delay setting on interface, what does it mean?

Answered Question
Aug 16th, 2009

Hi every body.

my book shows an output of " show int f0/0". The output also shows delay setting among other things. What does this delay mean or indicate?

Thanks and have a good day.

I have this problem too.
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Correct Answer by Joseph W. Doherty about 7 years 5 months ago

Bandwidth does mean the amount of information that can be passed during some time interval, and for networks, it's usually measured as bits per second.

The bandwidth setting on an interface often defaults to the maximum bandwidth supported by the physical interface but we can adjust bandwidth to represent the logical transmission rate. For example, with metro ethernet, the physical rate might be 10 Mbps or 100 Mbps, but we've only are provided 2 Mbps across the link. Setting bandwidth to 2 Mbps would indicate to other router functions the actual available bandwidth.

Delay has two primary factors, time for equipment to forward packets and physical distance. If you had two Ethernet links, one link 300 meters single wire, and the other 30 switches, each 10 meters apart, the physical distance (discounting distance within the switches) would be the same, but the switches will probably delay a frame as it crosses each switch (especially if switch is using a store and forward architecure). So, you'll likely see a delay difference between the two.

You could have a LL that provides the same delay as a typical (geostationary) satellite, however they would probably not be equal in length. This because signally progation speed is likely different between radio and wire, and it would be hard to find a wire of same length (about 71,400 km).

Correct Answer by Edison Ortiz about 7 years 5 months ago

Sarah,

From: http://www.cisco.com/en/US/docs/ios/interface/command/reference/ir_a1.html#wp1013415

The bandwidth command sets an informational parameter to communicate only the current bandwidth to the higher-level protocols; you cannot adjust the actual bandwidth of an interface using this command.

Note This is a QoS/Routing parameter only. It does not affect the physical interface.

Changing Bandwidth

For some media, such as Ethernet, the bandwidth is fixed; for other media, such as serial lines, you can change the actual bandwidth by adjusting hardware. For both classes of media, you can use the bandwidth command to communicate the current bandwidth to the higher-level protocols.

Bandwidth Inheritance

Before the introduction of the bandwidth inherit command option, when the bandwidth value was changed on the main interface, existing subinterfaces did not inherit the bandwidth value from the main interface. If the subinterface was created before the bandwidth was changed on the main interface, then the subinterface would receive the default bandwidth of the main interface, not the configured bandwidth. Additionally, if the router was subsequently reloaded, the bandwidth of the subinterface would then change to the bandwidth configured on the main interface.

The bandwidth inherit command controls how a subinterface inherits the bandwidth of its main interface. This functionality eliminates the inconsistencies related to whether the router has been reloaded and what the order was in entering the commands.

The no bandwidth inherit command enables all subinterfaces to inherit the default bandwidth of the main interface, regardless of the configured bandwidth. If a bandwidth is not configured on a subinterface, and you use the bandwidth inherit command, all subinterfaces will inherit the current bandwidth of the main interface. If you configure a new bandwidth on the main interface, all subinterfaces will use this new value.

If you do not configure a bandwidth on the subinterface and you configure the bandwidth inherit kbps command on the main interface, the subinterfaces will inherit the specified bandwidth.

In all cases, if an interface has an explicit bandwidth setting configured, then that interface will use that setting, regardless of whether the bandwidth inheritance setting is in effect.

Correct Answer by Joseph W. Doherty about 7 years 5 months ago

Delay is a metric assigned to an interface to represent time to cross the link, in microseconds. It's used by IGRP and EIGRP as one of those protocols' metrics.

What you're probably seeing for a delay value is a default based on interface type and perhaps (Ethernet) interface bandwidth. However, similar to the interface bandwidth command, you can change the value using the interface delay command. Then the show interface command should show the configured setting.

The usefulness of this metric might be understood using an example. Consider if you had two T1 serial interfaces, each with 1.5 Mbps, but one used a dedicated leased line but the other used a satellite link. Delay to transit the two interfaces would likely be much different although their bandwidths are the same. The delay metric might be used to indicate this difference.

