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INTERNETWORKING III

CHAPTER 5 STUDY GUIDE

 

 

1.     Routing refers to the process of choosing the best path over which to send packets and how to cross multiple physical networks. This is the basis of all Internet communication.

 

2.     In IP networks, the router forwards packets from the source network to the destination network based on the IP routing table. After the router determines which path to use, it can proceed with switching the packet. This means it accepts the packet on one interface and forwards it to another interface that is the next hop on the best path to the packet's destination.

 

3.     Entries in the routing tables contain an IP address of the next hop along the route to the destination. Each entry specifies 1 hop and points to a router that is directly connected, which means that it can be reached across a single network.

 

4.     A variety of metrics can be used to define the best path. Some routing protocols, such as Routing Information Protocol (RIP), use only one metric and that is hop count. And some routing protocols, such as IGRP, use a combination of metrics. The metrics most commonly used by routers are:

      Hop count The number of routers that a packet must go through to reach its destination.

      Bandwidth The data capacity of a link.

      Delay The length of time to move the packet from the source to destination.

      Load The amount of activity on a network resource.

      Reliability The error rate of each network link.

      Ticks The delay on a data link using IBM PC clock ticks.

      Cost An arbitrary value assigned by an administrator.

 

5.     After examining a packet's destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet and there is no default route, it typically drops the packet.

 

6.     Below is a list of protocols. Label the protocols as routed or routing and if they are routing protocols, label those as IGP (interior gateway protocols) or EGP (exterior gateway protocols)

 

PROTCOL

ROUTED/ROUTING

AppleTalk

Routed

APPN

Routing-IGP

BGP

Routing-EGP

EGP

Routing-EGP

EIGRP

Routing-IGP

IGRP

Routing-IGP

IPX

Routed

IS-IS

Routing-EGP

OSPF

Routing-IGP

RIP

Routing-IGP

TCP/IP

Routed

 

7.     Routers are capable of multiprotocol routing, which means they support multiple independent routing protocols, such as IGRP and RIP. This capability allows a router to deliver packets from several routed protocols, such as TCP/IP and IPX, over the same data links.

 

8.     Exterior routing protocols require the following information before routing can begin:

        A list of neighbor (also called peer) routers with which to exchange routing information

        A list of networks to advertise as directly reachable

 

9.     The optimal route - Refers to the ability of the routing protocol to select the best route. The best route depends on the metrics and metric weightings used to make the calculation. For example, one routing protocol might use the number of hops and the delay, but might weigh the delay more heavily in the calculation.

 

10.  Simplicity and efficiency - This is particularly important when the software implementing the routing protocol must run on a computer with limited physical resources.

 

11.  Robustness - Routing protocols should perform correctly in the face of unusual or unforeseen circumstances, such as hardware failures, high load conditions, and incorrect implementations. Because routers are located at network junction points, they can cause considerable problems when they fail. The best routing protocols are often those that have withstood the test of time and proven stable under a variety of network conditions.

 

12.  Rapid convergence - This is the speed and ability of a group of networking devices running a specific routing protocol to agree on the topology of a network after a change in that topology. When a network event, such as a change in a network's topology, causes routes to either go down or become available, routers distribute routing update messages. Routing update messages are sent to networks, thereby causing the recalculation of optimal routes and eventually causing all routers to agree on these routes. Routing protocols that converge slowly can cause routing loops or network outages.

 

13.  Flexibility -Routing protocols should quickly and accurately adapt to a variety of network circumstances. For example, assume that a network segment has gone down. Many routing protocols quickly select the next-best path for all routes that normally use a given segment. Routing protocols can be programmed to adapt to changes in network bandwidth, router queue size, network delay, and other variables.

 

14.  IGRP has a maximum hop count of 255, it defaults to 100, and it is usually set to 50 or less.

 

15.  Dynamic routing protocols can be supplemented with static routes where appropriate. For example, a gateway of last resort (that is, a router to which all unroutable packets are sent) may be designated. This router acts as a central storing place for all unroutable packets, ensuring that all messages are at least handled in some way.

 

 

16.  Below is a list of protocols. Label the protocols as distance vector, link state, or hybrid.

EIGRP

Hybrid

IGRP

Distance-vector

IS-IS

Link state

NLSP

Link state

OSPF

Link state

RIP

Distance-vector

 

17.  Each routing protocol must be configured separately. With any routing protocol, you must follow two basic steps:

1.     Create the routing process with one of the router commands.

2.     Configure the protocol specifics.

IGRP has the additional requirement of an autonomous system number.

 

18.  IGRP uses a combination of user configurable metrics. Name the metrics used.

      network delay

      bandwidth

      reliability

      load

 

19.  IGRP advertises three types of routes:

      Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise these routes. Additionally, subnet information is not included in IGRP updates, which poses a problem for noncontiguous IP subnets.

      System routes are routes to other major networks within the AS. The router derives system routes from directly connected network interfaces and system route information provided by other routers that use IGRP. These routes do not include subnetting information.

      Exterior routes are routes to networks outside the AS that are considered when identifying a gateway of last resort. The router chooses a gateway of last resort from the list of exterior routes that IGRP provides. The router uses the gateway of last resort if it does not have a better route for a packet and the destination is not a connected network. If the AS has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.

 

20.  IGRP provides a number of features that are designed to enhance its stability, including the following:

      Holddowns - When a router learns that a network is further away than was previously known, or it learns that the network is down, the route to that network is placed into holddown. During the holddown period, the route is advertised, but incoming advertisements about that network from any router other than the one that originally advertised the network's new metric are ignored. This mechanism is often used to help avoid routing loops in the network, but has the effect of increasing the topology convergence time.

      Split Horizons - This occurs when a router tries to send information about a route back in the direction from which it came and helps prevent routing loops.

      Poison Reverse Updates - Whereas split horizons should prevent routing loops between adjacent routers, these are intended to defeat larger routing loops. Increases in routing metrics generally indicate routing loops. These are then sent to remove the route and place it in holddown. A router poisons the route by sending an update with a metric of infinity to a router that originally advertised a route to a network. Poisoning the route can help speed convergence.

 

21.  For each path through an AS, IGRP records the segment with the lowest bandwidth, the accumulated delay, the smallest maximum transmission unit (MTU), and the reliability and load. A router running IGRP sends an IGRP update broadcast every 90 seconds. It declares a route inaccessible if it does not receive an update from the first router in the route within three update periods (270 seconds). After five update periods (450 seconds), the router removes the route from the routing table. IGRP uses flash update and poison reverse to speed up the convergence of the routing protocol. A flash update is the sending of an update sooner than the standard periodic update interval of notifying other routers of a metric change.