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RE:ddd
Internetworking functions of the network layer include network addressing and best path selection for traffic
In network addressing, one part of the address is used to identify the path used by the router and the other is used for ports or devices on the network
Routed protocols allow routers to direct user traffic; routing protocols work between routers to maintain path tables
Network discovery for distance-vectoe routing involves exchange of routing tables; problems can include slow convergence
For link-state routing, routers calculate shortest paths to other routers; problems can include inconsistant updates
Balanced hybrid routing uses uses attribs of both link state and distance-vector routing, and can apply paths to several protocols
Path determination for traffic going through a network cloud, occurs at the network layer. The path determination function enables a router to evaluate the available paths to a dest and to est the preffered handling of a packet
Routing services use network topology info when evaluating network paths. This info can be configured by the administer or collected through dynamic process running in the network
The net layer interfaces to networks and provides best-effort end-to-end packet delivery services to its user, the transport layer. The net layer uses the ip routing table to send packets from the source network to the dest network
After the router determines which path to use, it proceeded with forwarding the packet. It takes the packet that it accepted on one interface and forwards it to another interface ot port that reflects the best path to the packet's destination
11.1.3.1. Explain how routers route packets from source to destination.
To be truly practical, a network must consistently represent the paths available between the routyers.
Using these addresses the net layer can provide a relay connection that interconnects independent networks
The consistency of L3 addresses accross the entire network also improves bandwidth by preventing unnecessaruy broadcasts. Broadcasts invoke unnecessary process overheadd and waste capacity on any devices or links that do not need to receive the broadcast...
By using consistent end-to-end addressing to represent the path of media connections, the network layer can find a path to the dest without unneceesarily burdening the devices or links on the internetwork with broadcasts
11.1.4.1. Explain that both networks and individual hosts on a network must have addresses.
The 11.1.5.1. Explain best path selection and packet switching as the two basic processes of a router.
A router generally relays a packet from one data link to another, using two basic functions, a path
11.2.4.1. Explain why dynamic routing is necessary.
11.2.5.1. Explain what information is important in dynamic routing.
The success of dynamic routing depends on 2 basic router functions:
maintenance of a routing table
timely distribution of knowledge, in the form of routing updates, to other routers
Dynamic routing relies on a routing protocol to share knowledge among routers. A routing protocol defines the set of rules used by a router when it communications with neighboring routers. A routing protocol describes:
- how to send updates
- what knowledge is contained in these updates
- when to send this knowledge
- how to locate recipients of the updates
11.2.6.1. Explain how distances on network paths are determined by
various metrics.
When a routing algorithm updates A routing table, its primary objective is to determine the best info to include i the table. Each routing algorithm interprets what is best in its own way. The algorithm generates a number, called the metric value, for each path through the network. TypicLLy the smaller the metric number, the better the path.
You can calculate metrics based on a single characteristic of a path; you can calculate more complex metrics by combining several characteristics. The metrics most comm only used by routers are;
bandwidth- the capacity of a link
delay - the lenght of time required to move a packet along each linik from soutrce to dest
load - the amount of activity on a network resource such as a router
reliability - usually refers to the error rate of each network link
hop count - th # of routers a packet must pass through before reaching its dest
ticks - the delay on a data link using IBM pc clock ticks (approx 55 milliseconds)
cost- an arbritary value, usually based on bandwidth, monetaRY EXPENSE, OR OTHER measurements, that is assigned by a network administrator
11.2.7.1. Describe three classes of routing protocols.
Most routing algorithms can be classified as onre of two basic algorithms - distance vector or link state. The distance-vector routing approach determines the direction (vector) and distance to any link in the internetwork. The link-state (also called shortest path first) approach re-creates the exact topology of the entire internetwork (or at least the partition in which the router is situated)
The balanced hybrid approach combines aspects of link-state and dist-vect algos. The next several pages cover procedures and present techniques for minimizing the problems)
www.webproforum.com/vpn
11.2.8.1. Explain the concept of time to convergence.
The routing algo is fundamental to dynamic routing. Whenever the topology of a network changes because of groeth, reconfig, or failure, the network knowledge base must also change...
The knowledge needs to reflect an accurate, consistent view of the new topology. The view is called convergence . When all routers in a internetwork are operating with the same knowledge, the internetwork is said to have converged.
Fast convergence is a desireable network feature because it recduces the period of time in which routers would continue to make incorrect/wasteful routing decisions
11.3.1.1. Explain the concept of distance-vector routing.
Distance-vector based routing algos (alos known as Bellman-Ford) pass periodic copies of a routing table fronm router to router. These regular uopdates between routers communicate topology changes.
Each router receives a routing table from its directly neighboring routers. The algo eventually accumulates network distances so that it caN maintian a database of network topology info.
11.3.6.1. Explain the solution of defining a maximum.
Distance vector routing algos are self correcting but a loop problem can require a count to infinity. DVprots define infinity as a specific max#. This # refers toa routing metric
Split Horizon - will prevent certain routers from saying a path (incorrect) is available via another router
hold down timer -
11.4.1.1. Explain the concept of link-state routing.
LINK STATE- uses SPF algos, where they maintaIN a complex database of topology info. Link-state maintianins full knowledge of distant routers and how they interconnect
Link-state routing uses;
link-state advertisements (LSAs)
a topological databse
the SPF algo and the resulting SPF tree
a routing table of paths and ports to each network
11.4.2.1. Explain how link-state protocols exchange routing tables.
routers exchange LSAs with each other
each router in paralell with the others constructs a topological database consitsing of all LSAs
the SPF compoutes network reachability
the router lists its best paths and the ports to these dest nets. It also maintianins a db of topologies and status details
11.4.3.1. Explain how topology changes propagate through the
network of routers.
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