Advice Needed - Multiple WAN Links

Unanswered Question
Mar 27th, 2009


Can someone advice on this scenario,


I have three point-2-point link between site(A) and Site (B)

site(A)-------4Mb---------Site(B) (Vlan 5)

Site(A)-------3MB---------Site(B)(vlan 6 )

Site(A)-------8MB---------Site(C) (Vlan 7 )

What is the best option to use all links together, I m playing with ospf cost on each link.

Kindly advice

I have this problem too.
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Giuseppe Larosa Fri, 03/27/2009 - 03:13

Hello Ronald,

for the different speeds you should use the 8 Mbps link as primary link.

In case of failure of the primary link can be acceptable to use the other two in parallel having almost the same speed.

So my suggestion is (on both ends or each link in interface config mode)

ip ospf cost 200 for first two links

ip ospf cost 100 for last 8 Mbps link

Hope to help


Joseph W. Doherty Fri, 03/27/2009 - 05:07

The issue you face, OSPF only supports equal cost routing. Either you cost all ports the same, which will route traffic equally, or you do something like what Giuseppe suggests (where you normally will just use the 8 Mbps link, but will use the 4 and 3 Mbps links if the 8 Mbps links fails).

The choice is whether you think it best to normally guarantee flows can ramp up to 8 Mbps, but only have 8 Mbps, or flows might have 3, 4, or 8 Mbps, but there's 15 Mbps aggregate bandwidth available.


Other alternatives exist, but they get much more complex.

For example, you could run two GRE tunnels across the 8 Mbps link and route across 4 paths (4,3,4,4). You could defined secondary addresses on the 8 Mbps links and staticly route across 4 paths. You could use EIGRP which support unequal path bandwidths. Or, you might be able to use OER/PfR do dynamically load balance across the 3 physical paths.

ronald.ramzy Sat, 03/28/2009 - 23:11

Thanks All.

If I need to run EIGRP for this WAN Links.

What is the required setup to make it working as failover/load-balancing.

All required step-by-step would be very helpful and supported with a cisco link to understand those commands.

rpfinneran Sun, 03/29/2009 - 03:07

By default, VLAN interfaces have a delay of 100 usec's.

Now, we need to find the metric for each interface. I am assuming you're not going to change k values. Metric = 256(10^7/bandwidth) + 256(delay). So, you should get the following metrics...

Vlan 5 = 256(10^7/4000) + 256(100) = 665600

Vlan 6 = 256(10^7/3000) + 256(100) = 878848

Vlan 7 = 256(10^7/8000) + 256(100) = 345600

As parallel links, all routes here would be considered Feasible Successors. The Feasible distance would be 345600 (plus whatever on other side of links). Needless to say, it is pretty likely that a variance of 2 would allow all three links to be added to the routing table. So, the total configs necessary should be...

conf t


int vl5

bandwidth 4000

int vl6

bandwidth 3000

int vl7

bandwidth 8000



router eigrp


traffic-share balanced

variance 2

maximum-paths 3



Now, in the routing table, you should see all three routes to Site B (I am assuming you made a typo in the original post that says 8Mb goes to C).

Here is a good Cisco Page on this:



Giuseppe Larosa Sun, 03/29/2009 - 05:07

Hello Ryan, Ronald,

EIGRP provides a way to use unequal links.

Traffic will be sent as:

3/6 on best path, 2/6 on second path, 1/6 on third path

it is a form of wheighted load balancing

to be able to use also the third link variance should be set to 3

Hope to help


rpfinneran Sun, 03/29/2009 - 05:15

Good catch, Giuseppe...I did all that math and kind of skipped over the final calculation...I am a slacker today. Variance should be 3 as Giuseppe noted.

Giuseppe Larosa Sun, 03/29/2009 - 05:21

Hello Ryan,

you did the job and a good one.


but this thread and the other one about OSPF on FR and VPN links are related ?

Best Regards


ronald.ramzy Sun, 03/29/2009 - 23:11


These are two different sites and not related to each other...

I am looking at scenario to use the link effectively.


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