area . nssa translate type7 suppress-fa and the FA Address
The OSPF Forwarding Address Suppression in Translated Type-5 LSAs feature causes a not-so-stubby area (NSSA) area border router (ABR) to translate Type-7 link state advertisements (LSAs) to Type-5 LSAs, but use the address 0.0.0.0 for the forwarding address instead of that specified in the Type-7 LSA. This feature causes routers that are configured not to advertise forwarding addresses into the backbone to direct forwarded traffic to the translating NSSA ABRs For Example:
SW1#show ip ospf database external 2.2.2.0 OSPF Router with ID (9.9.9.9) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 58 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 2.2.2.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0xAC5A Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 192.168.24.2 --------- Forwarding address External Route Tag: 0
This could happen if you do not know how to reach the Forwarding address (192.168.24.2)
SW1#show ip route 192.168.24.2 % Network not in table SW1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 4.4.4.4 1 FULL/BDR 00:00:32 192.168.14.4 Vlan14 1.1.1.1 255 FULL/DR 00:00:37 192.168.100.1 Vlan100 10.10.10.10 1 FULL/BDR 00:00:36 192.168.100.10 Vlan100 SW1#show ip ospf database OSPF Router with ID (150.150.0.1) (Process ID 100) OSPF Router with ID (9.9.9.9) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 86 0x80000142 0x00BB4E 4 4.4.4.4 4.4.4.4 20 0x80000005 0x00EA1B 1 6.6.6.6 6.6.6.6 897 0x8000012B 0x005A47 3 9.9.9.9 9.9.9.9 25 0x80000143 0x006822 4 10.10.10.10 10.10.10.10 87 0x8000002B 0x00766C 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 192.168.14.10 9.9.9.9 26 0x80000001 0x00DB84 192.168.100.1 1.1.1.1 70 0x80000002 0x00F71A Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 2.2.2.0 4.4.4.4 16 0x80000001 0x00AC5A 0
The 192.168.24.2 was filtered, lets look at the External OSPF database
SW1#show ip ospf database external 2.2.2.0 OSPF Router with ID (150.150.0.1) (Process ID 100) OSPF Router with ID (9.9.9.9) (Process ID 1) Type-5 AS External Link States LS age: 23 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 2.2.2.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0xAC5A Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 192.168.24.2 --------- Forwarding address External Route Tag: 0 SW1#show ip route 192.168.24.0 % Network not in table SW1#ping 2.2.2.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) SW1#
We see that we still have the Forwarding Address there, even if the route is not present in the Routing Table. Not lets use the area . nssa translate type7 suppress-fa command
Serial0.42 from LOADING to FULL, Lo R4(config-router)#router ospf 100 R4(config-router)# R4(config-router)#area 42 nssa translate type7 suppress-fa R4(config-router)#
SW1#show ip ospf database external 2.2.2.0 OSPF Router with ID (150.150.0.1) (Process ID 100) OSPF Router with ID (9.9.9.9) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 8 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 2.2.2.0 (External Network Number ) Advertising Router: 4.4.4.4 LS Seq Number: 80000002 Checksum: 0x2564 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 --------- Forwarding address modified External Route Tag: 0 SW1#ping 2.2.2.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 59/62/67 ms SW1# SW1#show ip route 2.2.2.2 Routing entry for 2.2.2.0/24 Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 1 Last update from 192.168.14.4 on Vlan14, 00:02:09 ago Routing Descriptor Blocks: * 192.168.14.4, from 4.4.4.4, 00:02:09 ago, via Vlan14 Route metric is 20, traffic share count is 1 SW1#show ip route 0.0.0.0 % Network not in table SW1#
In short, if routers do not have the knowledge on how to reach the forwarding address, due to some kind of lsa filtering, you can suppress the FA route advertisement and suppress this value, this would results that the FA be equal to 0.0.0.0 which forces the use of the ASBR to reach the destination.
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Clear and precise information.