Presentaci

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EIGRP Fundamentals
EIGRP is called (by Cisco) an advanced
distance-vector protocol. It is a distance-vector
protocol, meaning it makes decisions based on
metric information it receives from its
neighbors. However it employs some of the
advanced mechanisms of the link-state
protocols. It establishes neighbor relationships
with adjacent routers, it sends only changes in
its updates, and it keeps a topology database.
For that reason, Cisco marketing often
calls EIGRP a hybrid protocol (though by strict
definition it is distance-vector).  
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• EIGRP vs. Traditional Distance Vector (RIP or
  IGRP)
• EIGRP has the following advantages over either
  RIP or IGRP
• l It supports classless interdomain routing
  (CIDR) and variable-length subnet masks
• (VLSM).
• l It sends routing updates only when network
  topology changes -- instead of its entire
• routing table at regular intervals.
• l Its convergence is very fast -- often
  instantaneous due to the topology database.
• Its algorithm offers complete protection from
  routing loops (well, almost).
• It is less CPU intensive.
• It supports IPX and AppleTalk. OSPF supports
  only IP.
• EIGRP supports unequal-cost load balancing.
• EIGRP (with properly designed summarization)
  does not propagate topology changes to every
  router in its area. This can save valuable
  bandwidth, especially in unstable or frequently
  fluctuating environments. Even with
  summarization, OSPF propagates network changes to
  all routers in an area.

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• EIGRP does have the following weaknesses
•  
• It is Cisco proprietary -- it does not
  interoperate with other vendors' devices. This,
  of course, is the big one. If you are working in
  a mixed environment, EIGRP doesn't make as much
  sense.
• l In a poorly designed network, EIGRP's
  stuck-in-active (SIA) mechanism can wreak havoc.
  In particular, one malfunctioning router can
  trigger a chain reaction that brings an entire
  network to its knees. OSPF handles the "one bad
  router" problem much better.
• However, if EIGRP is properly designed, this
  problem can be overcome

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EIGRP Basic Configuration One of EIGRP's
strengths is its ease of configuration -- to
configure EIGRP on a router (config) router
eigrp ASNUMBER (config-router) network
IPNETWORK   ASNUMBER Autonomous system of the
EIGRP network For example, Figure 1 displays a
router and its EIGRP configuration
S0/0
S0/1
10.1.17.0/24
10.1.18.0/24
ROUTERA
RouterA(config) router eigrp 90 RouterA(config-ro
uter) network 10.0.0.0
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Any interface that lies within the network
indicated in the network statement runs the EIGRP
process. Verify this with the SHOW IP EIGRP
INTERFACES exec command. For example, from Figure
1   RouterAsh ip ei int IP-EIGRP interfaces for
process 90 Xmit Queue Mean
Pacing Time Multicast Pending Int
Peers Un/Reliable SRTT Un/Reliable Flow
Timer Routes Se0/1 1 0/0 230
2/95 1235 0 Se0/0 1 0/0 1494
1/63 7531 0
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You can see that both interfaces S0/0 and S0/1
are active and have active peers. You can see
which peers with the SHOW IP PROTOCOL exec
command   ROUTERAgtSH IP PROT ROUTING PROTOCOL IS
"EIGRP 90" OUTGOING UPDATE FILTER LIST FOR ALL
INTERFACES IS INCOMING UPDATE FILTER LIST FOR ALL
INTERFACES IS DEFAULT NETWORKS FLAGGED IN
OUTGOING UPDATES DEFAULT NETWORKS ACCEPTED FROM
INCOMING UPDATES EIGRP METRIC WEIGHT K11, K20,
K31, K40, K50 EIGRP MAXIMUM HOPCOUNT 100 EIGRP
MAXIMUM METRIC VARIANCE 1 REDISTRIBUTING EIGRP
90 AUTOMATIC NETWORK SUMMARIZATION IS IN
EFFECT ROUTING FOR NETWORKS 10.0.0.0 ROUTING
INFORMATION SOURCES GATEWAY DISTANCE LAST
UPDATE (THIS ROUTER) 5 005255 10.1.18.1 90
005233 10.1.17.1 90 005233 DISTANCE INTERNAL
90 EXTERNAL 170   Note EIGRP automatically
summarizes network statements to their classful
state. For example, NETWORK 10.1.2.0 is
automatically summarized to NETWORK 10.0.0.0. IOS
release 12.0(4)T added the option of placing
wildcard bits to specifically identify subnets to
be included (and in turn subnets to be
excluded).  
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The EIGRP Route Selection Process   EIGRP uses a
best-metric method of determining which path to
take to a destination. Each link in a network
has a cost, or metric associated with it. Each
router sends the metrics for each of its routes
to its neighbors. The neighbors then compute a
total distance for a destination, and use that
distance to determine the best path. For example,
examine Figure 2.    
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• PC1 is sending a packet to PC2. Router A receives
  the packet, which has a destination network of
  LAN2. There are two possible paths to LAN2
• 1. Through Router B. Router B has reported a
  distance of 110 to get to LAN2. That, plus the 50
  to get to Router B equals a total distance of
  160.
• 2. Through Router C. Router C has reported a
  distance of 110 to get to LAN2. That, plus the
  100 to get to Router C equals a total distance of
  210.
• Since the path through Router B has a lesser
  distance, it is chosen as the best path to LAN2.
• Practical Route Selection
• While the details of how EIGRP selects routes are
  complex, when
• the default assumptions are followed, EIGRP will
• On links with relatively few hops, select the
  path that has the greatest minimum bandwidth
  (i.e., a T1-T1-T1 link is preferred over
  Ethernet-ISDN-Ethernet.
• l On links with relatively large numbers of hops,
  select the path with minimum delay.

