www.ciscopress.com switching basics and intermediate routing ccna 3 chapter 3
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Switching Basics and Intermediate Routing CCNA 3
Chapter 3
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EIGRP and Troubleshooting Routing Protocols - Introduction
• EIGRP (Enhanced IGRP) is a proprietary Cisco routing protocol based on IGRP– Supports VLSM and CIDR– Compared to IGRP:
• Faster convergence times• Improved scalability• Superior management of routing loops
• EIGRP is a hybrid routing protocol– Offers the best of distance vector and link-state
protocols
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EIGRP and Troubleshooting Routing Protocols – Introduction
• Features of EIGRP:– Partial updates– Neighbor discovery– Easier to configure than OSPF– Ideal choice for large, multiprotocol networks
built primarily with Cisco routers
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EIGRP and Troubleshooting Routing Protocols – Introduction
• Reasons network problems occur:– Commands are entered incorrectly– Access lists are constructed or placed
incorrectly– Routers, switches, or other network devices
are configured incorrectly– Physical connections are bad
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EIGRP Concepts
• Balanced hybrid routing protocols like EIGRP combine aspects of distance vector and link-state protocols– Uses distance vectors with more accurate metrics to
determine best paths to destination networks– Uses topology changes to trigger updates instead of
using periodic updates– Converges more rapidly than distance vector– Uses less bandwidth, memory and processor
overhead
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EIGRP ConceptsComparing EIGRP and IGRP
• EIGRP uses metric calculations similar to IGRP– Supports unequal-cost load balancing
• EIGRP has better:– Convergence properties– Operating efficiency– Network overhead
• The metric is the same (bandwidth and delay by default), but EIGRP has a weight assigned to the metric that is 256 times that for IGRP– Automatic redistribution between IGRP and EIGRP in
same AS
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EIGRP ConceptsComparing EIGRP and IGRP
• EIGRP has a maximum hop count of 224• EIGRP uses convergence technology called
Diffusing Update Algorithm (DUAL)– Guarantees loop-free operation at every instant
throughout a route computation– Allows all devices involved in a topology change to
synchronize simultaneously– Routers not affected by topology changes are not
involved in recomputations– The convergence time of DUAL rivals that of any
other existing routing protocol
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EIGRP ConceptsEIGRP Features
• EIGRP has rapid convergence times– Almost instantaneous
• Uses DUAL to achieve rapid convergence• Stores backup routes for destinations in routing
table• If no backup route available, queries neighbors• Queries are propagated until an alternate route is
found
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EIGRP ConceptsEIGRP Features
• EIGRP has low usage of network resources during normal operation– Only hello packets are sent on a stable
network• Uses EIGRP hello packets to establish
relationships with neighboring EIGRP routers• Builds a neighbor table from the hello packets it
receives from adjacent EIGRP routers
– Does not send period routing updates
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EIGRP ConceptsEIGRP Features
• EIGRP supports automatic (classful) route summarization at major network boundaries by default
• Manual route summarization can be configured on arbitrary network boundaries to reduce the size of the routing table
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EIGRP ConceptsEIGRP Terminology
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EIGRP ConceptsEIGRP Terminology
Routing Protocols Supported by EIGRP
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EIGRP ConceptsEIGRP Terminology
Contents of Tables Used by EIGRP
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EIGRP ConceptsEIGRP Terminology
• Reliable Transport Protocol (RTP) is a transport layer protocol that guarantees ordered delivery of EIGRP packets to all neighbors– Does not rely on TCP/IP to exchange routing
information, as EIGRP is protocol-independent– RTP is a proprietary protocol
• EIGRP can call on RTP to provide reliable or unreliable service as needed– Can simultaneously multicast and unicast to different
peers, which allows for maximum efficiency
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EIGRP ConceptsEIGRP Packet Types
• EIGRP uses hello packets– They discover, verify and rediscover neighbor routers
• Hello packets are always sent unreliably– No acknowledgement is transmitted– Hello packets are sent at a fixed interval, depending
on the interface’s bandwidth– Hello packets are sent to the multicast IP address
224.0.0.10– On low-speed networks (T1 or slower), hello packets
are sent every 60 seconds; on faster networks they are sent every 5 seconds
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EIGRP ConceptsEIGRP Packet Types
• Packets must be received within the hold time interval period to maintain a passive state, which is a reachable and operational status– If EIGRP does not receive a packet from a
neighbor within the hold time, it considers that neighbor down
– DUAL then reevaluates the routing table– By default, the hold time is three times the
hello interval
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EIGRP ConceptsEIGRP Packet Types
• EIGRP does not require that neighbors have the same hello intervals and dead intervals (OSPF does)– Neighbors learn about the times through the
