Description

This article explains how the 'ping mpls ldp' command uses the family inet MTU as maximum size for MPLS payload, independent of the configured family MPLS MTU value. 

Solution

Topology

mx1 ge-2/0/3.1113 --------- ldp --------- ge-2/1/3.1113 mx2 

user@mx1> show ldp neighbor
Address                             Interface       Label space ID     Hold time
2.2.2.2                             lo0.13          2.2.2.2:0         39

user@mx1> show interfaces ge-2/0/3.1113 extensive |match mtu
    Protocol inet, MTU: 1500 <-- inet MTU(default)
    Protocol mpls, MTU: 1480, Maximum labels: 5, Generation: 297, Route table: 6
    Protocol multiservice, MTU: Unlimited, Generation: 298, Route table: 6

user@mx2> show interfaces ge-2/1/3.1113 extensive |match mtu
    Protocol inet, MTU: 1500 <-- inet MTU(default)
    Protocol mpls, MTU: 1480, Maximum labels: 5, Generation: 310, Route table: 19
    Protocol multiservice, MTU: Unlimited, Generation: 311, Route table: 19

user@mx1> ping mpls ldp sweep 2.2.2.2
 88! 5044. 2568. 1328! 1948. 1640. 1484! 1564. 1524. 1504. 1496! 1508. 1504. 1500!
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 1500 bytes


The media MTU is set to 1000.  Therefore, family inet MTU is 982. 

user@mx1> show configuration interfaces ge-2/0/3
mtu 1000;

user@mx2> show configuration interfaces ge-2/1/3
mtu 1000;

user@mx1> show interfaces ge-2/0/3.1113 extensive |match mtu
    Protocol inet, MTU: 982 <-- inet MTU
    Protocol mpls, MTU: 962, Maximum labels: 5, Generation: 271, Route table: 6
    Protocol multiservice, MTU: Unlimited, Generation: 272, Route table: 6

user@mx2> show interfaces ge-2/1/3.1113 extensive |match mtu
    Protocol inet, MTU: 982 <--inet MTU
    Protocol mpls, MTU: 962, Maximum labels: 5, Generation: 284, Route table: 19
    Protocol multiservice, MTU: Unlimited, Generation: 285, Route table: 19

user@mx1> ping mpls ldp sweep 2.2.2.2
 88! 5044. 2568. 1328. 708! 1020. 864! 944! 1024. 984. 964! 976! 988. 984. 980!
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 980 bytes


The 'ping mpls ldp' command uses the family inet MTU as the maximum size for MPLS payload because an MPLS ping (ldp) is actually an IP packet sent to 127.0.0.1 that is forwarded within an LSP.  It is able to test the reachability and health of the forwarding plane. However, it may not be a valid tool for testing the family MPLS MTU in the backbone. 

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