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EIGRP Metrics EIGRP uses a combination of four
values to compute the distance to a destination.
The four values are Delay - the total delay (in
ms) to the destination Bandwidth - 107 divided
by the minimum bandwidth of any link in the path
to the destination Load - on a scale of 1-255,
how loaded the link is (255 is completely
saturated) Reliability - on a scale of 1-255,
how reliable the path is (255 is completely
reliable) The formula for computing the metric
is
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Where K1, K2, K3, K4, and K5 are constant
coefficients and BW (107 / minBandwidth)
256 minBandwidth is not the bandwidth on the
local outgoing interface. It is the lowest of any
bandwidth along the path to the
destination. Delay is measured in tens of
microseconds. This is different from the output
of the show interfaces command, which displays
delay in milliseconds. K2, K4, and K5 only exist
for backward compatibility with IGRP, and are set
to zero by default (actually, if K4 and K5 are
zero, their whole term is completely left out of
the equation). K1 and K3 are set to 1 by
default. This leaves us with the following
formula (107 / minBandwidth ) sum of
interface delays 256 EIGRP uses this formula
of metric computation to accomplish two
things 1. On routes of few hops, the route with
the greatest minimum bandwidth is
usually preferred. 2. On routes with many hops,
the route with the least total delay is usually
preferred.
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Once again, PC1 is attempting to talk to PC2.
Router A must determine the best path. The path
through Router B has the greatest available
bandwidth (with the delay being the same), so it
is chosen as the route to LAN 2. Metric delay and
bandwidth values are easy to determine. They are
based on the interface type and link speed. Table
1 illustrates standard Cisco values.
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Note When EIGRP advertises the metric values for
a route, it advertises two other values not
mentioned here MTU and hopcount. These are used
for purposes such as loop avoidance, but not for
determining a composite metric.
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EIGRP Terminology EIGRP defines four terms
Successor The next-hop router to a
destination, as reported by SHOW IP
ROUTE. Feasible Successor A router closer to
the destination than the current router. As we
will see, EIGRP not only tracks the successor,
but also other routers that may provide valid
paths to a destination in the event the successor
fails. Feasible Distance The current best
distance (metric) to a destination. In other
words, the metric through the successor. Reported
Distance The distance from the successor to the
destination network. These values are kept in the
EIGRP topology database. The topology database
contains all known routes to a destination, not
just the route in the routing table. It can be
examined with the SHOW IP EIGRP TOPOLOGY exec
command.
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To get to LAN2 (10.1.5.0) from RouterA Successor
10.1.18.1 (RouterB). This route has the least
distance, so it is the successor. This is the
route that would be found in the routing
table. Feasible Successor 10.1.17.1 (RouterC).
This is a possible route, but not the
best. Feasible Distance 7693056. This is the
distance of the route through the
successor. Reported Distance 5514496. This is
the distance from the successor (RouterB) to the
destination. In other words, the successor's
Feasible Distance to the destination. SHOW IP
EIGRP TOPOLOGY does not show ALL routes to a
destination, though. It only shows those that go
through feasible successors. What's the
difference? The path through a feasible successor
is guaranteed to be loop-free. If a router may
possibly cause a loop, it does not become a
feasible successor, and its route is not placed
in the topology database (it is, but it is not
used). How does EIGRP determine if a path is
loop-free? It uses EIGRP's DUAL Algorithm.
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The DUAL Algorithm The DUAL Algorithm, among
other things, is used to determine if an
alternate path to a destination is loop-free. It