exchange of hello packets– They use the information to make stable
relationships regardless of timer settings
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EIGRP ConceptsEIGRP Packet Types
• EIGRP routers use acknowledgement packets to indicate receipt of EIGRP packets during a reliable exchange– RTP provides reliable communication– Acknowledgement packets are hello packets without
data– Acknowledgement packets are unicast, hello packets
are multicast– Acknowledgements can be attached to other kinds of
EIGRP packets, such as reply packets
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EIGRP ConceptsEIGRP Packet Types
• EIGRP uses update packets when a router discovers a new neighbor– Unicast packets are sent to the new neighbor
so it can add to its topology table– More than one update packet may be needed
• Update packets are also used when a router detects a topology change– The router sends a reliable multicast to all
neighbors
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EIGRP ConceptsEIGRP Packet Types
• EIGRP routers use query packets when they need specific information from one or all neighbors
• If a router loses its successor route and cannot find a feasible successor:– DUAL places the route in active state– A query is multicast to all neighbors in an attempt to
locate a successor– Neighbors send replies indicating whether or not they
have a successor
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EIGRP ConfigurationBasic EIGRP Configuration
• The router eigrp and network commands create an EIGRP routing process:– Router(config)#router eigrp AS-number– Router(config)#network network-number
Simple EIGRP Network
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EIGRP ConfigurationBasic EIGRP Configuration
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EIGRP ConfigurationBasic EIGRP Configuration
Router A Command Descriptions from Previous Slide
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EIGRP ConfigurationBasic EIGRP Configuration
• When using serial links with EIGRP, the bandwidth setting must be configured on the interface– Otherwise EIGRP assumes the default bandwidth and
not the true bandwidth• If the link is slower, the routers might not converge
– To set the bandwidth use this syntax:• Router(config-if)#bandwidth kbps
• Cisco also recommends adding the following command to all EIGRP configurations:– Router(conf-router)#eigrp log-neighbor-changes
• Monitors stability of routing system
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EIGRP ConfigurationConfiguring EIGRP Summarization
Effect of Autosummarization is to Summarize at the Classful Boundary
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EIGRP ConfigurationConfiguring EIGRP Summarization
• In many instances, autosummarization is not the preferred option– Case of discontiguous subnets
• Autosummarization prevents routers from learning about discontiguous networks
• With autosummarization turned off, EIGRP routers will advertise subnets
• Use this command syntax to turn off autosummarization:
– Router(config-router)#no auto-summary
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EIGRP ConfigurationConfiguring EIGRP Summarization
• With EIGRP, a summary address can be manually configured on a per-interface basis (use the following syntax, all on one command line):– Router(config-if)#ip summary-address eigrp
autonomous-system-number ip-address mask administrative-distance
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EIGRP ConfigurationConfiguring EIGRP Summarization
Discontiguous Subnets With and Without Autosummarization
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EIGRP ConfigurationConfiguring EIGRP Summarization
• By default, EIGRP summary routes have an administrative distance of 5; can be configured for any distance from 1 to 255
Granular Routing Updates with Interface Summarization
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EIGRP ConfigurationConfiguring EIGRP Summarization
Using Interface Summarization with EIGRP
• Autosummarization is turned off with the
no auto-summary command
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EIGRP ConfigurationVerifying the EIGRP Configuration
Numerous show Commands Verify EIGRP Configuration
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Troubleshooting Routing Protocols
• A foundation for building a problem-solving process:– When analyzing a network failure, make a clear problem
statement– Gather the facts needed to help isolate possible causes– Consider possible problems based on gathered facts– Create an action plan based on potential problems– Implement the action plan– Analyze the results to determine if the problem has been solved– If not, create an action plan based on the next most likely
problem on the list– After the actual cause of the problem is identified, solve it
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Troubleshooting Routing Protocols
• Cisco routers provide integrated commands to help in troubleshooting:– The show commands help monitor installation
behavior, normal network behavior, and isolate problem areas
– The debug commands assist in the isolation of protocol and configuration problems
– TCP/IP tools such as ping, traceroute and Telnet help isolate the OSI layer where the problem exists
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Troubleshooting Routing ProtocolsTroubleshooting RIP
• The most common problem in RIP that prevents RIP routes from being advertised is VLSM– RIPv1 does not support VLSM– If RIP routes are not being advertised, check these:
• Layer 1 or Layer 2 connectivity issues• VLSM subnetting is configured (can’t be used with RIPv1)• Mismatched RIPv1 and RIPv2 routing configurations exist• Network statements are missing or incorrectly assigned• Outgoing interface is down• Advertised network interface is down
