Basic knowledge of the distance vector routing protocol RIP version 2

Because RIP is a distance vector routing protocol, it will NOT have any ideas about the current network topology.  It will learn routes directly from the update sent by its neighbors.

There are some importance facts about the RIP version 2:

1. RIPv2 is using UDP port 520 to transmit its request & response message.

2. On the broadcast network, RIPv2 uses the multicast address 224.0.0.9

3. Distance vector routing likely got the loop problem named "Counting to infinity".

4. There are two loop-prevent mechanisms for RIPv2: Split-Horizon and Poison-Reverse.

5. Update are sent every 30s, but we can use the trigger update.

Adding routes dynamically to the routing table or forwarding table

Manually controlling the routing table is easy and fun when your network is small with a few routers. When the number of routers increasing, it's almost impossible to do the STATIC way.

This time, we need a protocol so all routers can talk together to exchange the routing information. There are two kinds of that protocol Distance Vector & Link State routing protocol.

Adding routes manually to the routing table or forwarding table

You can always add routes into the routing table manually on every router even your home wifi routers by WEB interface or command lines.

By controlling the router's forwarding table manually, you will understand the routing process more.  By default, routers will put all subnets connected to its interface into the routing table. Thus you do NOT need to worry much about those connected routes.

You should only think about how the packet gets to the destination. The importance thing is this process is only ONE WAY. You should also think about how the packet gets back to its source.

Understand the forwarding table or routing table of a router

First, an incoming Layer-2 Frame to a router interface will be de-encapsulated to get the IP Packet inside it.  After that, the router will look at the IP Header to find a field named IP Destination.

Next, it will check that IP destination address with its Routing Table.

       1. If there is a matched, the router will re-encapsulated that packet into a Layer-2 Frame then forward that frame to the matched interface.

       2. If NOT, the router will drop that packet and do nothing.