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Cisco Bug: CSCus33977 - PMTUD uses stale interface information for MTU

Last Modified

Jul 21, 2018

Products (12)

  • Cisco Carrier Routing System
  • Cisco ASR 9922 Router
  • Cisco IOS XR Software
  • Cisco ASR 9010 Router
  • Cisco XR 12410 Router
  • Cisco ASR 9904 Router
  • Cisco XR 12404 Router
  • Cisco ASR 9006 Router
  • Cisco XR 12416 Router
  • Cisco ASR 9001 Router
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Known Affected Releases

4.2.1.BASE 5.3.2.BASE

Description (partial)

Symptom:On an IOS-XR router running 4.2.1 with path mtu discovery, if the source interface to reach the destination address changes then tcp continues to query for the MTU using the interface handle of the wrong interface. 

In the case in which the previous used interface has just been shutdown, it will query for the MTU of the wrong interface and continue to use it until the TCP session is reset. If the interface has the ipv4 or ipv6 addresses removed by removing the configuration, then the query for the MTU on the interface will fail. 

When the MTU query fails, path mtu discovery will use an MTU and MSS of 1240 as a default.  This results in the a subobtimal MSS being used on the TCP session and the MSS not getting increased base on the new and actual interface and path used to reach the destination.

Conditions:This issue is seen with established TCP in which there are multiple routes to the same destination and the route and source interface changes between interfaces while the tcp session remains established.
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