R4 is an ASBR and is redistributing the external prefix 2004:DB8::/64 into OSPF domain,therfore R4 creates and floods an LSA Type 5 for the external prefix as shown by the following output: R1#show ipv ospf database external OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 230 LS Type: AS External Link Link State ID: 0 Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0xDCE5 Length: 36 Prefix Address: 2004:DB8:: Prefix Length: 64, Options: None Metric Type: 2 (Larger than any link state path) Metric: 20 R2 and R3 create and flood an inter-area router LSA (LSA Type 4) to R1 and tell him how to reach the ASBR R4 as shown by the two LSAs displayed with the show ipv ospf database inter-area router command below, notice the Destination Router ID field which is the router-ID of R4: R1#show ipv ospf database inter-area router OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Inter Area Router Link States (Area 0) Routing Bit Set on this LSA LS age: 289 Options: (V6-Bit, E-Bit, R-bit, DC-Bit) LS Type: Inter Area Router Links Link State ID: 67372036 Advertising Router: 2.2.2.2 LS Seq Number: 80000001 Checksum: 0x2B73 Length: 32 Metric: 64 Destination Router ID: 4.4.4.4 Routing Bit Set on this LSA LS age: 289 Options: (V6-Bit, E-Bit, R-bit, DC-Bit) LS Type: Inter Area Router Links Link State ID: 67372036 Advertising Router: 3.3.3.3 LS Seq Number: 80000001 Checksum: 0xD8D Length: 32 Metric: 64 Destination Router ID: 4.4.4.4 R1 examines the external LSA 2004:DB8::/64 created by the ASBR R4,it looks that the Forward Address is not set then it calculates how to reach the ASBR. R1 examines the inter-area router LSAs for R4, created by the ABRs R2 and R3. After doing so, R1 knows that the ASBR R4 is reachable through the two ABRs. R1 calculates the best metric to reach the ASBR R4. The metric to reach the ASBR is equal to the metric R3s metric to reach the ABR, plus the ABRs metric to reach the ASBR listed in the LSA Type 4. The metric to the ASBR through R2 is:64+64=128 The metric to the ASBR through R3 is:64+64=128 Because the paths through the two ABRs R2 and R3 have the same cost toward the ASBR R4, R1 installs a load balancing to the external prefix 2004:DB8::/64: R1#show ipv6 route | s 2004:DB8::/64 OE2 2004:DB8::/64 [110/20] via FE80::C802:34FF:FE9C:8, Serial1/1 via FE80::C801:7FF:FE68:8, Serial1/0 To see The metric to the ASBR R4 and the ABRs R2 and R3, we can use the show ipv ospf border-routers: We can see that R4 (4.4.4.4) is reachable through two paths via serial1/0 and s1/1 interfaces with the same cost 128. R1#show ipv ospf border-routers OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Codes: i - Intra-area route, I - Inter-area route I 4.4.4.4 [128] via FE80::C801:7FF:FE68:8, Serial1/0, ASBR, Area 0, SPF 13 I 4.4.4.4 [128] via FE80::C802:34FF:FE9C:8, Serial1/1, ASBR, Area 0, SPF 13 i 2.2.2.2 [64] via FE80::C801:7FF:FE68:8, Serial1/0, ABR, Area 0, SPF 13 i 3.3.3.3 [64] via FE80::C802:34FF:FE9C:8, Serial1/1, ABR, Area 0, SPF 13 R1# Lets configure area 1 as not-so-stubby-area (NSSA): The ASBR R4 creates an LSA Type 7 and floods this LSA in the area 1: R4#show ipv ospf database nssa-external OSPFv3 Router with ID (4.4.4.4) (Process ID 1) Type-7 AS External Link States (Area 1) LS age: 297 LS Type: AS External Link Link State ID: 1 Advertising Router: 4.4.4.4 LS Seq Number: 80000001 Checksum: 0x7ABE Length: 52 Prefix Address: 2004:DB8:: Prefix Length: 64, Options: P Metric Type: 2 (Larger than any link state path) Metric: 20 Forward Address: 2034:34::4 When two NSSA ABRs receives an LSA Type 7, the ABR with a higher router-ID wins and translates this LSA into LSA Type 5,in this case R3 converts the LSA 7 to LSA 5 because it has a higher router-id than R2(3.3.3.3 and 2.2.2.2 respectively),below we can see that R1 receives an LSA Type 5 from R3 with the Advertising Router :3.3.3.3 R1#show ipv ospf database external OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 72 LS Type: AS External Link Link State ID: 0 Advertising Router: 3.3.3.3 LS Seq Number: 80000001 Checksum: 0x9C8B Length: 52 Prefix Address: 2004:DB8:: Prefix Length: 64, Options: None Metric Type: 2 (Larger than any link state path) Metric: 20 Forward Address: 2034:34::4 Lets see the routing table of R1, it installs an OE2 route through R3 (s1/1): R1#show ipv route | s 2004:DB8::/64 OE2 2004:DB8::/64 [110/20] via FE80::C802:34FF:FE9C:8, Serial1/1 How to explain this behavior? Lets see the LSA Type 5 received by R1 once again: Notice now the Forward Address is set to a non-zero value 2034:34::4: R4 does redistribution of the 2004:DB8::/64 prefix as a Type-7 LSA . In this case the forwarding address will be an interface attached to R4. R2 and R3 which are ABRs will retain the forwarding address when translating Type-7 to Type-5 LSA. From RFC 5340: Section A.4.7: If included, data traffic for the advertised destination will be forwarded to this address. It MUST NOT be set to the