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Cisco Bug: CSCum63417 - ASA should not allow interface MTU config greater than 9202/9198

Last Modified

Apr 16, 2020

Products (1)

  • Cisco ASA 5500-X Series Firewalls

Known Affected Releases

9.1(2)

Description (partial)

Symptom:
The ASA allows an interface MTU configuration of up to 65535:

ciscoasa(config)# mtu outside ?

configure mode commands/options:
  <64-65535>  MTU bytes

However, per the ASA command reference, the maximum allowed size of a jumbo frame is 9216:

http://www.cisco.com/en/US/docs/security/asa/command-reference/jk.html#wp1633967

"A jumbo frame is an Ethernet packet larger than the standard maximum of 1518 bytes (including Layer 2 header and FCS), up to 9216 bytes."

As a result, if a user sets the MTU to something higher than 9202 on a physical interface or 9198 on a sub-interface, packets are dropped by the ASA when the L2 header (and 802.1q tag if applicable) is added.

Conditions:
These drops occur if the MTU is set to a value greater than 9202 on a physical interface or 9198 on a sub-interface.
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