EIGRP Bandwidth Commands

I would like to have clear idea about the difference between the commands below, when used in EIGRP:
Bandwidth <Value>
Example : Bandwidth 256

Ip Bandwidth-percent EIGRP <EIGRP AS>  <Percent Value>

example : ip Bandwidth-percent EIGRP 1 60

when shoud I use one and not the other ?


Thank you
jskfanAsked:
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William MillerInventory/IT ConsultantCommented:
Directly from the Cisco article corresponding to your query:

https://www.cisco.com/c/en/us/td/docs/ios/12_2/iproute/command/reference/fiprrp_r/1rfeigrp.html

ip bandwidth-percent eigrp
 
To configure the percentage of bandwidth that may be used by Enhanced Interior Gateway Routing Protocol (EIGRP) on an interface, use the ip bandwidth-percent eigrp command in interface configuration mode. To restore the default value, use the no form of this command.
 
ip bandwidth-percent eigrp as-number percent
 
no ip bandwidth-percent eigrp as-number percent


bandwidth
 
Minimum bandwidth of the route in kilobits per second. It can be from 1 to 4294967295.


bandwidth (interface)
 
Sets a bandwidth value for an interface.
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Hemil AquinoNetwork EngineerCommented:
Here is something easy that would help you to understand the bandwidth of EIGRP and bandwidth percentage.

Let say you have R1, R2, R3.

R1 has a link bandwidth of 50 Mbps -- R3 has a a link bandwidth of 10 Mbps
R1 has a link bandwidth of 50 Mbps -- R2 has a  link bandwidth of 50 Mbps -- R3 has a link bandwidth of 20 Mbps

Which link do you think is the best to send traffic? From R1 to R2 to R3 right? that is because you are considering the bandwidth of each location. So when you use the clause  "Bandwidth 256 or anything" is because you are considering that link as a higher link with a reliable connection. Below my image, so you can have an idea.

EIGRP.png
Now, R3 has a network address of 9.9.9.9/32. Which path would you consider better to send traffic, from (R1 to R3) OR (R1,R2,R3)
depending of how do you configure the bandwidth, your router will take a link decision.  

Now, Ip Bandwidth-percent EIGRP:

In case you didn't know EIGRP despite of being the best routing protocol in my perspective, it takes 50% of the bandwidth when a query come up. Let me explain in terms you can understand.

If you are working in a company that has more than 50 router and one interface goes down EIGRP will do a SIA (Stuck In Active) meaning, I will query all the routers to find out if somebody has a path to reach the network that was broken. Using the 50 percent of it's bandwidth of each network. That's a lot of traffic with lots of bandwidth.

By default EIGRP has been configured with the bandwidth of 50% but you can lower that depending on you needs.

I hope this clarifies you.
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jskfanAuthor Commented:
Thank you for your explanation..

I know EIGRP uses by default 50% of the Bandwidth  (Though I am not sure why is different than OSPF ).

What I need to know in which scenario that you need to go to the interfce and type for instance bandwidth 256 and in which scenario you go to the interface and type for instance : ip Bandwidth-percent EIGRP 1 60

I know that They both have influence on the cost..but .when to use one command and not the other command
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JustInCaseCommented:
If you apply bandwidth command to interface - EIGRP and OSPF link cost (if using auto calculation) will be calculated accordingly. It will not limit bandwidth by itself in any way.

ip bandwidth-percent - will configure maximum amount of bandwidth can be used for EIGRP (it can go over 100%). The point is, that, if bandwidth value is to low (for example there are several subinterfaces on serial link) bandwidth percent over 100% may be needed to make EIGRP have enough bandwidth to properly function (if traffic forwarding is relaying on routing protocol and routing protocol is down all traffic will be dropped).

However, bandwidth command should not be used to influence EIGRP calculation, since in combination with QoS can/will limit traffic bandwidth on interface, and can also influence other routing protocols. It should actually be used only to match real interface bandwidth (for example interface itself is 100Mb, but ISP is providing 10Mb traffic in that case you should configure bandwidth on interface and QoS (typically shaping with priority queues), it is better for you to choose which traffic will be dropped, than ISP is randomly dropping your traffic).

PS
it is not subject of this topic, but for EIGRP traffic engineering changing delay would be right choice since only EIGRP is using it in calculations and also does not influence in any way delay on interface.
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jskfanAuthor Commented:
I am not sure why the concern about the Percentage of the Bandwidth is just with EIGRP, other protocols do not limit themselves to 50%.
I also do not understand how come EIGRP cannot detect the real bandwidth of the Serial Interface
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JustInCaseCommented:
Cisco on serial interface by default bandwidth is 1544 Kbit, but actually bandwidth is function of interface clock (DCE interface). Even when for example clock rate 64000 is configured under interface, bandwidth when show interface serial <port> is issued is still listed as 1544 Kbit. And, issue it is not just related to serial interface, the same is with Ethernet, on link to ISP for example negotiated speed is 100Mb, but actual speed that is provided by ISP maybe is 10Mb. So, until you manually configure bandwidth on interface, calculated cost in routing protocols for link is wrong and traffic may choose wrong path or do equal cost routing on interfaces with different "real" bandwidth. The same is with QoS if bandwidth is not matching reality, it will simply be wrong and can do more harm than good.

In case of serial cable, ISP is typically configuring clock rate on serial interface (DCE), on side of client there is no clock command present (DTE interface). With Ethernet ISP is policing traffic on their side, and on client side is seen just negotiated interface speed <10, 100, 1000 Mb> and it may be matching to actual speed provided by ISP... So, it is the same issue in both cases - auto calculated speed in OSPF and EIGRP on interface is wrong, since it does not match reality.
In case where we have 2 uplinks to ISP and both negotiated interface speeds are 100Mb, but in reality bandwidth provided by ISP on link1 is 50Mb and on link2 is 10 Mb is where it is important to "know" actual bandwidth on interface provided by ISP.

I don't know why Cisco decided to implement percentage of bandwidth command just in EIGRP. I can only guess that is result of implicit EIGRP traffic shaper to 50% by default, and that there were problems with routing on slow links or EIGRP was taking too much percentage of traffic on fast links. Generally, if EIGRP will use all free traffic for routing, or not having enough bandwidth on interface will lead to the same effect - there will be no other data flow on interface. Regarding OSPF, as far as I know, it does not have implicit traffic shaping, so I guess that's why there is no percentage of bandwidth command there.
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jskfanAuthor Commented:
Thank you Very much Guys
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JustInCaseCommented:
You're welcome.
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