I think you're confusing things. Finding and joining an AD domain doesn't rely on broadcast traffic, it relies on direct communication with a DNS server (hosting the AD FQDN) and a Domain Controller (which most times is the same server as the server hosting the DNS FQDN of the AD domain) for protocols such as DNS, LDAP, RPC, Kerberos, SMB, etc.

The communication between a client machine trying to join a domain and a domain controller is unicast, not broadcast.

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  • Perfect, that's what I thought might be the case after I posted this. Second question, any idea how DNS might direct to the other domains? It seems to be functioning that way on Domain2 but not Domain1 or Domain3. I'm used to working on one domain on one LAN.
    – Mike
    May 18, 2015 at 21:52
  • Sure, the DNS servers in one domain probably have conditional forwarders set up for the other domains (and vice versa or any combination thereof).
    – joeqwerty
    May 18, 2015 at 21:56

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