Advertising Per-Topology and Per-Algorithm Label Blocks

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Last updated 2016-04-07 (latest revision 2015-10-05)
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When segment routing is used in a network that is controlled by a link state IGP (such as ISIS or OSPF), each node in the network can be assigned one or more index numbers, known as "node-SIDs". The node-SIDs are unique within the network, and are known to all the nodes in the network. If an ingress node has a data packet to be sent to an egress node, the ingress node may select a node-SID corresponding to the egress node, and "translate" that node-SID to an MPLS label. The MPLS label represents a particular path to the egress node; the path is determined by applying a routing algorithm to a particular view of the network topology and a particular set of metric assignments to the links of that topology. The packet can then be forwarded by pushing the label on the packet's label stack and transmitting the packet to the next hop on the corresponding path to the egress node. This document compares two different procedures for translating a node-SID to the MPLS label that represents a path chosen by a particular algorithm operating on a particular topology. It also specifies the ISIS extensions needed to support one of the procedures (known as the "per-topology/per-algorithm label block" procedure).


Chris Bowers (
Hannes Gredler (
Uma Chunduri (

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)