BTW, here's a reference to an interface default delay cheat sheet: https://cisco.hosted.jivesoftware.com/message/24718

Correct Answer by Lucien Avramov about 7 years 5 months ago

Interesting the link does work for me.

Here is another one for the same document:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml

Correct Answer by Lucien Avramov about 7 years 5 months ago

This document explains you HOW to calculate the delay:

http://www.cisco.com/en/US/customer/tech/tk365/technologies_white_paper09186a0080094cb7.shtml

Delay is cumulative and added to the received delay value (total known on the peer who sent the update to you thru this interface.) When you send it out to your other peers, the delay value you send now reflects the new total (delay received plus delay on interface.)

So in this way delay can also influence route advertisements, just not on the interface the delay is defined on.

Correct Answer by Giuseppe Larosa about 7 years 5 months ago

Hello Sarah,

the delay parameter is an administrative parameter assigned statically by IOS on a per interface type basis.

the meaning should be the serialization delay in transmitting a frame over the interface.

the delay parameter is used by EIGRP in its composite metric calculation.

You can modify the delay parameter using

int f0/0

delay x

there has been a long thread about this some time ago where the question was how IOS assigns delays to interface and why subinterfaces inherit the delay value from main interface.

The latter makes sense if we think of it as the delay to put the frame on wire so it is reasonable subinterfaces inherit the same delay of main interface.

the real delay experienced on the link is a function of link usage and so varies over time: if the packet has to wait in a full queue the delay is bigger then that assigned by IOS to the interface.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sun, 08/16/2009 - 21:01

Hello Sarah,

the delay parameter is an administrative parameter assigned statically by IOS on a per interface type basis.

the meaning should be the serialization delay in transmitting a frame over the interface.

the delay parameter is used by EIGRP in its composite metric calculation.

You can modify the delay parameter using

int f0/0

delay x

there has been a long thread about this some time ago where the question was how IOS assigns delays to interface and why subinterfaces inherit the delay value from main interface.

The latter makes sense if we think of it as the delay to put the frame on wire so it is reasonable subinterfaces inherit the same delay of main interface.

the real delay experienced on the link is a function of link usage and so varies over time: if the packet has to wait in a full queue the delay is bigger then that assigned by IOS to the interface.

Hope to help

Giuseppe

Correct Answer
Lucien Avramov Sun, 08/16/2009 - 21:19

This document explains you HOW to calculate the delay:

http://www.cisco.com/en/US/customer/tech/tk365/technologies_white_paper09186a0080094cb7.shtml

Delay is cumulative and added to the received delay value (total known on the peer who sent the update to you thru this interface.) When you send it out to your other peers, the delay value you send now reflects the new total (delay received plus delay on interface.)

So in this way delay can also influence route advertisements, just not on the interface the delay is defined on.

sarahr202 Sun, 08/16/2009 - 21:43

Thanks Lavramov.

The link does not work . But you have explained it already very well and addressed my question.

Thanks and have a good night.

sarahr202 Sun, 08/16/2009 - 21:41

Thanks Giuseppe.

So can i say the delay is total time taken by a frame to be transmitted out of interface. That time time is calculated the from the moment the frame is placed in output buffer of interface to the time when it is fully transmitted out of interface. Is my concept correct ?

thanks Giuseppe and i believe it is a day over there so you have a good day.

Correct Answer
Joseph W. Doherty Mon, 08/17/2009 - 03:58

Delay is a metric assigned to an interface to represent time to cross the link, in microseconds. It's used by IGRP and EIGRP as one of those protocols' metrics.

What you're probably seeing for a delay value is a default based on interface type and perhaps (Ethernet) interface bandwidth. However, similar to the interface bandwidth command, you can change the value using the interface delay command. Then the show interface command should show the configured setting.

The usefulness of this metric might be understood using an example. Consider if you had two T1 serial interfaces, each with 1.5 Mbps, but one used a dedicated leased line but the other used a satellite link. Delay to transit the two interfaces would likely be much different although their bandwidths are the same. The delay metric might be used to indicate this difference.

