RSVP and Labels

RSVP signals the path for the TE tunnel, but it is also its task to carry the MPLS label so that the packets can be label-switched along the path of the TE tunnel. Look at Figure 8-9 to see the RSVP messages sent for the TE tunnel signaling.

Figure 8-9 RSVP for TE and Labels

Figure 8-9 RSVP for TE and Labels

The PATH messages carry a Label Request object. When the tail end router receives this Label Request object, it assigns a label to this TE tunnel LSP and advertises it to the upstream router (the penultimate hop router) in a Label object in the RESV message. This label is the incoming label in the LFIB of the tail end router. The upstream router receives the label from the tail end router and puts this label as the outgoing label in the LFIB for this TE tunnel LSP. The router assigns a label from the global label table to this TE tunnel LSP and sends it in a Label object in the RESV message to its upstream router. This label becomes the incoming label in the LFIB for this TE tunnel LSP. It continues like this until the RESV message reaches the head end router of the TE tunnel LSP. The fact that a label is advertised from tail end router to head end router, hop-by-hop, after being requested by the head end router indicates that TE tunnels use Downstream-on-Demand (DoD) label distribution.

NOTE Because RSVP with TE extensions takes care of the distribution of the MPLS labels, you do not need to configure Label Distribution Protocol (LDP) on the interfaces. Therefore, the MPLS network does not strictly need to have mpls ip on the interfaces, if TE is deployed. However, if you do not deploy TE to carry all traffic from ingress LSRs to egress LSRs, you need LDP to avoid unlabeled traffic in the core network. MPLS VPN traffic, for instance, needs to be labeled at all times in the core network.

The label that the tail end router advertises is the explicit NULL label in the RESV Label object. The receiving router that is running Cisco IOS—the penultimate hop LSR—interprets this as an implicit NULL label by default. So—by default—the packet that is sent on a TE tunnel from the penultimate hop router to the tail end router has no label or the top label is popped off. Therefore, penultimate hop popping (PHP) occurs. Figure 8-10 shows the packet forwarding for packets forwarded on the TE tunnel of Figure 8-9.

Figure 8-10 Packet Forwarding

Figure 8-10 Packet Forwarding

If you want the penultimate hop router to interpret the explicit NULL label as explicit NULL instead of implicit NULL, you must configure the following hidden command on the penultimate hop router:

mpls traffic-eng signalling interpret explicit-null verbatim

You use explicit NULL when you want to preserve the QoS information. Refer to Chapter 12, "MPLS and Quality of Service," for more information on this.

You can also configure the tail end router to advertise an implicit NULL label (label 3) in the Label object in the RESV message to the penultimate hop router. For this, you need the following command on the tail end router:

mpls traffic-eng signalling advertise implicit-null [acl-name | acl-number]

You can configure a standard access list or named access list to indicate to which peer routers to send the implicit NULL.

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