CCNP ROUTE OSPF STUB AREA

LAB ON YOUTUBE:

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OSPF Filtering Overview

OSPF is a link-state routing protocol

To calculate identical SPFs everyone must have the same input to SPF (the LSDB)

implies that filtering cannot be configured within an area

Inter-area filtering through

 stub areas

 LSA 3 filter

OSPF path selection order 

intra area routes O

inter area routes OIA

External Type 1 E1

External Type 2 E2

NSSA Type 1 N1

NSSA Type 2 N2

 Stub areas used to limit type of LSAs allowed to enter an area

  Intra Area routes (O) 

 LSA 1 & 2

 Inter Area routes (OIA)

 LSA 3 & 4

 External routes (E1 & E2)

 LSA 5

 NSSA external routes (N1 & N2)

 LSA 7

 All routers in the area must agree on the stub flag

OSPF stub areas:

 stub area 

 removes external routes (LSA 5)

 removes ASBR advertisement (LSA 4)

 ABR originates inter-Area default route (LSA3)

 Enabled on all routers in the area 

 area [area ] stub

any time when we are using the subnets and using the 

default route , we are loosing the visibility of the

network .(in case when we have multiple exit points)

Controlling NSSA redistribution

Redistribution routes on an NSSA router are originated as LSA 7
If NSSA ASBR is also ABR....
- Type 7 originated into NSSA
- Type 5 originated into Area 0

Type 7 originated can be suppressed 
- area [area] nssa no-redistribution
- send type 5 to area 0 but not Type 7 to NSSA


area1----asbr-------area0
 7        |         5
  |
 rip
 while redistributing rip , we can suppress type7 lsa.


 NSSA Translator Election:
 NSSA ABR translates Type 7 LSAs into Type 5 for Area0 advertisement
  if multiple ABRs, only one of them performs translation
  NSSA Translator Election chooses ABR with higher Router-id
  If Forwarding Address is non-zero, ABR need not be in the transit path.
   
 LSA 3 Filter
 Stub areas can only filter on LSA type
 ABRs can filter which summary LSAs (LSA 3) they generated bet areas
 Applied to process level of ABR
 area [area] filter-list prefix [prefix-list] [in | out]
  in/out allows for control of ABRs with more than 2 areas

Stub Area
Redistributed routes a.k.a. OSPF external routes are not advertised into a stub area. OSPF inter-area routes are advertised into the area, and the ABR will inject a default route into a stub area.

Totally Stubby Area
Totally Stubby takes it a step further… OSPF external routes and inter-area routes are not advertised into a Totally Stubby Area. The ABR injects a default route into a Totally Stubby Area.

Not So Stubby Area (NSSA)
This is where it gets somewhat tricky… OSPF external routes are not advertised into a Not So Stubby Area (NSSA) just like in a stub area. OSPF inter-area routes are advertised into an NSSA just like in a stub area. By default, the ABR will not inject a default route into the NSSA. The big difference though is that NSSA allows you to redistribute external routing information into the area. The ASBR generates a Type 7 LSA that contains the external routing information. The ABR receives each Type 7 LSA, translates it to a Type 5, and floods it into area 0.

Totally Not So Stubby Area (Totally NSSA)
Just like in a Totally Stubby Area, OSPF external routes and inter-area routes are not advertised into a Totally Not So Stubby Area (Totally NSSA). In this case the ABR will inject a default route into the Totally NSSA. Just like in NSSA, the ASBR generates a Type 7 LSA that contains the external routing information. The ABR receives each Type 7 LSA, translates it to a Type 5, and floods it into area 0.

Why is this important? The general design rule to follow with OSPF is to “separate complexity from complexity.” For example, you would want to separate a large hub and spoke topology from the rest of the network, so you would place the hub and spoke topology in a separate area or flooding domain. A failure of one of the spoke routers doesn’t impact the rest of the network outside of the area. Hiding or limiting topology information makes the network more stable, helps provide faster convergence, and allows for scalable OSPF routing design. Stub areas optimize this concept of reducing flooding and they effectively limit the size of the flooding domain.

The stub area types can be summarized as follows:

STUB TYPES	KEYWORDS	LSAs	DEFAULT INJECTED
STUB	AREAXSTUB	1,2,3	YES
TOTALLY STUBBY	AREAXSTUB NO-SUMMARY	1,2,DEFAULT OF 3	YES
NOT-SO-STUB	AREAXNSSA	1,2,3,7	NO
NOT-SO-TOTALLY STUBBY	AREAXNSSA NO-SUMMARY	1,2,DEFAULT OF 3,7	YES