SVI on 3750 won't load balance

[klchua]

Hi,

I have the following configuration. 

The L3 Switch (3750) is directly connected to 2 routers.  The 2 ports on the switch towards the router are in the same Vlan.  We have 2 static routes in the L3 switch pointing to both Routers.

I expected the switch to load balance multiple flows across the 2 links, but it seems that only 1 link will be used.  

From sh ip cef exact route, the result is load balancing.  But when traffic actually flows, and I have hundreds, they all go via one interface.

Converting the 2 ports on the switch to separate Routed ports then yielded the expected load balancing behaviour.

Regards,
KL

[Joseph W. Doherty]

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Posting

Could you expand upon the physical topology before you defined the links as routed?  Two links on same VLAN that can reach the same destination sounds like a L2 loop, could STP been blocking one of the ports?  (CEF routing probably showed both destinations reachable by the VLAN.)

[Jon Marshall]

KL

The only thing that springs to mind is ICMP redirects although that depends on the routers. Were the routers exchanging routes between themselves over the shared vlan ie. the vlan that the 2 routers and the 3750 switch were connected to ?

If so then the router that is not used may well be sending an ICMP redirect back to the 3750 to use the other router. An ICMP redirect would not be sent when the routers are not on the same subnet which is the case when you use routed ports.

Obviously the way to test that would be to disable ICMP redirects on the SVI interface and see what happens.

Joseph - i don't think it is a L2 loop because the routers won't be running STP so there are no loops as such.

Jon

[Joseph W. Doherty]

Disclaimer

The    Author of this posting offers the information contained within this    posting without consideration and with the reader's understanding that    there's no implied or expressed suitability or fitness for any  purpose.   Information provided is for informational purposes only and  should not   be construed as rendering professional advice of any kind.  Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In    no event shall Author be liable for any damages whatsoever  (including,   without limitation, damages for loss of use, data or  profit) arising  out  of the use or inability to use the posting's  information even if  Author  has been advised of the possibility of such  damage.

Posting

Joseph - i don't think it is a L2 loop because the routers won't be running STP so there are no loops as such.
Jon

Jon, agreed if the routers are directly connected, which would be implied by changing them to routed links.

[edit]

Re-reading original post, configuration is described as routers are directly connected, your statics are not directed at mHSRP IPs that are active on the same router?

[smogra]

Hi,

Issue the show etherchannel load-balance command in order to check the frame distribution policy.

You can determine which interface in the  EtherChannel forwards traffic, with the frame distribution policy as a  basis. Issue the remote login switch command to log in remotely to the Switch Processor (SP) console in order to make this determination. Then, issue the  test etherchannel load-balance interface port-channel number {ip | l4port | mac} [source_ip_add | source_mac_add | source_l4_port] [dest_ip_add | dest_mac_add | dest_l4_port]

command.

This command will tell you which interface is selected from etherchannel when traffic flows.

If same interface is selected all the time, try changing the districution policy. This should work.

Issue the port-channel load-balance {src-mac |  dst-mac | src-dst-mac | src-ip | dst-ip | src-dst-ip | src-port |  dst-port | src-dst-port | mpls} global configuration command in order to configure the load balancing.

Cheers

Sweta

Please rate the answer if it was useful.

[Jon Marshall]

What etherchannel ??