How to troubleshoot the Router?

INTRODUCTION:
WAN (wide area network) is a computer network which has broader vision than LAN,
such networks links cross metropolitan, regional or national boundaries. Or
normally it can be said that a network that uses routers and public communication
links are categorized to be a WAN. If comparing with Metropolitan area networks
(MAN), Campus area networks (CANs) which are usually confined to a room, building
or a city, WAN is expanded to international boundaries as well. The best known
example of Wan is Internet.
WAN can be used to connect LANs and other types of networks together, so that
computers and users at one location can be connected to the users and computers to
other locations. Many WANs are built for particular organization and some are
private as well. Other are built by Internet Service Provider (ISP), provide
connections from an organization's LAN to the Internet. WANs are often built by
using leased lines. At each end of leased line a router is connected to the LAN on
one side and a hub within the WAN on the other side. Packet switching and circuit
switching can be used for WAN connection instead of leased lines. Network
protocols including TCP/IP deliver transport and addressing functions.

WAN CONNECTIVITY AND DATABASE CONCEPTS:

For active WAN connectivity the concept of Monitoring and Troubleshooting Active
Directory Replication is very essential. This concept is elaborated in this
article. Replication is defined as a duplicate copy of similar data on the same or
a different platform or system. When we are using a directory service such as
Active Directory, the database of directory is being carried by all domain
controllers,so that when you want to contact a domain controller for use, there is
always a local copy available for use so that requests do not have to be sent over
the wide area network (WAN) repeatedly for efficient processing and higher speed.
When talking about Active Directory, replication takes place and if any change is
committed in the data then a copy of the Active Directory database is stored and
updated on all other participating domain controllers on network ,to ensure
active directory replication each copy of the database should be the same and
all domain controllers have to be synchronized.
In the following figure you can easily understand that how two sites are
connected using WAN link. Here you can see a common network with a domain
controller at each location. The main benefit of having a domain controller local
to your PC’s at each network side is to have requests are the PC’s for need of
services to speed up requests and in case of disaster recovery, which could happen
if WAN link is dropped, the local PCs are still able to find a local domain
controller to use. Keeping traffic off the wide area network (WAN) and containing
it to the local area network (LAN) is the best design practice you can implement.