In-depth OSPF forwarding address resolution

In-depth resolution of OSPF Forwarding addresses FA is short for Forwarding Address. FA is the field in the TYPE 5 LSA advertised by ASBR. Its role is to tell the router in the OSPF domain how to quickly reach the next hop address of the route advertised by LSA 5. To avoid ASBR as the next hop on the broadcast network of the OSPF internal router, and then the ASBR forwards the next hop to the correct one, resulting in additional routes. To put it simply, the FA field is similar to the "third-party next hop" concept in the BGP protocol and mainly plays a role in the broadcast sharing network.
Five types of lsa fa fields can be set to 0 or not. There are several rules: 1. When it is associated with the next hop of the introduced route (interconnect) FA is set to 0 when OSPF is not started for the interface. For example, in the figure above, R1 introduces the route of the direct connection interface E0/1, and OSPF is not started for this interface, therefore, the FA field in LSA of TYPE 5 is 0. Www.2cto.com 2. When all the following conditions are met, the FA field in TYPE 5 LSA is set to non-. On ASBR, OSPF is enabled for the interface associated with the next hop of the introduced external route, that is, it is within the range of the Network command. 2. On the ASBR, the interface associated with the next hop of the introduced external route cannot be configured with a passive interface; // The first two rules ensure that the LSA can be advertised. 3. On the ASBR, the OSPF network type configured with the interface that introduces the next hop of an external route cannot be P2P or P2MP. // Ensure that the network type is Broadcast or NBMA shared network. After the preceding conditions are met, the forwarding addresses of the five LSA types are the interface IP addresses connected to the next hop of the external route, for example, R2E0/0 and R3E0/0. How does FR affect OSPF route selection? 1. When calculating external routes, first check whether the ASBR of advertised TYPE 5 LSA exists; otherwise, ignore the LSA; 2. Check whether the FA address of TYPE 5 LSA is 0. If the value is 0, the route must be forwarded by ASBR. Therefore, the shortest path to ASBR metric is preferred. When there are multiple shortest paths, it is preferred to learn from the larger area ID. 3. If FA is not 0, the shortest path of the FA address metric is preferred.
In this figure, R4 and R1 are in Area 0, R1 learns to external routes through RIP redistribution, and R2 and R3 both run RIP. The intermediate switch is used to simulate a bus network. R1 E0/1 and R2 R3 E0/0 are all in the same network segment. The configuration of R4 is as follows: www.2cto.com R4 (config) # router ospf 1R4 (config-router) # router-id 4.4.4.4R4 (config-router) # passive-inter deR4 (config-router) # no passive-int e0/0R4 (config-router) # network 4.4.4.4 0.0.0.0 area 0R4 (config-router) # network 10.0.1.0 0.0.3 area 0R4 (config-router) # network 10.1.0.0 0.0.255 area 0
R1 configuration: R1 (config-if) # router ospf 1R1 (config-router) # router-id 1.1.1.1R1 (config-router) # passive-inter deR1 (config-router) # no passive-inter e0/0R1 (config-router) # no passive-inter e0/1R1 (config-router) # network 172.16.0.0R1 (config-router) # network 172.16.1.0 0.0.0.255 area 0R1 (config-router) # network 1.1.1.1 0.0.0.0 area 0R1 (config-router) # network 10.0.1.0 0.0.0.3 area 0R1 (config-router) # redistribute rip subnets metric 1000 // type 1 overhead 1000R2 for re-distributing RIP to OSPF is configured as follows:
R2 (config) # router ripR2 (config-router) # ver 2R2 (config-router) # no auto-summaryR2 (config-router) # network 172.16.0.0R2 (config-router) # network 10.0.0.0R2 (config-router) # network 2.0.0.0R2 (config-router) # redistribute ospf 1 metric 5R3 configuration: R3 (config-if) # router ripR3 (config-router) # ver 2R3 (config-router) # no auto-summaryR3 (config-router) # network 172.16.0.0R3 (config-router) # network 10.0.0.0R3 (config-router) # network 3.0.0.0 when the network segment 172.16.1.0 is not published to OSPF on R1 (no network 172.16.1.0 0.0.0.255 area 0), view the route on R4: r4 # show ip route ospfO E1 10.2.0.0/24 [110/1010] via 10.0.1.2, 00:03:21, Ethernet0/0O E1 10.3.0.0/24 [110/1010] via 10.0.1.2, 00:03:21, Ethernet0/0
View 5 types of LSA: www.2cto.com LS age: 271 Options: (No TOS-capability, DC) LS Type: AS External Link State ID: 10.2.0.0 (External Network Number) Advertising Router: 1.1.1.1 LS Seq Number: 80000003 Checksum: 0x6610 Length: 36 Network Mask:/24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 1000 Forward Address: 0.0.0.0 // the forwarding address is 0.0.0.0 External Route Tag: 0
