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Cisco Bug: CSCvs83955 - Cisco 9800 control packets not honoring Mobility PMTU

Last Modified

Aug 29, 2020

Products (10)

  • Cisco IOS
  • Cisco Catalyst 9800-40 Wireless Controller
  • Cisco Embedded Wireless Controller on Catalyst 9115AX Access Points
  • Cisco Embedded Wireless Controller on Catalyst 9120AX Access Points
  • Cisco Catalyst 9800-L-C Wireless Controller
  • Cisco Embedded Wireless Controller on Catalyst 9117AX Access Points
  • Cisco Catalyst 9800-L-F Wireless Controller
  • Cisco Catalyst 9800-80 Wireless Controller
  • Cisco Catalyst 9800-CL Wireless Controller for Cloud
  • Cisco Embedded Wireless Controller on Catalyst 9130AX Access Points

Known Affected Releases

16.12.1s 16.12.2s

Description (partial)

C9800 control packets are not honoring Mobility PMTU.
Hence, when Foreign controller is sending some big mobility packets (bigger than discovered PMTU) to the Anchor controller, the packets might get dropped in the path if fragments are not allowed or if DF bit is honored (packets go out from Foreign with DF bit set, so if a hop in the path has lower MTU than this packet, the hop wont be able to fragment this packet and hence it will just drop it, affecting the mobility anchor export of the client)

- Two mobility C9800 Wireless Controllers, one acting as Foreign controller and another as Anchor controller.
- CWA is used for a WLAN, which requires some mobility packets to be big between Foreign and Anchor (for example, the Mobility Anchor export packets are big due to attributes like redirect URL, ACL, AP Policy Tag, and others... packet could be around 1500B)
- Controllers perform PMTU and discover the appropriate MTU, as confirmed by 'show wireless mobility summary'.
- Controllers have a WAN link or similar link in the path with MTU lower than what WLCs are using to transmit their mobility packets.
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