Load Balance in OSPF and Multilink Group E1

Unanswered Question
Oct 13th, 2008


I have one customer running multilink group using 4 X E1 to one of their branch.

Currently their problem is when on of the E1 in the multilink flapping, the have to shut it manually because it affect other E1 links in the multilink.

Then I proposed to them to run 2 X Multilink group.

-------| <------------- E1 GROUP 1 ----------------> |--------|

R1----| <------------- E1 GROUP 1 ----------------> |----R2-|

-------| <------------- E1 GROUP 2 ----------------> |--------|

-------| <------------- E1 GROUP 2 ----------------> |--------|

I proposed them to run ospf to load balance the connection. But the root problem is, when 1 of the E1 link in one of the multilink flapping, we have to shut down the link.

Will the OSPF detects the flapping interface and reduce the cost of the multilink group that has failure interface?

Please shed a light in my day.


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Overall Rating: 4 (2 ratings)
satish_zanjurne Mon, 10/13/2008 - 21:27


1.You have one multilink interface , which has 4 E1 as member

2.So logically it is a single bundle or 1 multilink interface.

3.If the it is a branch & branch router is stub router , just use static route instead of OSPF.

4.Also can you mention what problem exactly the flapping E1 is causing

5.Even if it flaps , after having static route or connected route , there should not be any problem.

HTH...rate if helpful..

muhammadsafwan Mon, 10/13/2008 - 23:56

1. Currently it's running static route through the multilink interface.

2. When 1 of the 4 X E1 flapping, the multilink interface faces some interference. A lot of packet drops.

3. We need to shut down the failure interface to solve this.

4. If i not mistaken, the cause of the flapping interface is due to physical problem.

5. I want to load balance the multilink interface so that if one of the multilink flapping, OSPF will use the second multilink interface.


Giuseppe Larosa Tue, 10/14/2008 - 01:18

Hello Muhammad,

I would go further :

I would suggest to remove any multilink config, use each link as a distinct L3 link and have OSPF running on all links

In this way OSPF will detect what links are working and if one link flaps it will not interfere with the others

If there is a physical problem until it is fixed it would be better to not use multilink for the impact on other links

Hope to help


muhammadsafwan Tue, 10/14/2008 - 03:17

Hi Giuseppe,

That's a brilliant idea. I was thinking about that too..

For the physical part, i cannot interfere much since that's is their internal problem.

If one link failed or flapping, will ospf detect is as a lower cost link?

Giuseppe Larosa Tue, 10/14/2008 - 08:46

Hello Muhammad,

with every link as a standalone L3 link OSPF will see an OSPF adjacency fails on the not operating link and so it will not use it.

By using fast hellos the detection can be as fast as one second.

Actually in OSPF the lower cost is better (as for us when we try to buy something).

Here, OSPF will manage the 4 links as parallel paths to all destinations on other side and will use the currently working links.

Hope to help


Giuseppe Larosa Tue, 10/14/2008 - 20:47

Hello Muhammad,

actual load-balancing depends from the switching mode of router: with CEF enabled and normal flow based load balancing the rule is :

IP SA (last two bits ) EXOR IP DA (last two bits) EXOR hash

hash seed never changes until next router reload.

the result is an index from 00 to 11

You can determine the exact path a flow takes by using

sh ip cef exact-route

You could change this by enabling per packet load-sharing on the serial interfaces but this can be worse for VoIP for example because it increases jitter.

I would stay with flow based load balancing the four links usage is not perfectly the same but if there is enough number of different flows you are fine.

Hope to help


muhammadsafwan Tue, 10/14/2008 - 20:52

Hi Giuseppe?

How do we enable this load balance? Is it enable by default?

Is it by default per-destination load balance is enabled?

Giuseppe Larosa Tue, 10/14/2008 - 21:20

Hello Muhammad,

yes by default you can load balance on up to 4 parallel links using per-destination load balance you don't need to add anything just the OSPF config.

Hope to help


bagvanliju Tue, 10/14/2008 - 22:57

hai Giu,

Nice xplanation ....i ve got certain douts on load sharing ...which i was abut to put forward ...but ur xplanation cleared ....that ..thanks a lot ..i ave rated ...



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