BTW, here's a reference to an interface default delay cheat sheet: https://cisco.hosted.jivesoftware.com/message/24718

sarahr202 Tue, 08/18/2009 - 08:13

Hi Joseph.

Could you please explain to me what does bandwidth on interface mean? For example we have fast ethernet link between two hosts, does it mean 100Mb max can travel in one second.

In your case you gave a very good example about T1 connection via satellite.Delay is different for both connections because link to satellite is longer in length than the dedicated link T1. Is my understanding correct?

If i am correct, then if both links i.e dedicated link and statellite links are equal in length, then the delay would be the same assuming all others variables being equal.

Thanks a lot

Correct Answer
Edison Ortiz Tue, 08/18/2009 - 10:14

Sarah,

From: http://www.cisco.com/en/US/docs/ios/interface/command/reference/ir_a1.html#wp1013415

The bandwidth command sets an informational parameter to communicate only the current bandwidth to the higher-level protocols; you cannot adjust the actual bandwidth of an interface using this command.

Note This is a QoS/Routing parameter only. It does not affect the physical interface.

Changing Bandwidth

For some media, such as Ethernet, the bandwidth is fixed; for other media, such as serial lines, you can change the actual bandwidth by adjusting hardware. For both classes of media, you can use the bandwidth command to communicate the current bandwidth to the higher-level protocols.

Bandwidth Inheritance

Before the introduction of the bandwidth inherit command option, when the bandwidth value was changed on the main interface, existing subinterfaces did not inherit the bandwidth value from the main interface. If the subinterface was created before the bandwidth was changed on the main interface, then the subinterface would receive the default bandwidth of the main interface, not the configured bandwidth. Additionally, if the router was subsequently reloaded, the bandwidth of the subinterface would then change to the bandwidth configured on the main interface.

The bandwidth inherit command controls how a subinterface inherits the bandwidth of its main interface. This functionality eliminates the inconsistencies related to whether the router has been reloaded and what the order was in entering the commands.

The no bandwidth inherit command enables all subinterfaces to inherit the default bandwidth of the main interface, regardless of the configured bandwidth. If a bandwidth is not configured on a subinterface, and you use the bandwidth inherit command, all subinterfaces will inherit the current bandwidth of the main interface. If you configure a new bandwidth on the main interface, all subinterfaces will use this new value.

If you do not configure a bandwidth on the subinterface and you configure the bandwidth inherit kbps command on the main interface, the subinterfaces will inherit the specified bandwidth.

In all cases, if an interface has an explicit bandwidth setting configured, then that interface will use that setting, regardless of whether the bandwidth inheritance setting is in effect.

Correct Answer
Joseph W. Doherty Tue, 08/18/2009 - 10:39

Bandwidth does mean the amount of information that can be passed during some time interval, and for networks, it's usually measured as bits per second.

The bandwidth setting on an interface often defaults to the maximum bandwidth supported by the physical interface but we can adjust bandwidth to represent the logical transmission rate. For example, with metro ethernet, the physical rate might be 10 Mbps or 100 Mbps, but we've only are provided 2 Mbps across the link. Setting bandwidth to 2 Mbps would indicate to other router functions the actual available bandwidth.

Delay has two primary factors, time for equipment to forward packets and physical distance. If you had two Ethernet links, one link 300 meters single wire, and the other 30 switches, each 10 meters apart, the physical distance (discounting distance within the switches) would be the same, but the switches will probably delay a frame as it crosses each switch (especially if switch is using a store and forward architecure). So, you'll likely see a delay difference between the two.

You could have a LL that provides the same delay as a typical (geostationary) satellite, however they would probably not be equal in length. This because signally progation speed is likely different between radio and wire, and it would be hard to find a wire of same length (about 71,400 km).

canelillatunilla Thu, 05/05/2011 - 10:35

Hi,

I agree that delay has two primary factors, time for equipment to forward packets and physical distance.

The router does not know the physical distance of a link, so it can not take in account this to calculate the delay.

I think the default delay on an interface is based on the time for equipment to forward frames, time since the frame has completely entered on an input interface until the frame has completely left the output interface, and this default value it constant (without considering queue load, etc.)

Am I right?

Kind regards.

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