A Bridge operates at the data link layer. Bridges analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames toward their destination. A Router operates at the network layer. It receives instructions for forwarding packets between topologies and determine the most efficient path . Physical [repeater] Data Link [Bridge] Network [router]
Routers in a network regularly exchange information with each other. All routers maintain this information in routing tables.
Cost refers to the cost in terms of bandwidth of using a particular route. Hop count is the number of routers that process the packet in order to transport it to the destination device. The routing metric is calculated by adding the metrics associated with each route used by a packet to reach the destination network.
Typical Factors considered for Routing metrics <ul><li>Hop count </li></ul><ul><li>Bandwidth </li></ul><ul><li>Delay </li></ul><ul><li>Load </li></ul><ul><li>Reliability </li></ul><ul><li>Maximum Transfer Unit size </li></ul>
Dynamic routing protocols enable routers to • Dynamically discover and maintain routes • Calculate routes • Distribute routing updates to other routers • Reach agreement with other routers about the network’s topology
Administrative Distance <ul><li>It is a trustworthiness of the source of routing information. If router learns about the same network from multiple sources, it must decide which source is more trustworthy. </li></ul><ul><li>THE LOWER THE ADMINISTRATIVE DISTANCE, THE MORE TRUSTWORTHY THE SOURCE. </li></ul>
<ul><li>Dynamic routing protocols can be one of three types: </li></ul><ul><li>Distance vector: Routing by rumor! </li></ul><ul><li>Link-state: Uses Shortest Path Algorithm, builds a map of the network. Makes informed decisions </li></ul><ul><li>Hybrid: Uses a combination of the two methods </li></ul>
<ul><li>In Distance Vector routing, routers send copies of their entire routing tables or updates to their immediate neighbors at set intervals. </li></ul><ul><li>Each router builds a cumulative table of network distances from the routing information that it receives. </li></ul><ul><li>When a router receives a table, it adds its own distance “value” or vector to the table before propagating it to its immediate neighbors. </li></ul><ul><li>This is how routers learn about each other </li></ul>Distance vector routing
Factors affecting convergence after failure include • the routing protocol used • how often routing updates are sent • the timers used • the diameter of the network and its complexity
RIP <ul><li>Broadcasts its routing table every 30 seconds and allows 25 routes/ pkt. </li></ul><ul><li>RIP uses only “hop count” to determine the best path. The issue is if other paths exist with “higher B/W, less congestion …… Can you tell why this would be an issue </li></ul><ul><li>Cannot handle route with hop count > 15. </li></ul>
IGRP uses a composite routing metric. It is based on the following parameters: • bandwidth • delay • reliability • load • maximum transfer unit (MTU)
The standard algorithm used to implement IGRP enables only the bandwidth and the delay parameters by default. All IP routing protocols implemented by Cisco - including IGRP - can perform load balancing over their routes.
IGRP routing metrics distinguish between paths of different bandwidth. IGRP chooses the highest bandwidth path to the destination network. For example, IGRP chooses a path through a FDDI network over a path through a token ring network.
IGRP <ul><li>IGRP allows load balancing over equal and unequal metric paths </li></ul><ul><li>IGRP variance means that if one path is twice as good as other, the better path will be used twice as much </li></ul><ul><li>IGRP periodically scans all default routes and chooses one with the lowest metric as the default route </li></ul><ul><li>IGRP supports triggered updates to a change in status to reduce convergence time </li></ul>