Information Technology Reference
In-Depth Information
table. Hellos are multicast every 60 seconds on multipoint WA N interfaces (X.25, Frame
Relay, AT M ) with speeds less than 1.544 Mbps, inclusive. The neighbor holdtime is 180
seconds on these types of interfaces. To summarize, hello/holdtime timers are 5/15 sec-
onds for high-speed links and 60/180 seconds for multipoint WA N links less than 1.544
Mbps, inclusive.
Note:
EIGRP does not send updates using a broadcast address; instead, it sends them to
the multicast address 224.0.0.10 (all EIGRP routers). It also can send an updates using uni-
cast packets if the neighbor command is used.
EIGRP Metrics
EIGRP uses the same composite metric as IGRP, but the bandwidth (BW) term is multi-
plied by 256 for finer granularity. The composite metric is based on bandwidth, delay,
load, and reliability. MTU is not an attribute for calculating the composite metric.
EIGRP calculates the composite metric with the following formula:
EIGRP
metric
= {k1 * BW + [(k2 * BW)/(256 - load)] + k3 * delay} * {k5/(reliability + k4)}
In this formula, BW is the lowest interface bandwidth in the path, and delay is the sum of
all outbound interface delays in the path. The router dynamically measures reliability and
load. It expresses 100 percent reliability as 255/255. It expresses load as a fraction of 255.
An interface with no load is represented as 1/255.
Bandwidth is the inverse minimum bandwidth (in kbps) of the path in bits per second
scaled by a factor of 256 * 10
7
. The formula for bandwidth is
(256 * 10
7
)/BW
min
The delay is the sum of the outgoing interface delays (in tens of microseconds) to the des-
tination. A delay of all 1s (that is, a delay of hexadecimal FFFFFFFF) indicates that the
network is unreachable. The formula for delay is
[sum of delays] * 256
Reliability is a value between 1 and 255. Cisco IOS routers display reliability as a fraction
of 255. That is, 255/255 is 100 percent reliability, or a perfectly stable link; a value of
229/255 represents a 90 percent reliable link.
Load is a value between 1 and 255. A load of 255/255 indicates a completely saturated
link. A load of 127/255 represents a 50 percent saturated link.
By default, k1 = k3 = 1 and k2 = k4 = k5 = 0. EIGRP's default composite metric, adjusted
for s c aling fac tor s, is
EIGRP
metric
= 256 * { [10
7
/BW
min
] + [sum_of_delays] }
