Communication Across a Simple Routed Network

Take a look at Figure 8-1, where a single router connects to networks 192.168.0.0/16 and 10.0.0.0/8. By this point, you should recognize these network addresses as two of the private IP address ranges that are not valid on the public Internet. The router is a simple 2-port Ethernet router, with interface E0 connected to the 192.168.0.0 network, and interface E1 connected to the 10.0.0.0 network. Both interfaces on the router have already been assigned their IP addresses, 192.168.0.1 and 10.0.0.1 respectively. Using an IP address that ends in “.1” for router interfaces is fairly common. It is not a rule, but rather a convention. You may or may not choose to follow this convention, but it does make the IP address of a router interface easier to remember.

Figure: A simple routed internetwork.

Believe it or not, without configuring anything else on this router, it is already capable of routing data between network 192.168.0.0 and network 10.0.0.0. How is this possible? Well, the first thing you need to understand is that after assigning a Cisco router IP addresses, it knows about the networks that it is directly connected to, and the command ip routing is enabled by default. In other words, the router is aware that interface E0 is connected to the 192.168.0.0 network, and that interface E1 is connected to network 10.0.0.0. A router will automatically add both of these networks to its routing table. If our router receives a packet destined for the 192.168.0.0 network on interface E1, it will know to forward it out interface E0. In the same way, interface E0 knows that any traffic destined for network 10.0.0.0 should be forwarded out interface E1. A router always knows how to get to the networks to which it is directly connected, without any additional configuration. Things get a little more complex when a router needs to get to networks that are not directly connected.

Each network in this example also has a single host configured, as shown in Figure 8-2. Notice that each host has an IP address, subnet mask, and default gateway value assigned. The default gateway IP address is that of the local router interface on the host’s network. Hosts will forward traffic to the router when they calculate that the destination host is not on their local network.

Figure: Simple routed network with 1 host on each network.

In this example, each network is a Layer 2 broadcast domain, running Ethernet. Both networks are also unique Layer 3 IP networks, as shown by their logical layout. On most (but not necessarily all) networks, each broadcast domain will be associated with a unique IP network (or subnet) address.

There are cases, however, where the preceding statement is not necessarily true. For example, a company may run out of IP addresses and choose to map two subnets to a single broadcast domain. Making things work would then involve adding a second IP address to the local router interface. Even through hosts will be in the same broadcast domain, the router will still need to route packets between the different subnets. I didn’t say it was efficient, but it is something that you may come across.

For the purpose of illustration, I’m going to assume that Host A wishes to communicate with Host B. In order for these two hosts to communicate, our router will obviously need to be involved. Let’s take a look at how the communication process occurs when Host A attempts to create an HTTP session with Host B.

1. The first step in the process is the creation of an HTTP request at the Application Layer. In this case, Host A is requesting a web page from Host B. After formatting the HTTP request, the data is passed down to the Transport layer. Remember, our interaction is happening over TCP/IP.
2. Once the Transport Layer on Host A receives the data, it will add header information that will include the source and destination TCP ports. In this case, the destination TCP port will be 80, and the source port some number above 1024. Once this is complete, the segment will be passed to the Network Layer.
3. At the Network Layer, the IP header will be added. This header will include a variety of information, but most importantly the source and destination IP addresses. The source address is 192.168.0.99, and the destination address is 10.0.0.100. The next step is passing the packet down to the Data Link layer.
4. Recall the ANDing process looked at in Chapter 5. This is the process that a host uses to determine whether a destination host is local or remote. In this case, the ANDing results will be different, which means that the destination is on a remote network. When a destination is remote, the packet cannot be sent directly to that host. Instead, it must be first sent to a router.
5. Pay particular attention to this step. Host A knows that it needs to send this packet to its local router, but the router is not the ultimate destination IP address – 10.0.0.100 (Host B) is. In order to get this data to the router, our host must frame the packet with the destination MAC address – in this case, the MAC address associated with the router’s E0 interface. To obtain this MAC address, it will send out an ARP request looking for the MAC address associated with 192.168.0.1. Once the router responds, Host A will frame the packet. In this case, the source address is the MAC address of Host A, and the destination address is the MAC address of the router’s E0 interface.
6. Ultimately, this frame will reach interface E0 on the router. The router will notice that the frame is destined for its MAC address, and as such it will process the frame. After calculating the frame’s CRC, it will strip off the frame header and pass the resulting packet up to the Network layer At this point, the router will notice that the destination IP address is not it’s own. When a router receives a packet that is not destined for it specifically, it looks in its routing table to see whether it has a route defined to the destination network. In this case, the router sees that it is directly connected to network 10.0.0.0/8, and also determines that it should forward the packet via interface E1. Before it can forward the packet, however, it has to pass it back down to the Data Link Layer for re-framing.
7. At the Data Link layer, the router will re-frame the packet with a new header, meaning that source and destination MAC addresses will need to be added. In this case, the source MAC address is now the MAC address of interface E1 on the router. The destination MAC address will be obtained via an ARP request to IP address 10.0.0.100. Once Host B replies with its MAC address, this will be added to the frame as the destination MAC address. The router will then calculate a new CRC for the frame, and forward it through interface E1 as a series of bits.
8. Eventually the frame will arrive at Host B. By inspecting the destination MAC address, Host B will recognize that the frame is meant for it, calculate the CRC, strip off the framing, and pass it up to the Network Layer.
9. At the Network Layer, Host B will also recognize its IP address as the destination. This means that it has to process the packet further. In the IP header, it will see that the data should be passed to TCP at the Transport layer.
10. At the Transport Layer, Host B will look at the destination TCP port, and recognize that the data contained in the segment should be sent to TCP port 80. This is the port on which the web server application is listening for connections.
11. The web server on Host B will process the HTTP request contained in the data. Ultimately it will need to send a reply, where the whole process happens again, in reverse.

If those twelve steps seem like an awful lot of work to pass data between two hosts, you’re right. However, this is the way things work when you want to communicate between hosts on a routed network. Think of it this way – we are effectively using a Layer 3 protocol (IP) to communicate between different Layer 2 networks. In this case, both of those networks were Ethernet, but that won’t always be the case. For example, what if Host B had resided on a Token Ring network? In that case, our router would require one Ethernet interface and one Token Ring interface. When the data was sent to the router from Host A, it would have been framed for Ethernet. After the router stripped off the framing and passed the data up to the Network layer, it would determine that the packet needed to be forwarded out the Token Ring interface, where it would need to be framed for Token Ring. As even this very basic example suggests, a router is doing a great deal of work to every packet it encounters – not only does it have to make a forwarding decision based on the destination IP address, but it also has to reframe every single packet that it forwards on its way. By this point, you should be recognizing some of the reasons as to why routing is typically much slower than switching.

You should also recognize that although packets are re-framed at the router, the actual source and destination IP addresses never change. In fact, the only thing that the router touches for certain in the IP header is the time-to-live (TTL) value. Recall that a router will always decrement the TTL on an IP packet that it forwards by one, which ultimately ensures that packets don’t end up being forwarded around a network forever. Once its TTL expires, a packet is discarded.

A router may further alter an IP packet under special circumstances, such as when the maximum transmission unit (MTU) of connected networks is different. For example, imagine that the networks connected were Ethernet and Token Ring. Token Ring has a much larger maximum frame size than Ethernet. When a frame is received on a Token Ring interface, the packet it contains may be much larger than what Ethernet can handle (recall that the MTU of an Ethernet frame is only 1518 bytes). In this case, the router will “chop up” or fragment the packet into a number of smaller packets, then reframe each and send them on their way. Again, it becomes clear that there is more to what a router does than initially meets the eye.