EIGRP sends updates only when necessary and sends them only to neighboring routers. There is no periodic update timer.
EIGRP uses hello packets to learn of neighboring routers. On high-speed networks, the default hello packet interval is 5 seconds. On multipoint networks with link speeds of T1 and slower, hello packets are unicast every 60 seconds.
The holdtime to maintain a neighbor adjacency is three times the hello time: 15 seconds. If a router does not receive a hello within the holdtime, it removes the neighbor from the table. Hellos are multicast every 60 seconds on multipoint WAN interfaces 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 seconds for high-speed links and 60/180 seconds for multipoint WAN 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 any updates using unicast packets if the neighbor command is used.
EIGRP Metrics
EIGRP uses the same composite metric as IGRP, but the bandwidth (BW) term is multiplied 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:
EIGRPmetric = {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% 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 × 107. The formula for bandwidth is
(256 × 107)/BWmin
The delay is the sum of the outgoing interface delays (in tens of microseconds) to the destination. 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% reliability, or a perfectly stable link; a value of 229/255 represents a 90% 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% saturated link.
By default, k1 = k3 = 1 and k2 = k4 = k5 = 0. EIGRP’s default composite metric, adjusted for scaling factors, is
EIGRPmetric = 256 × { [107/BWmin] + [sum_of_delays] }
BWmin is in kbps, and sum_of_delays is in tens of microseconds. The bandwidth and delay for an Ethernet interface are 10 Mbps and 1 ms, respectively.
The calculated EIGRP BW metric is
256 × 107/BW = 256 × 107/10,000
= 256 × 1000
= 256,000
The calculated EIGRP delay metric is
256 × sum of delay = 256 × 1 ms
= 256 × 1000 × 1 microseconds
= 256,000 microseconds
Table 3-9 shows some default values for bandwidth and delay.
Table 3-9 Default EIGRP Classic Metric Values
| Media Type | Delay | Bandwidth | Metric |
| 10 Gigabit Ethernet | 10 μs | 10,000 Mbps | 512 |
| Gigabit Ethernet | 10 μs | 1,000 Mbps | 2816 |
| Fast Ethernet | 100 μs | 100 Mbps | 28,160 |
| Ethernet | 1000 μs | 10 Mbps | 281,600 |
| T1 (1.544 Mbps) | 20,000 μs | 1544 kbps | 2,170,031 |
| 64 kbps | 512,000 μs | 64 kbps | 53,107,200 |
The metric weights subcommand is used to change EIGRP metric computation. You can change the k values in the EIGRP composite metric formula to select which EIGRP metrics to use. The command to change the k values is the metric weights tos k1 k2 k3 k4 k5 subcommand under router eigrp n. The tos value is always 0. You set the other arguments to 1 or 0 to alter the composite metric. For example, if you want the EIGRP composite metric to use all the parameters, the command is as follows:
router eigrp n
metric weights 0 1 1 1 1 1