Was going though some of the Blog like (IPexpert,http://lostintransit.se), as per the blog below the category to choos Forward address in OSPF database:
" first it will choose the highest loopback that is on the ASBR and that is also being advertised into the area. If there is no loopback, it will go with the highest interface that is running OSPF and also advertised into the NSSA".
While doing practice i found one wierd ouput which opposes the selection of NSSA forward address of Highest Physical interface ID as per blog. Below is the excercise i have done and as per my excercise " It Chose the interface with lower IP address ".
Can any one help me out to overcome my observation....
RFC 1587 that codifies the NSSA area type has the following to say about the forwarding address selection:
If the network between the NSSA AS boundary router and the adjacent
AS is advertised into OSPF as an internal OSPF route, the forwarding
address should be the next hop address but if the intervening network
is not advertised into OSPF as an internal OSPF route, the forwarding
address should be any one of the router's active OSPF interface
Note that the RFC 1587 does not mandate which exact address out of potentially many IP addresses an NSSA ASBR has will be used as the forwarding address. Hence, according to the standard, this is left to individual implementors' choice.
Support for updated NSSA RFC 3101 has been added only to most recent IOSes. Even this RFC is not more specific in this regard:
When a router is forced to pick a forwarding address for a Type-7
LSA, preference should be given first to the router's internal
addresses (provided internal addressing is supported). If internal
addresses are not available, preference should be given to the
router's active OSPF stub network addresses. These choices avoid the
possible extra hop that may happen when a transit network's address
is used. When the interface whose IP address is the LSA's forwarding
address transitions to a Down state (see [OSPF] Section 9.3), the
router must select a new forwarding address for the LSA and then re-
originate it. If one is not available the LSA should be flushed.
So even though different articles out there about Cisco's NSSA may state different things, RFCs as the authoritative source do not mandate any particular way of selecting the highest or the lowest ASBR's IP address. The only mandated requirement is that if the network connecting the ASBR and the next hop of the redistributed routes is added to OSPF as an internal network, the next hop will be preserved. Otherwise, any IP address on the ASBR's active interfaces advertised in OSPF shall be used.
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