Interface MTU and Frame Handling

The interface Maximum Transmission Unit (MTU) impacts ingress and egress frames differently on a Cisco IOS device. This is because the MTU represents the largest payload size that can be transmitted without fragmentation on a particular interface. Here’s how it affects frames in each direction:

Ingress Frames

  • Behavior: When a frame arrives at an interface, the MTU doesn't directly discard or fragment the incoming frame. Instead:
    • The interface accepts the frame if it is smaller than or equal to the MTU of the receiving interface.
    • If the frame exceeds the MTU but is still within the maximum frame size the interface can physically handle (e.g., when jumbo frames are enabled on a GigabitEthernet interface), it is accepted but may trigger errors depending on the network layer.
  • Fragmentation:
    • Fragmentation cannot take place on ingress. An incoming frame must be fully received before processing it, and before applying any fragmentation or other processes on it. If the payload size of an incoming packet exceeds the MTU, nothing can be done, and it is simply dropped. This will take place even if the do not fragment (DF) bit is set in the IPv4 header.

Egress Frames

  • Behavior: When a frame is being transmitted from an interface:
    • The MTU is a hard limit for the payload size that can be sent out of the interface.
    • If the packet size exceeds the MTU:
      • For Layer 3 packets, fragmentation occurs to split the payload into smaller packets that fit within the MTU if the DF bit is not set.
      • If fragmentation is disabled (i.e. the DF bit is set), the packet is dropped, and an ICMP message is sent (if applicable).
  • Key Difference: On egress, the MTU actively dictates whether a packet can be transmitted as-is or requires fragmentation. Upon egress, a packet can be manipulated, processed, and thus fragmented to conform to the MTU.

For more information about MTU, take a look at these NetworkLessons notes:

https://networklessons.com/cisco/ccie-routing-switching/pppoe-mtu-troubleshooting-cisco-ios