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eigrp feasibility condition

I've worked with the simulator and my lab all weekend trying to come up with some way to get two links of unequal value to load balance over EIGRP. I can not for the life of me get EIGRP to put both links in the IP table because the AD of the slow link is so far away from the FD of the fast link, short of making them equal values. This is very much true to real life and it bothers me that I can not make it work. I'm less concerned about testing now and more concerned about uncovering a truth.

No mater the values used for BW and Delay on either link, the difference between FD/AD for any given route is negligable. And because the Feasibility condition says AD must be less than successor's FD, that pretty much means you could never have, say, a 2:1 ratio on your links...and that means a variance of anything over 1:1 (equal cost) is impossible to achieve. That means unequal cost load balancing is nothing but a theory that can not be achieved in practical terms, am I correct? Is anyone really doing this?

How on earth does one get the AD to be less than the FD and still maintain a true ratio of the link speed?

6 REPLIES

Re: eigrp feasibility condition

I agree that i haven't seen in real life the use of variance for unequal cost loadbalancing but for testing you can always modify the metric weights to just choose the delay for metric calculation and avoid bandwidth

router eigrp

metric weights 0 0 0 1 0 0

Remember you will have to configure this on all your routers though

Though the varinace parameter works and installs the route in the RT i am very doubtful about the ratio that is used to send traffic across these links. Maybe someone can test the share ratio and confirm

Narayan

New Member

Re: eigrp feasibility condition

ah, sneaky...everything I've read in my CCNP prep for BSCI says don't play with metrics. I'm going to try what you said. How dangerous would that be in the real world? I don't want to think that some day someone will look at what I did and say, "what was she thinking???"

Re: eigrp feasibility condition

This method is best suited for labs

In the real world it is not feasible avoiding bandwidth for metric calculation :-)

Narayan

New Member

Re: eigrp feasibility condition

if you change the metrics you have to change it on all routers. if the K values don't match you won't get neighbor relationships in EIGRP.

New Member

Re: eigrp feasibility condition

Well I read in a Cisco white paper (http://www.cisco.com/warp/public/103/19.html) that the ratio is derived for each feasible link in the table by taking the highest FD to the actual FD for each link.

But let me ask something else because even in my lab this just isn't working.

Topology looks like this:

SWA >> R1 R3 << SWB

SWA >> R2 R4 << SWB

So A and B each have two paths by which to find each other. The top route (R1/R3) is 15 MB link. The bottom route (R2/R4) is 6MB.

This is the config I've placed on switch B and routers 3 and 4:

Switch B

router eigrp 64

network 10.64.0.0 0.0.255.255

no auto-summary

variance 3

Router R3

router eigrp 64

redistribute bgp 1 metric 15037 200 255 1 1500

network 10.64.0.0 0.0.255.255

no auto-summary

!

router bgp 1

no synchronization

bgp log-neighbor-changes

network 10.64.0.0 mask 255.255.0.0

network 69.138.62.244 mask 255.255.255.252

redistribute static

neighbor 69.138.62.245 remote-as 65000

neighbor 69.138.62.245 timers 12 36

no auto-summary

Router R4

router eigrp 64

redistribute bgp 6401 metric 6144 200 255 1 1500

network 10.64.0.0 0.0.255.255

no auto-summary

!

router bgp 6401

no synchronization

bgp log-neighbor-changes

network 10.64.0.0 mask 255.255.0.0

network 172.20.39.244 mask 255.255.255.252

redistribute static

neighbor 172.19.39.245 remote-as 1803

neighbor 172.19.39.245 timers 12 36

neighbor 172.19.39.245 route-map SETPATH out

no auto-summary

This config yeilds the following FD/AD values at Switch B:

P 10.1.0.0/16, 1 successors, FD is 221696, tag is 65000, serno 1431

via 10.64.1.3 (221696/221440), Vlan1

via 10.64.1.2 (467968/467712), Vlan1

Now you will notice that the bottom path has an AD so high it will NOT ever be feasible.

I've played with the values of Delay and BW and now, per your suggestion the weights. There is nothing short of EQUAL metrics that will cause FD of slow link to meet Feasibility condition.

Yesterday I played with injecting BGP into EIGPR on routers with equal metrics then changing delay/bw on the links between the router and the switch. It made absolutely no change in the calculation what so ever.

Question: I thought if I changed the bw/delay between the links eigrp would pick that up but it doesn't...how is that?

Gold

Re: eigrp feasibility condition

It's almost always best to have equal cost bandwidth links, with different delays, to make variance work. Generally speaking, this only works in the field when you have a long path, not a single link, where the minimum bandwidth ends up being the same, although the local bandwidth is different, and you can adjust the delay to make it balance the way you want. I normally only see this work when you have more than two links, as well.

HTH....

Russ

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