CCNA Complete Guide 2nd Edition.pdf - Cisco Learning Home

MTU and Fragmentation
- Maximum Transmission Unit (MTU) defines the maximum amount of network layer data that
a data link layer frame can carry; or the largest packet size that an interface can handle.
This ensures that an IP packet that encapsulates a TCP segment will fit into a single frame.
The default MTU value on Ethernet interfaces is 1500 bytes.
- The following information can be gathered with a packet sniffer, eg: Ethereal, Wireshark:
Max application data size 1460 bytes
Max TCP segments size 1480 bytes
Max IP packets size 1500 bytes
Max Ethernet frames size 1514 bytes [1]
TCP and IP headers are 20 bytes in length.
[1] 14 bytes = 6 bytes Source MAC address + 6 bytes Destination MAC address + 2 bytes Type.
The actual Ethernet frame size should be 1518 bytes (1514 bytes + 4 bytes FCS).
- Routers are unable to forward a packet out an interface if the packet is larger than the MTU.
Fragmentation is the process of breaking an IP packet into smaller pieces (≤ MTU).
Fragmented packets are reassembled before being passed to the transport layer protocol.
- Fragmentation is performed at network layer (L3) using the 16-bit Identification, 3-bit Flags,
and 13-bit Fragmentation Offset bits in the IP header. The 3-bit Flags field contains the
Don’t fragment bit, More fragments bit, and a reserved bit.
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