simple load-balance question

Unanswered Question
Aug 5th, 2009

how do we understand if routers are load balancing the data transmition?

Configuration:

there are 2 routers (A and B)at the center and 2 routers (C and D) are at the remote site.

A is connected to C (E1)

B is connected to D (E1)

OSPF is the routing protocol.

i know OSPF do load-balance as default but can i see it? how do i make sure these guys are load-balancing?

thanks for helping :)

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Jon Marshall Wed, 08/05/2009 - 10:55

Easiest way i know is to use traceroute which shows you the hops along the way so you can see if it uses one hop then the other.

Jon

Joseph W. Doherty Wed, 08/05/2009 - 10:55

Perhaps a simple question, but also perhaps not a simple answer. Yes, OSPF, by default, does equal cost load balancing, but we don't know how traffic gets to your routers, or OSPF topology costing. For instance, assuming your two (E1) WAN links are costed the same, traffic that starts on router A likely sees its (WAN) connection to router C as a lessor cost path then also using router B and would not use that path. However, if there were another router before router A and B, or links were costed from router A to make both WAN paths alike, both paths would be used. (A show IP route would show multiple paths for a destination if it see OSPF equal cost paths.)

[edit]

Ah, Jon provides the simple answer! (You would need, however, to insure the traceroute generates sufficient probes to hit muliples paths - most default to 3.)

ohalnet53 Wed, 08/05/2009 - 21:42

hi joseph :)

thanks for replying

2 routers at the center are connected to each other via a multilayer switch, OSPF is running on that switch too.

for router A and B (routers at the center); output of #show IP route , shows both paths.

in this case can i be sure it load-balances the data ?

Joseph W. Doherty Thu, 08/06/2009 - 03:12

"in this case can i be sure it load-balances the data ?"

It should, if the L3 switch can't be bypassed. I.e., you don't have hosts on the same subnet as shared between the L3 switch and the WAN routers.

If you run traceroute from the L3 switch, as Jon suggested, you should see it trace both paths.

Lastly, if you note packet counts along both paths, over a day or so, they'll likely be somewhat even.

Jon Marshall Thu, 08/06/2009 - 03:15

"Ah, Jon provides the simple answer!"

You can always rely on me for the simple things :-)

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