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2: Link-State IGP Migrations


Link-State IGP Migrations

The foundation of a mid- or big-size topology in current networks is the IGP (Interior Gateway Protocol) that performs core routing and constitutes the basic pillar for further protocols and application.

As per their original definition back in the late 1980s, IGPs interconnect devices inside a common network domain. The basic idea at that time was that IGPs would transport all routes within those boundaries and an EGP (Exterior Gateway Protocol), the precursor for the well-known BGP (Border Gateway Protocol), would populate those advertisements between Autonomous Systems (ASs).

We have seen how BGP has been developed as a truly scalable multiprotocol concept and was the best fit to package route information, to control the flow of route advertisements and withdrawals, and to transport those information constructs, not only among ASsbut also within the boundaries of a domain. BGP is a hard-state protocol, in which received routes remain present unless a withdrawal is sent or the session with the peer is dropped, without requiring regular routing updates, as opposed to the IGPs. It would be inconceivable nowadays to transport Internet or other external (or internal) routes over an IGP; it would simply not scale.


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