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Cisco Networking Academy
Chapter 6 Review
Configuring EIGRP
IGRP/EIGRP
Similarities:
• share the same distance-vector routing technology
• use the same composite routing metric
• support all the same distance vectors, and their mathematical weights
• EIGRP also uses the IGRP variance feature to provide unequal-cost load
balancing.
Differences:
• There is only one minor difference in the algorithm that calculates the
composite metric: The IGRP metric is 20 bits long, whereas the EIGRP
metric is 32 bits long
IGRP/EIGRP Redistribution
IGRP doesn't have any concept of internal and external routes
EIGRP routes that are translated and redistributed into an IGRP
network are treated as native IGRP routes.
Automatic redistribution between IGRP and EIGRP will occur
only if the two protocols are configured with the same autonomous
system (AS) number
If they have different AS numbers, they will assume that they are part of
different networks
EIGRP
A Cisco proprietary routing protocol
Features:
• Rapid Convergence
• Efficient use of bandwidth
• Support for VLSM and CIDR
• Independence from Routed Protocols
• Multiple network-layer support
EIGRP Technologies
Neighbor discovery and recovery
EIGRP routers maintain routing tables by periodic communication to:
• Dynamically learn of new routers that join their network
• Identify routers that become either unreachable or inoperable
• Rediscover routers that had previously been unreachable
Neighbor discovery/recovery process consists of periodically transmitting a
small hello packet to neighbors
• establishes the relationship between immediate neighbors (known as an
adjacency)
– used to exchange routing metrics supported and other information
EIGRP Technologies
Reliable Transport Protocol (RTP)
used to transport all EIGRP message types through a network
• reliable packets require explicit acknowledgement
– Update
– Query
– Reply
• unreliable packets do not require explicit acknowledgement
– Hello
– ACK
(Cont.)
EIGRP Technologies
Reliable Transport Protocol (RTP)
can support both multicasting and unicasting
• Multicast packets are delivered to multiple, specific destinations simultaneously
using a group address
• Unicast packets are explicitly addressed to a single destination
• can support both multicast and unicast transmissions simultaneously for
different peers.
EIGRP Technologies
Diffusing Update Algorithm (DUAL)
Finite-state machine
• tracks all routes advertised by neighbors and uses the composite metric of each
route to compare them
• selects loop-free path using a successor and remember any feasible successors
– If successor lost - use feasible successor
– If no feasible successor - query neighbors and recompute new successor
EIGRP Technologies
Protocol-specific modules
EIGRP can be easily retrofitted to support any new routed protocols that
may be developed by just adding another protocol-specific module
Each protocol-specific module is responsible for all functions related to its
specific routed protocol
EXAMPLE
• The IP-EIGRP module is responsible for:
– Sending and receiving EIGRP packets that bear IP data
– Notifying DUAL of new IP routing information that is received
– Maintaining the results of DUAL's routing decisions in the IP routing table
– Redistributing routing information that was learned by other IP-capable routing
protocols
EIGRP Neighbor Table
Each EIGRP router maintains a neighbor table that lists adjacent
routers
there is a neighbor table for each protocol that EIGRP supports
used to support reliable, sequenced delivery of packets
• One field in each row of the table is populated with the last sequence number of
the packet received from that neighbor
• EIGRP uses this field for two purposes:
– Sequence numbers are used to acknowledge specific packets that were delivered
reliably
– This field can also alert EIGRP to packets being received out of sequence.
EIGRP Neighbor Table
Key elements:
Neighbor address - the network-layer address of the neighbor router
Queue count - the number of packets waiting in queue to be sent
• if this value is constantly higher than zero, then there may be a congestion
problem at the router
• a zero means that there are no EIGRP packets in the queue
Smooth Round Trip Timer (SRTT) - the average time it takes to send and
receive packets from a neighbor
• used to determine the retransmit interval (RTO)
Hold Time - the interval to wait without receiving anything from a neighbor
before considering the link unavailable
EIGRP Routing Table
EIGRP chooses the best (successor) routes to a destination from
the topology table and places these routes in the routing table
contains the lowest-metric routes that DUAL calculated for all known
destinations
can contain up to six routes to each destination
A separate routing table is maintained for each routed protocol that
EIGRP is configured to support.
EIGRP Topology Table
The information needed to calculate a set of distances, and vectors,
to all known and reachable destinations
Each EIGRP router maintains a topology table for each configured
network protocol
Entries in a topology table can be either in an active or passive state
• A passive route is one that is stable and available for use
• An active route is one currently being recomputed.
