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Community Member

Load Balancing


We have a Remote location connected with Primary Leased line(Point to Point Link)and also a ISDN backup link for primary. Both the links are configured in OSPF network. Now my problem is whenever I making ISDN link up even though Primary link is up for load sharing purpose, load sharing is not happening. I verified the cost of the both links and found the same. And both are in the same Area. Can somebody help me out??????


Re: Load Balancing

Cost of the links might be the same, but it's cost of the complete path that is taken into account when determining which path to take. What does 'sh ip route ' say?


Re: Load Balancing

The first issue is : Is the ISDN call active when you want to use load-balancing ?

Moreover, ISDN is quite slow and costly, why you require load-balancing between a LL and ISDN ?

Can you provide the "sh ip route" & "sh ip ospf data" ?

If you find there are two routs for the same destination in routing table, it should work.

If the ospf cost are the same, the load-balancing should be active.

And, how do you classify it is not load-balancing ? How many hosts at the remote side ? If it is per-destination load-balancing, same destination will fall into the same path.

Hope this helps.

Re: Load Balancing

Hi Krishna,

I guess when you say Load Sharing, you mean to say during the times of congestion, you want to take the extra load using the ISDN BRI.

If I'm correct, then I guess first you should check is the SRTT on both the links individually and if the difference between them is too high, then its really difficult to achieve what you are trying to achieve.

As if a data stream is divided across the links there would be increased jitter on the stream and OSPF will not use the same for load balancing (or load sharing).

Kind Regards,

Wilson Samuel

Hall of Fame Super Gold

Re: Load Balancing


I think that it would help if we knew some details of your environment. Is the ISDN configured as OSPF demand circuit or some other way? Does the leased line configure the ISDN as backup interface? To clarify can you provide the config?

Also I assume that the leased line and the ISDN originate on the same router. Do they also terminate on a single router or does leased line terminate on one and ISDN on another? Can you clarify this?



Community Member

Re: Load Balancing

Hi rick,

ISDN is configured as Point to Multipoint network. And it is used for backup for leased line.And also both are terminated in the same router. Actaully we want to use this ISDN for load-sharing purpose whenever leased line bandwidth is fully loaded.


Community Member

Re: Load Balancing

Hi Samuel,

yes, we awant to use ISDN at time time of Leased line congestion.

Wht is SRTT???



Re: Load Balancing

SRTT = A calculated smoothed round-trip timeout.

Check here :

Could you please provide the config. and the bandwidth of the LL ?

You are looking for something like bandwidth on-demand routing. As I know, only the multilink PPP can do it by triggered by the link load and setup another channel. In this case, I believe it can't do it.

You may need to manual redirect some traffic to the ISDN, e.g. policy-based routing or setup static route for the traffic to pass through ISDN.

Hope this helps.

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