states "A path is loop-free if the reported
distance is less than the current feasible
distance." If the reported distance is greater
than the current feasible distance, the path may
(but not necessarily) have a loop, and,
therefore, it is not a feasible successor. But if
the reported distance is less than the feasible
distance, there cannot be a loop. For
example Router A's best path to LAN2 goes
through Router B. Therefore Router B is the
successor. Is Router C a feasible successor?
(Remember that a path must be guaranteed to be
loop-free for a router to become a feasible
successor.) Yes, Router C is a feasible
successor. Its reported distance, 110, is less
than the current feasible distance of 160.
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EIGRP Hello Packets EIGRP Routers do not
exchange routing information until they form a
neighbor relationship. To do this, EIGRP uses its
version of the HELLO protocol. Periodically
(every 5 seconds for highspeed interfaces) an
EIGRP router multicasts a hello packet out of its
configured interfaces. HELLOs flow only from
primary IP addresses EIGRP does not form
neighbors with secondary addresses. EIGRP
routers that receive these packets will attempt
to form a neighbor relationship with the sender
(provided they are compatible -- common
autonomous system numbers, K values, etc.). Once
the neighbor relationship, or adjacency, is
established, the two will exchange full routing
information. Thereafter, they will only exchange
routing information when a change is detected.
However, they continue to send and receive hello
packets.
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EIGRP Hello Packets The hello protocol is not
used just to find neighbors it is also used to
detect when neighbors drop. EIGRP keeps track of
the hello packets it receives from its neighbors
and if it doesn't hear from a neighbor for a
certain amount of time, it will drop the neighbor
relationship. That causes all information learned
from that neighbor, such as routing and topology
information, to be dropped as well. If the router
subsequently hears from the neighbor again, they
will reestablish the adjacency and re-exchange
routing information
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EIGRP uses two timers to accomplish its
goals Hello Timer The Hello timer specifies
how often hello packets are sent. The default is
5 seconds for most interfaces. The exceptions are
low-speed NBMA interfaces (such as frame relay)
which default to 60 seconds. The default hello
time can be modified with the IP HELLO-INTERVAL
EIGRP interface configuration command. Hold
Timer The Hold timer specifies how long a router
will wait without hearing from a neighbor before
clearing the adjacency. It defaults to 3 times
the Hello timer (15 seconds for most interfaces
and 180 seconds for NMBA interfaces). Having the
hold time equal to three times the hello timer
allows for a hello packet or two to be lost (for
whatever reason) without clearing the adjacency.
The default hold time can be modified with the ip
hold-time eigrp interface configuration command.
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EIGRP Hello and Hold Timers The hold timer is
actually specified by the neighbor router's hello
timer, which is sent in the hello packet. This
allows neighbors with differing timers to operate
successfully. For example
When Router A establishes its adjacency with
Router B, it informs Router B that its hello time
is 5 seconds. Router B then sets its hold timer
for Router A to be 15 seconds -- allowing it to
quickly detect a downed link. Router A, in turn,
will wait 180 seconds, due to Router B's longer
hello time.
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EIGRP Packet Format Instead of riding an
established protocol, like TCP or UDP, EIGRP
designers developed their own reliable protocol
to carry EIGRP messages. This packet format
carries all messages, from hello packets to
routing updates.
EIGRP ensures reliable communication using a
sequencing and acknowledgement mechanism similar
to TCP. Unlike TCP though, EIGRP uses a fixed
window size of 1. Type/Length/Value fields carry
the packet data (usually routing updates or
queries).
