OSPF - LSA TYPE 4 AND 5

Details on how the OSPF LSA 4 (ASBR-Summary) and LSA 5 (External) operate and what information they contain. We'll take special attention into how the calculations are made to reach external routes.

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External LSA Type 5

They describe networks that are external to the OSPF domain

Generated by the ASBR

Have "basically" the following information: 
External Network (link-id) / Network Mask
Router-id of the ASBR
Metric
Type of metric (Type 1 y 2)
Type 2: The nearest exit to the external network
Type 1: The exit with the lowest total metric (taking into account the internal cost)

Within the area where the ASBR resides, the metric is calculated:
Metric of the ASBR to the network + Metric to reach the ASBR 

Outside of the area where the ASBR resides, the metric is calculated: 
Metric of the ASBR to the network + Metric of the ABR to the ASBR + Metric to reach the ABR

ASBR Summary LSA Type 4
Summarizes the connectivity information to the ASBR for the other areas
Generated by the ABR

Composed of: Router-id of ASBR + Metric of the ABR to the ASBR 

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