Guide To TCPIP, Second Edition

1
Guide To TCP/IP, Second Edition

• Chapter 10
• Routing In The IP Environment

2
Topics

• Routing and routing tables
• Routing protocols
• distance vector
• link-state protocols
• Routing Characteristics
• convergence
• split horizon
• poison reverse
• TTL
• black holes
• Interior gateway protocols
• RIP
• OSPF
• EIGRP

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More Topics

• BGP (an exterior gateway protocol)
• Routing Management
• policy considerations
• load balancing
• maintaining a network map
• Classless vs. class-oriented protocols
• Choosing a routing protocol for your WAN
• Router connections on internal and external
  networks
• Securing routers and routing protocols
• Router troubleshooting concepts, tools, and
  techniques

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Routing Tables

• Memory-based router database
• Database entries are called routes
• Network addresses
• Next hop (next router towards destination)
• Metrics
• Vendor-specific information
• A list of all the networks that the router can
  reach

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Cisco Routing Table
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How Routing Tables Are Used

• Router reads destination address from IP Header
• Looks in routing table for a network match
• If a match is found the packet is sent to the
  corresponding next hop router

7
Placing Entries in a Routing Table

• Direct cable connection to network
• Manually configured
• Provides administrative control
• Simple and Secure
• Lots of work and error-prone for large number of
  entries
• Dynamically
• Network Layer Reachability Information (NLRI)
  conveyed via a routing protocol

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Advantages and Disadvantages of Dynamic Routing
Protocols

• Easy to maintain
• Single point of failure that attackers can
  exploit
• May take a long time to discover new routers
• Protocols can be complex
• Lack of control router decides which paths to
  take

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Routing vs Routed Protocols

• Routing protocols
• exchange routing information
• Routing Information Protocol (RIP)
• Open Shortest Path First (OSPF)
• Routed protocols
• Layer 3 protocols that are used to get packets
  through an internetwork
• Internet Protocol (IP)
• Internetwork Packet Exchange (IPX)

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Routing Protocol Groups

• Routing domains
• Autonomous systems (AS) controlled by an
  organization to cope with domain differences
• Interior gateway protocols (IGPs)
• Used inside AS
• Exterior gateway protocols (EGPs)
• Use between AS
• Protocol Functionality
• Distance vector
• Link-state

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Distance Vector Protocols

• Broadcast entire routing table every 30 seconds
• Convergence (when all router tables are accurate)
  is a function of time
• Route by Rumor Routers know of routes, but
  not existence of routers
• Routing decisions are based on distance to next
  hop
• Chatty and inefficient
• Examples RIP, IGRP, BGP

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Setting Up Routing Tables in RIP

• Routers boot and defines own distance vector as 0
• Routers calculate cost to connected links
• Each router announces its distance vector info on
  all connected links
• Each router updates its routing table with new
  distance vector info
• Triggered updates are generated to other directly
  connected networks
• Periodic updates are broadcast or multicast

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Distance Vector Protocols Use Hop Counts
14
Routing Loops

• Both routers believe the best path to a network
  is through the other router
• Routers pass packets back and forth until TTL
  expires

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Loop-Avoidance Schemes

• Count to infinity
• Advertise infinite number of hops to a router
• Does not prevent loops, but limits damage
• Network diameter
• number of hops across network is artificially
  limited
• IP header TTL field
• Defines lifetime of a packet
• Split horizon
• No advertising network on same interface route
  learned about
• Poison reverse
• advertise route as unreachable

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Link-State Protocols

• Generates information about directly connected
  neighbors only
• 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 used to determine optimal path
  thru network
• Do not broadcast entire routing tables

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Link-State Processes

• Meet neighbors with Hello process
• Transmit link-state advertisements (LSAs)
  containing list of neighbors and cost to each
  neighbor
• Routers build picture of network based upon LSAs
• Picture converted into forwarding tables
• Link-State summaries periodically broadcast
• Routers can request updates

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Link-State Hello Process
19
Routing Characteristics

• Route convergence
• Loop-free path to all networks
• Split horizon
• Limits route advertisements
• Poison reverse
• Assigning costs to routes to prevent routing
  loops
• Time to Live (TTL)
• Network layer header field limits packet life
• Multicast vs broadcast update behavior
• Broadcasts cannot traverse routers

