Thursday 29 September 2016

Understanding OSPF stub, NSSA, totally stub and totally NSSA's

Stub Areas
OSPF makes use of stub areas to control the advertisements of routes into an area - it does this by designating a stub interface on an ABR (Area Border Router.) By doing this you are able to supress external route advertisements through the ABR (keeping LSA flooding to a minimum) otherwise the routing table could get very large - instead of advertising the external routes the ABR will advertise itself as a default route instead.

For example OSPF has several LSA types:

Type 1: Router LSA – This LSA is generated by each router in each area it is present – the LSA contains the router’s ID.

Type 2: Network LSA -  These LSA’s are generated by the DR (Designated Router) and contain the router-id of the DR and all of the routers attached to the transit network – they are only flooded within the originating area only. They are not generated on non-broadcast / point to point links since a DR will not be elected.

Type 3: Summary LSA – These LSA’s are generated by the ABR (Area Border Router) – they represent all of the connected networks in the current area and the adjacent area to it and are passed to other ABR’s to inform them of these networks.

Type 4: Summary ASBR – These LSA’s are generated by ABR’s and contain routes to ASBR (Autonomous System Boundary Routers.) The link-state ID is that of the ASBR being advertised.

ASBR (Autonomous System Border Router):
This router is elected to communicate between different autonomous systems.

Type 5: External LSA – This type of LSA is generated by ASBR’s and contain routes networks that are not part of the AS (autonomous system.) – for example another OSPF or EIGRP domain.

Type 6: Multicast LSA – Used for multicast applications.

Type 7: NSSA External LSA – These LSA’s allow you to inject external routes into a NSSA (Not-so-stubby-area) – usually Type 5 LSA’s do this job, but they are not allowed in stub areas so we used Type 7 instead.

Stub Areas do not accept type 5 (external) LSA’s or ASBR’s – meaning that they can’t accept external routes.

Totally Stub Areas
These areas block type 3 (summary LSA’s), type 5 (external LSA’s) and do not accept ASBR’s – like a normal stub area external routes are not accepted with the addition of type 3 LSA’s as well – meaning that the ABR will simply (and only) present a default gateway (of itself) to the devices on the network.

NSSA (Not so stubby area)
NSSA’s are the same as stub areas although they allow ASBR’s to be present within the area – since stub areas do not allow type 5 (external LSA’s) we need to use type 7 (NSSA external LSA’s) LSA’s instead.

Totally NSSA
This type of area block type 3 (summary) and 5 (external) LSA’s and allows ASBR in the same area.

Route redistribution
Route redistribution is the process of redistributing routes from one OSPF domain to another or routing protocol to another – for example EIGRP to OSPF – I will cover this process in a later tutorial.


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