guide to tcp/ip, second edition1 guide to tcp/ip, second edition chapter 10 routing in the ip...

Guide to TCP/IP, Second Edition 1

Guide To TCP/IP, Second Edition

Chapter 10

Routing In The IP Environment


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


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


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


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


Understanding Routing (cont.)


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


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)


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)


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


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)


Distance Vector Routing Protocols (cont.)


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


Loop-Avoidance Schemes

• Count to infinity

• Network diameter

• IP header TTL field

• Split horizon

• Poison reverse


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


Link-State Routing Protocols (cont.)


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


Routing Characteristics (cont.)

• ICMP router advertisements– Allow hosts to passively learn about available

routes

• Areas, autonomous systems, and border routers


Interior Gateway Protocols

• RIP– RIPv1, RIPv2– Distance vector routing protocol– UDP based– UDP port 520

• RIPv2– Support for variable-length subnets


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


RIPv1 (cont.)


RIPv2

• Support of variable-length subnet masks• Basic authentication• Multicasts routing updates• RIPv2 packet format

– Command– Version– Reserved– Address Family– Authentication Type


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


RIPv2 (cont.)


Open Shortest Path First

• Link-state routing protocol

• Non-proprietary

• Configurable metrics

• Multicast Hello packets

• Establish adjacencies


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)



• 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)



• OSPF header fields– Version Number Field

– Type Field

– Packet Length Field

– Router ID Field

– Area ID Field

– Checksum Field

– AuType Field

– Authentication


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


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


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


Border Gateway Protocol


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


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


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


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



• 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



• 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



• 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



• 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



• 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



• 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

guide to tcp/ip, second edition1 guide to tcp/ip, second edition chapter 10 routing in the ip...

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