By default, next-hop MPLS is disabled. BGP uses the default BGP decision process and native IP forwarding to build BGP EMCP routes. Only IP routing tables are used to resolve routes. When next-hop MPLS is enabled, BGP uses the following configuration information to determine the cost of a BGP shortcut.
Next-hop MPLS is enabled without an option—LSP with a fixed metric of one is used to resolve the routes.
Next-hop MPLS is enabled with the LSP metric comparison option—BGP compares the LSP metrics and uses the metric as the IGP cost for the next hop.
Next-hop MPLS is enabled with the
follow-igp metrics option—BGP uses IGP metrics instead of the LSP metrics. When BGP resolves the next hop with LSP, it uses the native IGP cost for that next hop, and ignores the LSP metric of a MPLS tunnel. Then the IGP cost of each next hop is compared, and only paths with the lowest values are considered for ECMP.
