AIT 622 by wulinqing


									                                             Assignment #5
                                          AIT622 – Summer 2008

Due: Tuesday, July 8


1. Although simple, why did the initial Internet core router system become impractical?

It became impractical because all routers had to coordinate their default route and a packet could travel
through all n routers when traveling from source to destination, very impractical.


2. What is an autonomous system? In the current Internet, what are the AS’s?

An autonomous system is a group of networks and routers controlled by a single administrative authority. In
the internet each large ISP is an autonomous system.

3. What is the function of an exterior gateway protocol? Name one such protocol.

An exterior gateway protocol is any protocol used to pass network reachability information between two
autonomous systems. An example is Border Gateway Protocol BGP.

4. The text says that, technically, BGP is not a routing protocol but rather a reachability protocol. Why?

Because an exterior gateway protocol like BGP only propagates reachability information, a receiver can
implement policy constraints, but cannot choose a least cost route. A sender must only advertise paths that
traffic should fallow.

5. Because BGP uses a reliable transport protocol, KEEPALIVE message cannot be lost. With this being
the case, does it make sense to specify a keepalive interval as one-third of the hold timer value? Why or why

Yes, because of the constraints posed upon BGP. In no case can BGP speaker make the KEEPALIVE
interval les than one second and in no case can the hold timer be less than three seconds. The time
constraints allow for a continuous connection.


6. What is the function of an interior gateway protocol? Name three such protocols.

An IGP is a generic description that refers to any protocol that interior routers use when they exchange
routing information. Three examples of IGP are Routing Information Protocol (RIP), HELLO, and Open

7. What problem does the split horizon update technique attempt to overcome and how does the technique
work? Name two other techniques that can be used to help overcome the same problem.
Split horizon update attempts to overcome the slow convergence problem by not allowing routers to
propagate information about a route back over the same interface from which the route arrived. Two other
techniques used to solve the same problem are hold down and poison reverse.

8. What routing metric does RIP use? What routing metric does Hello use? Despite the limitations of the
metric used by RIP, why does this metric still remain popular?

RIP uses a hop-count metric. Hello reports an estimate of delay to the destination. The limit RIP imposes
affects large autonomous systems or autonomous systems that do not have a hierarchical organization; also
RIP does not react quickly to slight differences in delay, thus avoiding a two-stage oscillation effect.

9. What type of protocol is OSPF? What is the main distinguishing characteristic of OSPF?

OSPF is a interior gateway protocol. OSPF is open source, includes type of service routing, provides load
balancing, allows a site to partition its networks and routers into subnets called areas, specifies that all
exchanges between routers can be authenticated, includes support for host-specific, subnet-specific, classful
network-specific routs, accommodates multi-access networks like Ethernet, uses hardware multicast
capabilities, allows managers to describe a virtual network topology, and allows routers to exchange routing
information learned from other sites.

10. What are two consequences of using default routes for most routers?

First, it means that local routing errors can go undetected. Second, on the positive side, using default routes
whenever possible means that the routing update messages exchanged by most routers will be much smaller
that they would be if complete information were included.
11. Below is a partially completed hierarchy for understanding the algorithms, protocols, and paradigms that are presented in the text (much like
the one a student might make for a study guide!). Underneath, there is a list of terms; put the letter for each term in one of the “leaf” boxes (i.e.,
the empty boxes at the end of each branch). All terms will be used; each term will appear in only one box.

                                                               Algorithms, Protocols, & Paradigms


                          Interior                         Exterior                                 Routing/Forwarding           Group Membership
                                                                                    NOTE: in this homework exercise you are only
                                                                                    completing the boxes for the left side of the
                                                                                    hierarchy. That is, only the boxes which fall under
                                                                                    the main heading of “Unicast”. The
     DV              Modified DV        Link State                                  “Multicasting”
                                                                                Broadcast & Prune side will be done in a later
  (hop count)          (delay)           (SPF)                                      homework exercise. All of the terms given should
                                                                                    be used –        Demand-Driven       Broadcast
                                                                                   Data-Driven with four terms and four boxes it is
                                                                                    probably safe to assume that one term will go in
                                                                                    each box!

   B                  C                 A                  D

       A. OSPF            B. RIP            C. Hello        D. BGP
          (Ch. 15)           (Ch. 15)           (Ch. 15)        (Ch. 14)
Other “study guide” types of things you should know (but don’t need to answer as part of the homework!)

1. Describe how the D-V routing update process works as related to Figure 13.4 in the text.

Distance Vector, v identifies a destination, d is the distance to that destination.

2. Describe the primary class of alternatives to distance-vector routing algorithms.

3. Understand the extra hop problem (pages 251-252).

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