How do applications know there's asynchronous routing, why do they care?

[cscbrannent]

We made did a campus LAN upgrade recently, during which we migrated from a mixed EIGRP/OSPF IGP to a multi-area OSPF redistributing to/from BGP on our WAN. We have two entry points on different sides of the campus with two different AS' numbers, so not running iBGP. We accidentally introduced asynchronous routing to our WAN, which we bandaided with static routes. We're working on changing the routing - using the same AS and running iBGP between the two routers to the WAN, sending community strings changing the provider's local pref, etc.

Question is: Why did the asynchronous routing - going out router1, coming back router2 - why did it matter to the applications? How do the applications know there's asynchronous routing? What immediately broke for us was XEN desktops, and Microsoft Remote Desktop. I can understand how the network would be able to tell with something like URPF, but why does it matter to applications and how do they know?

[Richard Burts]

You have not given us enough detail about your network and about the problem for us to understand the issue and give you reasonable explanation.

I would say in general that applications do not know and do not care about asymmetric routing. They care about whether they can send and receive their application data.

I would guess that either some device was doing stateful inspection and rejected traffic when it did not arrive on the expected interface. Or that your network is doing address translation and if the response does arrive on the interface through which it left that the translation could not work.

If you want better answers from us you need to give us more information to work with.

HTH

Rick

Sent from Cisco Technical Support iPhone App

[Joseph W. Doherty]

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Posting

Like Rick, end hosts shouldn't normally care what's the path taken by their traffic, and path used is generally "invisible" to the end host unless it enables a "debugging" option like recording the route.

As to why async routing would break your transfers, well Rick lists a good one, some device using flow state analysis that expects to see traffic in both directions.  I've also seen other issues such as packets being delivered out of sequenced or lost on a different path that can cause problems.

[cscbrannent]

Sorry I didn't get back to this post - there's no stateful devices.  each Lan side source and destination are layer3 switches with normal 3800 routers connecting to WAN.  OSPF on the inside, BGP to PE MPLS.  Reverse on the other side - MPLS PE-BGP-OSPF Layer3 switching.

No configured NAT, RPF, or Stateful inspection.

Some apps broke, some did not.  Notable apps which broke are Xen desktop and Microsoft RDP.