Asymmetric routing

[mirzaakberali]

Hi Gentleman,

I am struggling to understand what is Asymmetric routing and scenarios in which it occurs.

Any post on this will be appreciated

[Collin Clark]

Asymmetric routing is when a packet returns on a patch that is different from a path that the traffic was sent. This can be seen in normal situations when there are multiple paths to/from a destination. It can also be seen in misconfiguration situations such as a server having two NIC's for load balancing and it's instead routing between them.

Hope that helps.

[Istvan_Rabai]

Hi Ali,

One addition to the previous post:

Asymmetric routing (traffic going one path and coming back another path) would cause problems if e.g there are stateful firewalls inspecting your traffic:

Traffic going out from your autonomous system will trigger a stateful firewall to create the respective entries in its database, but traffic coming back may try to traverse another firewall where the respective entries are not created and therefore these packets would be discarded.

Another issue with asymmetric routing is troubleshooting. It's not easy to troubleshoot traffic that is taking different paths in different directions.

One more issue with asymmetric routing is about designing load balancing and QoS.

It's a hard thing to plan what bandwidths you should purchase from your provider(s) and what QoS policies to put in place if you cannot predict your traffic behavior.

All this can cost you some additional money, packet delays or packet losses in certain situations.

I know this is very general, but your question is also somewhat general.

Cheers:

Istvan

[Fabio Semperboni]

Hi,

I have written an article to explain how asymmetric routing works and how to eliminate it in some situations.

http://www.ciscozine.com/2012/04/15/unicast-flooding-due-to-asymmetric-routing/

Cheers