Guide To TCP/IP, Second Edition

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Guide to TCP/IP, Second Edition

1

Guide To TCP/IP, Second Edition

Chapter 10


Routing In The IP Environment


Guide to TCP/IP, Second Edition

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Objectives


Understand how basic routing works, how routing
tables help control this behavior, and the various
types of routing protocols, including distance
vector and link
-
state protocols


Describe various routing characteristics, including
route convergence, split horizon, poison reverse,
TTL, and black holes, and understand why they
are necessary


Understand the interior gateway protocols, such as
RIP, OSPF, and EIGRP

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Objectives (cont.)


Describe how to manage routing on an
internetwork, including policy
considerations, load balancing, and
maintaining a network map


Describe factors involved in choosing a
routing protocol for your wide area network

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Objectives (cont.)


Describe router connections on internal and
external networks, including the Internet


Understand the importance of securing
routers and routing protocols


Explain basic router diagnostic
troubleshooting concepts, tools, and
techniques

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Understanding Routing


Routing table


Database that lives in the memory of the router


Entries in this database are known as “routes”


Network addresses


“next hop”


Metrics


Vendor
-
specific information


Compilation of information about all the networks that
the router can reach

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Understanding Routing (cont.)


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Understanding Routing (cont.)


The routing table is used as follows:


Router reads destination address


Looks in the Network field of its routing table
for a match


Finds a match, then it sends the packet to the
corresponding next hop

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How Entries Are Placed in the Routing Table


Route entries can be placed in a routing
table in three basic ways:


Direct connection


Manually configured


Exact configuration


Simple and secure


Dynamically with a routing protocol


Network Layer Reachability Information (NLRI)

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Routing Protocols and Routed Protocols


Routing protocols are used to exchange routing
information


Routing Information Protocol (RIP)


Open Shortest Path First (OSPF)


Routed protocols are Layer 3 protocols that are
used to get packets through an internetwork


Internet Protocol (IP)


Internetwork Packet Exchange (IPX)

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Grouping Routing Protocols


Two primary ways to group routing
protocols


Routing domains or autonomous systems


Interior gateway protocols (IGPs)


Exterior gateway protocols (EGPs)


Functionality of routing protocols


Distance vector


Link
-
state

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Distance Vector Routing Protocols


Route by rumor


Periodically broadcast entire routing table


Considered “chatty”


Routing decisions are based on network
distance


Routing Information Protocol (RIP)


Interior Gateway Routing Protocol (IGRP)


Border Gate Protocol (BGP)

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Distance Vector Routing Protocols (cont.)


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Routing Loops


One router believes the best path to a
network is via a second router, and at the
same time, the second router believes the
best path to that network is through the first
router


TTL

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Loop
-
Avoidance Schemes


Count to infinity


Network diameter


IP header TTL field


Split horizon


Poison reverse

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Link
-
State Routing Protocols


Generates information about directly connected
neighbors


Flood network (update) with information


Update only when a link change is detected


All routers have identical view of network
topology


Convergence time is short


Dijkstra algorithm


Build adjacencies with neighbor routers

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Link
-
State Routing Protocols (cont.)


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Routing Characteristics


Route convergence


Loop
-
free path to all networks


Time to Live (TTL)


Defined in the Network layer header


Multicast vs broadcast update behavior

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Routing Characteristics (cont.)


ICMP router advertisements


Allow hosts to passively learn about available
routes


Areas, autonomous systems, and border
routers

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Routing Characteristics (cont.)


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Routing Characteristics (cont.)


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Routing Characteristics (cont.)


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Interior Gateway Protocols


RIP


RIPv1, RIPv2


Distance vector routing protocol


UDP based


UDP port 520


RIPv2


Support for variable
-
length subnets

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RIPv1


Broadcast routing table every 30 seconds


Does not support non
-
default subnet masks


A RIP packet can have up to 25 networks


RIPv1 packet format:


Command


Version


Reserved (or Zero)


Address Family Identifier


IP Address


Metric

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RIPv1 (cont.)


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RIPv1 (cont.)


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RIPv2


Support of variable
-
length subnet masks


Basic authentication


Multicasts routing updates


RIPv2 packet format


Command


Version


Reserved


Address Family


Authentication Type

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RIPv2 (cont.)


RIPv2 packet format (cont.)


Authentication


Plain text and encrypted passwords


Address Family Identifier


Route Tag


Internal route entry, external route entry


IP Address


Subnet Mask


Next Hop


Metric

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RIPv2 (cont.)


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Open Shortest Path First


Link
-
state routing protocol


Non
-
proprietary


Configurable metrics


Multicast Hello packets


Establish adjacencies

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Open Shortest Path First (cont.)


Dijkstra algorithm


Designated router (DR)


Backup designated router (BDR)


Link State Advertisements (LSA)


LSA multicast to DR (224.0.0.6)

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Open Shortest Path First (cont.)


