EIGRP bandwidth percent

By default EIGRP uses 50% of the bandwidth reported by the IOS software, but what if there’s no bandwidth set on a serial link and it defaults to 1.5m with clock rate set to 64000, how much bandwidth would EIGRP traffic use?

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Netstudent

If it default to 1.5Mb/s then it uses half of that. 750Kb/s

It doesn't matter if the BW is manually set or if it is left to default, it will use 50% unless you change that. Sometimes the bandwidth can be set artificially low for routing policy reasons. IN these cases the bandwidth percentage can be configured to more than 100%.

Configuring artificially low BW fools the EIGRP metric but also gives EIGRP access to more than the artificial bandwidth for updates and what not.

singh8281

Wouldn't that congest the link during convergence, no traffic other than routing updates would travel for a short time. Is this kind of behavior used in production networks?

Netstudent

EIGRp should only converge at startup and if something on the network changes. IN a production network, there shouldn't be a lot of changes occuring during production hours.

singh8281

So there's no hard rule that bandwidth must match the clock rate. You would only use the bandwidth to influence the routing metrics then?

marlon23

clock rate doesnt need to be equal to bandwidth, for example you can have T1 clocked to 1,5Mb & deal with ISP for a 1Mb of bandwidth. Whole EIGRP bandwidth percent feature was usable when hi-speed link was 56k

tech-airman

singh8281 wrote:

By default EIGRP uses 50% of the bandwidth reported by the IOS software, but what if there’s no bandwidth set on a serial link and it defaults to 1.5m with clock rate set to 64000, how much bandwidth would EIGRP traffic use?

singh8281,

What is the default bandwidth for a serial interface on the DCE end without a clock rate set?

Paul Boz

There's a lot of misinformation going around about EIGRP and bandwidth consumption.

People seem to think that EIGRP constantly consumes 50% of the bandwidth of a link and that's simply not the case. EIGRP can use up to 50% of a link during times of convergence, but that really depends on how the network has been changed and how large the network is. If you have a small network you won't even notice much bandwidth consumption during times of convergence.

It's not good practice to "fool" EIGRP by manipulating bandwidth values either. The best method or doing this is to change the delay.*

Source - Routing TCP/IP Vol 1

singh8281

tech-airman wrote:

singh8281,

What is the default bandwidth for a serial interface on the DCE end without a clock rate set?

wouldn't it be 1.544, the default for serial wic-2t?

singh8281

marlon23 wrote:

clock rate doesnt need to be equal to bandwidth, for example you can have T1 clocked to 1,5Mb & deal with ISP for a 1Mb of bandwidth. Whole EIGRP bandwidth percent feature was usable when hi-speed link was 56k

What is the actual purpose of the clock rate then? and under what conditions would you set the clock lower or higher than the actual link speed.

tech-airman

singh8281 wrote:

tech-airman wrote:

singh8281,

What is the default bandwidth for a serial interface on the DCE end without a clock rate set?

wouldn't it be 1.544, the default for serial wic-2t?

singh8281,

Questions:

1.544 what?
Which router model do you have the "serial wic-2t" inserted into?
Which IOS version are you running on that router?

singh8281

tech-airman wrote:

singh8281 wrote:

tech-airman wrote:

singh8281,

What is the default bandwidth for a serial interface on the DCE end without a clock rate set?

wouldn't it be 1.544, the default for serial wic-2t?

singh8281,

Questions:
1.544 what?

Which router model do you have the "serial wic-2t" inserted into?

Which IOS version are you running on that router?

1544 Kb/s

2611xm, 12.3 advent

why is that relevant??

tech-airman

singh8281 wrote:

tech-airman wrote:

singh8281 wrote:

tech-airman wrote:

singh8281,

What is the default bandwidth for a serial interface on the DCE end without a clock rate set?

wouldn't it be 1.544, the default for serial wic-2t?

singh8281,

Questions:
1.544 what?

Which router model do you have the "serial wic-2t" inserted into?

Which IOS version are you running on that router?

1544 Kb/s

2611xm, 12.3 advent

why is that relevant??

singh8281,

According to the "Cisco IOS Interface and Hardware Component Command Reference, Release 12.3" webpage, for the command "clock rate", the defaults are "No clock rate is configured." So if by default the clock rate is not configured, what is the bandwidth of the serial interface?

Source:

Cisco IOS Interface and Hardware Component Command Reference, Release 12.3 - http://www.cisco.com/en/US/docs/ios/12_3/interface/command/reference/int_a1g.html#wp1142413

APA

specifying the bandwidth also directly relates to how often EIGRP sends out hello packets....

LAN Link - every 5 secs (10Mbps +)
WAN Link - every 60 secs (less than 10Mbps)

singh, without specifying the bandwidth on on interface... the default line rate of the interface is used..... so in your case 1544Kbps... so EIGRP would use 50% of this....

Setting bandwidth is more apparent when you have a 5Mbps connection to a Service provider.... and port speed is forced to 10Mbps fixed or 100Mbps fixed... If you don't specify the bandwidth command then EIGRP will think the total bandwidth of the link is line-rate (10Mbps or 100Mbps) when it is actually 5Mbps.... So if needed it would attempt to use 50% of that link during convergence (5Mbps or 50Mbps) plus as I stated above it would be sending out unnecessary hello packets every 5 secs.... due to thinking the links bandwidth is at line rate....

Clockrate has nothing to do with bandwidth it is there on serial connections for routers to synchronize their flows between each other.... hence why without a clockrate coming from the DCE side of a serial connection the link is never successful...

Hope this helps

gojericho0

Just wanted to add this as well since this also tends to be a misconception. The bandwidth command has nothing to do with the actual speed of the link. Its just used for reference (calculation of metric for example). I always keep my bandwidth command the same as the actual bandwidth of the link because its more intuitive and doesn't cause confusion.

singh8281

A.P.A wrote:

Clockrate has nothing to do with bandwidth it is there on serial connections for routers to synchronize their flows between each other.... hence why without a clockrate coming from the DCE side of a serial connection the link is never successful...

Hope this helps

If a link is clocked at 128000 and bandwidth is set to 64 on both routers, at what rate would data packets flow. I would assume the rate at which bandwidth is set.

I googled it and this is what i came up with

http://techrepublic.com.com/5208-11190-0.html?forumID=40&threadID=192872&messageID=1993843

APA

Yes I worded my previous post incorrectly..... apologies for that

On a serial-link the clock rate command is the defined bandwidth of the link and is only set at the DCE end......

Bandwidth command is just a variable which applies to help dynamic metric calculation and other routing calculations in no way is it a guarantee of the links bandwidth... Althought to ensure logical and correct configuration you should match this command with the actual links bandwidth if the bandwidth is not exactly line-rate...

There are also certain functions the bandwidth command plays a part in... (e.g max-reserved-banwidth for qos on an interface is 75% of the detected bandwidth, to ensure a correct figure you should ensure the bandwidth command is matched to the actual bandwidth you have subscribed for)

Cheers,