LSA Type 1 - All SPF routers advertise their connected interfaces
LSA Type 2 - All DR routers advertise their connected interfaces for each different subnet
LSA Type 3 - Advertise by an ABR's, traverse areas and contain short description on their networks.
LSA Type 4 - Advertise by an ABR and point an ASBR host
LSA Type 5 - External routes (hence redistribution from other routing protocols) which have been injected by an ASBR into OSPF AS
LSA Type 7 - External routes (LSA Type 5) which have been translated to Type 7 and then advertised by an ASBR which reside in an NSSA area
NSSA area is similar to a stub area, an area which receive only Type 1,2 and 3 LSA's but in an NSSA also Type 7 (which are actually converted Type 5 LSA's) can traverse. In this way we can configure a stub area with external route connection.
Totally NSSA area, again similar to Totally stub area which allows type 1 and 2 and in the Totally NSSA also type 7.
Route Designators:
Route designators are the letters which can be found left to prefix routes when doing show ip route on a Cisco device,
E1 express external route with a full total cost and that mean the cost from the router (the one that you have typed show ip route) to the ASBR, Which has advertised the external route, and the cost or metric from the ASBR to the final destination, hence the advertised prefix/route.
E2, which is default on Cisco IOS, express only the cost/metric from the ASBR to the final destination. No cost calculation occurs regarding the way from the ASBR to the router.
N1 and N2 are similar to E1 and E2 equally beside the fact that both belong to an ASBR in an NSSA area.
And now finally we are ready to talk about Forwarding Address Suppression, take the following Lab for example:
R3, R4, R5 and R6 are all configured in an NSSA area, While R4,R5 and R6 redistribute their Loopback interface into OSPF, these networks are advertised as LSA Type 7 into area 1 and traverse into area 0, through R3, as LSA Type 5.
While we want to minimize unnecessary route advertisement and LSA flooding I have configured R3 to summarize the prefixes that connect R3 to R4,R5 and R6 (200.1.3x.0), this has been accomplished by the command area 1 range 200.1.3x.0 255.255.255.0 not-advertise on R3. But doing so has raised new problem - R6, for example, was advertised, by LSA Type 4, as the ASBR for prefix 6.0.0.0/8 and R3 has advertised this prefix to area 0 with LSA Type 5 that pointing R6 IP address as the forwarding address!
R2#show ip ospf database external 4.0.0.0
OSPF Router with ID (2.2.2.2) (Process ID 1)
Type-5 AS External Link States
LS age: 147
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 4.0.0.0 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000004
Checksum: 0xF3A9
Length: 36
Network Mask: /8
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 200.1.34.4
External Route Tag: 0
So if R2 for example doesn't know a way to 200.1.34.4 he can't send traffic for network 4.0.0.0/8.
The solution - using Forwarding address suppression which helps us to keep small route tables in one hand and still be able to reach external routes in the other.
Configuring on R3 - the ABR the following commands:
R3(config-router)#area 1 range 200.1.34.0 255.255.255.0 not-advertise
R3(config-router)#area 1 range 200.1.35.0 255.255.255.0 not-advertise
R3(config-router)#area 1 range 200.1.36.0 255.255.255.0 not-advertise
R3(config-router)#area 1 nssa translate type7 suppress-fa
The first three lines are doing summarize on the ABR for the following prefixes while not-advertise keep them from being advertised to area 0, If you recall this is one of the methods for filtering OSPF LSA type 3.
The last line change the LSA Type 7 forward address from its original address to 0.0.0.0, then convert it to LSA type 5 and forward it to area 0:
R2#show ip ospf database external 4.0.0.0
OSPF Router with ID (2.2.2.2) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 209
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 4.0.0.0 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000005
Checksum: 0x513B
Length: 36
Network Mask: /8
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
And by that allows R2 to reach prefix 6.0.0.0/8 although he doesn't know the IP address of the ASBR.
Configuring this feature causes the router to be noncompliant with RFC 1587. Also, suboptimal routing might result because there might be better paths to reach the destination's forwarding address. This feature should not be configured without careful consideration and not until the network topology is understood.
Cisco doc link: http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ftoadsup.html
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