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MTU setting on LSQ interface does not take effect

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Article ID: KB27282 KB Last Updated: 03 Jul 2013Version: 1.0
Summary:
This article explains how setting the MTU on the LSQ interface does not take effect.

Symptoms:
MPLS/IP MTU setting on the LSQ sub interfaces does not take effect even after the commit succeeds.

Cause:
In order to change the MPLS/IP MTU size on the LSQ bundle, we need to set the MRRU size on the remote side. By default, the MRRU is set to 1504. The MRRU indicates the maximum packet size the remote side can handle. The local MTU gets adjusted accordingly by reducing encapsulation overhead from the configured remote MRRU

Solution:
By default, the family inet MTU of LSQ interface is set to 1500. The family MPLS MTU is derived from family Inet MTU and is set to 1488. When the customer has a requirement to operate at non-default MTUs for inet and MPLS, values derived would depend on the remote MRRU value. While setting the local MTU for inet and MPLS, check is performed to ensure that remote MRRU is greater than local MTU values. In normal operating conditions, default MTU values are checked against the remote MRRU value and are retained the same if the above mentioned condition is met. If the customer wants to have a higher value of MTU than it has to be specifically configured on the local side still meeting the condition of being lower than the remote MRRU. If for some reason the locally configured MTU is higher than the remote MRRU, local MTU will be adjusted automatically to match the remote MRRU.

NOTE: When we have requirement to configure a MTU value on the LSQ interfaces, we need to set the MRRU on the remote side considering the encapsulation overhead with respect to IP/MPLS. The following lab test illustrates the behavior of MPLS/IP MTU settings on the LSQ bundle with mlppp encaplustion.

Topology:

|R1|<----------->|R2|


On R2, we have the MPLS MTU set to 1552 and it does not take effect, because the the remote MRRU is set to default value.
So, it takes the remote MRRU(1504) and get adjusted by reducing the 12 bytes(Maximum MPLS label = 3).

MPLS MTU = remote MRRU - 12 (3 labels)
MTU = 1504 - 12 = 1492

lab@R2# show interfaces lsq-0/3/0
unit 0 {
     encapsulation multilink-ppp;
     mrru 1564;
     family inet {
         address 13.13.13.2/24;
     }
     family mpls {
         mtu 1552;
     }
}
lab@R2# run show interfaces lsq-0/3/0.0 extensive | match MRRU
MRRU 1564
Remote MRRU 1504
MRRU exceeded 0
 
lab@R2# run show interfaces lsq-0/3/0.0 extensive | match MTU
Protocol inet, MTU: 1500, Generation: 198, Route table: 0
Protocol mpls, MTU: 1492 [Adjusted], Generation: 199, Route table: 0         << It gets adjusted with the remote MRRU = 1504
Flags: Is-Primary, User-MTU, MTU-Protocol-Adjusted

On R1, we have the MPLS MTU set to 1552 and on R2 I have the MRRU set to 1560. Now the local MTU does take effect because the remote MRRU is higher than the local MPLS MTU in this case.
It gets adjusted with the remote MRRU. 

lab@R1# show interfaces lsq-0/1/0
unit 0 {
    encapsulation multilink-ppp;
    family inet {
        address 13.13.13.1/24;
    }
    family mpls {
        mtu 1552;
    }
}  

lab@R1# run show interfaces lsq-0/1/0.0 extensive | match MTU
Protocol inet, MTU: 1500, Generation: 199, Route table: 0
Protocol mpls, MTU: 1552, Maximum labels: 3, Generation: 200, Flags: User-MTU  

lab@R1# run show interfaces lsq-0/1/0.0 extensive | match MRRU
MRRU 1504
Remote MRRU 1564
MRRU exceeded 0  This is an expected behavior with mlppp encapsulation.

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