BGP LS, BGP Link-State is used to distribute Link state information and traffic engineering attributes from the network nodes to the Centralized TE controller. RSVP-TE has been providing resource allocation and providing an LSP with the distributed path computation algorithm (CSPF) for decades. It requires topology information from the network and only link-state IGP protocols such as OSPF and IS-IS can carry the topology information required for the controller to set up a shortest from each node to each destination prefix. In order to overcome Bin Packing, Dead Lock, or Network-wide optimal traffic engineering, centralized controllers have been used for a long time. Because with the distributed computation for Traffic Engineering, the above issues might arise.
RFC 7752 specifies the details of North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP. PCE (Path Computation Element) is an SDN controller which provides optimal path computation in Multi Area and Multi AS (Autonomous System) deployments. It requires Link State and Traffic Engineering attributes such as Link coloring, SRLG, reserved bandwidth, etc., from the network.
Link state IGP protocols (OSPF, IS-IS) can be used for this purpose but they are considered chatty and non-scalable, thus BGP with the new NLRI for the Link state was defined to carry IGP link-state information to the controller. RFC 7752 contains two parts:
- New BGP link-state Network Layer Reachability Information – BGP-LS NLRI defines three objects – links, nodes, and prefixes. We can reconstruct IGP topology with the combination of Node and Link objects. IP prefix objects provide network reachability information.
- New BGP path attribute (BGP-LS attribute) that encodes properties of link, node, and prefix objects, such as IGP metrics information as well.
We recommend you take a look at this video which explains the history of BGP-LS, its use case, and its usage of it in real networks. Ethan Banks and the inventor of the technology, Hannes Gredler are discussing it in the video.
