- Areas are identified by a 32-bit Area ID. The Area ID may be expressed either as a decimal number or in dotted decimal, and the two formats may be used together on Cisco routers. The choice usually depends on which format is more convenient for identifying the particular Area ID.
area 0 and area 0.0.0.0 are equivalent, as are area 16 and area 0.0.0.16, and area 271 and area 0.0.1.15.
router ospf 1
network 155.1.146.1 0.0.0.0 area 0.0.0.0
network 155.1.147.1 0.0.0.0 area 0.0.1.16
LSA filtering is done in two ways:
- Area Types
- LSA 3 filtering see filtering below
note:
- When only a single area network is used, it doen not have to be area 0.
- The rule is that all areas must connect to the backbone; therefore, a backbone area is needed only if there is more than one area.
1. Backbone (transit) - carries all LSA types.
2. Normal Area - carry all LSA types, type1, type2, type3, type4, type5
3. Stub Areas - Type 4 ASBR summary LSAs and 5 AS External LSAs are not flooded into stub areas.
- Still receives inter-area type 3 LSA's.
- ABRs at the edge of a stub area use network summary (type 3) LSAs to advertise a single default route (0/0) into the area, for destination external to the AS.
- The ABR will advertise this default route with a cost of 1.
- This default cost can be changed with the "area default-cost" command.
- Configured on ALL routers in the stub area with "area stub" command.
- will not accept external routes, accept anything internal and default route.
- LSA view: type1, type2, type3 and 0/0 route
Stub area restrictions:-
- All routers in a stub area must have identical link-state databases, and agree to be stub.
- To ensure this condition, all stub routers will set a flag (the E-bit) in their hello packets to zero.
- They will not accept hellos with E=1. (If the E-bit = Evil-bit, then Stub-Area = Holy-Area)
- Virtual links cannot be configured within, nor transit, a stub area.
- No router within a stub area can be an ASBR or perform any type of redistribution, including static and connect.
command:
- Configures attached area 1 as a stub area, required on all area routers
- Shows the default route in the routing table as 'O*IA 0.0.0.0/0'
router ospf 99
area 1 stub 4. Totally Stubby Areas (cisco specific)
- Uses a default route to reach not only destinations external to the autonomous system but also
all destinations outside the area.
- The ABR of Totally Stubby Area will block all summary LSAs with the exception of a single type3 LSA to advertise a default route (0/0).
- Configured with "area stub no-summary", which is necessary only at the ABR/s; the internal routers use the standard stub area configuration.
- LSA VIEW: type 1 and type 2 + 0/0 default route.
- does not allow any type 3 to come in. ( all other internal area routes are blocked)
- one single type 3 LSA default route from ABR.
- it can reach other areas but dont have the detail of the network because it only allows default route.
command:
- Configures attached area 2 as a totally stubby area, only needed on ABR's
- Shows the default route in the routing table as 'O*IA 0.0.0.0/0'
router ospf 99
area 1 stub no-summary5. NSSA (Not-So-Stubby Areas)
- An area that allows redistributed while retaining the characteristics of a stub area to the rest of the AS.
- No type 4 and 5 LSA's, but redistribution is allowed, ie AS-external routes.
- The ASBR in an NSSA will originate type 7 LSAs to advertise the external destinations.
- These NSSA external LSAs are flooded throughout the NSSA but are blocked at the ABR.
- The NSSA ASBR has the option of setting or clearing the P-bit.
- If the NSSA's ABR receives a type 7 LSA with the P-bit set to one, the type 7 LSA translates into a type 5 LSA before being flooded to other areas.
- If the P-bit is set to zero, no translation will take place and the destination in the type 7 LSA will
not be advertised outside of the NSSA.
- Configured on ALL routers in the NSSA area with "area nssa".
- Biggest difference to a stub area, redistribution is allowed, and no default route by default is sent into the area.
- With NSSA, the ABR does not originate a default route.
- To originate a default into a NSSA area, use the command "area nssa default-originate".
- LSA view: type1, type2, type3, type7
command:
- Configures attached area 3 to be nssa, required on all area routers
- NO default route is automatically generated.
router ospf 99
area 3 nssa- Configures attached area 4 to nssa, only needed on ABR's to generate the default
- Shows the default route in the routing table as 'O*N2 0.0.0.0/0'
router ospf 99
area 4 nssa default-information-originate
area 4 nssa default-information-originate
6. Totally NSSA
- ABR makes an NSSA totally stubby, with 'area nssa no-summary'.
- Removes inter-area (type 3) LSA’s.
- Removes external (type 4 and type 5) LSA’s.
- The ABR originates a default as 'O*IA'.
- Configured with 'area nssa no-summary', which is necessary only at the ABR; the internal routers use the standard NSSA area configuration.
- Allows redistribution into NSSA (LSA 7).
LSA view: type1, type2, type7 and 0/0 route
commands:
- Configures attached area 5 to totally-nssa, only needed on ABR's
- Shows the default route in the routing table as 'O*IA 0.0.0.0/0'
router ospf 99
area 5 nssa no-summary
- Configures attached area 6 to totally-nssa with default redistribution disabled
- Will show the type 3 default route in the routing table as 'O*IA 0.0.0.0/0'
router ospf 99
area 6 nssa no-redistribution no-summary
additional notes:
- All routers in a STUB or NSSA must agree on the STUB or NSSA flag. It is the ABR(s) of the stub or NSSA area that determines if it is totally-stubby or totally-NSSA by adding the keyword "no-summary" onto the stub/nssa command.
- The ABR generates the type 4 LSA. If the area is configured as a stub area, the ABR filters the type 5 LSAs(generated by the ASBR) and does not generate a type 4 LSA. So, technically, an OSPF stub configuration only explicitly filters type 5 LSAs, but it implicitly filters type 4 LSAs as well as there is no need for the ABR to generate a type 4 LSA.
- When an ABR is also an ASBR and is connected to a NSSA, the default behaviour is to advertise the
redistributed routes into the NSSA.
- This redistribution can be turned off by adding the 'no-redistribution' keyword to the "area nssa" command.
- Suppress OSPF forwarding address in translated type-5 LSAs
- This is used when an NSSA ABR translates type 7 LSAs to type 5 LSAs, but use the 0.0.0.0 as the forwarding address instead of the address specified in the type 7 LSA.
- Routers which are configured not to advertise forwarding addresses into the backbone, will directly forwarded traffic to the translating NSSA ASBRs.
======================================================
AREA TYPE STOPS TYPE5 STOPS TYPE3 ALLOW TYPE7
stub yes no no
totally stubby yes yes no
NSSA yes no yes
totally NSSA yes yes yes
======================================================
area type options:
[stub] blocks type 4 and type 5 LSA's
[no-summary] blocks type 3 LSA's except the default route type 3 LSA
[nssa] blocks type 4 and type 5 LSA's, but allows type 7 redistribution
[no-redistribution] blocks type 7 LSA's
command:
- Changes the cost of the default route advertised by the ABR. (default = 1)
router ospf 99
area 1 default-cost {cost}
- Configures a nssa, allowing type 3, blocking type 4, 5 and 7
- Will show the type 7 default route in the routing table as 'O*N2 0.0.0.0/0'
router ospf 99
area 7 nssa no-redistribution default-information-originate
- Suppresses the inclusion of a forwarding address when translated into type 5 LSAs
router ospf 99
area 8 nssa translate type7 suppress-fa
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