OSPF load balancing problem

[Manjunath MR]

Hi,

Our physical connection is as below. 3 GE links are connected from L3 Switch-A to L3 Switch-B. OSPF is running B/W SW-A and SW-B.

SW-A  ---------3GE links-------SW-B

Maximum paths 3 is configured to utilize all 3 links(As LACP is not supported here for link aggregation).

Out of 3 links, 1 link is utilizing 100% and another 2 links utilization is 60% each.

if we use 4 links it works fine.all links will carry equal traffic. but not with 3 links.

OSPF Doesn't support equal load balance with 3 links??? or any other configurations need to be done??

Thanks

[Mark Malone]

Hi

yes It should it support up to 4 links and more on newer ios versions , once the links are of equal cost and metrics to the same dest match

does the show ip route x.x.x.x ......................for each route match equally

do the routes appear in the route table as equal ?

[Manjunath MR]

Hi,

Yes, show ip route x.x.x.x  shows 3 routes for each route.

yes the routes appear in the route table as equal.

[Mark Malone]

if the costs and route type are the same and routes are in the table  it should be choosing to LB , is there any diff between the routes when its working for the 4 links and not the 3 , does the LSDB show any diff for the routes that are to part of the LB

If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.

[Julio E. Moisa]

Hi, Just a question why LaCP is not support?, by design or the IOS is not supported? what about PaGP?

[Julio E. Moisa]

By default the load balacing is made per destination, this a default behavior, you could make load balancing per packet but it could generate lose of packet sequence. Take in consideration that.

Please check this link:

http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/pplb.html

As Joseph mentioned you could check the IP Cef

Show ip cef <destination>

or 

Show ip cef exact-route <source IP> <destination IP>

To verify your paths.

[Masoud Pourshabanian]

Hello,

Are there only one source and one destination(subnet) advertised by OSPF? If you can break the advertised IP addresses, there will be more routes in the routing tables so you may get better load sharing.

Masoud

[rikshit4aggarwal]

OSPF looks at bandwidth of Outgoing interfaces toward destination. Do all 3 paths have equal BW??

Regards,

Rikshit

[Manjunath MR]

YES, all 3 paths have equal BW.

[Joseph W. Doherty]

You might just be seeing a "luck of the draw" situation.

If you're using CEF, I believe its deterministic, i.e. a flow with specific attributes will be mapped to the same outbound link.  If such, it just might be your traffic distributes better across 4 links than 3 links.

I recall some CEF implementations have a configuration option to impact how CEF chooses a link but unable to find a reference.

[Joseph W. Doherty]

Ah, I found some additional information.  You might find this TechNote of some interest:

http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/116376-technote-cef-00.html