Date created: 03/02/15 16:09:51. Last modified: 02/28/19 10:04:10

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)          |               |
+--------------------------------+----------------------------+-----------------------+--------------------------+---------------+
| Alcatel TiMOS (7210-SASX)      | Y                          | ?                     | 9014                     | 9000          |
| Cisco IOS (7600) | Y | Y | 9000 | 9000 | | Cisco IOS-XE (CSR1000v sub-int)| Y | ? | 9000 | 9000 | | Cisco IOS-XR (ASR9K) | N | Y | 9014 | 9000 | | Huawei VRP (CX600) | N | ? | 9000 | 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            |
+--------------------------------+----------------------------+-------------------------------------+
| Alcatel TiMOS (7210-SASX)      | 9000                       | 8958 (-14 Ethernet, -20 IP, -8 ICMP)|
| Cisco IOS (7600) | 9000 | 9000 | | Cisco IOS-XE (CSR1000v sub-int)| 9000 | 9000 | | Cisco IOS-XR (ASR9K) | 9014 | 9000 (-14 Ethernet) | | Juniper JUNOS (MX480) | 9000 | 8958 (-14 Ethernet, -20 IP, -8 ICMP)|
| Huawei VRP (CX600) | 9000 | 8972 (-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  | L2 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-XE (CSR1000v) | 1500 (default) | 1500 | 1518 | | Cisco IOS-XR (ASR9K) | 1514 (default, inc Headers)| 1500 (-14 Ethernet, -4 VLAN) | 1518 | +----------------------------+----------------------------+-------------------------------------+----------------------------+

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