Do anyone know how Cisco define the delay in different interface? For an optinmum EIGRP network, the bandwidth & delay should set to the real value & let the EIGRP do the rest. But how to "calculate" the delay? Is it the round trip delay for a one-byte packet or a 100-byte packet? Any idea? Tks a lot!
If you are going to influence EIGRP, do it with the delay command and not bandwidth, as bandwidth will affect other protocols as well (eg OSPF).
Delay itself is an integer that specifies the delay in tens of microseconds for an interface or network segment. It can be 0 or any positive number that is a multiple of 39.1 nanoseconds. It is also a static figure and not measured dynamically. It is usually presented (eg show interface, sh ip route, show ip eigrp top x.x.x.x) in microseconds.
The default delay of an interface depends on the interface (time it takes to place a packet on the media):
fastethernet - 100 microsec. (delay command = 10)
HSSI - 20000 microsec (delay command = 2000)
16Mb Token ring - 630 microsec (delay command =63)
We have a regional network here with a lot of alternative routes within Asia region. We want to control the traffic to use all the paths more cost effective & force the traffic go thru the regional hub sites before route back to the headquarter. When we want to set the delay, we almost need to calculate the metrix with the EIGRP formular at different sites. If we know how Cisco definite the delay, then the change at the major link & will not affect other routing.
From page 84 of my book "High Availability Networking with Cisco"... when using default K parameters (which you should do unless you have VERY good reasons not to...), the total cost of an EIGRP route is the total delay of all hops in tens of microseonds times 256 plus (256 times 10 to the 10 divided by the bandwidth in bits/second of the slowest link in the path). Many people forget that while all delays count, only the slowest link affects the bandwidth part of the metric.
As for why I talk about EIGRP metrics in a book about network availability... adjustment of delay and bandwidth values is sometimes required to ensure that alternate routes are feasible successors to preferred routes. Alternate routes which are feasible successors are put into service with no delay when link failure is discovered, alternate routes which are not can not be utilized until after running the route calculation algorithm, which translates into downtime.
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