@Salah_IT_Nerd Regarding Why Cost of Loopbacks is not added to metric, I looked at original OSPFv2 is RFC2328 and New OSPFv3 IPv6 and v4 are in RFC 2740 and updated 5340. OSPFv3 has improved database calculation by changing LSA type 1 and 2 and adding LSA 8 and 9. In OSPFv3, Address info has been removed from Types 1 and 2; but added or moved to new Type 8,9 LSA.
RFC5340#section-4.4.3.9 says "If the interface type is point- to-multipoint or the interface is in the state Loopback, the global scope IPv6 addresses associated with the interface (if any) are copied into the intra-area-prefix-LSA with the PrefixOptions LA-bit set, the PrefixLength set to 128, and the metric set to 0. Otherwise, the list of global prefixes configured in RTX for the link are copied into the intra-area-prefix-LSA by specifying the PrefixLength, PrefixOptions, and Address Prefix fields. The Metric field for each of these prefixes is set to the interface's output cost.
This "metric 0" is also listed in OSPFv2, " Router-LSAs If the state of the interface is Loopback, add a Type 3 link (stub network) as long as this is not an interface to an unnumbered point-to-point network. The Link ID should be set to the IP interface address, the Link Data set to the mask 0xffffffff (indicating a host route), and the cost set to 0. (RFC2328#page-12, 12.4.1). Note that Cisco IOS does not follow this here, which brings me to next section.
Following is not included in OSPFv3 but exists in OSPFv2 section 9 " [ Loopback ] The interface will be unavailable for regular
data traffic. However, it may still be desirable to gain information on the quality of this interface, either through sending ICMP pings to the interface or through something like a bit error test. For this reason, IP packets may still be addressed to an interface in Loopback state. To facilitate this, such interfaces are advertised in router-LSAs as single host routes, whose destination is the IP interface address.[4]
Further OSPFv2 Section C.7 "Host route parameters" mentioned that for such Host Routes "the actual configured cost in many cases is unimportant (i.e., will have no effect on routing). This could be why they changed Loopback metric 0 to 1.
Regards, ML
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