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Beginner

forwarding by flow? (vs packet)

Can layer 3 switches/routers make forwarding decisions on a flow by flow basis as opposed to packet by packet?  For example if I have two cross-connected dist switches and two cross-connected core switches with "full mesh" layer 3 connectivity between them (using something like OSPF) I want to avoid asymmetric routing.

By "full mesh" I mean:

given Dist sw/rtr A and B (cross connected)

and Core sw/rtr A and B (cross connected)

Dist A has one connection to Core A and one to Core B

Dist B has one connection to Core A and one to Core B

In the following diagram the two devices on the top are Core and the two on bottom are Dist (or vice versa).

4connectmesh.jpg

3 REPLIES 3
Frequent Contributor

forwarding by flow? (vs packet)

Actually correct me if I am wrong but STP will block some paths and that should take care of that too.

Beginner

forwarding by flow? (vs packet)

these are layer 3 connections, so no STP

VIP Expert

forwarding by flow? (vs packet)

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Posting

I believe Cisco L3 switches, will by default, route per flow.  (Also true for true Cisco software routers running later IOS versions.)

As to avoid asymmetrical routing, you would need to cost routes such that a particular destination would also take the same path.  As, again, Cisco L3 devices generally route all packets of the same flow along the same path (in one direction), multipath isn't normally an issue except for "stateful" packet monitoring watching for returned packets.

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