When you enable next-hop MPLS with the follow-IGP metrics option, BGP uses the native IGP metric for the BGP next hop resolved through IP route table, instead of the MPLS LSP metrics. BGP ignores all LSP metrics in the BGP decision process. However, the MPLS metric remains the same and is not overridden. Then the IGP cost of each next hop is compared, and only paths with the lowest values are considered for ECMP.
The advantage of using the IGP cost in a network is when this cost is significant throughout the local domain and all routing protocols, and you want to use it as a tie-breaker rather than use MPLS-specific metric value. This option works best when the MPLS LSP metrics follow the IGP cost, providing full advantage of both routing protocols and MPLS to select the best path. The advantage of doing this is that the BGP decision process on IGP cost is purely based on the IGP cost to a different next hop. MPLS tunnel is treated as a relay service.
Consider the following when enabling the use of IGP metrics instead of LSP metrics:
Be aware and ensure that the IGP costs are consistent across the network and that you want to rely on the IGP cost to determine where to send the traffic.
When combined with the BGP install-igp-cost command, you can change the route cost from BGP MED to IGP cost and is used when BGP routes are added to the RIB Manager.
When combined with a BGP outbound policy for route set metric-type internal command, you can set IP over MPLS routes using IGP metric to send out as the BGP MED value.
Note
The previous item is primarily for BGP to set the route MED value as the IGP cost and advertise the route to a neighbor. Configuring the use of IGP metrics is based on BGP routes resolving the next hop to MPLS LSP tunnel. A route map is required to set the BGP MED value to the IGP metric by the set metric-type internal command.Note
Mixed ECMP of LSP and native IGP paths is not supported. For a given prefix, if there are multiple BGP next hops with equal IGP cost, BGP prefers the next hop using the LSP path over the IGP path.