Question

- When a packet arrives at a router's interface, what information does the router require in order to make a forwarding decision?

- Why does the router not keep track of every packet that it helps deliver? Use examples to justify your answer.

- Are there any instances in which a router should keep track of the packets it forwards?

Answer 1

A)

When router receives any packet it has to remove Layer 2 header information present on packet(Example:In Ethernet, source and destination Mac address present on L2 header). Once router remove L2 information it looks for Layer 3 information available on packet that is source and destination IP address.For moving L3 packet between interfaces, router checks destination address and finds longest-prefix match in IP routing table to find outgoing interface. In IPv4 router uses longest mask to identify best routing entry for forwarding packet.

B)

When a packet arrives at a router, the router examines the IP address put there by the IP protocol layer on the originating computer. The router checks it's routing table. If the network containing the IP address is found, the packet is sent to that network. If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router. Hopefully the next router will know where to send the packet. If it does not, again the packet is routed upwards until it reaches a NSP backbone. The routers connected to the NSP backbones hold the largest routing tables and here the packet will be routed to the correct backbone, where it will begin its journey 'downward' through smaller and smaller networks until it finds it's destination.

Basic router functionality is comprised of three major components:

• Routing process

• Routing protocols

• Routing table

The routing process is the actual movement of IP packets from one port to another and the routing table holds the information used by the routing process. Routing protocols such as the Routing Information Protocol (RIP) or Open Shortest Path First (OSPF) are used to communicate with other routers and may end up “installing” routes in the routing table for use by the routing process. When a router is configured, the routing table is constructed by bringing interfaces up and providing the interfaces with IP addresses.

C)There are two choices:

Pass the packet on

Normally, if the packet passes the criteria in the packet filtering configuration, the router will forward the packet on towards its destination, just as a normal router (not a packet filtering router).

Drop the packet

The other action to take is to drop the packet if it fails the criteria in the packet filtering configuration.