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Maximum transmission unit (MTU) defines the largest size of packets that an interface can transmit without the need to fragment. IP packets larger than the MTU must go through IP fragmentation procedures. Cisco ATM router interfaces support an MTU between 64 and 17966 bytes. Each interface supports a default maximum packet size. For example, the maximum value is 9288 bytes on both the ATM interface processor (AIP) and network processor module (NP), and 4470 bytes on the PA-A3 and PA-A2 port adapters. This document reviews the default MTU values for ATM interfaces and clarifies when a router increments the AAL5 Oversized SDUs and the AAL5 length violation counters. Why are MTUs 4470 Bytes? Most Cisco ATM router interfaces use a default MTU size of 4470 bytes. This number was chosen to match exactly Fiber Distributed Data Interface (FDDI) and High-Speed Serial Interface (HSSI) interfaces for autonomous switching. Use the mtu command in interface configuration mode to configure a non-default value. Note that subinterfaces support a value that is different from the main interface as long as the main interface's value is as large as, or larger than the largest subinterface MTU. [miễn là giá trị MTU của cạc chính lớn hơn (as large as) hoặc lớn hơn giá trị lớn nhất của SUBINTERFACE MTU] subMTU là giá trị các cổng khác 1500 <= subinterfaces <4470 as long as miễn là 7200#show interface atm 3/0 ATM3/0 is up, line protocol is up Hardware is ENHANCED ATM PA Internet address is 1.1.1.1/8 MTU 4470 bytes, sub MTU 1500, BW 149760 Kbit, DLY 80 usec, reliability 255/255, txload 1/255, rxload 1/255 Use the show atm interface atm command to view the currently configured value. 7200#show atm interface atm 3/0 Interface ATM3/0: AAL enabled: AAL5 , Maximum VCs: 4096, Current VCCs: 2 Maximum Transmit Channels: 0 Max. Datagram Size: 4528 PLIM Type: SONET - 155000Kbps, TX clocking: LINE Cell-payload scrambling: ON sts-stream scrambling: ON 8359 input, 8495 output, 0 IN fast, 0 OUT fast, 0 out drop Avail bw = 155000 Config. is ACTIVE