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Beginner

NCS 540 and MTU

Hi,

I am testing NCS 540.

I connect it to ME-3800 and OSFP between both.

I had problem with MTU.

On the ME-3800, the MTU on the interface = 1530 (Header MPLS )

On the NCS 540, I must configurer 1544 (1530 + 14).

Have you an idea ?

 

 

 

 

 

2 ACCEPTED SOLUTIONS

Accepted Solutions
Highlighted
Cisco Employee

There is a difference in how ios-xr calculates the MTU

 

you can review this document

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-xr-software/116350-trouble-ios-xr-mtu-00.html

 

Thanks

 

View solution in original post

Highlighted

This is the classic XR-XE MTU difference.

From the document referenced below:

"

MTU characteristics in Cisco IOS XR software are:

 

MTU config and show commands, at L2 and L3, include the header size of their corresponding layer. For instance, the mtu command that configures the L2 MTU includes 14 bytes for an Ethernet interface (without dot1q), or 4 bytes for Point-to-Point Protocol (PPP) or high-level data link control (HDLC). The ipv4 mtu command includes 20 bytes of the IPv4 header.

 

The MTU of a higher layer must fit within the payload of the lower layer. For example, if the interface MTU of a non-dot1q Ethernet interface is the default of 1514 bytes, then higher layer protocols such as Multiprotocol Label Switching (MPLS) can have a maximum MTU of 1500 bytes on that interface. This means you can fit only a 1500 byte MPLS frame (including labels) inside the Ethernet frame. You cannot configure a 1508 byte MPLS MTU on that interface if you want to allow two MPLS tags on top of a 1500 byte IPv4 packet. In order to transmit a 1508 byte MPLS frame on an Ethernet interface, the interface MTU must be increased to 1522, or higher value, in order to ensure that the L2 interface payload is large enough to carry the MPLS frame.

In the classical Cisco IOS software (not the Cisco IOS XR software), the interface mtu command configures the L2 payload size, but does not include the L2 header. This is different from the Cisco IOS XR software which includes both the L2 and L3 overhead in the interface mtu command. The L3 MTU commands, as in the case of the ipv4 mtu command, configure the maximum packet size of that protocol which includes the L3 header. This is similar to the case of Cisco IOS XR software.

 

The default interface MTU in the Cisco IOS XR software must allow the transport of a 1500 byte L3 packet. Therefore, the default MTU is 1514 bytes for a main Ethernet interface and 1504 bytes for a serial interface.

"

 

Show IPv4 Int will provide the lowdown:

RP/0/RSP0/CPU0:DPS9K#show ipv4 int hu0/0/0/0 | i MTU
Thu Sep 26 10:10:37.675 CDT
MTU is 9216 (9202 is available to IP)

 

Here's the official document on XR MTU behavior:

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-xr-software/116350-trouble-ios-xr-mtu-00.html

 

View solution in original post

2 REPLIES 2
Highlighted
Cisco Employee

There is a difference in how ios-xr calculates the MTU

 

you can review this document

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-xr-software/116350-trouble-ios-xr-mtu-00.html

 

Thanks

 

View solution in original post

Highlighted

This is the classic XR-XE MTU difference.

From the document referenced below:

"

MTU characteristics in Cisco IOS XR software are:

 

MTU config and show commands, at L2 and L3, include the header size of their corresponding layer. For instance, the mtu command that configures the L2 MTU includes 14 bytes for an Ethernet interface (without dot1q), or 4 bytes for Point-to-Point Protocol (PPP) or high-level data link control (HDLC). The ipv4 mtu command includes 20 bytes of the IPv4 header.

 

The MTU of a higher layer must fit within the payload of the lower layer. For example, if the interface MTU of a non-dot1q Ethernet interface is the default of 1514 bytes, then higher layer protocols such as Multiprotocol Label Switching (MPLS) can have a maximum MTU of 1500 bytes on that interface. This means you can fit only a 1500 byte MPLS frame (including labels) inside the Ethernet frame. You cannot configure a 1508 byte MPLS MTU on that interface if you want to allow two MPLS tags on top of a 1500 byte IPv4 packet. In order to transmit a 1508 byte MPLS frame on an Ethernet interface, the interface MTU must be increased to 1522, or higher value, in order to ensure that the L2 interface payload is large enough to carry the MPLS frame.

In the classical Cisco IOS software (not the Cisco IOS XR software), the interface mtu command configures the L2 payload size, but does not include the L2 header. This is different from the Cisco IOS XR software which includes both the L2 and L3 overhead in the interface mtu command. The L3 MTU commands, as in the case of the ipv4 mtu command, configure the maximum packet size of that protocol which includes the L3 header. This is similar to the case of Cisco IOS XR software.

 

The default interface MTU in the Cisco IOS XR software must allow the transport of a 1500 byte L3 packet. Therefore, the default MTU is 1514 bytes for a main Ethernet interface and 1504 bytes for a serial interface.

"

 

Show IPv4 Int will provide the lowdown:

RP/0/RSP0/CPU0:DPS9K#show ipv4 int hu0/0/0/0 | i MTU
Thu Sep 26 10:10:37.675 CDT
MTU is 9216 (9202 is available to IP)

 

Here's the official document on XR MTU behavior:

https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-xr-software/116350-trouble-ios-xr-mtu-00.html

 

View solution in original post

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