IPv6 Unspecified Address (0:0:0:0:0:0:0:0) or an IPv6 Link-Local Address (Prefix FE80/10). While OSPFv3 routes are normally installed with link-local addresses, an OSPFv3 implementation advertising a forwarding address MUST advertise a global IPv6 address. Section A.4.8: A global IPv6 address MUST be selected as forwarding address for NSSA-LSAs that are to be propagated by NSSA area border routers. The selection should proceed the same as OSPFv2 NSSA support [NSSA] with additional checking to ensure IPv6 link-local address are not selected. Therfore the forwarding address selection will be: The highest IP address from among the loopback interfaces on which OSPF is activated If there are no loopback interfaces, then the IP address of the physical interface on which OSPF is activated. R1#show ipv ospf database external OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 72 LS Type: AS External Link Link State ID: 0 Advertising Router: 3.3.3.3 LS Seq Number: 80000001 Checksum: 0x9C8B Length: 52 Prefix Address: 2004:DB8:: Prefix Length: 64, Options: None Metric Type: 2 (Larger than any link state path) Metric: 20 Forward Address: 2034:34::4 So what the router R1 will do? It looks the best intra-area or inter-area routes to reach this Forward Address 2034:34::4. Lets see the routing table of R1. R1 has an inter-area route through R3 to the Forward Address 2034:34::4: R1#show ipv route | s 2034:34::/64 OI 2034:34::/64 [110/128] via FE80::C802:34FF:FE9C:8, Serial1/1 After finding the best path via R3 to reach the Forward Address, R1 will use this path to reach the external prefix 2004:DB8::/64: R1#show ipv route | s 2004:DB8::/64 OE2 2004:DB8::/64 [110/20] via FE80::C802:34FF:FE9C:8, Serial1/1 What if we announce the loopback of R1 in the NSSA area? R4(config-if)#int lo1 R4(config-if)#ipv address 44::4/64 R4(config-if)#ipv ospf 1 area 1 Lets verify ; Now the loopback IPv6 44::4 is chosen instead. So since the loopback has a lower IPv6 but still is preferred we can see that loopbacks are preferred in the selection over the physical addresses: . R1#show ipv ospf database external OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 26 LS Type: AS External Link Link State ID: 0 Advertising Router: 3.3.3.3 LS Seq Number: 80000002 Checksum: 0xFC6E Length: 52 Prefix Address: 2004:DB8:: Prefix Length: 64, Options: None Metric Type: 2 (Larger than any link state path) Metric: 20 Forward Address: 44::4 R1 looks the best inter-area or intra-area routes to reach the FA 44::4 and finds that it has a load balancing via two inter-area routes through R2 and R3: R1#show ipv route | s 44:: OI 44::4/128 [110/128] via FE80::C802:34FF:FE9C:8, Serial1/1 via FE80::C801:7FF:FE68:8, Serial1/0 Finally we can see that R2 does load sharing across its links R2 and R3 toward the external prefix 2004:DB8::/64: R1#show ipv route | s 2004:DB8::/64 OE2 2004:DB8::/64 [110/20] via FE80::C802:34FF:FE9C:8, Serial1/1 via FE80::C801:7FF:FE68:8, Serial1/0 R1# What happen if we suppress the Forward Address in the LSA Type 5 created by R3? To suppress the FA there is the The OSPF Forwarding Address Suppression feature by using the area 1 nssa translate type7 suppress-fa command. The OSPF Forwarding Address Suppression in Translated LSA Type 5 feature causes a the NSSA ABR R3 to translate the LSA Type 7 to LSA Type 5, but use the zero value for the forwarding address instead of that specified in the LSA Type 7. This feature causes R3 not to advertise forwarding addresses into the backbone to direct forwarded traffic to the translating NSSA ABR R3. Lets verify : R3(config-rtr)#area 1 nssa translate type7 suppress-fa Lets see the LSA Type created by R3. We can see that the FA is not set,it disappears: R1#show ipv ospf database external OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 153 LS Type: AS External Link Link State ID: 0 Advertising Router: 3.3.3.3 LS Seq Number: 80000003 Checksum: 0xF6CD Length: 36 Prefix Address: 2004:DB8:: Prefix Length: 64, Options: None Metric Type: 2 (Larger than any link state path) Metric: 20 As a result R1 looks the best path to reach the NSSA ABR R3 and finds that its through the interface s1/1 which is directly connected to R3: R1#show ipv route | s 2004:DB8::/64 OE2 2004:DB8::/64 [110/20] via FE80::C802:34FF:FE9C:8, Serial1/1 The cost to reach the NSSA ABR R3 is 64 which the cost of the serial 1/1 interface as shown by the show ipv ospf border-routers command: R1#show ipv ospf border-routers OSPFv3 Router with ID (1.1.1.1) (Process ID 1) Codes: i - Intra-area route, I - Inter-area route i 2.2.2.2 [64] via FE80::C801:7FF:FE68:8, Serial1/0, ABR/ASBR, Area 0, SPF 15 i 3.3.3.3 [64] via FE80::C802:34FF:FE9C:8, Serial1/1, ABR/ASBR, Area 0, SPF 15 After suppressing of the Forward Address 44::4 in the LSA type 5, R3 tells to R1 to direct forwarded traffic of the external prefix 2004:DB8::/64 to the translating NSSA ABR R3.
Posted on: Thu, 13 Nov 2014 17:46:09 +0000