Each computer and router interface maintains an ARP table for Layer 2 communication The ARP table is only effective for the broadcast domain (or LAN) that it is connected to The router also maintains a routing table that allows it to route data outside of the broadcast domain.

Each ARP table contains an IP-MAC address pair The routing tables knows the network IP address for reachable networks, the hop count or distance to those networks, and the interface the data must be sent out to get to the destination network. The routing tables also track how the route was learned (in this case either directly connected [C] or learned by RIP [R])

The Layer 2 switch can only recognize its own local MAC addresses and cannot handle Layer 3 IP addresses When a host has data for a non-local IP address, it sends the frame to the closest router, also known as its default gateway The host uses the MAC address of the router as the destination MAC address

The router examines the Layer 3 destination address of the packet to make the forwarding decision Just as a Layer 2 switch keeps a table of known MAC addresses, the router keeps a table of IP addresses known as a routing table

A Layer 2 device can handle a reasonable number of unorganized MAC addresses, because it will only have to search its table for those addresses within its segment. Routers need to handle a greater volume of addresses.

Protocols used at the network layer that transfer data from one host to another across a router are called routed or routable protocols Routed protocols transport data across a network. Routing protocols allow routers to choose the best path for data from source to destination

Routers use routing protocols to exchange routing tables and share routing information.

Path determination occurs at the network layer Path determination enables a router to compare the destination address to the available routes in its routing table, and to select the best path The routers learn of these available routes through static routing or dynamic routing

Routers can make decisions based on the load, bandwidth, delay, cost, and reliability of a network link. Routers use routing protocols to build and maintain routing tables that contain route information

Routing protocols fill routing tables with a variety of route information Routing tables contain the information necessary to forward data packets across connected networks Routers communicate with one another to maintain their routing tables through the transmission of routing update messages

Routers keep track of important information in their routing tables, including the following: Protocol type Destination/next-hop associations Routing metric Outbound interfaces

The following are the metrics that are most commonly used by routing protocols: –Bandwidth –Delay –Load –Reliability –Hop count –Ticks –Cost

An autonomous system is a network or set of networks under common administrative control

The distance-vector routing approach determines the distance and direction, vector, to any link in the internetwork Routing Information Protocol (RIP) Interior Gateway Routing Protocol (IGRP) Enhanced IGRP (EIGRP)

Link-state routing protocols send periodic updates, known as link-state refreshes Link-state routing protocols respond quickly to network changes sending trigger updates only when a network change has occurred When a route or link changes, the device that detected the change creates a link-state advertisement (LSA) concerning that link Open Shortest Path First (OSPF) Intermediate System-to-Intermediate System (IS-IS)

RIP is a distance vector routing protocol that uses hop count as its metric to determine the direction and distance to any link in the internetwork RIP selects the path with the least number of hops RIP cannot route a packet beyond 15 hops

IGRP is a distance-vector routing protocol developed by Cisco IGRP was developed specifically to address problems associated with routing in large networks that were beyond the range of protocols such as RIP IGRP can select the fastest available path based on delay, bandwidth, load, and reliability IGRP also has a much higher maximum hop count limit than RIP