Windows Server Troubleshooting - Routing

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Troubleshooting routers is common in networks. Both dedicated routers such as Cisco routers and software such as Windows can perform the routing functions. Routers are used to connect multiple networks together to form an internet. Note the appearance of the router icon.

Windows servers and workstations know the address of the router to be the Default gateway address configured in the network properties. Routers work with the information in the network layer protocol to do routing. The router determines whether each message is

  • a local message or
  • one destined for some other network in the internet.

Local messages are not forwarded by a router.

10.1.1.40            
 
Router does not forward local message
 
   
To 10.1.1.41
               
             

Messages destined for other networks are forwarded by a router.

10.1.1.40            
 
Router does forward remote message
 
   
To 11.1.1.41
         
To 11.1.1.41
   
             

The network layer includes network addresses that the router can use to forward a message in the direction of the destination network. In the case of IP, the netid network identifier is the front part of the IP address.  The following diagram shows how routers only need to understand up to the network layer to do their routing function.

Client               Server
HTTP               HTTP
TCP   Router       Router   TCP
IP   IP IP   IP IP   IP
Ethernet   Ethernet PPP   PPP Ethernet   Ethernet
                             

The following diagram shows the IP and MAC addresses for a packet as it is passed from one network to another through two routers. You can see that the IP packet stays consistent but the Ethernet frame is replaced by the routers as they pass the IP packet from one network to another.

A           B

IP address: A to B
MAC address: A to R1
R1
IP address: A to B
MAC address: R1 to R2
R2
IP address: A to B
MAC address: R2 to B
                     
             

RIP Routing Information Protocol

Routers learn about each other and other networks through a routing protocol. RIP is the simplest routing protocol and is usually the default with routers. Routers use routing tables to determine where to send forwarded messages. Routers know the network addresses of all networks that they are physically connected to because each network interface required a network address configuration. Routers do not have any information about remote networks configured by default. Although manual configuration is possible, most routers use a routing protocol to automatically discover the location of remote networks. With a routing protocol, routers can automatically reconfigure themselves in the internet topology changes. 

RIP is rather simplistic. RIP routers broadcast their known routing tables on all network connections. The routing table is initialized with the network addresses of all interfaces. Initially, remote networks are not known, but as RIP messages from remote routers are received, the router add to its knowledge of remote networks.

Initially, routers only have knowledge of their directly connected network addresses in their routing tables.

   
Routing table
10.1.0.0
10.2.0.0
   
Routing table
10.2.0.0
10.3.0.0
 
10.1.0.0
10.3.0.0
 
10.2.0.0
           
10.2.0.0
Network 10.1.0.0  

Network 10.2.0.0

    Network 10.3.0.0

Once they have learned about the remote network addresses from RIP messages, they can update their router tables. The update router information will be broadcast so that other routers will learn about these updates also.

   
Routing table
10.1.0.0
10.2.0.0
10.3.0.0
   
Routing table
10.2.0.0
10.3.0.0
10.1.0.0
 
10.1.0.0
10.3.0.0
 
10.2.0.0
    10.3.0.0
           
10.2.0.0
10.1.0.0     
Network 10.1.0.0  

Network 10.2.0.0

    Network 10.3.0.0