1587200023_CH11.pdf
(
814 KB
)
Pobierz
648843114 UNPDF
C
H
A
P
T
E
R
11
Hybrid: Enhanced Interior
Gateway Routing Protocol
(EIGRP)
primarily
to address many of the limitations of IGRP and RIP. As WANs were growing, so was the
need for a routing protocol that would use efficient address space on WAN links, as well as
the LAN networks. OSPF was available, but the CPU-intensive tasks that it had to perform
often overloaded the small processors of many edge or remote routers of that time. The con-
figuration was also more complex than that of RIP or IGRP. A routing protocol was needed
that could support VLSM and that could scale with large internetworks, yet that was less
CPU-intensive than OSPF. In 1994, Cisco answered the call by releasing Enhanced IGRP
in Cisco IOS Software Release 9.21. Today, EIGRP is used as the routing protocol on many
large government and commercial internetworks. It has proven to be very stable, flexible,
and fast. In addition to these characteristics, the ease of EIGRP configuration makes it one
of the most popular routing protocols among network engineers.
Enhanced Interior Gateway Routing Protocol (IGRP)
EIGRP can be referred to as a hybrid protocol. It combines most of the characteristics of
traditional distance vector protocols with some characteristics of link-state protocols.
Specifically, EIGRP is “enhanced” by using four routing technologies:
•
Neighbor discovery/recovery
•
Reliable Transport Protocol (RTP)
•
DUAL finite-state machine
•
Protocol-dependent modules
This chapter covers these technologies, as well as the operation and configuration of
EIGRP.
As internetworks grew in scale and diversity in the early 1990s, new routing protocols were
needed. Cisco developed
670
Chapter 11: Hybrid: Enhanced Interior Gateway Routing Protocol (EIGRP)
Technical Overview of EIGRP
EIGRP offers many advantages over other routing protocols, including the following:
•
—EIGRP is a classless routing protocol and carries the subnet
mask of the route in its update.
•
—By using the concept of feasible successors, defined by DUAL,
EIGRP is capable of preselecting the next best path to a destination. This allows for
very fast convergence upon a link failure.
•
—Under normal operation, only hellos and partial updates are
sent across a link. Routing updates are not flooded and are processed only
periodically.
•
—EIGRP does not send a full routing update; it sends only
information about the changed route.
•
—Through the use of VLSM and a complex composite metric, EIGRP
networks can be vast in size.
•
—EIGRP supports hierarchical network design, but it does not
require the strict configuration guidelines, such as the ones needed for OSPF.
•
Automatic route summarization
—EIGRP will perform automatic summarization
on major bit boundaries.
•
—As of Cisco IOS Software Release 11.3, EIGRP can be
configured to perform MD5 password authentication on route updates.
Looking at this list, it becomes evident why EIGRP has become a popular routing protocol.
It provides many of the enhancements of OSPF, without the strict configuration guidelines. It
could be argued that EIGRP’s weakest point is that it is a Cisco-proprietary protocol, but with
the aid of redistribution, this point becomes moot.
MD5 route authentication
EIGRP is a classless routing protocol. It directly interfaces to IP as protocol 88. EIGRP uses
the multicast address of 224.0.0.10 for hellos and routing updates instead of an all-hosts
broadcast like RIP uses. EIGRP also employs a system of hello and hold timers to maintain
neighbors. Aside from the initial routing update, partial routing updates are sent only when
network topology changes occur. The updates are also bounded, which means that updates
are sent only to pertinent routers. Like IGRP, EIGRP uses a composite metric to calculate
the best path to a destination. The sections that follow take a closer look at how EIGRP
makes use of metrics, neighbors, reliable transport, and DUAL in its operation.
NOTE
Early releases of EIGRP had stability issues over low-speed serial links and problems
maintaining many neighbors. Cisco significantly enhanced EIGRP with Cisco IOS
Software Releases 10.3(11), 11.0(8), and 11.1(3)— early releases of EIGRP are sometimes
referred to as EIGRP version 1. Cisco currently ships routers with IOS 12.0 and above.
Support for VLSM
Rapid convergence
Low CPU utilization
Incremental updates
Scalable
Easy configuration
Technical Overview of EIGRP
671
EIGRP Metrics
EIGRP uses metrics in the same way as IGRP. Each route in the route table has an associated
metric. EIGRP uses a composite metric much like IGRP, except that it is modified by a multi-
plier of 256. Recall from Chapter 10, “Distance Vector Protocols: Interior Gateway Routing
Protocol (EIGRP),” that bandwidth, delay, load, reliability, and MTU are the submetrics.
