Load balancing in OSPF with ISDN

[bapatsubodh]

Router A and Router B are having a leased line connection between them. In addition to this there is a ISDN BRI connectivity between them. Both routers are given loopback 0. addresses. OSPF is running on both the routers in which directly connected networks and loopback address are published using network command. Static connected routes are redistributed. It is required to use ISDN BRI connectivity to share the traffic load between Router A and Router B. to avoid the congestion of traffic on leased line between Router A and B. Can it be configured to form a parallel path to working leased line ? Can it form OSPF neighbor relationship ?If yes how it can be tested that load sharing is taking place ?

Thanx in advance.

Any similar link on Csico.com is

appreciable

Subodh

[ariela · Accepted solution]

Hi Subodh,

OSPF needs equal cost for load balancing.

Cost is calculated with this formula:

108 / bandwidth (in bits per second [bps])

See that:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t6

change ISDN path cost like leased line.

Check 'show ip osfp interface':

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094056.shtml

See the example:

Router1# show ip ospf interface ethernet 0

Ethernet0 is up, line protocol is up

Internet Address 10.10.10.1/24, Area 0

Process ID 1, Router ID 192.168.45.1, Network Type BROADCAST, Cost: 10

HTH

Andrea

[Richard Burts]

I would offer a note of caution about this. Andrea is right that OSPF needs equal cost paths to do load balancing or load sharing. If you manipulate the cost of the leased line and/or the ISDN to make them equal cost then OSPF should put both paths into the routing table and the router should use both paths to forward traffic. This may or may not turn out to be a good thing. Consider that if there are two paths in the routing table from OSPF the router will attempt to share traffic on both links equally. The possibility exists that you will constrain the throughput of the leased line to match the throughput of the ISDN.

HTH

Rick

[bapatsubodh]

Hi

Mr. Andera ,

Thanx a lot it help us lots to solve the problem.

I have confugured one channel from PRI line, gave the cost same as that of interface we were trying to back up. Assigned the loopback address. And it is working FINE.

Similarly is it possible to use a redundant leased circuit, this circuit has a path to the destination via another location. In case of original link fails this path is taken by packets. Can I use this path to load balance with the original path. Problem is this path is of same bandwidth as that of original path.( so cost is same ) In this case how do I configure OSPF to divert some traffic to this redundant path.

Thanx in advance.

Subodh