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CCNP 1 v3.0 Module 5
EIGRP
Cisco Networking Academy
© 2003, Cisco Systems, Inc. All rights reserved.
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Objectives
• EIGRP concepts
• EIGRP configuration
• Troubleshooting Routing protocols
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EIGRP Design Features
• Rapid Convergence
• Efficient use of bandwidth
• Support for VLSM and CIDR
• Multiple network-layer support
• Independence from routed protocols
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Comparing EIGRP with IGRP
• Comparisons between EIGRP and IGRP fall into
the following major categories:
Compatibility mode – IGRP and EIGRP are compatible
with each other.
Metric calculation – EIGRP scales the metric of IGRP by a
factor of 256. That is because EIGRP uses a metric that is
32 bits long, and IGRP uses a 24-bit metric.
Hop count – IGRP has a maximum hop count of 255 and
EIGRP has a maximum hop count limit of 224.
Automatic protocol redistribution – EIGRP automatically
redistributes IGRP routes from the same AS
Route tagging – EIGRP will tag routes learned from IGRP
or any outside source as external because they did not
originate from EIGRP routers. IGRP cannot differentiate
between internal and external routes.
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EIGRP and IGRP Metric Calculation
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Using EIGRP with IGRP
When configuring EIGRP it is only
necessary to configure the classful
network number of the subnet the
interface belongs to.
If you would like to configure EIGRP for an individual
subnet then you must use the wildcard mask
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EIGRP Concepts
• Every EIGRP router maintains a topology
table for each configured network protocol
(IP, IPX).
• All learned routes to a destination are
maintained in the topology table.
• The best routes from the topology table
are installed into the routing table (similar
to OSPF).
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EIGRP Technologies
•
Neighbor discovery and recovery (Hello Packets)
•
Reliable Transport Protocol (Layer 4)
•
DUAL finite-state machine algorithm (Routing Decisions)
–
Diffusing Update Algorithm
•
Protocol-dependent modules (PDMs)
•
By forming adjacencies, EIGRP routers:
1. Dynamically learn of new routes that join their
network
2. Identify routers that become either
unreachable or inoperable
3. Rediscover routers that had previously been
unreachable
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Building Neighbor Tables
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Discover Routes
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EIGRP Technologies
Four key technologies set EIGRP apart
from IGRP
1. Neighbor discovery and recovery
2. Reliable Transport Protocol (RTP)
3. PDMs
4. DUAL finite-state machine (FSM)
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Neighbor Discovery/Recovery
• EIGRP routers establish adjacencies with
neighbor routers by using small hello
packets.
• An EIGRP router assumes that, as long as
it is receiving hello packets from known
neighbors, those neighbors (and their
routes) remain viable.
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12
Reliable Transport Protocol
• RTP (Reliable Transport Protocol)
transport-layer protocol
• EIGRP is protocol-independent; that is, it
doesn’t rely on TCP/IP to exchange routing
information the way RIP, IGRP, and OSPF do.
• To stay independent of IP, EIGRP uses its own,
proprietary transport-layer protocol to guarantee
delivery of routing information: RTP. (Don’t
confuse with Real-Time Protocol)
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RTP
• Supports reliable and unreliable delivery
• Supports unicasting and multicasting
• With RTP, EIGRP can multicast and
unicast to different peers simultaneously,
which allows for maximum efficiency.
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Protocol Dependent Modules
• PDM (Protocol-dependent module)
• EIGRP is modular
• Different PDMs can be added to EIGRP as
new routed protocols are enhanced or
developed:
IPv4, IPv6, IPX, and AppleTalk
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PDMs
Each PDM is responsible for all functions related to
its specific routed protocol.
The IP-EIGRP module is responsible for the
following:
– 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
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Diffusing Update Algorithm (DUAL)
• The DUAL finite state machine (FSM) is the
routing decision process for EIGRP.
• DUAL tracks all routes advertised by all
neighbors.
• DUAL uses the distance information to select the
best routes to install into the routing table.
• This distance, or metric is the feasible distance.
• DUAL is the EIGRP distance vector algorithm
part of the hybrid routing protocol.
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DUAL FSM
DUAL selects alternate routes quickly by
using the information in the EIGRP
topology table.
If a link goes down, DUAL looks for a
feasible successor in its neighbor and
topology tables.
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EIGRP Successors and Feasible Successors
• A successor is a neighboring router used for
packet forwarding that has the least cost path to
a destination, based on the feasible distance,
that is guaranteed not to be part of a routing
loop.
• Feasible successors are viewed by a router as
neighbors that are downstream with respect to
the destination.