It can be found that when the forwarding address is 0.0.0.0, the Calculation of external routing overhead is the overhead to reach ASBR + External overhead = 10 + 1000 = 1010 when R1 releases the 172.16.1.0 network segment to OSPF (network 172.16.1.0 0.0.0.255 area 0) view routes on R4: R4 # show ip route ospfO E1 10.2.0.0/24 [110/1020] via 10.0.1.2, 00:00:16, Ethernet0/0O E1 10.3.0.0/24 [110/1020] via 10.0.1.2, 00:00:16, ethernet0/0 LS age: 172 Options: (No TOS-capability, DC) LS Type: AS External Link State ID: 10.2.0.0 (External Network Number) Advertising Router: 1.1.1.1 LS Seq Number: 80000004 Checksum: 0x6610 Length: 36 Network Mask:/24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 1000 Forward Address: 172.16.1.2 // the forwarding address is the IP External Route Tag: 0 LS age: 2796 Options: (No TOS-capability, DC) LS Type: AS External Link State ID: 10.3.0.0 (External Network Number) Advertising Router: 2.2.2.2 LS Seq Number: 80000002 Checksum: 0x4E24 Length: 36 Network Mask:/24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 1000 www.2cto.com Forward Address: 172.16.1.3 // the forwarding Address is the interface IP External Route Tag: 0 Link connected: a Stub Network class 1 LSA :( Link ID) Network/subnet number: 172.16.1.0 (Link Data) Network Mask: 255.255.255.0 Number of TOS metrics: 0 TOS 0 Metrics: 10
When the forwarding address is not 0, the calculation of the external route overhead is the overhead to reach the forwarding address + External overhead = 10 + 10 + 1000 = 1020 configure the passive interface (R1 (config-router) on R1) # passive-interface e0/1): Routing Bit Set on this lsa ls age: 19 Options: (No TOS-capability, DC) LS Type: AS External Link State ID: 10.2.0.0 (External Network Number) Advertising Router: 1.1.1.1 LS Seq Number: 80000005 Checksum: 0xF26 Length: 36 Network Mask:/24 Metric Type: 1 (Comparable directly to link s Tate metric) TOS: 0 Metric: 1000 Forward Address: 0.0.0.0 // All forwarding addresses of 5 LSA types change to 0.0.0.0 External Route Tag: 0. What will happen if there are seven LSA forwarding addresses?

Region 2 is the NSSA region, and RIP is re-released to OSPF. What is the forwarding address of the seven LSA types generated by RT2? After RT6 performs 7 to 5, what will happen to the forwarding address? For the configuration of this topology, see the previous blog: If the http://tangfangxiao.blog.51cto.com/2116646/632997 172.16.24.0/30 CIDR block is not released to OSPF, will the forwarding address of 7 LSA change to 0.0.0.0 like 5? If yes, after 7 to 5 is performed on RT6, the forwarding address is 0.0.0.0. Does this mean that the IP address reaches RT6 or the ASBR of RT2? Therefore, the forwarding address of LSA 7 cannot be 0.0.0.0 or 0.0.0.
When 172.16.24.0/24 is not published to OSPF, the forwarding address is Router-id. LS age: 324 Options: (No TOS-capability, Type 7/5 translation, DC) LS Type: AS External Link State ID: 172.16.4.0 (External Network Number) Advertising Router: 2.2.2.2 LS Seq Number: 80000001 Checksum: 0xB1BC www.2cto.com Length: 36 Network Mask:/24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 1000 Forward Address: 2.2.2.2 // the forwarding address is not 0 External Route Tag: 0
I re-specify the Router-id and restart the OSPF process: R2 (config-router) # do show ip ospf Routing Process "ospf 1" with ID 10.10.10.10 // The router-id is 10.10.10.10 Supports only single TOS (TOS0) routes Supports opaque LSA Link State ID: 172.16.4.0 (External Network Number) Advertising Router: 2.2.2.2 LS Seq Number: 80000002 Checksum: 0x0000c Length: 36 Network Mask:/24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 1000 Forward Address: 10.10.10.10 // the forwarding Address is 10.10.10.10 External Route Tag: 0
On RT6, the forwarding address of LSA after 7-class to 5-class will not change: LS age: 12 Options: (No TOS-capability, DC) LS Type: AS External Link // Class 5 LSA Link State ID: 172.16.4.0 (External Network Number) Advertising Router: 6.6.6.6 // advertise the Router LS Seq Number: 80000001 Checksum: 0x5FE8 Length: 36 Network Mask: /24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 www.2cto.com Metric: 1000 Forward Address: 10.10.10.10 // the forwarding Address remains unchanged
The other forwarding addresses are the same as those of the five LSA types. You can verify them and I will not write them! Summary: if the five types of LSA meet the following three conditions at the same time: a) This external route is introduced, the corresponding outbound interface enables OSPFb, the corresponding outbound interface is not set to passive-interfacec). The Type 5 LSA generated when the OSPF network Type of the corresponding outbound interface is broadcast, its FA address is equal to the next address of the introduced external route. Otherwise, the forwarding address of Type 0.0.0.0 (ASBR) 7 LSA is not 0.0.0.0, the following three conditions are met: the next address of the external route, and the opposite is the Router-id. Blog from the mortal world