Routing Loops on Layer 3 Distribution Switch

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Jan 23rd, 2009
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We have two L3 distribution switches (SA26-LAN-R1 and SA26-LAN-R2) that connects to two 7206vxr (SA26-MAN-R1 and SA26-MAN-R2). From the two routers, I can trace to any destination prefix without problems. From SA26-LAN-R2, I can trace to any destiniation prefix without problems. But on SA26-LAN-R1, when I trace to the same destination prefix the packets just loops around a couple of times, then sends it off. On SA26-LAN-R2, if I run the "show ip route" it shows one route. But if I run the same command on SA26-LAN-R1 it shows four different routes.

Attached is a diagram of the network, SA26-LAN-R1 config, sample show IP route to, and outputs trace. Any assistance on why there's a routing loop would be greatly appreciated.

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mlund Fri, 01/23/2009 - 04:30
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I don't see it looping, it just loadbalance the traffic between parallell paths to the destination.

By default every trace step is done 3 times, and if equal cost paths exists, they all will be used. That's why different ip adresses shows up in the result.


Joseph W. Doherty Fri, 01/23/2009 - 05:25
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If it't what Mikael describes, if you use extend trace route, you can increase the number of packets and better see alternation between paths.

davidhuynh5 Fri, 01/23/2009 - 10:25
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Thanks. So how do I stop it from using multiple routes? I just want one route like SA26-LAN-R2.

Joseph W. Doherty Fri, 01/23/2009 - 11:13
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Assuming its equal cost OSPF or static routes (I haven't fully studied your configs), you would need different metrics/costs for the different paths. However, using different equal cost paths is something we often try to achieve, not avoid except for some special reason. Why don't you want to use multiple paths?

davidhuynh5 Sat, 01/24/2009 - 07:35
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One of the guy at work, who is a CCIE, stated that it will cause asymetrical routing, which is bad. I assumed that he knows what he's talking about since he's CCIE. Is the current load balancing on a per packet basis? I guess I'm trying to understand it better. Thanks

Joseph W. Doherty Sat, 01/24/2009 - 15:50
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Asymetrical routing could lead to unicast flooding, which is bad, but that's not a given; much depends on topology. You might ask your CCIE to explain why he believes asymetrical routing is bad.

Mohamed Sobair Sat, 01/24/2009 - 08:04
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The reason is that you have 4 equal-cost paths to the destind Network. which shown from the trace output and the (Sh IP route) command.

If required a single preferred path, then increasing the Cost of the undesired pathes would result in taking your preferred path.



davidhuynh5 Sat, 01/24/2009 - 10:28
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What does the * next to one of the routes mean? Is it the preferred path?

mangesh.kamble Sat, 01/24/2009 - 10:44
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Hi David,

The * according to me, means it is the best path for any next packet towards that destination. You can test the same by using your Trace results as in that also we are sending some packets.




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