Hi @xine32,
The benefit of choosing an area link as primary over the other for a connection between 2 ABRs is that one of your links can be selected as the preferred path for the area where it was configured as secondary if the main path in that specific area is not available.
Let's see the below example:
If R1 wants to reach R3's loopback IP 3.3.3.3, the the shortest path in area 100 is to reach it is through interface lt-0/0/0.1, the "show route" command confirms this.
labroot@R1:R1> show route 3.3.3.3
inet.0: 13 destinations, 13 routes (13 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
3.3.3.3/32 *[OSPF/10] 00:01:09, metric 1
> to 192.168.1.2 via lt-0/0/0.1
The traceroute confirms that the destination IP 3.3.3.3 is one hop away
labroot@R1:R1> traceroute 3.3.3.3
traceroute to 3.3.3.3 (3.3.3.3), 30 hops max, 40 byte packets
1 3.3.3.3 (3.3.3.3) 3.495 ms 0.805 ms 0.567 ms
What happens if interface lt-0/0/0.1 goes down for any reason?
The the traffic will look for the shortest intra-area path in area 100, that path will be R1-R4-R2-R3.
The show route confirms this
labroot@R1:R1> show route 3.3.3.3
inet.0: 12 destinations, 12 routes (12 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
3.3.3.3/32 *[OSPF/10] 00:10:35, metric 3
> to 192.168.1.10 via lt-0/0/0.5
The traceroute confirms that the destination IP 3.3.3.3 is 3 hops away
labroot@R1:R1> traceroute 3.3.3.3
traceroute to 3.3.3.3 (3.3.3.3), 30 hops max, 40 byte packets
1 192.168.1.10 (192.168.1.10) 1.194 ms 0.577 ms 0.428 ms
2 192.168.1.17 (192.168.1.17) 0.480 ms 0.597 ms 0.447 ms
3 3.3.3.3 (3.3.3.3) 0.640 ms 0.752 ms 0.589 ms
If we configure lt-0/0/0.3 to participate in area 100 it will become part of the best path to reach the destination IP 3.3.3.3, this because the interface lt-0/0/0.3 will generate type 1 LSAs for area 100 as well.
The traceroute confirms that the traffic is now going through interface lt-0/0/0.3
Original Message:
Sent: 05-21-2024 22:48
From: xine32
Subject: OSPF Multi-area Adjacency
Hi,
I started learny JNCIS-SP certification. Has I remember, last time I did my time I did my CCNP, an interface can't belong more than one interface, it can be a feature added after 2016 maybe but doesn't matters. I seen now, it somethings been standardized after initial release of OSPF.
My question is what would be the benefit of choosing an area as primary over the other for a connection between 2 ABRs, I notice that Cisco is doing the same using different command than Juniper but getting a similar result at the end, for compatibility purpose I suppose. There only one physical connection between the 2 routers, so how to properly choose the Primary area and Secondary area(s) ?
Thank you
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Xine
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