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Cisco Bug: CSCvv11079 - ISR 4K - EM7455: LTE module linux default MTU 1500 need to be changed to 2000

Last Modified

Sep 30, 2020

Products (1)

  • Cisco 3G Wireless WAN

Known Affected Releases

16.9

Description (partial)

Symptom:
ISR with 4G LTE SIM (cellular modem card) if when the negotiated n/w MTU is not honoured by ISP (for example; modem negotiation is at 1500 bytes, but Carrier allows > 1500 byte packet to reach modem (no fragmentation)) then the modem will lock up and not pass traffic.  To recover from this state; would need to reload the cellular modem card in the ISR.

Conditions:
Seen only in very specific environments where the carrier supports 2K MTU and is not honouring the ISR SIM MTU and also has the backhaul network with MTU >2K to allow packets to return greater than the MTU of the ISR SIM MTU.
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