cost of directly connected routes

[sarahr202]

Hi every body

I have few questions

Please consider the following example.

R1-serial area 0------R2-----fastethernet

R2 has following table.

C    199.199.199.0/24 is directly connected, Serial0/0

C    10.0.0.0/8 is directly connected, FastEthernet0/0

R1 has following table.

C    199.199.199.0/24 is directly connected, Serial0/0

O IA 10.0.0.0/8 [110/74] via 199.199.199.2, 00:05:39, Serial0/0

Consider  10.0.0.0/8,   which is network  on R2's fastethernet.  The cost to reach 10.0.0.0/8  from R1 is 65  which means R2's cost to reach its directly connected network (10.0.0.0/8)  must be 10.

So  can  I deduce from the above   the  cost of directly connected fast etherne off R2 is 10  ?

Ospf cost =  100M/10M= 10

+=============================================

Please consider the following routing table of an ABR.

C 10.10.10.0/24 directly connected  fastethernet

0  10.10.20.0/24 (110/15) via  200.200.200.2 s0

0  10.10.30.0/24  ( 110/25)  via 200.200.200.2 s0

Letsay all these routes are present in area o

If I want to summarize these routes as:

area 0 range 10.0.0.0 255.0.0.0

What would be the default cost of this summary route?  ( Book says   the default cost =  the least cost among subnets being summarized )

Thanks and have a great weekend

[manju.cisco · Accepted solution]

Hi,

The default cost of a summary route will be the lowest cost among the subnets being summarized..........

In your case, since the network 10.10.10.0/24 which is on your FastEther0/0 has the cost of 10 (which is the lease cost among subnets being summarized), you would see the cost for 10.0.0.0/8 on R1 as 74 [WAN Cost (64) + Fa0/0 cost (10)].

The above is the default behaviour, which is w.r.t RFC1583, as so the book says the same. 

If you like to make use of RFC2328, which says to use the max cost among subnets being summarized, then you can issue the command "no compatible rfc1583" on the ABR (R2) under ospf process.

Then the cost for 10.0.0.0/8 on R1 would appear as 89 ( 64 + 25). The 25 is the max cost among the subnets summarized.

You may like to go through this document which explains this better.

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t28

Hope this helps.

Thanks.