Download Guide to TCP/IP, Third Edition

Guide to TCP/IP, Third
Edition
Chapter 10:
Routing in the IP Environment
Objectives
• Understand how basic routing works
• Describe various routing characteristics
• Understand interior gateway protocols, such as
RIP, OSPF, and EIGRP
• Understand BGP, an exterior gateway protocol
• Describe how to manage routing on an in-house
internetwork
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Objectives (continued)
• Be aware of the challenges associated with
redistributing a network from a classless protocol to
a class-oriented protocol
• Describe factors involved in choosing a routing
protocol for your wide area network
• Describe router connections on internal and
external networks, including the Internet
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Objectives (continued)
• 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
– Compilation of information about all the networks
that the router can reach
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How Entries Are Placed in the Routing
Table
• Route entry can be placed in a routing table in
three basic ways
– Through direct connection
– It can be manually configured
– An entry can be placed in a routing table is
dynamically, by using a routing protocol
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Routing Protocols and Routed
Protocols
• Routing protocols
– Used to exchange routing information
– Routing Information Protocol (RIP) and OSPF are
routing protocols
• Routed protocols
– Layer 3 protocols that are used to get packets
through an internetwork
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Grouping Routing Protocols
• Interior gateway protocols (IGPs)
– Routing protocols used inside a routing domain are
called interior
• Exterior gateway protocols (EGPs)
– Routing protocols used to connect these routing
domains
• Distance vector and link-state
– Used to communicate
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Distance Vector Routing Protocols
• RIP
• Interior Gateway Routing Protocol (IGRP)
• Border Gateway Protocol (BGP) also is a distance
vector routing protocol
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Link-State Routing Protocols
• Differ from distance vector routing protocols in two
ways
– They do not route by rumor
– They do not periodically broadcast their entire tables
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Routing Characteristics
• Network
– Converged when all the routers know a loop-free
path to get to all other networks
– Ideally should be in a converged state
• Split horizon
– Used to speed up the process of convergence and
resolve the counting-to-infinity problem
• Poison reverse
– Technique for assigning costs to routes designed to
prevent routing loops
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Routing Characteristics (continued)
• Time to Live
– Ensures that packets cannot loop endlessly through
a network
– Defined in the Network layer header
• Multicast Versus Broadcast Update Behavior
– Routers can be configured to forward multicasts
– RIPv1 sends broadcast updates
– RIPv2 can send multicast updates
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ICMP Router Advertisements
• Some routers
– Can be configured to send periodic ICMP Router
Advertisement packets
• Periodic ICMP Router Advertisements
– Allow hosts to learn passively about available routes
• Unsolicited ICMP Router Advertisements
– Sent periodically to the all-hosts multicast address
224.0.0.1
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Black Holes
• Occurs on a network when
– ICMP is turned off and
– Router discards packets without sending any
notification about its actions
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Areas, Autonomous Systems, and
Border Routers
• OSPF utilizes areas
– To reduce the number of entries in the link-state
database
• OSPF specification
– Defines the need for a backbone area, Area 0
• Autonomous systems (ASs)
– Groups of routers under a single administrative
authority
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Interior Gateway Protocols
• Used to exchange routing information within an AS
• Also are referred to as intra-domain routing
protocols
• RIP
–
–
–
–
Basic distance vector routing protocol
Two versions: RIPv1 and RIPv2
Communications are UDP based
RIP-based routers send and receive datagrams on
UDP port number 520
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Open Shortest Path First
• Defined in RFC 2328
• The premier link-state routing protocol used on
TCP/IP networks
• Based on
– Configurable values (metrics) that may be based on
network bandwidth, delay, or monetary cost
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Open Shortest Path First (continued)
• Six basic 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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Enhanced Interior Gateway Routing
Protocol
• Interior Gateway Routing Protocol (IGRP)
– Developed in the 1980s by Cisco Systems
– Updated in the early 1990s (Enhanced Interior
Gateway Routing Protocol)
• Enhanced Interior Gateway Routing Protocol
– Integrates the capabilities of link-state routing into a
distance vector routing protocol
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Border Gateway Protocol
• Exterior gateway protocols (EGP)
– Used to exchange routing information between
separate autonomous systems
– Defined in RFC 904
– Border Gateway Protocol (BGP) replaces EGP
routing
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Border Gateway Protocol (continued)
• Offers three types of routing operations
– Inter-autonomous system routing
– Intra-autonomous system routing
– Pass-through autonomous system routing
• When configured for intra-autonomous system
routing
– BGP routers are located within the same AS
• Pass-through autonomous system routing
– Enables BGP peer routers to exchange routing
information across an AS that does not support BGP
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Managing Routing on an In-House
Internetwork
• IP routing protocols have a shortcoming
– They do not discriminate between users, types of
traffic, and so on
– They only know how to get to a network
– Policy-based routing addresses this shortcoming
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Hybrid Networks
• What happens when you redistribute a network
from a classless protocol to a class-oriented
protocol that only understands the major network
address but not the VLSM you’re using?
• How can you convert the bandwidth, delay, load,
reliability, and MTU size of EIGRP and IGRP into
the simple hop counts that RIP uses?
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Routing On and Off a Wide Area
Network
• If the network in question is relatively small
– Consider using no routing protocol
• Frame relay and ATM
– Use virtual circuits
• So one physical interface can actually be logically
partitioned into several logical interfaces.
• If your company also uses Novell’s IPX protocol
– You may have to run one routing protocol for IP and
another routing protocol for IPX
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Mobile Users
• Mobile IP
– Defined by the IETF in RFCs 2003 through 2006 and
RFC 3220 (which obsoletes RFC 2002)
• Local Area Mobility
– Cisco proprietary feature that is similar to Mobile IP
– Operates by using the routing table
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Routing To and From the Internet
• BGPv4
– The exterior routing protocol in use on the Internet
– Should only be used by networks that connect to
multiple Internet providers
– Tracks hops between pairs of autonomous systems
instead of tracking hop counts for actual routers
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Securing Routers and Routing
Behavior
• Securing routers
– Turn off unnecessary services
– Shut down unnecessary listening ports
– Configure strong access security to prevent
tampering
– Secure physical access to the boxes
• Securing routing protocols
– Requires cooperation from the protocols themselves
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Troubleshooting IP Routing
• ROUTE
– View the host’s local routing table, and add and
remove route entries
• PING
– Sends ICMP Echo messages and test connectivity
• TRACERT
– Sends ICMP echoes with incrementally increasing
TTL values to identify the path to a destination
• PATHPING
– Utility used to discover path from host to destination
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Summary
• Routing protocols and routers
– Provide a mechanism that can forward traffic from a
sender’s subnet to an intended receiver’s subnet
• Routers
– Depend on various routing protocols to manage the
packet forwarding process
• Distance vector routing protocols such as RIP
– Provides a crude metric of routing cost
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Summary (continued)
• The OSPF protocol
– Supports much more sophisticated routing structures
that break up a network into routing areas
• Routing characteristics
– Help to determine what kinds of routing protocols to
use in specific applications
• Managing routing on a complex network means
– Understanding how and when to use exterior and
interior routing protocols
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Summary (continued)
• Router tables define the topology and behavior of
IP networks
– Essential to manage router security and updates as
safely as possible
• Troubleshooting tools for inspecting and diagnosing
routing problems through
– Windows 2000, Windows Server 2003, and
Windows XP IP host include route, tracert, ping, and
pathping
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