Network Oddities - "The Specified Network is Not Available"

For about a week now, I've been running into some strange network issues.  Here are some of the oddities:

1) No network printers listed under Add a Printer

2) Error message "The Specified Network is Not Available" on several different file transfer programs (Always Sync)

3) Files over 1MB in size fail to transfer from one site to another

Is there a common denominator that could be causing this issue?  We are running our domain in a Microsoft Server 2008 R2 enviroment.

Any guidance would be appreciated, else I will likely be placing a call to Microsoft.

If any additional specific infromation is needed, please let me know as I know my description of this issue might be to general.

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Rob WilliamsCommented:
Check ipconfig /all of a connecting PC and verify that only internal DNS servers are listed and no external DNS servers such as n ISP or router.  Also run from a command line
nslookup  InternalDomainName.local
and verify it resolves the IP of your internal DNS server/s
Matt VCommented:
It sounds like you have a site to site connection in place?  What type of connection are you using, and how do the two sides link together (remote AD sites etc)?
Rob WilliamsCommented:
#1 and 2 are often due to name resolution issues such as your DNS server may not be available.  Also having an ISP or router IP as an alternate instead of just your internal DNS servers can cause random name resolution errors.

Your previous question was related to domain controller time issues.  If the time on the PC's does not match that of the server there can be authentication issues.

#3 to me sounds like an MTU issue, i.e. set to high.
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tgarrityAuthor Commented:

In AD Sites and Services we have the following setup:

We have 4 sub nets setup (one for each branch)

We have each branch listed with domain controllers listed under each

Under Inter-Site Transports, we have 1 Site Link setup with the Cost set to 100 and the Replication Level set to 180.

All 4 Sites are listed under this Site Link

On what device do I need to change the MTU on?

Thank you both!
Do I need to separate them out so I essentially have 4 Sites setup with 1 Site Link listed under each?
Rob WilliamsCommented:
With a site to site VPN best to set on the routers.  You could lower substantially just as a test to something like 1300 to see if any there is any improvement in the data transfers. If so you know you are onto something and you can tweak, if not just change it back.

In AD you should have multiple sites, the appropriate subnet associated with each, and the servers in their proper site folder.  In a hub and spoke like configuration there is only one possible link between sites, so it sounds like your AD is correct.
tgarrityAuthor Commented:
We have a MPLS setup between all four branches, so I do not have access to the providers edge routers.  

Thanks for the clarification on AD.

I still think there is a bigger issue at hand and I can't identify it.

Rob WilliamsCommented:
Are you sure DNS is properly configured as mentioned?
tgarrityAuthor Commented:
What would you recommend that I check in DNS?
tgarrityAuthor Commented:
Good call on the DNS issue.

Using a DC at branch A I can ping a DC at branch B

Using a DC at branch B I can ping a DC at branch A

Using a DC at branch B, NSLOOKUP fails for branch A

Using a DC at branch A, NSLOOKUP works for branch B

tgarrityAuthor Commented:
Whoops, I just saw that the DC at branch B had the wrong DNS IP address.
Rob WilliamsCommented:
Good to hear.  However as mentioned at first, DNS could cause #1 & #2 but doubtful it is related to #3 "Files over 1MB in size fail to transfer from one site to another"
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