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More on Routing Characteristics

• ICMP router advertisements
• Allow hosts to passively learn about available
  routes
• Black holes
• ICMP is turned off
• Path Maximum
• Transmission Unit black hole router
• Areas, autonomous systems, and border routers

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Split Horizon Restricts Advertisements
22
A Black Hole Router
23
Aggregating Networks
24
EGP Connects AS
25
Popular Routing Protocols

• Intra-domain routing protocols
• RIP
• OSPF
• Inter-domain routing protocols
• BGP

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RIP

• Versions 1 and 2
• UDP based
• RIP routers send and receive datagrams on UDP
  port 520

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RIPv1

• Broadcast routing table every 30 seconds
• Support class A, B, C subnet masks
• Each RIP packet can contain info for up to 25
  networks
• RIPv1 packet format
• Command
• Version
• Reserved (or Zero)
• Address Family Identifier
• IP Address
• Metric

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RIPv1 Packet format
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RIPv2

• Supports
• variable-length subnet masks
• basic authentication
• multicasts routing updates

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RIPv2 Packet Fields

• Command
• Version
• Reserved
• Address Family
• Authentication Type
• Plain text passwords
• Address Family Identifier
• Route Tag
• info obtained internally or externally to routing
  area
• IP Address
• Subnet Mask
• Next Hop
• Metric

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RIPv2 Packet Format
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Open Shortest Path First

• Link-state routing protocol
• Open or non-proprietary
• Configurable metrics (bandwidth, delay, or
  monetary constraints)

33
OSPF Architecture
34
OSPF Procedures

• Multicast Hello packets
• Establish adjacencies
• Dijkstra algorithm for optimal path determination
• Designated router (DR)
• Alleviate some overhead
• Informs other routers of LSAs
• Backup designated router (BDR)
• Other routers multicast LSAs to DR on 224.0.0.6

35
OSPF Procedures Diagrammed
36
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) Stub areas do not accept Type 5
  LSAs

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OSPF header fields

• Version Number Field
• Type Field
• Packet Length Field
• Router ID Field
• Area ID Field
• Checksum Field
• AuType Field Authentication Type
• Authentication

38
Standard OSPF Header
39
Enhanced Interior Gateway Routing Protocol (EIGRP)

• Developed by Cisco in 1980s (IGRP)
• Updated to EIGRP in early 1990s
• Integrates distance vector and link-state
  capabilities

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Border Gateway Protocol

• Exchanges routing information between separate
  autonomous systems
• Three types of routing operations
• Inter-autonomous system routing
• Intra-autonomous system routing
• Pass-through autonomous system routing (for AS
  that does not support BGP)

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Border Gateway Protocol Design
42
Managing Routing On A In-House Internetwork

• Routing protocols
• Do not discriminate
• between users
• types of traffic
• Load balancing is managed on a flow basis not a
  per-packet basis
• Control network paths with Policy-based routing
  based on
• Type of applications protocol
• Link preferences

43
Hybrid Networks

• Network growth or mergers may force supporting
  multiple routing protocols
• Redistribution
• Requires careful attention to network design and
  configuration
• RIP simplicity conflicts with EIGRP complexity
• Solutions often vendor specific

44
Choosing a Routing Protocol

• Several small offices use static routing or RIP
• Hub and spoke
• Use On-Demand Routing
• Distance vector protocols not recommended on
  frame relay or ATM links
• Multiprotocol
• IP and IPX
• Use Ciscos EIGRP or upgrade and dump IPX
• Mobile Users generate mismatched IP addresses
• Mobile IP uses ICMP Router Discovery for location
  identification
• Local Area Mobility (LAM) uses routing table
  proxies

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Routing To And From The Internet

• BGPv4 is current exterior routing protocol on
  Internet
• Requires a major hardware investment
• Only networks that connect to multiple ISPs
  should use BGP

46
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
• Do not use Telnet to remotely configure your
  routers because it sends plain text login names
  and passwords
• Use secure routing protocols if possible (RIP
  lacks adequate authentication services)

47
Troubleshooting IP Routing

• At a minimum test connectivity by
• Viewing the routing tables
• using PING
• using TRACERT

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Chapter Summary

• Routing protocols and routers provide a mechanism
  that can forward traffic from a senders subnet
  to an intended receivers 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

52
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
• Its 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, its 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