OSPF Load balancing over unequal cost paths

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Feb 12th, 2009
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From our HQ we have to links to our datacentre located remotley. One is a direct 100mb link and the other is a 1000mb link connection to a another site which then has a 1000mb link to the datacentre.


OSPF is routing traffic over the two 1000mb connections as this is the 'best' route even though the 100mb link is a direct connection.


This means the 100mb link is idle until we have a link failure elsewhere.


Can OSPF be reconfigured so that all links are used? i.e some traffic via the 1000mb links and some via the 100mb link? In affect, unequal cost routing?


Is this an OSPF feature or would we need to use EIGRP as our routing protocol?


One other consideration is we run a fully routed network with only a small amount of layer 2 traffic at the datacentre local site.

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paolo bevilacqua Thu, 02/12/2009 - 03:50
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No, OSPF does not support routing over unequal costs paths.

You will find that in practice, one 1gb indirect link is much better than one 100mbps direct.

You will also find that the unequal cost feature of EIGRP has few if any users in real world, and can present issues that never appear with OSPF.

Joseph W. Doherty Thu, 02/12/2009 - 04:29
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"Can OSPF be reconfigured so that all links are used? i.e some traffic via the 1000mb links and some via the 100mb link? In affect, unequal cost routing?"


Yes and no.


You can configure link cost such that OSPF will see both links as equal, then OSPF will attempt a 50/50 flow split. (If running TCP, you should see higher bandwidth utilization peeks on the gig paths, but overall utilization should average 50/50.)


Or, you could configure multiple tunnels over the gig path, and route between them and the 100 Mbps path. You'll likely run out of maximum paths (6?) and still not achieve 10:1 ratio. If maximum paths is 6, should obtain 5:1 split.


Or, depending on your equipment, you might be able to run OER/PfR and have it dynamically load balance across the two paths in proportion to their bandwidths. Neither OER/PfR directly (yet) support OSPF (or EIGRP), but they can inject statics into OSPF.


"Is this an OSPF feature or would we need to use EIGRP as our routing protocol? "


OSPF, no, EIGRP yes, atlhough don't disregard Paolo's warning about it being uncommon configuration with EIGRP. From past posts on the subject, I recall the concern is whether EIGRP can do it well especially at high speed.


With such a bandwidth difference, I do agree with Paolo to just use the gig path and retain 100 Mbps path for failure. Yes, it seems wasteful to leave the 100 Mbps path idle, but it only adds 10% to your total bandwidth, and unless the gig path is very busy (and if it were you would want more than 100 Mbps) using it will likely only slow traffic on that path.

Richard Burts Thu, 02/12/2009 - 04:35
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David


An alternative that you might consider would be to leave the routing over the links as it is and let OSPF prefer the faster indirect path and use the direct path as a backup and then to configure Policy Based Routing so that certain types of traffic (perhaps Email, perhaps web traffic, perhaps something else that makes sense in your environment) are sent over the direct path. I have implemented this at customer sites and it works fairly well.


HTH


Rick

Joseph W. Doherty Thu, 02/12/2009 - 04:42
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As excellant suggestion especially if there's notable latency difference betweent the two paths!


BTW: PfR can effectively do dynamic PBR and adjust traffic by class and realized performance, not just link load.

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