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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 linkstate 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 Concepts
Comparing 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 Concepts
Comparing 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 Concepts
EIGRP 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 Concepts
EIGRP 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 Concepts
EIGRP 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 Concepts
EIGRP Terminology
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EIGRP Concepts
EIGRP Terminology
Routing Protocols Supported by EIGRP
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EIGRP Concepts
EIGRP Terminology
Contents of Tables Used by EIGRP
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EIGRP Concepts
EIGRP 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 Concepts
EIGRP 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 Concepts
EIGRP 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 Concepts
EIGRP 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 Concepts
EIGRP 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 Concepts
EIGRP 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 Concepts
EIGRP 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 Configuration
Basic 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 Configuration
Basic EIGRP Configuration
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EIGRP Configuration
Basic EIGRP Configuration
Router A Command Descriptions from
Previous Slide
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EIGRP Configuration
Basic 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 Configuration
Configuring EIGRP Summarization
Effect of Autosummarization is to
Summarize at the Classful Boundary
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EIGRP Configuration
Configuring 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 Configuration
Configuring 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 Configuration
Configuring EIGRP Summarization
Discontiguous Subnets With and Without
Autosummarization
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EIGRP Configuration
Configuring 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 Configuration
Configuring EIGRP Summarization
Using Interface Summarization with EIGRP
• Autosummarization is turned off with the
no auto-summary command
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EIGRP Configuration
Verifying 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 Protocols
Troubleshooting 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:
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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 Protocols
Troubleshooting 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 Protocols
Troubleshooting 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 Protocols
Troubleshooting EIGRP
• Reasons why EIGRP might not work properly:
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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 Protocols
Troubleshooting EIGRP
Using debug ip eigrp to Troubleshoot
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Troubleshooting Routing Protocols
Troubleshooting EIGRP
The debug ip eigrp Output Fields
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Troubleshooting Routing Protocols
Troubleshooting 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 Protocols
Troubleshooting 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 Protocols
Troubleshooting 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 Protocols
Troubleshooting OSPF
To Provide Detailed Output Relating to OSPF, Use the
debug ip ospf packet command
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Troubleshooting Routing Protocols
Troubleshooting 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:
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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
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