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EIGRP Load balancing..

Unanswered Question
Jul 6th, 2003
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Hi,

I am using two 800 router on one site, both are connected through 256kb link through 3700 router on main site. Now both 800 router are connected on same network and they are used for redendency perpose. Presentlly HSRP is running between both 800 ADSL router. Also Eigrp is running between all three routers. Now HSRP is working fine. but only one 800 router through all the traffic is going. I configured the variece command on 800 router but still it's showing only one route( 3700 router's lan subnet) in routing and topology table. It's showing redendent path for both 800's serial interfaces. How can I enable load sharing between those two 800 routers to reach 3700's local subnet.


Thanks,

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hbaerten Sun, 07/06/2003 - 22:58
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This can't be done with eigrp, and this is logical if you think about it: router A would send half of its traffic to router B but since router B also loadbalances it would send half of the traffic back to router A; creating unwanted results.


What you could do, is configure 2 hsrp groups, one for which router A is active and one for which router B is active. Then assign the second hsrp address as default gateway on half of the devices on the network. It's not ideal but I think it is the closest you can get to what you want without adding extra routers.


This page describes this in more detail:

Load Sharing with HSRP

http://www.cisco.com/warp/public/619/7.html


hth

Herbert

mustafa.mail Mon, 07/07/2003 - 02:10
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Hi,

Even I tried after disabling HSRP, but still it's not working.. Genrally load balancing should work with varience command in Eigrp ..


Thanks,

Mustafa

hbaerten Mon, 07/07/2003 - 22:48
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Generally yes, but in this case no. In eigrp terms, the reported distance of the standby router (B) is larger than the feasible distance of the active router (A) so A will never select the route via B.

Again, this is logical because otherwise there would be a loop if A uses a route via B and B uses a route via A.


I don't understand why you disabled HSRP, my suggestion was to implement a 2nd HSRP group for which B is active and A is standby. Have a look at the examples in the URL I gave.


The only other option I see is to add a second layer of (ethernet-to-ethernet) routers between your LAN and the 800's. So they will do hsrp on the lan side and eigrp to the 800's. The new routers will then have 2 routes via the 800's and will load balance over them.


regards,

Herbert

kkalaycioglu Tue, 07/15/2003 - 04:17
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To overcome EIGRP feasibility condition check and perform nonequal cost load balancing you can change the EIGRP metric of the 256K serial interface on one of the 800 router. In the interface conf mode apply bandwidth or delay command (I reccomend bandwidth at first). Take a sh interface output of the interface and read the bandwidth value and then change it to a value like 250 K or 260 K and then check the EIGRP routes.

ruwhite Tue, 07/15/2003 - 04:27
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I would never use the bandwidth, especially over lower speed serial links, to modify the metric of an eigrp route, since pacing is dependant on the bandwidth. I'd rather use delay, even though you have to use larger numbers to make the impact felt in the metric.


Second, I'm not certain that variance is going to help here, since all the traffic is being sent to one router through hsrp, or through the default gateway setting on the attached hosts? I'd look into GLBP instead of HSRP for this application, or use two HSRP groups, pointing half the hosts to each one, as suggested by an earlier poster.


Russ.W

kkalaycioglu Tue, 07/15/2003 - 05:31
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I completely agree about "bandwidth" command but I wrote it because setting the bandwidth is more straightforward compared to delay (difference between the delay value read on sh int output and the value entered with "delay" command).

For the second point let's call the 800s as A and B and the 3700 as C. Let's assume that we set the bandwidth value of the serial interfaces of A and B to 256K (equal values), EIGRP variance is set to 2 and A is the HSRP active router. In this case All outbound traffic from the local network behind A and B routers will first come to router A and EIGRP will always choose AC route because BC < AC feasibility cond required for nonequal cost loadbalancing is not met (because BC=AC). If we make a slight difference between AC and BC metrics (using delay or bandwidth, of course delay is the more appropriate way)router A receiving the traffic from the LAN can distribute outbound traffic between directions directly to C and C via B. This is my way of thinking using EIGRP mechanisms. Regards.


ruwhite Tue, 07/15/2003 - 13:51
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Yep--by setting the delay just a little lower on one link, you can force which one of the two routers will be the feasible sucessor of the other, and then load share between the two links using variance. It's a little odd, though, since you're pushing traffic at one of the 800's, then putting it back on the same wire to the other 800 to forwarding across the other link.


It seems easier to put the traffic on the right router in the first place, but that may not be an option here.


Russ.W


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