MTU & MSS - jibingl/CCNA-CCNP GitHub Wiki

Relationship

                      <--------------------------- IP MTU ------------------->
                      <--------------------- Ethernet MTU ------------------->
                                                 <--------- TCP MSS --------->
         +------------+------------+-------------+---------------------------+------------+
FRAME    | Eth-Header |  IP-Header |  TCP-Header |        Data payload       | Eth-trailer|
         +------------+------------+-------------+---------------------------+------------+
BYTES          14           20            20                 1460                  4

Bytes	Size Range	Default (IP/IPv6)	Description
IP MTU	68 - 9600	1500	The maximum size of an entire IP packet without fragmentation, or drop if DF bit is set in the IP header.
Ethernet MTU	64 - 9600	1500	The maximum Ethernet payload size, excluding Ethernet header, without dropped.
TCP MSS	28 - 9560	536/1220	Maximum TCP payload size, excluding TCP header, without fragmentation.

• IP MTU can smaller than Ethernet MTU, otherwise frame will be dropped in layer 2.
• Cisco router defines MSS in a local link as 536 bytes while 1460 bytes for remote connection.

Frame Size with Default MTU

Following table shows size of different VLAN frames with a default IP MTU (1500 bytes).

-	Size (Bytes)	Description
Untagged Frame Size	1518	Standard Ethernet frame including headers and trailer.
VLAN-Tagged Frame Size	1522	Standard frame (1518) plus the 4-byte VLAN tag.
QinQ (Double Tagging)	1526	Standard frame plus two VLAN tags (4 + 4 bytes).

---

Packet Unit Size

Packet Unit	Minimum Size	Maximum Size	Reasons
Ethernet frame	64 Bytes	1518 Bytes	When encapsulated in Ethernet, the IP packet has a minimum 46-byte payload (if an 802.1Q tag is absent). Ethernet MTU is 1500 bytes (or 1522 with VLAN tagging).
IPv4 packet	20 Bytes	65535 Bytes	Theoretically, 16-bit Total Length field in an IPv4 header can store values from 0 to 65535. As IP header is 20 bytes which defines the minimal.