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Cisco SPRI 300-510 study materials
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Cisco SPRI 300-510: MPLS and Segment Routing Implementation (LDP, MPLS TE, SR-MPLS, SRv6)

Cisco SPRI 300-510 · Work through your exam, one topic at a time.

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MPLS and Segment Routing Implementation (LDP, MPLS TE, SR-MPLS, SRv6)

What this domain tests

MPLS and Segment Routing Implementation covers label distribution with LDP, traffic engineering with RSVP-TE, and the segment-routing alternatives SR-MPLS and SRv6. The exam expects you to know how labels are allocated and forwarded, how TE paths are constrained, and where SR removes protocol dependencies.

LDP

  • Discovery uses UDP 646; sessions use TCP 646. The transport address, often a loopback, must be reachable.
  • LDP binds labels to IGP prefixes, not to BGP routes unless labeled unicast is configured.
  • Liberal label retention keeps bindings from non-best next hops; ordered control waits for the downstream binding before advertising.
  • PHP lets the penultimate router pop the label; implicit null is label 3 and explicit null is label 0.
  • LDP-IGP synchronization prevents traffic from being blackholed when LDP converges more slowly than the IGP.

MPLS TE

  • RSVP-TE signals unidirectional tunnels; CSPF computes the path from TED data.
  • Path options can be dynamic or explicit; affinity, bandwidth, and setup/hold priority constrain the path.
  • autoroute announce puts the tunnel into the IGP; forwarding adjacency advertises it as a link.
  • FRR uses a backup tunnel for link or node protection; make-before-break avoids traffic loss during reoptimization.

Segment routing

  • SR-MPLS assigns a prefix SID index; label = SRGB base + index. The default SRGB is often 16000–23999. Node SIDs are globally significant; adjacency SIDs are local.
  • OSPF/IS-IS advertise SID extensions; LDP and RSVP are not required. A mapping server can map LDP prefixes to SIDs.
  • SRv6 uses 128-bit SIDs formatted as locator:function and endpoint behaviors such as End, End.X, End.DX4, and End.DT4. It uses the IPv6 routing header, not MPLS labels.

Worked example

SRGB 16000–23999 and loopback 10.255.255.4/32 with prefix-SID index 4 produce label 16004. If the SRGB is changed to 20000–27999, the same index produces 20004. The index is the stable value; the label is derived. An adjacency SID is locally allocated and is not a node SID, so it must not be used to reach the loopback.

Traps

  • LDP is up but a prefix has no label because the IGP has no route to it.
  • PHP hides the egress label; explicit null preserves it for QoS.
  • A TE tunnel is not in the IGP until autoroute announce or forwarding adjacency is configured.
  • Inconsistent SRGBs break SR forwarding.
  • An SRv6 locator that is not advertised in IPv6 routing cannot be reached.