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The Neighbor and Topology Databases EIGRP keeps
track of two databases of information the
neighbor database and the topology
database. Neighbor Database Neighbors are found
with the hello protocol. When a router exchanges
hello packets and establishes a neighbor
relationship, it stores information about the
neighbor in the neighbor database. Examine the
neighbor database with the SHOW IP EIGRP
NEIGHBORS exec command.
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Topology Database EIGRP Routers store not only
the best route to a destination, but up to five
alternate routes as well. These, along with their
associated information (feasible distance,
reported distance, etc.) The topology database
actually contains ALL routes to a destination,
though only successors and feasible successors
are displayed with the show ip eigrp topology
command. To see all of the routes (including
non-feasible successors), use the show ip eigrp
topology all-links command.
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EIGRP Features EIGRP contains a number of
features that enhance its scalability and
efficiency. Two of them are route summarization
and load balancing. Route Summarization Despite
all of its strengths (or perhaps because of
them), EIGRP is a fairly CPU- and memory
intensive protocol. It gets particularly bad
during link failure conditions. It is possible to
overload the largest of CPUs with EIGRP
processing alone. Route summarization is one of
the tools provided to relieve some of this
burden. Example two major networks (10.0.0.0 /8
and 172.16.0.0 /16) meet at Router D (the entire
network is one autonomous system).
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Without summarization, every 10.0.0.0 subnet
appears in the routing table of Routers X, Y and
Z. For example, the following is the routing
table from Router Z ROUTERZSH IP ROUTE CODES
C - CONNECTED, S - STATIC, I - IGRP, R - RIP, M -
MOBILE, B - BGP D - EIGRP, EX - EIGRP EXTERNAL, O
- OSPF, IA - OSPF INTER AREA N1 - OSPF NSSA
EXTERNAL TYPE 1, N2 - OSPF NSSA EXTERNAL TYPE
2 E1 - OSPF EXTERNAL TYPE 1, E2 - OSPF EXTERNAL
TYPE 2, E - EGP I - IS-IS, L1 - IS-IS LEVEL-1, L2
- IS-IS LEVEL-2, - CANDIDATE DEFAULT U -
PER-USER STATIC ROUTE, O ODR GATEWAY OF LAST
RESORT IS NOT SET 172.16.0.0/24 IS
SUBNETTED, 4 SUBNETS D 172.16.0.0 90/2195456
VIA 172.16.2.2, 000824, SERIAL0 D 172.16.1.0
90/2681856 VIA 172.16.2.2, 000824, SERIAL0
90/2681856 VIA 172.16.3.1,
000824, SERIAL1 C 172.16.2.0 IS DIRECTLY
CONNECTED, SERIAL0 C 172.16.3.0 IS DIRECTLY
CONNECTED, SERIAL1 10.0.0.0/24 IS
SUBNETTED, 6 SUBNETS D 10.1.8.0 90/8253696 VIA
172.16.2.2, 000824, SERIAL0 D 10.1.5.0
90/2198016 VIA 172.16.2.2, 000824, SERIAL0 D
10.1.19.0 90/6049536 VIA 172.16.2.2, 000824,
SERIAL0 D 10.1.18.0 90/8228096 VIA 172.16.2.2,
000824, SERIAL0 D 10.1.17.0 90/11561472 VIA
172.16.2.2, 000824, SERIAL0 D 10.1.16.0
90/6049536 VIA 172.16.2.2, 000824, SERIAL0
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Routers X, Y, and Z really only need to know to
send all packets destined for any 10.0.0.0
network to Router D. This can be accomplished
with route summarization. Instead of advertising
the individual 10.0.0.0 /24 networks to the
172.16.0.0 network, Router D summarizes them all
into one route 10.0.0.0 /8. This one route is
then propagated throughout the 172.16.0.0
network. After summarization, the routing table
on Router Z looks like this ROUTERZSH IP
ROUTE CODES C - CONNECTED, S - STATIC, I - IGRP,
R - RIP, M - MOBILE, B - BGP D - EIGRP, EX -
EIGRP EXTERNAL, O - OSPF, IA - OSPF INTER AREA N1
- OSPF NSSA EXTERNAL TYPE 1, N2 - OSPF NSSA
EXTERNAL TYPE 2 E1 - OSPF EXTERNAL TYPE 1, E2 -
OSPF EXTERNAL TYPE 2, E - EGP I - IS-IS, L1 -
IS-IS LEVEL-1, L2 - IS-IS LEVEL-2, - CANDIDATE
DEFAULT U - PER-USER STATIC ROUTE, O
ODR GATEWAY OF LAST RESORT IS NOT SET
172.16.0.0/24 IS SUBNETTED, 4 SUBNETS D
172.16.0.0 90/2195456 VIA 172.16.2.2,
004047, SERIAL0 D 172.16.1.0 90/2681856 VIA
172.16.2.2, 004047, SERIAL0
90/2681856 VIA 172.16.3.1, 004047, SERIAL1 C
172.16.2.0 IS DIRECTLY CONNECTED, SERIAL0 C
172.16.3.0 IS DIRECTLY CONNECTED, SERIAL1 D
10.0.0.0/8 90/2198016 VIA 172.16.2.2, 000009,
SERIAL0
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EIGRP SummarizationAutomatic

