OSPF area ip addressing and summarization

Unanswered Question
Nov 13th, 2008
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Hi,


I've been playing around with a three area ospf network in Packet Tracer 5. I've had some success, but would like to make sure that my ip addressing is sound.


I have subnetted 10.0.0.0 using a /30 mask. Each area will be summarized using 255.255.255.240 allowing four subnets per area, the addresses below are grouped accordingly:


Unused

10.0.0.0

10.0.0.4

10.0.0.8

10.0.0.12


Area 0

10.0.0.16 (10.0.0.1 - 10.0.0.2 usable)*

10.0.0.20

10.0.0.24

10.0.0.28


Area 1

10.0.0.32

10.0.0.36

10.0.0.40

10.0.0.44


Area 2

10.0.0.48

10.0.0.52

10.0.0.56

10.0.0.60


I have managed to get the above addressing scheme working in Packet Tracer 5, although I haven't tried summarizing yet.


1. Given that I plan to summarize each area, am I correct leaving the first first four subnets unused?


1. Does it make sense to use this ip addressing scheme?


Thanks,


Phil

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Giuseppe Larosa Thu, 11/13/2008 - 12:12
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Hello Phil,

in real world there are users in client Vlans with /24 or /25 subnets masks.


However, as an exercise you are free to make your address plan in this way.

the important concept is that an OSPF area allows to take advantage of a careful address plan: you need to avoid to mix up smaller subnets in different areas because the ABR routers at area boundary are the only ones that can summarize.

From this point ov view your plan is correct


Hope to help

Giuseppe


pkoritsas Thu, 11/13/2008 - 13:50
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Hi Giuseppe,


Am I right in thinking the first group of subnets are unusable because if summarized they would be summarized to the network address 10.0.0.0 which would cause problems?


Thanks,

Phil


ruwhite Fri, 11/14/2008 - 05:57
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No--they should be perfectly fine. Both OSPF and routers can discern between 10.0.0.0/8 and 10.0.0.0/24, for instance.


:-)


Russ

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