The cisco IP routing process for all routers are almost the same, sometimes it seems kind of simple and seamlessly doesn’t change, regardless of the size of network you have. As your network grows larger and larger you might only need to add two lines of command to integrate the new routers into the network depending on the routing protocol use in configuring the router.
For an example, we’ll use the diagram as drawn below to describe in details step by step what happens when computer “A” wants to communicate with computer “B” on a different network.
The communication goes like this: From our diagram above when a user from computer B pings computer A from a command prompt the following process take place. Ping let say 192.168.88.4 which is host address
A packet is generated on computer A using the IP and ICMP Network layer protocols.
Since IP addresses works with the ARP protocol to determine what network this packet is destined for by looking at the IP address and the subnet mask of Host. Since this is a request for a remote host, which means it is not destined to be sent to a host on the local network, the packet must be sent to the router so that it will be routed to the correct remote network.
For Host A to send the packet to the router, it must know the hardware address of the router’s interface located on the local network.
Remember that the Network layer will hand the packet and the destination hardware address to the Data Link layer for framing and transmitting on a local host. To get the hardware address, the host looks in a location in memory called the ARP cache.
If the IP address has not already been resolved to a hardware address and is not in the ARP cache, the host sends an ARP broadcast looking, for the hardware address of IP address
192.168.88.1. This is why the first Ping usually times out, and the other four are successful. After the address is cached, no timeouts usually occur.
The router responds with the hardware address of the Ethernet interface connected to the local network. The host now has everything it needs to transmit the packet out on the local network to the router. The Network layer hands down the packet it generated with the ICMP echo request (Ping) to the Data Link layer, along with the hardware address of where the host wants to send the packet. The packet includes the IP source address and the destination IP address, as well as the ICMP specified in the Network layer protocol field.
The Data Link layer creates a frame, which encapsulates the packet with the control information needed to transmit on the local network. This includes the source and destination hardware addresses and the type field specifying the Network layer protocol
The Data Link layer of Host A hands the frame to the Physical layer, which encodes the 1s and 0s into a digital signal and transmits this out on the local physical network.
The signal is picked up by the router’s Ethernet 0 interface, and the interface synchronizes on the digital signal preamble and extracts the frame. The router’s interface, after building the frame, runs a CRC and, at the end of the frame, checks the FCS field to make sure that the CRC matches and no fragmentation or collisions occurred.
The destination hardware address is checked. Since this will be a match, the type field in the frame will be checked to see what the router should do with the data packet. IP is, of course, in the type field, and the router hands the packet to the IP protocol running on the router. The frame is discarded, and the original packet that was generated by Host A now is in the router’s buffer.
IP looks at the packet’s destination IP address to determine if
the packet is for the router. Since the destination IP address is
192.168.88.2 , the router determines from the routing table that
192.168.88.0 is directly connected network on interface Ethernet 1. The router places the packet in the buffer of interface Ethernet 1. The router needs to create a frame to send the packet to the destination host. First, the router looks in the ARP cache to determine whether the hardware address has already been resolved from a prior communication. If it is not in the ARP cache, the router sends an ARP broadcast out Ethernet 1 to find the hardware address of 192.168.88.2.
Host B responds with the hardware address of its network interface card with an ARP reply. The router’s Ethernet 1 interface now has everything it needs to send the packet to the final destination. However, the most important thing here is that even though the frame’s source and destination hardware address changed at every interface of the router the message was sent to and from, the IP source and destination addresses never changed. The packets were never modified nor changed at all; only the frame changed.
Host B receives the frame and runs a CRC. If that checks out, it discards the frame and hands the packet to IP. IP will then check the destination IP address. Since the IP destination address matches the IP configuration of Host B, it looks in the protocol field of the packet to determine what the purpose of the packet is.
Since the packet is an ICMP echo request, Host B generatesl a new ICMP echo-reply packet with a source IP address of Host B and a destination IP address of Host A. The process starts all over again, except that it goes in the opposite direction. However, the hardware address of each device along the path is already known, so each device only needs to look in its ARP cache to determine the hardware address of each interface.
If you had a much larger network, the process would be the same, with the packet simply going through more hops before it finds the destination host.