• Purpose Smaller routing tables, smaller updates,
  query boundary
• Autosummarization
• On major network boundaries, subnetworks are
  summarized to a single classful (major) network
• Autosummarization is turned on by default

172.16.X.X
172.17.X.X
172.16.0.0/16
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Configuring Summarization
(config-router)
no auto-summary

• Turns off autosummarization for the EIGRP process

(config-if)
ip summary-address eigrp ltas-numbergt ltaddressgt
ltmaskgt

• Creates a summary address to be generatedby this
  interface

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Summarizing EIGRP Routes
192.168.4.2
172.16.1.0
A
10.0.0.0
S0
C
World
172.16.2.0
B
router eigrp 1
network 10.0.0.0network 192.168.4.0
router eigrp 1
network 10.0.0.0
!int s0 ip address 192.168.4.2 255.255.255.0
network 172.16.0.0
no auto-summary
ip summary-address eigrp 1
172.16.0.0 255.255.0.0
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EIGRP Load Balancing By default, EIGRP load
balances over equal-cost links. Router A has two
paths to LAN 1 through Router B and through
Router C. Since both paths have the same
composite metric, they are used alternatively. In
other words, they are evenly load balanced. The
precise load balancing flow depends on the packet
switching mechanism used by the router
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Unequal Cost Load Balancing
Router A has two paths to LAN 1. However, since
the metrics are not equal, only one path appears
in the routing table (the one through Router C).
However, with the VARIANCE router configuration
command, the load can be shared over the two
links. (config) router eigrp
ASNUMBER (config-router) variance
MULTIPLIER The multiplier is the factor by which
the lesser route must be within the primary
route. For example, if the multiplier is 2 and
the primary route metric is 100, the secondary
route must have a metric of 200 or less to be
used for load balancing.
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Variance Example
10
20
10
10
Network Z
(config)
25
20
variance 2

• Router E will choose Router C to get to Network Z
  because FD 20
• With variance of 2, Router E will also choose
  Router B to get to Network Z (20 10) lt (2 x
  FD)
• Router D will not be used to get to Network Z (45
  gt 40)