OSPF's configuration verification about Forwarding-Address
Personal understanding, time relationship, the writing is relatively messy, please forgive me.
Topology:
R2 is ASBR. On R1, we see that the FA of LSA5 class sent by R2 is not zero, which is a unicast address.
Key configurations:
[R1] dis cur
#
Sysname R1
#
Router id 91.1.1.1
#
Interface GigabitEthernet0/0/0
Ip address123.1.1.1 255.255.255.0
#
Interface LoopBack0
Ip address1.1.1.1 255.255.255.0
#
Ospf 10
Area 0.0.0.0
Network1.1.1.1 0.0.0.0
Network123.1.1.1 0.0.0.0
#
Ip route-static 0.0.0.0 0.0.0.0GigabitEthernet0/0/0
#
Dis cur
#
Sysname R2
#
Router id 92.2.2.2
#
Interface GigabitEthernet0/0/0
Ip address 123.1.1.2 255.255.255.0
Undo icmp redirect send
#
Interface LoopBack0
Ip address 2.2.2.2 255.255.255.0
#
Ospf 10
Import-route static
Area 0.0.0.0
Network 2.2.2.2 0.0.0.0
Network 123.1.1.2 0.0.0.0
#
Ip route-static 3.3.3.3255.255.255.255 123.1.1.3
#
[R3] dis cur
#
Sysname R3
#
Router id 93.3.3.3
#
Interface GigabitEthernet0/0/0
Ip address 123.1.1.3 255.255.255.0
#
Interface LoopBack0
Ip address 3.3.3.3 255.255.255.0
#
Ip route-static 0.0.0.0 0.0.0.0GigabitEthernet0/0/0
#
The main configuration is written on R2, where
Interface GigabitEthernet0/0/0
Ip address 123.1.1.2 255.255.255.0
Undo icmp redirect send
Turning off ICMP redirection only means that there is not much connection between IP redirection and OSPF's FA.
The other thing is to note that one of the non-zero conditions for an FA address is to generate a unicast address for it:
Ip route-static 3.3.3.3255.255.255.255 123.1.1.3
This 123.1.1.3 is the unicast address generated for FA, which will be written into the FA of the LSA5 class to guide the optimal routing of R1.
The following is a comparison between before and after R2 wrote the 123.1.1.3 address to FA:
[R1] dis ospf lsdb ase
OSPF Process 10 with Router ID 91.1.1.1
Link State Database
Type: External
Ls id: 3.3.3.3
Advrtr: 92.2.2.2
Ls age: 86
Len: 36
Options: E
Seq#: 80000001
Chksum: 0xb57
Netmask: 255.255.255.255
TOS 0 Metric: 1
E type: 2
Forwarding Address: 0.0.0.0
Tag: 1
Priority: Medium
[R1] dis ospf lsdb ase
OSPFProcess 10 with Router ID 91.1.1.1
Link State Database
Type: External
Ls id: 3.3.3.3
Advrtr: 92.2.2.2
Ls age: 11
Len: 36
Options: E
Seq#: 80000001
Chksum: 0x9c45
Netmask: 255.255.255.255
TOS 0 Metric: 1
E type: 2
Forwarding Address: 123.1.1.3
Tag: 1
Priority: Medium
When R1 receives the LSA5 address sent by R2 of ASBR with FA not 00:00, R1 first encapsulates ARP and goes to Ethernet to find the corresponding MAC address of FA. When receiving the reply from the other party, the encapsulation of IP packet succeeds and the following communication is OK. As shown in the demonstration of wireshark packet capture, the communication packet of ping3.3.3.3 on R2 is captured:
