route selection in OSPF

[Shiv]

Hi all, I got an issue with route selection in OSPF. If I have 5 intra-area routes all of equal cost, what that JUNOS does...I have ECMP enabled through a policy under routing-options..I expect it to do a load-balancing per-flow..Am I right?

Cheers

[JuniperEngineer]

In Junos the OSPF load balancing is per destination by default.

You can change it to per flow by using the following configuration (NOTE: per-packet == per-flow)

routing-options {      

forwarding-table {          
export load-balancing-policy;      }  }  

policy-options {      

policy-statement load-balancing-policy {          
then {              
load-balance per-packet;          }   }  }

 

I hope this helps 

[Thai]

Traffic pattern will vary depending on the ASIC.  Internet Processor I ASIC is truely per packet while II's are per flow.

http://www.juniper.net/techpubs/software/junos/junos84/swconfig84-routing/id-10480125.html#id-10480125

[GuyDavies]

Hi Shiv,

It also depends upon the spread of the sources/destinations.  The actual hash algorithms used to decide which of the equal cost paths to use are proprietary and not published.  However, I can say that if you have a very small cluster of sources and a very small cluster of destinations, there is a possibility that the balancing will not be very... well... balanced.  With a reasonable spread of src/dst IP addresses, you'll get a fair (but never perfect) balance. 

Rgds,

Guy