OSPF Load Balancing

[leoandino]

Hi.
I have an L3 switch with ospf connected to two DNS servers. Each DNS server propagates an IP listener by OSPF.

I understand, that OSPF having routes of equal cost, should balance the load by default, but this is not happening. So, how do I configure the L3 Switch to perform load balancing of the same path in OSPF?

For the load balancing to work in OSPF, should it have a lot of traffic?

Switch Capture:

O E2 210.0.0.4 [110/20] via 30.0.0.2, 00:39:07, Vlan30
                        [110/20] via 20.0.0.3, 00:39:07, Vlan20

Topology:

 SWITCH L3--------->DNS server VLAN20

                   --------->DNS srver VLAN 30

Verifying the tests, from both DNS servers, only one server receives responses from one of the routes.

What I did was:

add "maximum-paths 2" in OSPF

Vlan20 and Vlan 30 interface  add the "ip load-sharing per-packet"

and  enable IP cef.

Thanks you!

[Mark Malone]

hmm i think the lb is only for when you have say 2 equal cost paths going to a single ospf switch in front then it will utilize both links , i dont think it will LB to a server like that maybe im wrong but ive never used it like that , you dont even need the maximum paths command to get ospf to lb as its default mechanism once paths are equal , if you trace to that server does it go by both hops in the output ?

you could try ip load-sharing per packet but that command can cause sensitive applications like voice to have issues

[Peter Paluch]

Hi Mark, Leo,

The ip load-sharing per-packet command is typically not even available on multilayer switches, so this is most likely not a feasible approach.

Back to the issue at hand, for ECMP, Cisco's CEF implementation takes both the source and destination IP address into account when selecting a particular path toward destination. You can test the path the packet would be forwarded through using the

show ip cef exact-route source-IP destination-IP

command. The source-IP and destination-IP are, obviously, the source and destination IP addresses of the packet being handled (they can be entirely fictious, though here, the destination is obviously 210.0.0.4). Note that for a particular [Source,Destination] pair, CEF will always select the same path. With the fixed destination 210.0.0.4, you would need to send packets from multiple sources to see that they are being load-shared across the multiple paths.

What I suggest is just testing various source addresses in the show ip cef exact-route command, and then trying to actually configure them and use them to see if the traffic gets finally spread to both servers as expected.

Best regards,
Peter

[leoandino]

Hi.

Thanks for your answer.

perform the test of the command, and it is shown that it divides into traffic:

SW1 # sh ip cef exact-route 60.60.60.10 210.0.0.4
60.60.60.10 -> 210.0.0.4: Vlan30 (next hop 30.0.0.2)
SW1 # sh ip cef exact-route 60.60.60.4 210.0.0.4
60.60.60.4 -> 210.0.0.4: Vlan20 (next hop 20.0.0.3)

But in practice it does not work. :(

Is it necessary for the OSPF ECMP that the routes to the same destination are in different vlan?