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[MX] 'ping mpls rsvp sweep' command uses the family inet MTU as maximum size for MPLS payload for the 1st hop LSP

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Article ID: KB34756 KB Last Updated: 13 Jul 2019Version: 1.0
Summary:

This article provides an example of how the MTU can vary for LSPs, depending on the configuration.

Symptoms:
The 'ping mpls rsvp sweep' command uses the family inet MTU as the maximum size for MPLS payload for the 1st hop LSP. However, the same command uses the family mpls MTU for the 2nd, 3rd hop LSPs, as shown in the example below:

Topology:

Configuration:

User@MX> show configuration |display set 
set logical-systems LSYS-2 interfaces xe-2/0/1 unit 0 family inet mtu 1500
set logical-systems LSYS-2 interfaces xe-2/0/1 unit 0 family inet address 1.1.12.2/24
set logical-systems LSYS-2 interfaces xe-2/0/1 unit 0 family mpls
set logical-systems LSYS-2 interfaces xe-2/0/2 unit 0 family inet mtu 1500
set logical-systems LSYS-2 interfaces xe-2/0/2 unit 0 family inet address 1.1.23.2/24
set logical-systems LSYS-2 interfaces xe-2/0/2 unit 0 family mpls
set logical-systems LSYS-2 interfaces lo0 unit 2 family inet address 2.2.2.2/32
set logical-systems LSYS-2 protocols ospf traffic-engineering
set logical-systems LSYS-2 protocols ospf area 0.0.0.0 interface lo0.2 passive
set logical-systems LSYS-2 protocols ospf area 0.0.0.0 interface xe-2/0/1.0 interface-type p2p
set logical-systems LSYS-2 protocols ospf area 0.0.0.0 interface xe-2/0/2.0 interface-type p2p
set logical-systems LSYS-2 protocols rsvp interface xe-2/0/1.0
set logical-systems LSYS-2 protocols rsvp interface xe-2/0/2.0
set logical-systems LSYS-2 protocols mpls interface xe-2/0/1.0
set logical-systems LSYS-2 protocols mpls interface xe-2/0/2.0
set logical-systems LSYS-3 interfaces xe-0/0/0 unit 0 family inet mtu 1500
set logical-systems LSYS-3 interfaces xe-0/0/0 unit 0 family inet address 1.1.34.3/24
set logical-systems LSYS-3 interfaces xe-0/0/0 unit 0 family mpls
set logical-systems LSYS-3 interfaces xe-2/0/3 unit 0 family inet mtu 1500
set logical-systems LSYS-3 interfaces xe-2/0/3 unit 0 family inet address 1.1.23.3/24
set logical-systems LSYS-3 interfaces xe-2/0/3 unit 0 family mpls
set logical-systems LSYS-3 interfaces lo0 unit 3 family inet address 3.3.3.3/32
set logical-systems LSYS-3 protocols ospf traffic-engineering
set logical-systems LSYS-3 protocols ospf area 0.0.0.0 interface lo0.3 passive
set logical-systems LSYS-3 protocols ospf area 0.0.0.0 interface xe-2/0/3.0 interface-type p2p
set logical-systems LSYS-3 protocols ospf area 0.0.0.0 interface xe-0/0/0.0 interface-type p2p
set logical-systems LSYS-3 protocols rsvp interface xe-2/0/3.0
set logical-systems LSYS-3 protocols rsvp interface xe-0/0/0.0
set logical-systems LSYS-3 protocols mpls interface xe-2/0/3.0
set logical-systems LSYS-3 protocols mpls interface xe-0/0/0.0
set logical-systems LSYS-4 interfaces xe-0/1/0 unit 0 family inet mtu 1500
set logical-systems LSYS-4 interfaces xe-0/1/0 unit 0 family inet address 1.1.34.4/24
set logical-systems LSYS-4 interfaces xe-0/1/0 unit 0 family mpls
set logical-systems LSYS-4 interfaces lo0 unit 4 family inet address 4.4.4.4/32
set logical-systems LSYS-4 protocols ospf traffic-engineering
set logical-systems LSYS-4 protocols ospf area 0.0.0.0 interface lo0.4 passive
set logical-systems LSYS-4 protocols ospf area 0.0.0.0 interface xe-0/1/0.0 interface-type p2p
set logical-systems LSYS-4 protocols rsvp interface xe-0/1/0.0
set logical-systems LSYS-4 protocols mpls interface xe-0/1/0.0
set interfaces xe-0/0/0 mtu 9500
set interfaces xe-0/1/0 mtu 9500
set interfaces xe-2/0/0 mtu 9500
set interfaces xe-2/0/0 unit 0 family inet mtu 1500
set interfaces xe-2/0/0 unit 0 family inet address 1.1.12.1/24
set interfaces xe-2/0/0 unit 0 family mpls
set interfaces xe-2/0/1 mtu 9500
set interfaces xe-2/0/2 mtu 9500
set interfaces xe-2/0/3 mtu 9500
set interfaces lo0 unit 1 family inet address 1.1.1.1/32
set protocols ospf traffic-engineering
set protocols ospf area 0.0.0.0 interface lo0.1 passive
set protocols ospf area 0.0.0.0 interface xe-2/0/0.0 interface-type p2p
set protocols rsvp interface xe-2/0/0.0
set protocols mpls label-switched-path to-LSYS-2 to 2.2.2.2
set protocols mpls label-switched-path to-LSYS-2 primary to-LSYS-2
set protocols mpls label-switched-path to-LSYS-3 to 3.3.3.3
set protocols mpls label-switched-path to-LSYS-3 primary to-LSYS-3
set protocols mpls label-switched-path to-LSYS-4 to 4.4.4.4
set protocols mpls label-switched-path to-LSYS-4 primary to-LSYS-4
set protocols mpls path to-LSYS-2
set protocols mpls path to-LSYS-3
set protocols mpls path to-LSYS-4
set protocols mpls interface xe-2/0/0.0

