Introduction
Enhanced IGRP (EIGRP) is a
classless, enhanced distance-vector protocol that gives us a real edge over
another Cisco proprietary protocol, Interior Gateway Routing Protocol (IGRP).
That’s basically why it’s called Enhanced IGRP. Like IGRP, EIGRP uses the
concept of an autonomous system to describe the set of contiguous routers that
run the same routing protocol and share routing information. But unlike IGRP,
EIGRP includes the subnet mask in its route updates. And as you now know, the
advertisement of subnet information allows us to use VLSM and summarization
when designing our networks!
EIGRP is sometimes referred to as a
hybrid routing protocol because it has characteristics of both distance-vector
and link-state protocols. For example, EIGRP doesn’t send link-state packets as
OSPF does; instead, it sends traditional distance-vector updates containing
information about networks plus the cost of reaching them from the perspective
of the advertising router. And EIGRP has link-state characteristics as well-it
synchronizes routing tables between neighbors at startup, and then sends specific
updates only when topology changes occur. This makes EIGRP suitable for very
large networks. EIGRP has a maximum hop count of 255 (default 100). There are a
number of powerful features that make EIGRP a real standout from IGRP and other
protocols. The main ones are listed here:
·
Cisco proprietary protocol
·
First released in 1994 with IOS version
9.21
·
Includes all features of IGRP
·
Classless routing protocol(same as RIPv2 and OSPF)
·
Support
for VLSM/CIDR
·
Metric (32 bit) : Composite Metric (BW +
Delay + load + MTU + reliability
·
Administrative distance is 90 for internal
EIGRP 170 for External
EIGRP.
·
Updates are through Multicast (224.0.0.10
)
·
Max Hop count is 255 (100 by default)
·
Hello packets are sent every 5 seconds
·
Convergence rate is fast
·
Support authentication (MD5)
·
Support
for IP, IPX, and AppleTalk via protocol-dependent modules
·
Support
for summaries and discontinuous networks
·
Efficient
neighbor discovery
·
Communication
via Reliable Transport Protocol (RTP)
·
Best
path selection via Diffusing Update Algorithm (DUAL)
·
Supports equal and unequal cost load
balancing
Disadvantages of EIGRP
·
Works only on Cisco Routers
·
More resource (CPU,RAM) required
·
Complex configuration
·
More advanced knowledge required
To become a
neighbor, the following conditions must be met:
ü
The
router must hear a Hello packet from a neighbor.
ü
The EIGRP autonomous system (AS) must be the
same.
ü Identical metrics (K-values) must be the same.
EIGRP Metric
Calculation
EIGRP uses a
composite metric which can be based on the following metrics:
ü
Bandwidth (K1) – Slowest link in
the route path, measured in kilobits
ü
Load (K2) – Cumulative load of all outgoing
interfaces in the path, given as a fraction of 255
ü
Delay of the Line (K3)
– Cumulative delay of all outgoing interfaces in the path in tens of
microseconds
ü
Reliability (K4) – Average
reliability of all outgoing interfaces in the path, given as a fraction of 255
ü
MTU (K5) – The smallest Maximum Transmission Unit
in the path. The MTU value is actually never used
to calculate the metric only
Bandwidth and Delay are used by default.
By
default, only Bandwidth and Delay
of the Line are used. This is identical to IGRP, except
that EIGRP provides a more granular metric by multiplying the bandwidth and
delay by 256. Bandwidth and delay are determined by the interfaces that lead
towards the destination network.
By
default, the full formula for determining the EIGRP metric is:
[10000000/bandwidth
+ delay] * 256
The
bandwidth value represents the link with the lowest bandwidth in the path, in kilobits. The delay
is the total delay of all outgoing interfaces in the path.
As
indicated above, each metric is symbolized with a “K” and then a number. When configuring EIGRP metrics, we actually
identify which metrics we want EIGRP to consider. Again, by default, only
Bandwidth and Delay are considered. Thus, using on/off logic:
K1 = 1, K2
= 0, K3 = 1, K4 = 0, K5 = 0
If
all metrics were set to “on,” the full formula for determining the EIGRP metric
would be:
[K1 * bandwidth * 256 + (K2 * bandwidth) /
(256 - load)
+ K3 * delay * 256] * [K5 / (reliability +
K4)]
Remember, the “K” value is either set to on (“1”) or off (“0”).
Configuring EIGRP Metrics
EIGRP
allows us to identify which metrics the protocol should consider, using the
following commands:
Router(config)# router eigrp 10
Router(config-router)# metric weights 0 1
1 1 0 0
The
first command enables the EIGRP process
for Autonomous System 10. The second actually identifies which EIGRP
metrics to use. The first number (0)
is for Type of Service, and should always be zero. The next numbers, in order,
are K1 (1), K2 (1),
K3 (1), K4 (0),
and K5 (0). Thus, we are instructing EIGRP to use
bandwidth, load, and delay to calculate the total metric, but not reliability
or MTU.
Our formula would thus be:
[K1 *
bandwidth * 256 + (K2 * bandwidth) / (256 - load)
+ K3 * delay * 256]
The
actual values of our metrics (such as bandwidth, delay, etc.) must be configured
indirectly. We can adjust the bandwidth of an interface:
Router(config)# int s0/0
Router(config-if)# bandwidth 64000
Router(config-if)# ip
bandwidth-percent eigrp 10 30
However,
this command does not actually dictate the physical speed of the interface. It merely controls
how EIGRP considers this interface. Best practice is to set the
bandwidth to the actual physical speed of the interface.
The
ip bandwidth-percent eigrp command
limits the percentage of bandwidth EIGRP can use on an interface. The
percentage is based on the configured bandwidth value.
By default, EIGRP will use up to 50% of the bandwidth of an
interface. The above command adjusts this to 30% for Autonomous System 10.
If adjustments to the
EIGRP metric need to be made, the delay metric (in tens of microseconds) on an
interface should be used:
Router(config)# int s0/0
Router(config-if)#
delay 10000
Metric
settings must be identical on the connecting
interfaces of two routers; otherwise they will not form a neighbor
relationship.
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