[j-nsp] Junos Load Balancing Behavior

[Tim Eberhard]

Srx's, assuming you're running in flow mode will not load balance as of today. The forwarding table will show two routes, but it will only pick one.

This has been discussed here previously, a quick google search of ECMP and SRX should help. 

Good luck, sorry to give you the bad news..
Tim Eberhard

On Feb 2, 2012, at 11:01 AM, Devin Kennedy <devinkennedy415 at hotmail.com> wrote:

> Hello:
> 
> 
> 
> I'm looking for some insight on the load balancing behavior that Junos uses
> by default.  We are certifying our Junos platform CE routers (SRX, MX10,
> M7i) and not seeing what we expected given the documentation we have.  
> 
> 
> 
> According to the Juniper docs and the old JNCIP study guide, OSPF will
> automatically load balance if there are two equal cost routes.  And indeed
> in the routing table we have default route advertised via OSPF to a CE
> router which shows two next hops (one to each of two PE's).  
> 
> 
> 
> juniper at SRX240-5> show route 0/0 exact 
> 
> 
> 
> inet.0: 23 destinations, 23 routes (23 active, 0 holddown, 0 hidden)
> 
> + = Active Route, - = Last Active, * = Both
> 
> 
> 
> 0.0.0.0/0          *[OSPF/150] 20:45:21, metric 112, tag 13979
> 
>                      to 10.7.122.1 via ge-0/0/6.0
> 
>> to 10.7.122.2 via ge-0/0/6.0
> 
> 
> 
> However in the forwarding table there is only one next-hop shown and when
> testing traffic flows we don't see any load balancing by default.  
> 
> 
> 
> juniper at SRX240-5> show route forwarding-table destination 0/0    
> 
> Routing table: default.inet
> 
> Internet:
> 
> Destination        Type RtRef Next hop           Type Index NhRef Netif
> 
> default            user     0                    ulst 262142     2
> 
>                              80:71:1f:c0:3c:81  ucst   584     4 ge-0/0/6.0
> 
> default            perm     0                    rjct    36     4
> 
> 0.0.0.0/32         perm     0                    dscd    34     2
> 
> 
> 
> Routing table: __master.anon__.inet
> 
> Internet:
> 
> Destination        Type RtRef Next hop           Type Index NhRef Netif
> 
> default            perm     0                    rjct   517     1
> 
> 0.0.0.0/32         perm     0                    dscd   515     1
> 
> 
> 
> Everything goes across the one next hop only (the one with the > in front of
> it).  We have to add an export policy to the routing-options
> forwarding-table stanza to get it to work.  
> 
> 
> 
> This is from the Junos documentation for OSPF for version 10.4:
> 
> 
> 
> "When several equal-cost routes to a destination exist, traffic is
> distributed equally among them."  
> 
> 
> 
> http://www.juniper.net/techpubs/en_US/junos10.4/topics/concept/ospf-routing-
> overview.html
> 
> 
> 
> Shouldn't the load balancing work by default as the documentation would lead
> one to believe?  Does anyone have any insight into this?  Is the
> documentation incorrect and you actually are required to always add a
> load-balancing export policy in order to get the desired load-balancing
> behavior?
> 
> 
> 
> 
> 
> 
> 
> Best Regards,
> 
> 
> 
> Devin J Kennedy
> 
> Juniper Engineer - AT&T Labs
> 
> 
> 
> 
> 
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