Yu Cheng (Jade)
ICS 351
Prelab Report 4
October 07, 2008
[Question 1]
Provide the command that configures a Linux PC as an IP router.
On a Linux system, IP forwarding is enabled when the file
/proc/sys/net/ipv4/ip_forward contains a 1 and disabled when it contains a 0.
This change is not permanent, however, it will be lost when the system is
rebooted. The command to make this change is:
on PC# `echo "1" > /proc/sys/net/ipv4/ip_forward`
[Question 2]
What are the main differences between a distance vector routing protocol and a
link state routing protocol? Give examples for each type of protocol.
Routing Protocols allow routers to dynamically advertise and learn routes,
determine which routes are available and which are the most efficient routes to
a destination. There are two major types of algorithms for IP routing: Distance
Vector Routing and Link State Routing.
Basically, Distance Vector protocols determine the best path on how far the
destination is, while Link State protocols are capable of using more
sophisticated methods taking into consideration link variables, such as
bandwidth, delay, reliability and load. How does a router determine wheter
datagrams to a particular host can be directly delivered through one of its
interfaces?
The IP Distance Vector routing protocols still in use today are: Routing
Information Protocol (RIP v1 and v2) and Interior Gateway Routing Protocol
OSPF, IS-IS and EIGRP are some of the link-state routing protocols in use today.
[Question 3]
What are the differences between an intradomain routing protocol (also called
interior gateway protocol, or IGP) and an interdomain routing protocol (also
called exterior gateway protocol, or EGP)? Give examples for each type of
protocol.
IGP is used within a single autonomous system. It has a single network
administration. The nodes, networks share the unique routing policy. IGP makes
best use of network resources. The interior gateway protocols can be divided
into two categories: 1) Distance-vector routing protocol and 2) Link-state
routing protocol. Examples provided in Question 2 such as RIP, IGRP, OSPF are
IGP protocols.
EGP is used among different autonomous systems. The networks in the system have
their independent administrative entities. The communication is between
independent network infrastructures. Border Gateway Protocol (BGP) is a recent
exterior gateway protocol.
[Question 4]
Which routing protocls are supported by the software package Zebra?
Zebra is a routing software package that provides TCP/IP based routing services
with routing protocols support such as RIP, OSPF and BGP. In addition to
traditional IPv4 routing protocols, Zebra also supports IPv6 routing protocols.
Zebra also supports special BGP Route Reflector and Route Server behavior.
[Question 5]
In the Zebra software package, the processes ripd, ospfd, and bgpd deal,
respectively, with the routing protocls RIP, OSPF, and BGP. Which role does the
process zebra play?
The process zebra changes the kernel routing table and redistributes routes
between different routing protocols. The ripd daemon handles the RIP protocol,
while ospfd is a daemon which supports OSPF version 2. bgpd supports the BGP-4
protocol.
[Question 6]
Describe how a Linux user accesses the processes of Zebra (zebra, ripd, ospfd,
bgpd) to configure routing algorithm parameters?
Each daemon has it's own configuration file and terminal interface. When we
configure a static route, it must be done in zebra configuration file. When we
configure BGP network it must be done in bgpd configuration file. To resolve the
problem, Zebra provides integrated user interface shell called vtysh. vtysh
connects to each daemon with UNIX domain socket and then works as a proxy for
user input.
[Question 7]
What is the main differece between RIP version 1 (RIPv1) and RIP version 2
(RIPv2)?
The main difference between RIPv1 and RIPv2 is that RIPv2 enables the use of a
simple authentication mechanism to secure table updates. More importantly, RIPv2
supports subnet masks, a critical feature that is not available in RIPv1. To
describe the difference in more detail they are:
1. RIPv2 allows for password authentication of the routing table updates
between routers
2. RIPv2 allows for the use of a 16 bit "route tag" that can identify
individual routes and imported routes (from other protocols), or be used
in other ways by implementors of the protocol.
3. In subnetted IP networks, routers running RIPv1 cannot determine the
configured subnet mask (as opposed to the "native" or "natural" one)
because RIPv1 messages do not carry that information. RIPv2 rectifies this
situation by specifying a 32 bit subnet mask field.
4. RIPv2 has a "next hop" field that can identifiy another router on the
local network as the best next hop in the path to the destination being
advertised. Specifying a value of 0.0.0.0 in this field indicates that
routing should be via the originator of the RIP advertisement, the normal
behavior for RIPv1.
[Question 8]
Explain what it means to run RIP in passive mode.
Passive routers listen and update their routes based on advertisements, but do
not advertise; active routers advertise their routes (reachability information)
to others. Typically, routers run RIP in active mode, while hosts use passive
mode.
[Question 9]
Explain the meaning of triggered updates in RIP.
Triggered updates allow a RIP router to announce changes in metric values almost
immediately rather than waiting for the next periodic announcement. The trigger
is a change to a metric in an entry in the routing table. For example, networks
that become unavailable can be announced with a hop count of 16 through a
triggered update. Note that the update is sent almost immediately , where a time
interval to wait is typically specified on the router. If triggered updates
were sent by all routers immediately, each triggered update could cause a
cascade of broadcast traffic across the IP internetwork.
Triggered updates improve the convergence time of RIP internetworks but at the
expense of additional broadcast traffic as the triggered updates are propagated.
[Question 10]
Explain the concept of split-horizon in RIP.
Split horizon helps reduce convergence time by not allowing routers to advertise
networks in the direction from which those networks were learned. The only
information sent in RIP announcements are for those networks that are beyond the
neighboring router in the opposite direction. Networks learned from the
neighboring router are not included.
Split horizon eliminates count-to-infinity and routing loops during convergence
in single-path internetworks and reduces the chances of count-to-infinity in
multi-path internetworks.
[Question 11]
What is an autonomous system (AS)? Which roles do autonomous systems play in the
internet?
Autonomous system (AS) is a collection of connected IP routing prefixes under
the control of one or more network operators that presents a common, clearly
defined routing policy to the Internet. Each autonomous system is managed
independently with respect to BGP
[Question 12]
What is the AS number of you institution? Which autonomous system has AS number
1?
University of hawaii's AS number is: AS6360. I found it by searching "whois
hawaii.edu", which gave me a page: http://cqcounter.com/whois/?query=hawaii.edu
The AS # 1 is Level 3 Communications, Inc. 1025 Eldorado Blvd.
I found it at a online AS number loopup: http://enc.com.au/itools/aut-num.php
[Question 13]
Explain the terms stub AS, multihomed AS, and transit AS?
A stub Autonomous System refers to an AS that is connected to only one other AS.
A multihomed Autonomous System is an AS that maintains connections to more than
one other AS. This allows the AS to remain connected to the Internet in the
event of a complete failure of one of their connections. However, this type of
AS would not allow traffic from one AS to pass through on its way to another AS.
A transit Autonomous System is an AS that provides connections through itself to
other networks. That is, network A can use network B, the transit AS, to connect
to network C.