 A backup route (feasible successor) is retained
• can have multiple feasible successors for a destination
EIGRP Topology Table
Field
Description
Bandwidth
The bandwidth of the slowest interface in the path to a destination is used to
calculate the route composite metric
Total delay
contains the sum total of delay values in that route
Reliability
The reliability of the path
Load
The load level of the path
MTU
the size of the smallest maximum transmission unit (MTU) supported by the
router interfaces in the path
Reported distance
the distance reported by an adjacent neighbor to a specific destination
(does not include the distance between this router and the adjacent neighbor)
Feasible distance
the lowest calculated metric to each destination
Route source
The source of the route is the identification number of the router that
originally advertised that route
EIGRP Route Tagging
EIGRP classifies routes as either internal or external
Internal routes are those that originated within an EIGRP network
External routes were either learned from a different routing protocol or are
static routes that have been injected into EIGRP through redistribution
• External routes include the following information and are identified in the
topology table
– The identification number (router ID) of the EIGRP router that
redistributed that route into the EIGRP network
– The number of the Autonomous System where that route's destination
resides
– The protocol used in that external network
– The cost or metric received from that external protocol
– A tag that can be administratively set and used in route filtering
EIGRP Packet Types
Hello Packets
used to discover (or rediscover) and track other EIGRP routers in the
network
hello packets are transmitted at fixed intervals known as hello intervals
• default interval on relatively low-bandwidth (T1 or less) multipoint
circuits (such as multipoint Frame Relay, ATM, and X.25 circuits)
–
60 seconds
• default interval on higher-bandwidth interfaces including point-to-point
serial links, multipoint circuits with bandwidth greater than T1, and LANs
– 5 seconds
EIGRP Packet Types
Acknowledgment Packets
used to acknowledge receipt of any EIGRP packet that requires reliable
delivery
a hello packet without any data
sent as a unicast packet to a single, specific IP address
EIGRP Packet Types
Update Packets
used to convey routing information to known destinations
• Updates sent to a new neighbor provide a complete dump of topological data
– update packets will be directly addressed to that neighbor
• Updates would be sent whenever a change in either topology or link metric
occurred
– update packets would use an IP multicast address to forward to multiple neighbors
simultaneously
updates contain only the information that has changed, when it has changed
EIGRP Packet Types
Query Packets
used whenever a router needs specific information from one or all of its
neighbors
only sent when a destination becomes active
can be both unicast and multicast
Reply Packets
used to respond to a query
are unicast directly back to the originator of the query
Configuring EIGRP - IP
To enable EIGRP and define the autonomous system
router(config)#router eigrp autonomous-system-number
• autonomous-system-number is the number that identifies the autonomous
system
• used to indicate all routers that belong within the internetwork (value must
match all routers within the internetwork)
To indicate which networks are part of the EIGRP autonomous
system on this router
router(config-router)#network network-number
• network-number is the network number that determines which interfaces of the
router are participating in EIGRP and which networks are advertised by the
router
The network command configures only connected networks
Configuring EIGRP - IP
When using serial links, it is important to use the bandwidth on the
interface to change the bandwidth used for calculating routing
metrics
router(config-if)#bandwidth kilobit
• If you do not change the bandwidth for these interfaces, EIGRP assumes that
the bandwidth on the link is of T1 speed
• If the link is slower, the router may not be able to converge, routing updates
might become lost, or suboptimal path selection may result
Configuring EIGRP - IPX
To enable IPX routing
router(config)#ipx routing
To define EIGRP as the IPX routing protocol
router(config-ipx-router)#ipx router {eigrp autonomous-system-number |
rip}
• If IPX EIGRP is selected, an autonomous system number must be specified
(must be the same for all IPX EIGRP routers in the network)
Indicate which networks are part of the EIGRP autonomous
system
router(config-router)#network network-number
Show Commands - IP
Command
Description
show ip eigrp neighbors
Displays neighbors discovered by EIGRP
show ip eigrp topology
Displays the EIGRP topology
show ip route eigrp
show ip protocol
ces-id
ro
p
ces-id
ro
p
show ip eigrp traffic
Displays the current EIGRP entries in the
routing table
Displays the parameters and current state of
the active routing protocol
Displays the number of EIGRP packets
sent/received.and statistics
Show Commands - IPX
Command
Description
show ipx route
Displays the contents of the IPX routing table
show ipx eigrp neighbors
Displays the IPX neighbors discovered by
EIGRP
show ipx eigrp topology
Displays the EIGRP topology table
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Cisco Networking Academy
End Review