MTU & MSS - jibingl/CCNA-CCNP GitHub Wiki

Relationship

                      <--------------------------- IP MTU ------------------->
                      <--------------------- Ethernet MTU ------------------->
                                                 <--------- TCP MSS --------->
         β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
FRAME    β”‚ Eth-Header β”‚  IP-Header β”‚  TCP-Header β”‚        Data payload       β”‚ Eth-trailerβ”‚
         β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜
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 theΒ 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.