Interop MTUs

Inter-op MTU sizes. The "Usable MTU" represents the maximum size for the payload part of an Ethernet frame.

+----------------------------+----------------------------+-----------------------+--------------------------+---------------+
| Device Tested On           | Includes Ethernet headers  | Includes MPLS headers | Configured interface MTU | Usable L3 MTU |
|                            |    in interface setting    |                       | (PHY interface)          |               |
+----------------------------+----------------------------+-----------------------+--------------------------+---------------+
| Cisco IOS (7600)           | Y                          | Y                     | 9000                     | 9000          |
| Cisco IOS (ME3600) EVC     |                            |                       |                          |               |
| Cisco IOS-XR (ASR9K)       | N                          | Y                     | 9014                     | 9000          |
| Juniper JUNOS (MX480)      | N                          | Y                     | 9014                     | 9000          |
+----------------------------+----------------------------+-----------------------+--------------------------+---------------+

Inter-op ping command sizes. The IOS ping command will account the Ethernet headers, IOS-XR command expects them to be removed, Junos expects all headers upto the ICMP payload to be removed.

+----------------------------+----------------------------+-------------------------------------+
| Device Tested On           |    MTU size on phy         |    Max size usable with             |
|                            |    interface               |    "ping" cmd and df-bit            |
+----------------------------+----------------------------+-------------------------------------+
| Cisco IOS (7600)           | 9000                       | 9000                                |
| Cisco IOS (ME3600) EVC     |                            |                                     |
| Cisco IOS-XR (ASR9K)       | 9014                       | 9000 (-14 Ethernet)                 |
| Juniper JUNOS (MX480)      | 9000                       | 8958 (-14 Ethernet, -20 IP, -8 ICMP)|
+----------------------------+----------------------------+-------------------------------------+

In the table below we can see the impact of tagged VLAN sub-interfaces. IOS ignores Ethernet headers including VLAN tags in the MTU value set at an interface (meaning that it always offers the layer 3 protocol 1500 bytes of transport, even if one configures a dot1q tag, IOS sends 1518 byte frames instead of 1514 byte frames). IOS-XR has the same behaviour as IOS.

+----------------------------+----------------------------+-------------------------------------+-------------------------+
| Device Tested On           |    MTU size on phy         |    Max size usable with "ping" cmd  | Packet size on the wire |
|                            |    interface               |    and df-bit from VLAN sub-int     |                         |
+----------------------------+----------------------------+-------------------------------------+-------------------------+
| Cisco IOS (7200)           | 1500 (default)             | 1500                                | 1518                    |
| Cisco IOS (7600)           | 1500 (default)             | 1500                                | 1518                    |
| Cisco IOS-XR (ASR9K)       | 1514 (default, inc Headers)| 1500 (-14 Ethernet, -4 VLAN)        | 1518                    |
+----------------------------+----------------------------+-------------------------------------+-------------------------+

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