CEF session flow

Answered Question
Jun 9th, 2008

Hi all,

Is CEF per-destination really session-based?

I attached a diagram and I have a question about the flow of network traffic.

If someone initiates a connection to a system from a site in the MPLS cloud destined to a CRD server on the 6509s, would the traffic be routed back into the MPLS cloud (router) in the same direction that it came in from?

Or does it risk asymmetrical routing? Is that bad , if it does?

FYI - The connections with the 'delay' object are back-up routes.

Thanks for the help.

-MIke

Correct Answer by n.nandrekar about 8 years 8 months ago

Hi!

The return path that the traffic will take totally depends on the routing information present. The return traffic can take a different path. CEF does nothing but install the routing information in the hardware. It doesn't have any intelligence of its own.

If both source and destination are in eigrp/ospf domain, it is quiet likely that the same path will be chosen as best path for both the source and destination. But in case BGP is involved, there are many attributes that can influnce the path/return path selection. The LSPs of MPLS too are unidirectional. So the return traffic will be through a different LSP and that can take a different path.

Asymetric routing can cause different delays in different directions. If you consider the paths for delay-sensitive traffic during the design, it shouldnt cause any problems. Asymetric routing can rather help in utilizing the redundant connections better and can be used to an advantage.

Regards,

Niranjan

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Correct Answer
n.nandrekar Mon, 06/09/2008 - 20:18

Hi!

The return path that the traffic will take totally depends on the routing information present. The return traffic can take a different path. CEF does nothing but install the routing information in the hardware. It doesn't have any intelligence of its own.

If both source and destination are in eigrp/ospf domain, it is quiet likely that the same path will be chosen as best path for both the source and destination. But in case BGP is involved, there are many attributes that can influnce the path/return path selection. The LSPs of MPLS too are unidirectional. So the return traffic will be through a different LSP and that can take a different path.

Asymetric routing can cause different delays in different directions. If you consider the paths for delay-sensitive traffic during the design, it shouldnt cause any problems. Asymetric routing can rather help in utilizing the redundant connections better and can be used to an advantage.

Regards,

Niranjan

msrohman Tue, 06/10/2008 - 06:45

Thanks for the info, Niranjan.

The MPLS connections have the same latency & bandwidth for the circuits.

The routes in the 4506 switch are equal-cost.

In you opinion, how would you go about attaching WAN/MPLS connectivity to a data-center. My company is big on resiliency. I'd rather have an active/active and utilize both circuits.

Thanks again!

n.nandrekar Tue, 06/10/2008 - 09:55

Hi!

I really dont think I would be the right person to answer this question of yours. It would require greater knowledge of the topology, requirements, devices etc. Also might depend on what kind of mpls connectivity, supported features on the release you are running. There are pretty many things that you can deploy including pseudowire-redundancy, fast-reroute etc. in mpls, as well as manipulating the prefered paths using bgp attributes. Any decision should be worked from the exact requirements, design and available hardware perspective. Also I really dont have enough design/deployment experience to comment on this.

Regards,

Niranjan

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