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OSPF load balancing and distribution

hugh2_nguyen
Beginner
Beginner

Hi,

I have OSPF running on my Cisco 3825 router and I am looking for for a way to simultaneously transmit data using all three available routes instead of transmitting over only one route; i.e. the OSPF route.

Would you please give me inputs on a solution or how to configure it. Thanks.

Hugh

9 Replies 9

Reza Sharifi
Hall of Fame
Hall of Fame

Hi Hugh,

OSPF uses the metric a Cost, which is related to the bandwidth. If there are equal cost paths (the same bandwidth on both multilinks), OSPF installs both routes in the routing table.

See this link:

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml#

HTH

Reza

Reza, thanks for the advice and the link.

I'm wondering if there is way to do load balancing/distribution with unequal cost paths.

Hugh

Not with OSPF. EIGRP is able to do unequal cost load balancing.

Jon

Jon,

Thanks for the input. If Cisco does not support unequal cost load balancing for Cisco OSPF router, I am hoping there might be some third party solution out there.

Regards,

Hugh

I think the OSPF equal cost limitation might be part of the standard, not limited to Cisco. BTW, if multiple paths are almost identical, you can just cost them the same and then (equally) route across them.

Something that is specific to Cisco, which supports dynamic routing (including OSPF) load balancing (not just routing "static" load balancing) is Cisco's PfR with PIRO support. This requires a very recent IOS and Cisco platform that supports it.

Joe, thanks. Cisco doc states that PfR operates based on dynamic policy-based routing (PBR). I'm wondering how this will affect user configured QoS policies.

"Cisco doc states that PfR operates based on dynamic policy-based routing (PBR). I'm wondering how this will affect user configured QoS policies."

It can, but PBR is not a requirement. From what you've described, an OER/PfR configuration that only accounts for actual link load, and balances it (including for different bandwidth links), is all you might need. Both OER and PfR support many additional features, which might be useful or useless to you.

As to QoS policies, basic OER/PfR configuration just directs traffic across links, i.e. no need to adjust QoS policies. However, PfR can take into account QoS policies and make decisions on such criteria. For example, VoIP that's is encountering drops might need to be moved to another path while some backup experiencing drops doesn't have the same need. (These are some of the advanced features available within those technologies, but again, probably not necessary if your only interest is optimal link utilization.)

Mohamed Sobair
Rising star
Rising star

Hi,

If the paths has equal costs to the destination , of course having the same OSPF route type, then the load sharing is performed and I dont think you need OER / PFR.

Unlike IGP, OER/PFR distribute trafiic based on characterstics other than Normal IGP Cost like packet loss and delay of the links.

with PFR, its also valid to load share, but you need to know that it distributes the traffic based on other criterias.

HTH

Mohamed

"If the paths has equal costs to the destination , of course having the same OSPF route type, then the load sharing is performed and I dont think you need OER / PFR. "

The question that was posed, where I suggested OER/PfR, was in reference to unequal cost links while using OSPF. Even if you do have routing load sharing, whether equal or unequal cost, neigher takes into account actual link load, as can OER/PfR. If the links carry many concurrent flows, routing load sharing usually is "good enough". If there are few flows, especially if one is bandwidth heavy and link capacity filled by it, OER/PfR can direct other flows to not saturated link, resulting in a better load utilization than provided just by routing load sharing.

"Unlike IGP, OER/PFR distribute trafiic based on characterstics other than Normal IGP Cost like packet loss and delay of the links.

with PFR, its also valid to load share, but you need to know that it distributes the traffic based on other criterias."

Yes, they can, but much depends on the configuration. I believe much is optional except perhaps if OER/PfR determines destination is unreachable across a path.

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