Area ID Change Functionality ..

Changing area-id for an area is useful for link state routing protocols in order to merge two areas into one or to split an area into several areas.  
– An area address is a variable length quantity.  
– An area can have multiple area addresses. Neighboring IS’s will not form an adjacency unless they have a single area address in common.  This is quite useful for IP networks that are transitioning from one area address to another, merging two areas into one or even to split an area into several pieces.   
– Seamless transition of area addresses for an area is easier in IS-IS, e.g. initially an area can have area adress A, then the set {A, B} and when the new area address B is recognized by all the routers in the area, old area address A can be removed.  


– In OSPF each area has a single ID, a 4-byte quantity.   
– OSPF does not have the ability to merge and split areas dynamically as IS-IS has, though partitioned backbone can be repaired by using virtual link. But it should be ensured that the area through which virtual link is configured is having full routing information, i.e. it should not be a stub area.   
– Area-id can not be changed dynamically in case of OSPF.   


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