OAM

LSPs are unidirectional tunnels. When an LSP ping is sent, the echo request is transmitted via the tunnel and the echo response is transmitted via the vanilla IP to the source. Similarly, for a p2mp-lsp-ping, on the root, the echo request is transmitted via the mLDP P2MP tunnel to all leafs and the leafs use vanilla IP to respond to the root.

The echo request for mLDP is generated carrying a root Target FEC Stack TLV, which is used to identify the multicast LDP LSP under test at the leaf. The Target FEC Stack TLV must carry an mLDP P2MP FEC Stack Sub-TLV from RFC 6388 or RFC 6512. See Figure 1.

Figure 1. ECHO Request Target FEC Stack TLV

The same concept applies to inter-AS and non-segmented mLDP. The leafs in the remote AS should be able to resolve the root via GRT routing. This is possible for inter-AS Option C where the root is usually in the leaf RTM, which is a next-hop ASBR.

For inter-AS Option B where the root is not present in the leaf RTM, the echo reply cannot be forwarded via the GRT to the root. To solve this problem, for inter-AS Option B, the SR OS uses VPRN unicast routing to transmit the echo reply from the leaf to the root via VPRN.

Figure 2. MVPN Inter-AS Option B OAM

As shown in Figure 2, the echo request for VPN recursive FEC is generated from the root node by executing the p2mp-lsp-ping with the vpn-recursive-fec option. When the echo request reaches the leaf, the leaf uses the sub-TLV within the echo request to identify the corresponding VPN via the FEC which includes the RD, the root, and the P2MP-ID.

After identifying the VPRN, the echo response is sent back via the VPRN and unicast routes. There should be a unicast route (for example, root 10.0.0.14, as shown in Figure 2) present in the leaf VPRN to allow the unicast routing of the echo reply back to the root via VPRN. To distribute this root from the root VPRN to all VPRN leafs, a loopback interface should be configured in the root VPRN and distributed to all leafs via MP-BGP unicast routes.

The OAM functionality for Options B and C is summarized in Table 1.

Notes:

1. For SR OS, all P2MP mLDP FEC types respond to the vpn-recursive-fec echo request. Leafs in the local-AS and inter-AS Option C respond to the recursive-FEC TLV echo request in addition to the leafs in the inter-AS Option B.

   For non inter-AS Option B where the root system IP is visible through the GRT, the echo reply is sent via the GRT, that is, not via the VPRN.

2. This vpn-recursive-fec is a Nokia proprietary implementation, and therefore third-party routers do not recognize the recursive FEC and do not generate an echo respond.

   The user can generate the p2mp-lsp-ping without the vpn-recursive-fec to discover non-Nokia routers in the local-AS and inter-AS Option C, but not the inter-AS Option B leafs.