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Open Shortest Path First (cont.)


Six types of LSAs


Type 1 (Router Links Advertisement)


Type 2 (Network Links Advertisement)


Type 3 (Network Summary Link Advertisement)


Type 4 (AS Boundary Router Summary Link
Advertisement)


Type 5 (AS External Link Advertisement)


Type 7 (Not So Stubby Area Networks Advertisement)

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Open Shortest Path First (cont.)


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Open Shortest Path First (cont.)


OSPF header fields


Version Number Field


Type Field


Packet Length Field


Router ID Field


Area ID Field


Checksum Field


AuType Field


Authentication

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Open Shortest Path First (cont.)


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Enhanced Interior Gateway Routing
Protocol (EIGRP)


Developed by Cisco in 1980s (IGRP)


Updated to EIGRP in early 1990s


Hybrid routing protocol


Integrates distance vector and link
-
state
functions

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Enhanced Interior Gateway Routing
Protocol (EIGRP) (cont.)


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Border Gateway Protocol


Exchange routing information between
separate autonomous systems


BGP offers three types of routing operations


Inter
-
autonomous system routing


Intra
-
autonomous system routing


Pass
-
through autonomous system routing

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Border Gateway Protocol


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Securing Routers And Routing Behavior


Turn off unnecessary services, shut down
unnecessary listening ports


Configure strong access security to prevent
tampering, and, of course, secure physical access
to the boxes


Assign secure encrypted passwords


Telnet sends unencrypted login names and
passwords


Use secure routing protocols

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Troubleshooting IP Routing


Test connectivity with the following tools


ROUTE: Used to view the host’s local routing
table, and add and remove route entries


PING


TRACERT


PATHPING

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Chapter Summary


Routing protocols and routers provide a
mechanism that can forward traffic from a
sender’s subnet to an intended receiver’s subnet


Generally, routers depend on access to tables of
information that describe known routes and
default routers so that traffic can be directed
properly within any given internetworked
environment, or forwarded outside that
environment for delivery elsewhere

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Chapter Summary (cont.)


Routers depend on various routing protocols to manage the
packet forwarding process


Interior routing protocols are designed for use within
autonomous routing domains, such as those that fall under
the purview and control of a single company or
organization


Exterior routing protocols provide a means whereby
routers belonging to multiple companies or organizations
can safely and securely forward data and manage routing
information amongst the parties involved in a common
connection

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Chapter Summary (cont.)


Distance vector routing protocols such as RIP represent the
oldest and simplest type of routing protocols, in which the
number of router transitions (called hops) provides a crude
metric of routing cost, and where no routing loops should
occur as part of the routing topology


Link
-
state routing protocols like OSPF provide more
sophisticated routing metrics and controls, and not only
can deal with multiple routes between a sender and
receiver, but can also use more powerful route metrics to
balance loads across such links, or failover from less
expensive to more expensive routes, as needed

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Chapter Summary (cont.)


The OSPF protocol supports much more sophisticated
routing structures that break up a network into routing
areas to help optimize routing tables and behavior


In addition, OSPF recognizes special categories of routing
areas, such as a backbone area (where all individual areas
interconnect) and autonomous systems, which represent
individual routing regions that fall under specific
administrative and management control


In such cases, area border routers may connect separate
routing areas to the backbone, or to other routing areas

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Chapter Summary (cont.)


Routing characteristics, which determine how long it takes
route information (and changes) to stabilize within a group
of routers that share information, help to determine what
kinds of routing protocols to use in specific applications


One important characteristic is convergence (how long it
takes routing protocols to calculate optimal routes
following updates), including techniques such as split
horizon, poison reverse, and Time to Live settings


Other important characteristics include information update
mechanisms, Router Advertisements, and how routing
domains may be logically subdivided to help manage
complexity and reduce router traffic

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Chapter Summary (cont.)


Managing routing on a complex network means
understanding how and when to use exterior and interior
routing protocols, and how to establish the right kinds of
connections between multiple routing domains


Private WAN links, Internet connections, and Mobile IP
users all require special handling where routing is
concerned to make sure that systems and services behave
as required


It’s especially important to understand how and when
interior routing protocols, such as OSPF, must interoperate
with exterior routing protocols, such as BGP

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Chapter Summary (cont.)


Because router tables define the topology and
behavior of IP networks, it’s essential to manage
router security and updates as safely as possible


For those reasons, using strong passwords and
secure links to access and update routers and their
configurations is absolutely essential

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Chapter Summary (cont.)


Key troubleshooting tools for inspecting and
diagnosing routing problems through a Windows
2000 and Windows XP IP host include
ROUTE,TRACERT, PING, and PATHPING


To become as proficient in managing routes and
routers as possible, make yourself completely
familiar with these command
-
line utilities