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10 Gig EIGRP metric

I have recently connected a 10 Gig connection from the local telco between two sites on 6509's. These two sites also have a 1 Gig links between them. When I connected the 10 Gig link I expected the 10 gig link to be the preferred route, but after looking at the routes I noticed that both links have the same EIGRP path cost. Also the minimum BW for both links is 1000000Kb or 1Gb. Why?

11 REPLIES
Cisco Employee

Re: 10 Gig EIGRP metric

Hi,

EIGRP uses the minimum bandwidth on the path to a destination network and the total delay to compute routing metrics.

Do a show ip interface Gig and TenGig and this will show you the delay value. Most likely the delay values are going to be the same in your case.

Also, do a "show ip eigrp topo xxx.xxx.xxx.xxx" and look at the minimum bandwidth value and this should point you in the right direction on where the bottleneck is. In case if you want the routes to prefer 10G path over 1G, then you can bump up the delay value under 1G and the route should take the 10G path. The delay value is only for EIGRP Metric Calculation and does not affect the amount of time it takes the data to reach the destination.

Please refer the below link for detail explanation on EIGRP Metrics

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

Let me know if it helps.

Regards,

Arul

New Member

Re: 10 Gig EIGRP metric

Arul,

thanks for responding. I am aware of the metric computation and that it uses bw and delay to calculate the metric. My issue is that the minimum bw for both links is the same, this is the problem and I don't understand why the minimum bw for a 10gig link would be 1000000 Kb (1Gb) and not the full 10000000Kb (1Gb).

Re: 10 Gig EIGRP metric

Hi,

I think this is because EIGRP calculates the bandwidth based on the following formula 1E7/BW in KB as reference. For a 10G ehthernet or OC3 this results in fraction and hence i think it uses the maximum bandwidth as 1G for all calculations.

In OSPF we have ospf auto-cost reference bandwidth command which can be used to change the reference bandwidth to calaculate the costs for OC3 and 10G links. i do not see any similar command available for EIGRP though.

HTH

Narayan

Re: 10 Gig EIGRP metric

Hi,

you can manually configure bandwidth or delay in the GE interface to be worse than GE. This should lead to the desired routing. Be aware however, that "bandwidth" is also used for QoS policies (CBWFQ, LLQ, percentage based policer, shaper) and could have negative side effects on your traffic priorization.

Regards, Martin

New Member

Re: 10 Gig EIGRP metric

Thanks for the great responses. I did the calculation and do not receive a fraction for some reason. I come up with 1 for 10G and 10 for 1G. I have already manually set the bw for the 10G link to be 10000000Kb and this still give me a minimum bw of 1000000Kb for the link. That is the whole problem the metric is correct because the minimum bw for both links is the same. It is incorrectly assigning a minimum bw of 1000000Kb(6 zeros) to the 10G link. Shouldn't the minimum bw for the 10G link be 10000000Kb(7 zeros). Could it be an issue with the IOS? Does it use the bw of the physical link or the entire path?

Re: 10 Gig EIGRP metric

Hi,

EIGRP uses

1) the minimum bandwidth along the path

2) the sum of all delays along the path

Regards, Martin

Gold

Re: 10 Gig EIGRP metric

CSCdx36932 is what you want.... EIGRP currently divides the minimum bandwidth by 10^7, so if the bandwidth is over that, the result is always 1.

HTH

:-)

Russ

New Member

Re: 10 Gig EIGRP metric

I can understand what EIGRP does with the bw to calculate the metric, but the minimum bw for the 10Gig link should be 10 Gigs not 1Gig. I guess somewhere along the path from site a to z the telco has a 1 Gig link and that is why I am getting a minimum bw of 1Gig and not 10Gig on the 10Gig link. The metric ends up being the same because for some reason the minimum bw for the 10gig link is 1000000Kb(1Gb). I guess I'll have to talk to the telco. They are providing the 10 Gig link from site A to Z.

Thanks

Re: 10 Gig EIGRP metric

If iam not wrong, EIGRP uses the configured bandwidth on the interface and not the physical link bandwidth.

The reason why the minimum bandwidth is not exceeding 1 Gig should either be an IOS bug or EIGRP protocol itself has a limitation on the configurd Bandwidth.

can you change the configured bandwidth of any Fastethernet interface to 10GE and check the minimum bandwidth reported by EIGRP?

Narayan

New Member

Re: 10 Gig EIGRP metric

I have configured the bandwidth manually on the 10Gig interface and it still gives me only 1Gig. That is exactly what I thought(EIGRP limitation or IOS bug), but nobody seems to have experienced this problem before and I can't find any documentation that says so. I guess I'll have to open a TAC case. Thanks

New Member

Re: 10 Gig EIGRP metric

The minimum BW parameter is not for the 10Gb link,, but it's for the PATH between two routers.  It means that, somewhere along the path between the two networks, there exists a 1Gb link.  So, no matter whether you are traversing the 1Gb or 10Gb link, the network topology still contains a 1Gb link.  It could be the LAN connection for a network you were checking path metrics for, or it could be the Loopback interface of one of the routers, if that was your destination network.

Arul gave excellent guidance on adjusting the delay metric of the 1Gb link in order to favor the 10Gb link.

-rb

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