Default Routing

[Tushar Gawai]

What's the selection process between two default routes?

Which path User will take to reach Server? ISP1 or ISP2?  &   WHY ?

 

[Georg Pauwen · Accepted solution]

Hello,

with two default routes, load balancing will happen. That means, based on the source and destination IP, the first flow will take one default route, the next flow the other default route, and so on...

[Tushar Gawai]

Thought so.  Can you explain, what do you mean " based on the source & destination IP " ?

[Georg Pauwen]

Hello,

actually, I think the default is per destination load sharing. You can test this by doing a traceroute to two different destinations, it should alternate the paths it takes.

If for some reason, you need to change the default (per destination) to per packet (which is not recommended), you need to enable that on the interfaces:

--> ip load-sharing per-packet

[Tushar Gawai]

Hello Georg,

In this case, there is only one source & destination. If left it to default, CEF will forward traffic per flow. That means user will take path via either ISP1 or ISP2.  

Lets say user takes ISP1 for first time until the session gets terminated. Will it take ISP2 the next time? 

Is there anything to do with outgoing  higher IP or higher port ?

[Georg Pauwen]

Hello,

good question. I did some testing, and IP addressing of the interfaces does not seem to matter, just the destination.

If you want to check which path a session between a source and destination address is assigned, use the:

show ip cef exact-route source-ip-address destination-ip-address

command.