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How to resolve Exchange 2013 DR server Database Copy Status Displays unknown in the ECP console

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You might have come across a situation when you have Exchange 2013 server in two different sites (Production and DR). After adding the Database copy in ECP console it displays Database copy status unknown for the DR exchange server. Issue is strange though but both Exchange logs are replicated.
Synopsis
You might have come across a situation when you have Exchange 2013 server in two different sites (Production and DR). Initially after adding the ECP console it displays Database copy status unknown for the DR exchange server. The Issue is strange if both Exchange servers communicate and the logs have been replicated, but the database copy shows Unknown.
png1.png 
When you click view details there it shows Unknown Error.   
 
Cause
The root cause is due to Network Devices and Server MTU values. Here is an explanation.

Larger MTU support called jumbo frames may be configured on the router. Commonly the router configuration supports a MTU value sized 1500 or larger (the networking equipment like switches and routers can support larger MTUs). But when your ISP doesn’t allow this, it reduces the MTU size to 1400 and if your server and router has 1515 as MTU value, the large MTU traffic is trying to pass through a small tunnel, so it doesn’t get completely through. Hence the status of the Exchange database copy status shows Unknown. The issue doesn’t occur if both Exchange servers are in the local LAN.

Workaround
This solution below is an workaround, you need to request the ISP to increase the MTU value. But most cases it is a tedious process to explain to the ISP and get it done and some ISP even doesn't allows. As a workaround following steps will provide sync.

This scenario shows both sites with a router MTU value 1500, both Exchange mailbox servers have MTU value 1515, and ISP supports MTU value 1474.  The solution is to reduce the MTU value on both the routers and servers to less than ISP provided value. In this case we use 1400. The router part is taken care of by changing the MTU size to 1400. The following steps explains changing in Windows 2012 / 2008 servers.
 
Procedure:
Open an elevated command prompt and type:
 

netsh interface ipv4 show subinterfaces

Open in new window

image.gifimage.gif and press enter.
 

Idx  Met  MTU         State      Name
-------------------------------------------------------------
 1   50   4294967295  connected  Loopback Pseudo-Interface 1
13   10   1514        connected  Local Area Connection 1
14   10   1514        connected  Local Area Connection 2
18    5   1500        connected  Local Area Connection 3

Open in new window

image.gifimage.gif Note: Existing MTU value on Both servers
png2.pngTo reset the MTU to the Ethernet default of 1400 bytes:
 

netsh interface ipv4 set subinterface 13 mtu=1400 store=persistent
netsh interface ipv4 set subinterface 14 mtu=1400 store=persistent
netsh interface ipv6 set subinterface 13 mtu=1400 store=persistent
netsh interface ipv6 set subinterface 14 mtu=1400 store=persistent

Open in new window

image.gifimage.gif Reboot the server if necessary, in this case we have rebooted the server to set the Exchange working properly. Now the Exchange DB copy status shows status healthy.
 
Summary :
This document provides an workaround solution to fix the Exchange 2013 database copy unknown status. After changing the MTU value it reduces the sending and receiving tunnel in to the same size which solved the issue. Now the database copy on the DR exchange server displays healthy. 
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by:Ganesh Kumar A
Updated in summary, objectives, subject.
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