• A successor route is chosen from a list of
feasible successors
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DUAL FSM
• Feasible successors are routes that represent
the next lowest-cost paths to a destination
without introducing routing loops.
• Feasible successor routes can be used in case
the existing route fails; packets to the
destination network are immediately forwarded
using the feasible successor, which at that point,
is promoted to the status of successor.
• If a feasible successor is not found, the route is
flagged as Active, or unusable at present.
• Once a feasible successor is found the route is
placed back into the Passive state.
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Calculating a Feasible Successor
• Reported distance is the total metric along
a path to a destination network as
advertised by an upstream neighbor.
• A feasible successor is a path whose
reported distance is less than the feasible
distance.
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EIGRP Successors and Feasible Successors
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Feasible Successor Route Selection Rules
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Route Tagging with EIGRP
• All external routes are included in the topology
table and are tagged with the following
information:
Identification number, known as router ID, of the EIGRP
router that redistributed the route into the EIGRP
network
AS number of the destination
Protocol used in that external network
Cost or metric received from that external protocol
Configurable administrator tag
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Data Structure
The five EIGRP packet types are as follows:
1. Hello (used to discover, verify, and
rediscover neighbor routers)
2. Acknowledgment
3. Update
4. Query
5. Reply
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EIGRP Packet Types
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EIGRP Hello Packets
• 5-second hello:
– broadcast media, such as Ethernet, Token Ring, and FDDI
– point-to-point serial links, such as PPP or HDLC leased circuits,
Frame Relay point-to-point subinterfaces, and ATM
– point-to-point subinterfaces
– high bandwidth (greater than T1) multipoint circuits, such as ISDN
PRI and Frame Relay
– Holdtime 15 seconds (3xhello)
• 60-second hello:
– multipoint circuits T1 bandwidth or slower, such as Frame Relay
multipoint interfaces, ATM multipoint interfaces, ATM
– switched virtual circuits, and ISDN BRIs
– Holdtime 180 seconds (3xhello)
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EIGRP Hello packets
• If a neighbor is not heard from for the
duration of the hold time, EIGRP considers
that neighbor down, and DUAL must step
in to reevaluate the routing table.
– By default, the hold time is three times the
hello interval, but an administrator can
configure both timers as desired.
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Default Hello Intervals and Hold Times for EIGRP
The default hold time is three times the hello interval.
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EIGRP Hello packets
• Unlike OSPF routers, EIGRP routers do
not need to have the same hello intervals
and hold down intervals.
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Acknowledgement Packets
• Acknowledgement packets, which are
“dataless” hello packets, are used to
ensure reliable communication.
– Unlike multicast hellos, acknowledgement
packets are unicast.
– acknowledgements can be made by
piggybacking on other kinds of EIGRP
packets, such as reply packets.
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Update Packet
Update packets are used when a router
discovers a new neighbor.
– An EIGRP router sends unicast update
packets to that new neighbor so that it can
add to its topology table.
– More than one update packet may be needed
to convey all of the topology information to
the newly discovered neighbor.
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Update Packet
• Update packets are also used when a
router detects a topology change. In this
case, the EIGRP router sends a multicast
update packet to all neighbors alerting
them to the change.
• All update packets are sent reliably.
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Query and Reply Packets
• EIGRP routers use query packets
whenever it needs specific information
from one, or all, of its neighbors.
A reply packet is used to respond to a query.
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Query and Reply Packets
• If an EIGRP router loses its successor and
cannot find a feasible successor for a route,
DUAL places the route in the active state.
– the router multicasts a query to all neighbors,
searching for a successor to the destination network.
– Neighbors must send replies that either provide
information on successors, or indicate that no
successor information is available.
• Queries can be multicast or unicast, while
replies are always unicast. Both packet types are
sent reliably.
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EIGRP Configuration
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EIGRP Route Summarization
• EIGRP automatically summarizes routes at the classful
boundary.
• The two routers below will auto-summarize to each other
because the 2.1.1.0/24 and 2.2.2.0/24 networks are
separated by a 10.1.1.0/30 network.
• This separation of subnets from one major network
number by a different major network number is known as
discontiguous subnets.
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EIGRP Automatically Summarizes Based
on Class
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Turning off Auto-Summarization
• To turn off auto-summarization:
Router(config)# router eigrp 1
Router(config-router)# no auto-summarization
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Manual Summarization with EIGRP
With EIGRP, a summary address can be manually configured by
configuring a prefix network.
Manual summarization of EIGRP routes is done at interface configuration
mode.
Router(config-if)# ip summary-address eigrp 1 2.1.0.0 255.255.0.0
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EIGRP Automatically Summarizes Based
on Class
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Verifying EIGRP
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EIGRP debug Commands
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