User@MX> show mpls lsp ingress
Ingress LSP: 3 sessions
To              From            State Rt P     ActivePath       LSPname
2.2.2.2         1.1.1.1         Up     0 *     to-LSYS-2        to-LSYS-2
3.3.3.3         1.1.1.1         Up     0 *     to-LSYS-3        to-LSYS-3
4.4.4.4         1.1.1.1         Up     0 *     to-LSYS-4        to-LSYS-4
Total 3 displayed, Up 3, Down 0

User@MX> show interfaces xe-2/0/0 |match mtu
  Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None,
    Protocol inet, MTU: 1500 <------------------------ family inet MTU
      Flags: Sendbcast-pkt-to-re, User-MTU
    Protocol mpls, MTU: 1488, Maximum labels: 3 <----- family mpls MTU
    Protocol multiservice, MTU: Unlimited

User@MX> show interfaces xe-2/0/1 |match mtu
  Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None,
    Protocol inet, MTU: 1500
      Flags: Sendbcast-pkt-to-re, Is-Primary, User-MTU
    Protocol mpls, MTU: 1488, Maximum labels: 3
    Protocol multiservice, MTU: Unlimited

User@MX> show interfaces xe-2/0/2 |match mtu
  Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None,
    Protocol inet, MTU: 1500
      Flags: Sendbcast-pkt-to-re, User-MTU
    Protocol mpls, MTU: 1488, Maximum labels: 3
    Protocol multiservice, MTU: Unlimited

User@MX> show interfaces xe-2/0/3 |match mtu
  Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None,
    Protocol inet, MTU: 1500
      Flags: Sendbcast-pkt-to-re, User-MTU
    Protocol mpls, MTU: 1488, Maximum labels: 3
    Protocol multiservice, MTU: Unlimited

User@MX> show interfaces xe-0/0/0 |match mtu
  Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None,
    Protocol inet, MTU: 1500
      Flags: Sendbcast-pkt-to-re, Is-Primary, User-MTU
    Protocol mpls, MTU: 1488, Maximum labels: 3
    Protocol multiservice, MTU: Unlimited

User@MX> show interfaces xe-0/1/0 |match mtu
  Link-level type: Ethernet, MTU: 1514, MRU: 1522, LAN-PHY mode, Speed: 10Gbps, BPDU Error: None,
    Protocol inet, MTU: 1500
      Flags: Sendbcast-pkt-to-re, Is-Primary, User-MTU
    Protocol mpls, MTU: 1488, Maximum labels: 3
    Protocol multiservice, MTU: Unlimited

 

Pings:

User@MX> ping mpls rsvp sweep to-LSYS-2
 100! 5052. 2576. 1340! 2580. 1960. 1652. 1496! 1576. 1536. 1516. 1508. 1504. 1500!
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 1500 bytes <----------- family inet MTU

User@MX> ping mpls rsvp sweep to-LSYS-3
 100! 5052. 2576. 1340! 2580. 1960. 1652. 1496. 1420! 1500. 1460! 1480! 1492. 1488! 1496. 1492.
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 1488 bytes <----------- family mpls MTU

User@MX> ping mpls rsvp sweep to-LSYS-4
 100! 5052. 2576. 1340! 2580. 1960. 1652. 1496. 1420! 1500. 1460! 1480! 1492. 1488! 1496. 1492.
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 1488 bytes <----------- family mpls MTU
Solution:

By default, penultimate hop popping (PHP) is configured for LSP "to-LSYS-2", since it is directly connected and no labels are exchanged. 

Per design of PHP, packets with an IP header are sent; hence, the family inet MTU is used.

If "ultimate-hop-popping" is configured, labels are exchanged even in direct connections, and the family mpls MTU is used, as expected.

User@MX# set protocols mpls ultimate-hop-popping

[edit]
User@MX# show |compare
[edit protocols mpls]
+   ultimate-hop-popping;

[edit]
User@MX# commit
commit complete

User@MX> ping mpls rsvp sweep to-LSYS-2
 100! 5052. 2576. 1340! 2580. 1960. 1652. 1496. 1420! 1500. 1460! 1480! 1492. 1488! 1496. 1492.
--- lsp ping sweep result---
Maximum Transmission Unit (MTU) is 1488 bytes <----------- family mpls MTU
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