SR-MPLS OAM

SR-MPLS operation, administration, and maintenance (OAM) monitors LSP connectivity and rapidly detects failures. It is mainly implemented through ping and tracert functions.

During an SR-MPLS ping or tracert test, MPLS Echo Request packets are forwarded based on MPLS labels in the forward direction, and MPLS Echo Reply packets are returned over a multi-hop IP path. You can specify a source IP address for a ping or tracert test. If no source IP address is specified, the local MPLS LSR ID is used as the source IP address. The returned MPLS Echo Reply packets use the source IP address in the MPLS Echo Request packets as their destination IP address.

SR-MPLS BE Ping

On the network shown in Figure 1, PE1, P1, P2, and PE2 all support SR-MPLS. An SR-MPLS BE tunnel is established between PE1 and PE2.
Figure 1 SR-MPLS BE ping/tracert
The process of initiating a ping test on the SR-MPLS BE tunnel from PE1 is as follows:
  1. PE1 initiates a ping test and checks whether the specified tunnel is of the SR-MPLS BE IPv4 type.
    • If the tunnel type is not SR-MPLS BE IPv4, PE1 reports an error message indicating a tunnel type mismatch and stops the ping test.
    • If the tunnel type is SR-MPLS BE IPv4, PE1 continues with the following operations.
  2. PE1 constructs an MPLS Echo Request packet encapsulating the outer label of the initiator and carrying destination address 127.0.0.0/8 in the IP header of the packet.
  3. PE1 forwards the packet to P1. After receiving the packet, P1 swaps the outer MPLS label of the packet and forwards the packet to P2.
  4. Similar to P1, P2 swaps the outer MPLS label of the received packet and determines whether it is the penultimate hop. If yes, P2 removes the outer label and forwards the packet to PE2. PE2 sends the packet to the Rx/Tx module for processing.
  5. PE2 returns an MPLS Echo Reply packet to PE1 and generates the ping test result.

SR-MPLS BE Tracert

On the network shown in Figure 1, the process of initiating a tracert test on the SR-MPLS BE tunnel from PE1 is as follows:
  1. PE1 initiates a tracert test and checks whether the specified tunnel is of the SR-MPLS BE IPv4 type.
    • If the tunnel type is not SR-MPLS BE IPv4, PE1 reports an error message indicating a tunnel type mismatch and stops the tracert test.
    • If the tunnel type is SR-MPLS BE IPv4, PE1 continues with the following operations.
  2. PE1 constructs an MPLS Echo Request packet encapsulating the outer label of the initiator and carrying destination address 127.0.0.0/8 in the IP header of the packet.
  3. PE1 forwards the packet to P1. After receiving the packet, P1 determines whether the TTL–1 value in the outer label of the packet is 0.
    • If the TTL–1 value is 0, an MPLS TTL timeout occurs, and P1 sends the packet to the Rx/Tx module for processing.
    • If the TTL–1 value is greater than 0, P1 swaps the outer MPLS label of the packet, searches the forwarding table for the outbound interface, and forwards the packet to P2.
  4. Similar to P1, P2 also performs the following operations:
    • If the TTL–1 value is 0, an MPLS TTL timeout occurs, and P2 sends the packet to the Rx/Tx module for processing.
    • If the TTL–1 value is greater than 0, P2 swaps the outer MPLS label of the received packet and determines whether it is the penultimate hop. If yes, P2 removes the outer label, searches the forwarding table for the outbound interface, and forwards the packet to PE2.
  5. PE2 sends the packet to the Rx/Tx module for processing, returns an MPLS Echo Reply packet to PE1, and generates the tracert test result.

SR-MPLS TE Ping

On the network shown in Figure 2, PE1, P1, and P2 all support SR-MPLS. An SR-MPLS TE tunnel is established between PE1 and PE2. The devices assign adjacency labels as follows:
  • PE1 assigns the adjacency label 9001 to the PE1-P1 adjacency.
  • P1 assigns the adjacency label 9002 to the P1-P2 adjacency.
  • P2 assigns the adjacency label 9005 to the P2-PE2 adjacency.
Figure 2 SR-MPLS TE ping/tracert
The process of initiating a ping test on the SR-MPLS TE tunnel from PE1 is as follows:
  1. PE1 initiates a ping test and checks whether the specified tunnel is of the SR-MPLS TE type.
    • If the tunnel type is not SR-MPLS TE, PE1 reports an error message indicating a tunnel type mismatch and stops the ping test.
    • If the tunnel type is SR-MPLS TE, PE1 continues with the following operations.
  2. PE1 constructs an MPLS Echo Request packet encapsulating label information about the entire tunnel and carrying destination address 127.0.0.0/8 in the IP header of the packet.
  3. PE1 forwards the packet to P1. After receiving the packet, P1 removes the outer label 9002 and forwards the packet to P2.
  4. P2 removes the outer label 9005 of the received packet and forwards the packet to PE2 for processing.
  5. PE2 returns an MPLS Echo Reply packet to PE1.

SR-MPLS TE Tracert

On the network shown in Figure 2, the process of initiating a tracert test on the SR-MPLS TE tunnel from PE1 is as follows:
  1. PE1 initiates a tracert test and checks whether the specified tunnel is of the SR-MPLS TE type.
    • If the tunnel type is not SR-MPLS TE, PE1 reports an error message indicating a tunnel type mismatch and stops the tracert test.
    • If the tunnel type is SR-MPLS TE, PE1 continues with the following operations.
  2. PE1 constructs an MPLS Echo Request packet encapsulating label information about the entire tunnel and carrying destination address 127.0.0.0/8 in the IP header of the packet.
  3. PE1 forwards the packet to P1. After receiving the packet, P1 determines whether the TTL–1 value in the outer label of the packet is 0.
    • If the TTL–1 value is 0, an MPLS TTL timeout occurs, and P1 sends the packet to the Rx/Tx module for processing.
    • If the TTL–1 value is greater than 0, P1 removes the outer MPLS label of the packet, buffers the TTL–1 value, copies the value to the new outer MPLS label, searches the forwarding table for the outbound interface, and forwards the packet to P2.
  4. Similar to P1, P2 also determines whether the TTL–1 value in the outer label of the received packet is 0.
    • If the TTL–1 value is 0, an MPLS TTL timeout occurs, and P2 sends the packet to the Rx/Tx module for processing.
    • If the TTL–1 value is greater than 0, P2 removes the outer MPLS label of the packet, buffers the TTL–1 value, copies the value to the new outer MPLS label, searches the forwarding table for the outbound interface, and forwards the packet to PE2.
  5. P2 forwards the packet to PE2, and PE2 returns an MPLS Echo Reply packet to PE1.
Parent Topic: Understanding Segment Routing MPLS
Copyright © Huawei Technologies Co., Ltd.
Copyright © Huawei Technologies Co., Ltd.
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