The maximum transmission unit (MTU) is the largest packet size an interface can forward without fragmentation. A standard MTU is 1,500 bytes. When a packet exceeds the MTU of an interface or router along its path, IPv4 can divide it into smaller fragments for the destination host to reassemble. section 7.1.5 section 7.4
Important operational and exam points:
- A router may fragment packets larger than the outgoing interface’s MTU, including packets as large as 9,000 bytes when a path supports only 1,500 bytes. section 7.1.5
- If the IPv4 Don’t Fragment (DF) bit is set and the packet exceeds the MTU, the packet is discarded instead of fragmented. section 7.1.5
- Fragmentation uses the Identification, Flags, and Fragment Offset fields so the destination can associate, order, and reassemble fragments. section 7.1.5
Checking MTU on a Cisco router
Use show ip interface to inspect detailed interface information. Representative output includes “MTU is 1500 bytes” for both GigabitEthernet0/0 and GigabitEthernet0/1. This confirms the interfaces’ forwarding size limit. section 7.3.2
R1# show ip interface #1 GigabitEthernet0/0 is up, line protocol is up #2 Internet address is 192.168.1.1/24 #2 Broadcast address is 255.255.255.255 #2 Address determined by setup command #2 MTU is 1500 bytes #2 . . . GigabitEthernet0/1 is up, line protocol is up #3 Internet address is 192.168.2.1/24 #3 Broadcast address is 255.255.255.255 #3 Address determined by setup command #3 MTU is 1500 bytes #3 . . . #1 Views more detailed information about R1’s interfaces #2 Information about G0/0 #3 Information about G0/1
Inspect representative Cisco interface output showing operational state, IP addressing, and an MTU of 1,500 bytes. section 7.3.2
