OSPF Forward Address Suppression
Share this page:The aim is to SUPRESS the address of the router that originated the Prefix. When the area is NSSA, and you want to CONTROL the remap process of the LSA7 to LSA5, but use 0.0.0.0 as the forwarding address instead of the one specified in the LSA7:
(config-router)#area 1 nssa translate type7 suppress-fa ?
default-information-originate Originate Type 7 default into NSSA area
no-redistribution No redistribution into this NSSA area
no-summary Do not send summary LSA into NSSA
Before the command has been applied the external (LSA5) subnet within the area 0 is seen as:
#sh ip ospf database external 6.0.0.0
OSPF Router with ID (1.1.1.1) (Process ID 1)
Type-5 AS External Link States
LS age: 557
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 6.0.0.0 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000003
Checksum: 0x1286
Length: 36
Network Mask: /8
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 200.1.36.6
External Route Tag: 0
While after the command has been implemented, we have:
#sh ip ospf database external 6.0.0.0
OSPF Router with ID (1.1.1.1) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 41
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 6.0.0.0 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000004
Checksum: 0x3952
Length: 36
Network Mask: /8
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 0.0.0.0 <- THE FORWARD ADDRESS HAD CHANGED
External Route Tag: 0
If you add “no-summary” to this command, LSA3 s are filtered, and the default route is advertised instead.
You can use the similar approach to NOT ADVERTISE THE SPECIFIC PREFIXES into the NSSA, but advertise only the defult route on the ABR. In this example the Area 1 is NSSA:
(config-router)#area 1 nssa default-information-originate no-summary
Area 1 (NSSA Area) will learn the Default Route as the LSA7 (N2):
#sh ip route
…
Gateway of last resort is 205.1.36.3 to network 0.0.0.0
O*N2 0.0.0.0/0 [110/1] via 205.1.36.3, 00:05:21, Serial1/0.63
1.0.0.0/32 is subnetted, 1 subnets
!!!Regardless of whether you´re using the “nssa default-information-originate” or the “nssa no-summary” command
in the OSPF Area, the Default Route will be injected into that area. The difference is the route type:
NSSA NO-SUMMARY
Gateway of last resort is 10.1.34.3 to network 0.0.0.0
O*IA 0.0.0.0/0 [110/65] via 10.1.34.3, 00:04:22, Serial1/0.43
NSSA DEFAULT-INFORMATION-ORIGINATE
Gateway of last resort is 10.1.35.3 to network 0.0.0.0
O*N2 0.0.0.0/0 [110/1] via 10.1.35.3, 00:00:22, Serial1/0.53
1.0.0.0/32 is subnetted, 1 subnets