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This Question is Answered 2 Helpful Answers available (2 pts)
5564 Views 4 Replies Latest reply: Sep 6, 2013 1:50 PM by Jose Dominguez RSS

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ip virtual-reassembly

Feb 18, 2011 8:42 AM

Subramaniya Karthic 184 posts since
Aug 3, 2010

In the documentation of  virtual-reassembly It was mentioned that the command should not be used for asymmetric paths..

I couldnot understand why would this make a difference in asymmetric paths.. Can someone come up with an explanation for the same

 

the link  that i mentioned is :

http://www.cisco.com/en/US/partner/docs/ios/sec_data_plane/configuration/guide/sec_virt_fr
ag_reassm_ps10591_TSD_Products_Configuration_Guide_Chapter.html

 

 

VFR Configuration Restriction

VFR should not be enabled on a router that is placed on an asymmetric path. The reassembly

process requires all of the fragments within an IP datagram. Routers placed in the

asymmetric path may not receive all of the fragments, so the fragment reassembly will

fail.

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    1. Feb 18, 2011 9:09 AM (in response to Subramaniya Karthic)
    Re: ip virtual-reassembly

    Great question.

     

    With ip virtual-reassembly, each IP datagram is associated with a managed timer. If a device does not receive all of the fragments within the specified time (which can be configured via the timeout seconds option), the timer will expire and the IP datagram (and all of its fragments) will be dropped.

     

    In an asymmetric network, some packets go one way, while other packets go a different way, and a single device would not see all the fragments, and the above timer would expire, causing a failure.

     

    Keith

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    3. Feb 19, 2011 12:26 AM (in response to Subramaniya Karthic)
    Re: ip virtual-reassembly

    Those are great questions!

     

    I would consider an asymmetric path to have some traffic between point A and B go 1 path, and some traffic between the same two points use a different path.    A transit router in ether one of these paths would only see some of the traffic, but not all of it, which may cause the problem with virtual reassembly.

     

    If our network was asymmetric in that all the traffic went outbound 1 path, and all the return traffic came back on a different path, I don't think that would cause a problem for virtual reassembly, but it could cause a problem with a stateful firewall who is tracking sessions.

     

    It seems pretty easy to get the terms confused, and I bet alot of the confusion is based on the context of what device or devices we are have in the path(s).  I would probably ask a few additional questions if the customer said they had asymmetric anything, just to clarify.

     

    Best wishes,

     

    Keith

  • Jose Dominguez 1 posts since
    Aug 18, 2010

    Follow up question: Does the assymetric network restriction apply even if the assymetry is based on two interfaces on the same router?

     

    Thx,

     

    José.

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