Like IGRP, EIGRP chooses a route based primarily on bandwidth and delay, or the composite
metric with the lowest numerical value. When EIGRP calculates this metric for a route, it calls
it the
feasible distance
•
—Bandwidth is expressed in units of kilobits. It must be statically config-
ured to accurately represent the interfaces that EIGRP is running on. For example, the
default bandwidth of a 56-kbps interface and a T1 interface is 1544 kbps. To accurately
adjust the bandwidth, use the
bandwidth
kbps
interface subcommand. Table 11-1
highlights some common bandwidth values.
•
—Delay is expressed in microseconds. It, too, must be statically configured to
accurately represent the interface that EIGRP is running on. The delay on an interface
can be adjusted with the
delay
time_in_microseconds
interface subcommand.
Common delay values are represented in Table 11-1.
•
—Reliability is a dynamic number in the range of 1 to 255, where 255 is
a 100 percent reliable link and 1 is an unreliable link.
•
—Load is the number in the range of 1 to 255 that shows the output load of
an interface. This value is dynamic and can be viewed using the
command. A value of 1 indicates a minimally loaded link, whereas 255 indicates a
100 percent loaded link.
show interfaces
•
—The maximum transmission unit (MTU) is the recorded smallest MTU value
in the path, usually 1500.
NOTE
Whenever you are influencing routing decisions in IGRP or EIGRP, use the metric of delay
over bandwidth. Changing bandwidth can affect other routing protocols, such as OSPF.
Changing delay affects only IGRP and EIGRP.
to the route. EIGRP calculates a feasible distance to all routes in the
network. The following list is a detailed description of the five EIGRP submetrics:
Bandwidth
Delay
Reliability
Load
MTU
672
Chapter 11: Hybrid: Enhanced Interior Gateway Routing Protocol (EIGRP)
Table 11-1 highlights the common metrics used.
Table 11-1
Common IGRP and EIGRP Metrics
Medium
Bandwidth
Delay
100-Mbps ATM
100,000 kbps
100
µ
s
Gigabit Ethernet
100,000 kbps
100
µ
s
Fast Ethernet
100,000 kbps
100
µ
s
FDDI
100,000 kbps
100
µ
s
HSSI
45,045 kbps
20,000
µ
s
16-Mbps Token Ring
16,000 kbps
630
µ
s
10-Mbps Ethernet
10,000 kbps
1000
µ
s
T1
1544 kbps
20,000
µ
s
DS-0
64 kbps
20,000
µ
s
56-kbps media
56 kbps
20,000
µ
s
EIGRP uses a composite metric (CM) that is derived from the five submetrics. When
EIGRP computes the composite metric, it uses a formula that involves five constants
or “k” values. The constant values have default value such as the following:
k1 = k3 = 1 and k2 = k4 = k5 = 0
By setting k2, k4, and k5 to 0, it essentially nullifies the submetrics of load, reliability, and
MTU. This is precisely why you should first use delay and then bandwidth when trying to
influence which routes EIGRP prefers. The formula EIGRP uses to calculate the composite
metric is as follows:
CM = 256
×
([k1
×
BW
mim
+ (k2
×
BW
mim
) / (256-LOAD) + k3
×
DELAY
sum
]
×
X)
where the following is true:
BW
mim
= 10
7
/ bandwidth_of_slowest_link
(delays_along_the_path)
X = k5 / (reliability + k4) if and only if k1<>1, if k1 = 1 then X = 1
=
Σ
sum
With the k values set at the default value you have
k1 = k3 = 1
k2 = k4 = k5 = 0
CM = 256
×
(BW
mim
+ DELAY
sum
)
NOTE
The router calculation of the composite metric will always differ slightly from the result
when it is performed by longhand. This is because of the way the router handles floating-
point mathematics; there will be slight rounding discrepancies.
DELAY
Plik z chomika:
Caveman85
Inne pliki z tego folderu:
Cisco.Press.Penetration.Testing.and.Network.Defense.chm
(14976 KB)
Cisco.Press.IS.IS.Network.Design.Solutions.chm
(3096 KB)
Lab3-5.exe
(112 KB)
Practice_Lab4_Section_2.pdf
(31 KB)
Practice_Lab5_Section_3.pdf
(1392 KB)
Inne foldery tego chomika:
• Kurs Cisco CCNA v 3.1
Administrowanie Sieciami Komputerowymi - mgr inż Paweł Pławiak
CCNA
CCNA - CCSP - CNAP - CCSP - Cisco
CCNA - CCSP - CNAP - CCSP - Cisco2
Zgłoś jeśli
naruszono regulamin