Configuring the Juniper Media, IP & MPLS MTU
Here I have some information on MTU Juniper, I had investigated it for a customer while ago. The actual frames transmitted also contain cyclic redundancy check (CRC) bits, which are not part of the media MTU. For example, the media MTU for a Gigabit Ethernet interface is specified as 1500 bytes, but the largest possible frame size is actually 1504 bytes; you need to consider the extra bits in calculations of MTUs for interoperability.
Encapsulation Overhead by Encapsulation Type
|
|
Encapsulation Overhead (Bytes) |
|
802.1Q/Ethernet 802.3 |
21 |
|
802.1Q/Ethernet Subnetwork Access Protocol (SNAP) |
26 |
|
802.1Q/Ethernet version 2 |
18 |
|
ATM Cell Relay |
4 |
|
ATM permanent virtual connection (PVC) |
12 |
|
Cisco HDLC |
4 |
|
Ethernet 802.3 |
17 |
|
Ethernet circuit cross-connect (CCC) and virtual private LAN service (VPLS) |
4 |
|
Ethernet over ATM |
32 |
|
Ethernet SNAP |
22 |
|
Ethernet translational cross-connect (TCC) |
18 |
|
Ethernet version 2 |
14 |
|
Extended virtual local area network (VLAN) CCC and VPLS |
4 |
|
Extended VLAN TCC |
22 |
|
Frame Relay |
4 |
|
PPP |
4 |
|
VLAN CCC |
4 |
|
VLAN VPLS |
4 |
|
VLAN TCC |
22 |
Default media MTU = Default IP MTU + L2 encapsulation overhead
Default IP MTU = Default media MTU - L2 encapsulation overhead
MPLS MTU = physical interface MTU - L2 encapsulation overhead - 12
If IP MTU is already set, so MPLS MTU = IP MTU + 20 bytes
IP MTU - payload without header!
Note: In other words, the formula used to determine the MPLS MTU is the following:
MPLS MTU = physical interface MTU - encapsulation overhead - 12