Types of Dynamic Routing Protocols 187
·
Bandwidth--Bandwidth refers to a link's available traffic capacity. Although bandwidth
is a rating of the maximum attainable throughput on a link, routes through links with
greater bandwidth are not necessarily better than routes that pass through slower links. As
an example, if a T-1 link is congested, the actual time required to send a packet to the
destination on the T-1 could be greater than a 64 K line.
·
Load--Load refers to the degree to which a network device is busy. Load can be calculated
in a variety of ways, including CPU utilization and packets processed per second.
·
MTU--The MTU setting acknowledges that media can support different sizes of packets.
As an example, Ethernet, with a few exceptions, supports a maximum of 1500 bytes, while
Token Ring can support much larger packets. MTU is used to avoid sending a frame that
is too large in cases where the IP fragment bit is set. Though MTU is an included
parameter, it has never been used in the calculation of metrics.
NOTE
The interstate highway (high-bandwidth) might look like the best and fastest path to take--until
you discover that several lanes are blocked (delay) due to an accident.
Use bandwidth and delay when determining the best path.
IGRP and EIGRP can base route selection on multiple metrics. IGRP and EIGRP use five
metrics to determine the best path:
·
Bandwidth
·
Delay
·
Reliability
·
Load
·
MTU
Some people use the following mnemonic to help remember these metrics: Big Dogs Really
Love Meat.
Table 6-2 shows the different metrics used by different routing protocols.
Table 6-2
Routing Protocol Metrics
Protocol
Metric
RIP V1
Hop count
IGRP
Bandwidth, delay, load, reliability, MTU
EIGRP
Bandwidth, delay, load, reliability, MTU
OSPF
Cost
87200333.book Page 187 Wednesday, August 22, 2001 2:37 PM