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Troubleshooting Routing ProtocolsTroubleshooting RIP
• Use these commands to help with RIP troubleshooting:– The debug ip rip command displays information on RIP routing
transactions– Use the no debug ip rip, undebug all, or u all commands to
turn off debugging
Sample RIP Network for Troubleshooting
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Troubleshooting Routing ProtocolsTroubleshooting RIP
Troubleshooting with debug ip rip
Might see entries such as:
RIP: broadcasting general request on Ethernet0
RIP: broadcasting general request on Ethernet0
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Troubleshooting Routing ProtocolsTroubleshooting EIGRP
• Reasons why EIGRP might not work properly:– Layer 1 or Layer 2 connectivity issues– AS numbers on EIGRP routers are mismatched– Link might be congested or down– Outgoing interface is down– Advertised network interface is down– Autosummarization is enabled on routers with discontiguous
subnets• Use the no auto-summary command to disable automatic network
summarization
• One of the most common reasons for a missing EIGRP neighbor is a failure on the actual link
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Troubleshooting Routing ProtocolsTroubleshooting EIGRP
Using debug ip eigrp to Troubleshoot
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Troubleshooting Routing ProtocolsTroubleshooting EIGRP
The debug ip eigrp Output Fields
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Troubleshooting Routing ProtocolsTroubleshooting EIGRP
• The debug ip eigrp command generates a lot of output, use it only when network traffic is light
• The debug eigrp fsm command is also used for EIGRP debugging– Displays information on DUAL feasible successor
maps– Helps engineers analyze packets that are sent and
received on an interface
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Troubleshooting Routing ProtocolsTroubleshooting OSPF
• The majority of problems with OSPF relate to formation of adjacencies and synchronization of link-state databases
Output from the debug ip ospf events Command
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Troubleshooting Routing ProtocolsTroubleshooting OSPF
• The debug ip ospf events output seen in the previous slide might appear if:– The subnet masks for routers on the same network do not match– The OSPF hello interval for the router does not match that
configured for a neighbor– The OSPF dead interval does not match that configured for a
neighbor
• If a router configured for OSPF routing does not see an OSPF neighbor on an attached network, perform these tasks:– Make sure both routers have been configured with the same IP
mask, OSPF hello interval, and OSPF dead interval– Make sure that both neighbors are part of the same area type
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Troubleshooting Routing ProtocolsTroubleshooting OSPF
To Provide Detailed Output Relating to OSPF, Use the debug ip ospf packet command
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Troubleshooting Routing ProtocolsTroubleshooting OSPF
Output Fields for the debug ip ospf packet Command
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Summary
• EIGRP characteristics– Interior Gateway Protocol– Quick convergence times with minimum traffic– Enhanced version of IGRP; Cisco proprietary protocol
• EIGRP and IGRP are compatible, with some differences– EIGRP offers multiprotocol support– EIGRP uses same metric as IGRP– EIGRP uses a multiplier of 256 which makes the EIGRP
metrics larger that IGRP metrics
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Summary
• EIGRP routers keep route and topology information in RAM– Information is saved in three tables
• Neighbor table lists adjacent routers• Topology table has all the routes in the AS• The routing table holds the best routes to a destination
• DUAL takes the information in the neighbor and topology tables and calculates the lowest cost to each destination– This is called the successor route– The backup route is called the feasible successor
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Summary
• EIGRP is a balanced hybrid routing protocol– Acts as a link-state protocol when updating
neighbors and maintaining routing information– Advantages include:
• Rapid convergence• Efficient use of bandwidth• Support for VLSM and CIDR• Support for multiple network layers• Independence from routed protocols
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Summary
• DUAL results in the fast convergence of EIGRP– Each router constructs a topology table
• The topology table identifies the routing protocol (EIGRP), the lowest cost of the route (the feasible distance)
• The cost of the route as reported by the neighbor (the reported distance)
• EIGRP commands vary depending on which routed protocol is used– The network command configures connected networks
• EIGRP automatically summarizes routes at classful boundaries prior to IOS Release 12.2(8)T– If discontiguous subnets exist, autosummarization must be
turned off
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Summary
• Manual summarization is done at the interface level with the ip summary-address eigrp
• The show ip eigrp command can verify the EIGRP configuration
• The debug ip eigrp command can display information on eigrp packets and troubleshoot EIGRP
• Troubleshooting can be approached with an eight-step troubleshooting methodology
• Network engineers rely on show and debug commands to troubleshoot routing protocols
• RIP, IGRP, EIGRP, and OSPF have their own set of debug commands tailored for troubleshooting