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Cisco Bug: CSCup60557 - F2 / F2e does not send ICMP unreachable for ip length 1501-1516 bytes

Last Modified

Feb 07, 2019

Products (8)

  • Cisco Nexus 7000 Series Switches
  • Cisco Nexus 7000 10-Slot Switch
  • Cisco Nexus 7000 4-Slot Switch
  • Cisco Nexus 7700 6-Slot Switch
  • Cisco Nexus 7700 18-Slot Switch
  • Cisco Nexus 7000 18-Slot Switch
  • Cisco Nexus 7000 9-Slot Switch
  • Cisco Nexus 7700 10-Slot Switch

Known Affected Releases

6.2(6a) 6.2(8) 7.1(0)D1(0.152)

Description (partial)

Routing between interfaces with F2 or F2e modules where the ingress interface has an MTU of 9216 and the egress interface has an MTU of 1500, with no 802.1Q trunking or fabricpath configured and ICMP unreachables enabled on the ingress interface, a N7K will not generate an ICMP unreachable for packets with an ip length between 1501 bytes and 1516 bytes with the DF bit set for path MTU discovery.

In other words, even though the MTU for the egress interface is 1500 Bytes, it will still hardware-switch the packets up to a 1516 byte IP payload for F2/F3, and up to a 1534 byte IP payload for F2e. This behavior is changed 6.2(10) and later such that any payload exceeding the MTU (i.e. 1501) will be software switched.

F-Series line cards and a discrepancy between the ingress and egress MTU.
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