Chapter 2 Section 4

Metric variables

Bandwith - data capacity of a link Delay- Length of time that is required to move a packet along each link from the source to the destination. delay depends on the bandwidth of intermediate links port queues at each router, network congestion and physical distance. cost- An arbitrary value that is assigned by a network administrator usually based on bandwidth administrator preference or other measurement. Hop count- the number of routers that a packet must travel through before reaching its destination.

SHOW IP ROUTE command

C - directly connected L- Local Routes S- Static routes S*- Static Default O- OSPF D -EIGRP, letter D stands for DUAL which is the algorithm used by EIGRP

What is routing?

Routing is the process that forwards data packets between networks or subnetworks using a TCP/IP Internet layer device, called a Router.

Directly Connected Networks

This entry comes from having router interfaces that are directly attached to network segments. this method is the most ceratin method of poulating a routing table. if the interface fails or is adminstratively shut down, the entry for that network is removed from the routing table. the administrative distance is 0 and therefore preempts all other entried for the desination network. entries wit the lowest admindistance are the most trusted source.

Cisco Express Forwarding

This is the third type of packet forwarding method used by Cisco Routers. It is the most recent and preferred Cisco IOS packet-forwarding mechanism which incorporates the best of the previous switching mechanisms. Generation of cache table entries is not packet-triggered but change-triggered. When something changes in the network topology the change is also reflected in the cache table. this makes Cisco express forwarding the fastest forwarding mechanism and the preferred choice. The show ip route command displays the routing table the router is using.

Default Routes

A default route is an optional entry that is used when no explicit path to a destination is found int eh routing table. the default route can be manually insterted or it can be populated from a dynamic routing protocol.

more on path determination.

A routing table holds only one entry per network. a router must be able to determine which routing protocol it should use if it has two identical routes to a network from two different sources. Multiple sources exist from having multiple dynamic routing protocols running and from static and default information being available. the process that runs on the router must be able to evaluate all the sources and select the best one to poplulate the routing table. a routing protocol uses different metrics to measure the distance and desirability of a path. administrative distance is the feature that routers use to select the best path when there are two or more routes to the same destination from two routing protocols. Administrative distance define the reliability of a routing protocol. each protocol is prioritized from most to least reliable with an administrative distance value.

More on Routing Tables

As part of path determination a router builds a routing table that identifies known networks and how to reach them. Each router has its own local routing table, populated from different sources. routing metrics vary depending on the routing protocol that is running in the router. all network addresses can be added dynamically or statically, any connected network is configured automatically. a routing table also includes direct connected or next hop association. when a router receives an incoming packet it uses the destination address and searches the routing table to find the best path. if no entry can be found, the router discards the packet AFTER sending an ICMP message to the source address of the packet. routers communicate with eachother and maintain their routing tables.

Distance Vector routing protocols

Distance vector routing protocols have information available that is similar to a road sign. it does not know the entire path to every netowrk sefment. the router only has to know the direction of the vector in which to send the packet. This method determines the direction(Vector) and the Distance(Hop Count) to any network. an example of a distance vector protocol is RIP which uses hop count as its routing metric.

Link State routing protocols

Link state routing protocols have a complete map of the area. each router builds its own internal map of the entire network topology in its link state(topology) database. each router on its own runs an SPF Algorithm to calculate the shortest path to all known destinations. an example is OSPF which uses cost as its routing metric.

What are the two most important functions of a router?

Path Determination and Packet Forwarding

what is path Determination?

Path determination is the protocol that a router uses to determine how its Routing table becomes populated and how it is aware of changes on the network from and to other routers. the first letter in each line of the routing table indicates which protocol was the source for the information.

Process Switching

Process switching is one of the three packet forwarding mechanisms that a router uses. it is the oldest forwarding mechanism that is available in Cisco routers. every packet requires a full look-up in the routing table which makes this method slow. typically obsolete from modern networks

three types of memory that a router has

RAM ROM and Flash. Ram is memory on the motherboard. Volatile Rom is read only on motherboard. non volatile. hard to manipulate. Flash non volitile easily manipulated

Dynamic Routes

Router learns dynamic routes automatically when a routing protocol is configured and a neighbor relationship to other routers is established. the information changes with changes in the network. larger networks require dynamic routing because there are usually many addresses and constant changes. these changes require updates to routi g tables across all routers in the network or connectivity is lost.

what is Packet Forwarding

Routers use the routing table to determine where to forward packets. the destination ip address in the packet determines the packet destination. they compare this address with their routing table. the result is a decision about which outgoing interface to use to send the packet out of the router. if routers do not have matching entry in their routing tables the packets are dropped.

Dynamic Routing Protocols

a dynamic routing protocol uses an algorithm that generates a number (metric) for each path through the network. sophisticated routing protocols can base route selection on multiple metrics combining them into a single metric. typically the lower the metric the better the path.

Static Routes

a system admin manually enters a static route. default admin distance for a static route is 1 therefore static routes will be included in the routing table unless there is a direct connection to that network. not suitable for large networks as they are not a scalable solution

Fast Swtiching

fast switching is another packet forwarding mechanism used by routers. In order to overcome the slow performance of process switching Cisco IOS platforms support several switching mechanisms that use a cache to store the most recently used destinations. The first packet whose destination is not found in the fast-switching cache is process-switched. and an entry is created in the cache. subsequent packets are switched in the interrupt code using the cache to improve performance.