Hello @yo8-20
"if only R1 loopback 0..." you want to summarize or restrict advertisement ?
Summarize one loopback ? Summarizing only one loopback interface might seem unusual in a such case. Generally, summarization is used to aggregate multiple networks or subnets to reduce the size of the routing table and optimize routing efficiency.
To go further with summarization:
To summarize routes in OSPF, you typically use the `area` command with the `range` option on the router that is the ABR between the areas being summarized. In your scenario, R2 is the ABR between Area 0 and Area 12. Assuming you want to summarize the loopback interfaces of R1 into a single summary route advertised into Area 0, here's how you can do it:
R2 Configuration
router ospf 1
area 10 range <summary_address> <mask>
Replace <summary_address> with the network address to be summarized, and <mask> with the corresponding subnet mask... For example, if R1's has 3 Loopback interfaces with IP addresses: 192.168.1.1/32, 192.168.1.10/32 and 192.168.1.250/24 and you want to summarize it into a single route ''192.168.1.0/32 to Area 10, you should configure:
router ospf 1
area 10 range 192.168.1.0 255.255.255.0
This configuration on R2 will summarize the routes within the specified range and advertise the summary route into Area 0. R3 will then only see this summarized route to reach R1's Loopback interfaces.
Best regards
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