Network Device Part 4: Routers
Whereas switches can only examine and forward packets based on the contents of the MAC header, routers can look further into the packet to discover the network for which a packet is destined. Routers make forwarding decisions based on the packet's network-layer header (such as an IPX header or IP header). These network-layer headers contain source and destination network addresses.
Local devices address packets to the router's MAC address in the MAC header. After receiving the packets, the router must perform the following steps:
1. Check the incoming packet for corruption, and remove the MAC header . The router checks the packet for MAC-layer errors. The router then strips off the MAC header and examines the network-layer header to determine what to do with the packet.
2. Examine the age of the packet. The router must ensure that the packet has not come too far to be forwarded. For example, IPX headers contain a hop count. By default, 15 hops is the maximum number of hops (or routers) that a packet can cross. If a packet has a hop count of 15, the router discards the packet.
IP headers contain a Time to Live (TTL) value. Unlike the IPX hop count, which increments as the packet is forwarded through each router, the IP TTL value decrements as the IP packet is forwarded through each router. If an IP packet has a TTL value of 1, the router discards the packet. A router cannot decrement the TTL value to 1 and then forward the packet.
3. Determine the route to the destination. Routers maintain a routing table that lists available networks, the direction to the desired network (the outgoing interface number), and the distance to those networks. After determining which direction to forward the packet, the router must build a new header. (If you want to read the IP routing tables on a Windows 95/98 workstation, type ROUTE PRINT in the DOS box.)
4. Build the new MAC header and forward the packet. Finally, the router builds a new MAC header for the packet. The MAC header includes the router's MAC address and the final destination's MAC address or the MAC address of the next router in the path.
You should try capturing the packets on each side of a router on your company's network. You will be able to see the change in the hop count or TTL value and the new MAC header. When you analyze a communication, you should examine the network layer to determine the actual source and destination of the packet.
What special feats can routers perform that switches cannot? Because routers operate at layer three of the OSI model, they support forwarding based on network addresses (as opposed to forwarding based on MAC addresses or VLAN designations). Routers can also forward packets based on the best known path (especially in the case of link state routers). In addition, routers can provide detailed filters based on the source and destination network address, as well as the source and destination process (as defined in the port number field in the network header).
Now routers are used to connect networks together and route packets of data from one net-
work to another. Cisco became the de facto standard of routers because of its high-quality
router products, great selection, and fantastic service. Routers, by default, break up a broad-
cast domain—the set of all devices on a network segment that hear all the broadcasts sent on
that segment.
There are two advantages of using routers in your network:
They don’t forward broadcasts by default.
They can filter the network based on layer 3 (Network layer) information (e.g.,
IP address).
Four router functions in your network can be listed as follows:
Packet switching
Packet filtering
Internetwork communication
Path selection
Unlike layer 2 switches, which forward or filter frames, routers (layer 3 switches) use logical addressing and provide what is called packet switching. Routers can also provide packet filtering by using access lists, and when routers connect two or more networks together and use logical addressing (IP or IPv6), this is called an internetwork. Last, routers use a routing table (map of the internetwork) to make path selections and to forward packets to remote networks.
