Default MTU Values

Table 1 shows the default MTU values which are dependent upon the (sub-) port type, mode, and encapsulation.

Table 1. MTU Default Values
Port Type	Mode	Encap Type	Default (bytes)
Ethernet	access	null	1514
Ethernet	access	dot1q	1518
Fast Ethernet	network	—	1514
Other Ethernet	network	—	9212¹
SONET path or TDM channel	access	BCP-null	1518
SONET path or TDM channel	access	BCP-Dot1q	1522
SONET path or TDM channel	access	IPCP	1502
SONET path or TDM channel	network	—	9208
SONET path or TDM channel	access	frame-relay	1578
SONET path or TDM channel	access	atm	1524

Note:

The default MTU for Ethernet ports other than Fast Ethernet is actually the lesser of 9212 and any MTU limitations imposed by hardware which is typically 16K.