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