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OSPF area summary command - Summarizing when two supernet is in two different areas.

Posted on 2016-10-18
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Last Modified: 2016-10-26
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I found this command in my network:

C1: Core switch 1
C2: Core switch 2
R1: Router 1
R2: Router 2
Block1: Distro block 1
Block2: Distro block 2

Core 1:
 area 100 range 10.148.0.0 255.255.128.0

Core 2:
 area 100 range 10.148.0.0 255.255.128.0

Both distribution blocks use subnets that fall under 10.148.0.0/17, but block 1 is in area 100, and block two is in area 102.

How big of an issue is summarizing if a subnet of the network is that is being summarized is in a different OSPF area?  There are no duplications anywhere, and the network is up and running.
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Question by:PeraHoman
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Predrag Jovic earned 500 total points
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Core 1:
 
area 100 range 10.148.0.0 255.255.128.0

Core 2:
 area 100 range 10.148.0.0 255.255.128.0
Since those two are summarizing the same area it is not problem since both advertise the same network range for area 100. However you need to be aware that qual cost path routing in OSPF can have unexpected results. Please read - How does OSPF use two Multilink paths to transfer packets?
On the drawing area 102 have 10.148.0.0/22 and area 100 have 10.148.0.0/17 in that case if ranges in areas 100 and 102 do not overlap (area 102 have IP address range 10.148.0.0/22 and area 100 do not have IP addresses from that IP address range) it is OK since IP address range (or summary) for area 102 is more specific than summarization for area 100.
When packet is compared to entries in routing table - the most specific route available will be selected.

According to your picture packets with IP destination address:
10.148.0.1 - 10.148.3.254 - will always be forwarded to area 102
10.148.4.0 - 10.148.127.254 - will always be forwarded to area 100
If IP routes are advertised correctly outside of areas (there is no reason not to be).
So, by itself it is not a problem since more specific route is always preferred.
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