Listen to this article if you do not want to read
Segment Routing traffic engineering (SRTE) database is very important in SRTE functionality having information about nodes, links, SR policies, prefixes which headend node or centralized SDN controller (SR-PCE) uses to compute and validate paths.Now, this database information can be received from IGP , PCEP or BGP-LS.
SRTE database on the headend node contains local area information received from IGP . To have multi-domain or multi-AS communication or traffic engineering, a centralized component called SR-PCE is required to have central view of topology and database of link state information of entire network .SR-PCE (SDN controller) gets all information using BGP-LS (BGP address family link state). SR-PCE consolidates all information received via BGP-LS to compute inter-domain paths required in SRTE.
BGP-LS originally was defined in RFC 7752, extensions has been done in it to carry additional information related to segment routing and performance parameters. Lets understand BGP-LS usage and applicability with the help of below sample topology, refer figure 1 below,
Each border router from each domain makes BGP-LS session with SR-PCE so that network topology and link state information is conveyed to central controller (SR-PCE). Local information of each domain comes from IGP usually (it can be BGP as well), IGP redistribute local information into BGP-LS and BGP-LS in turn convey it to SR-PCE using BGP NRLI.
IGP distribute neighbour and prefix information in the form of link state information to other nodes within domain, after that, each node forms logical topology map, computes shortest path tree (SPT) and calculates prefix shortest path reachability information. IGP distribute this information into BGP-LS which in turn abstract information into node, prefixes and links objects. Each of these objects is identified by NRLI (Network layer reachability information) as part of BGP update messages. Apart from these objects (Node, link , prefix), extension to NRLI has been done to support SR related parameters such as SR policies.
I hope this article will help to understand role of BGP-LS in SRTE and SDN.