lsa5 forward address,route selection in same area

[sarahr202]

Hi everybody

Please consider the following example:

rip network 10.0.0.0--------R1(asbr)--cost 1--------R2------cost 1------------------R3

                                                                                                                                      |

                                                                                                                                      |

                                                                                                                                 cost 64

                                                                                                                                      |

                                                                                                                                     R4

                                                                                                                                      |

                                                                                                                         rip network ( 10.0.0.0/8)

Above we have only single area 0 . R1 is redistributing 10.0.0.0/8 into ospf. with forward address:0:0:0:0

R4 is also redistributing the 10.0.0.0/8 but with forward addresss: 199.199.199.2

R3 routing table:

O 199.199.199.0( 110/64) via 200.200.200.4 ( R4).

.

LSA 5 from R1 (asbr) is shown below:

metric type 1

cost  3

route: 10.0.0.0/8

advertiser 1.1.1.1 ( R1)

LSA 5 from R4 is shown below:

metric type 1

cost 2

route 10.0.0.0/8

advertiser 4.4.4.4 ( R4)

forward address 199.199.199.2

The cost from R3 to reach 10.0.0.0 via R4 is 64 ( the cost to reach 199.199.199.2) +2 ( cost of the route 10.0.0.0 received in lsa) =68

The cost to reach 10.0.0.0/8 through R1 ( ASBR) is 2 ( the cost to reach R1 from R3)+ 3 ( cost of route 10.0.0.0 received in lsa)=5

What will R3 do? will it consider the cost of 10.0.0.0 received in lsa only to decide the best route ? Or will R3 consider the over all cost to reach 10.0.0.0 which is 68 via R4 and 5 via R1?

thanks and Happy holidays!!

[sarahr202]

I found the answer.

In a same area, if a router a receive a lsa 5 with forward address : some address and another lsa5 from a asbr with forward address: 0;0:0:0, ( assuming both lsa 5 carrying the same prefix,type 1), then the router computes the total cost by adding the advertised cost in lsa5 to the cost for reaching asbr /ip address specified in forward address. Router then picks the lesser cost.

In our case, R3 will choose the path via R1 because cost=2+3=5.

Now the question is what if we both lsa 5 are carrying prefix with different cost and but same type 2).

Will R3 simply pick the lesser cost advertised in lsa 5 without considering the cost to reach a particular ip address as specified in forward address or particular asbr?

In this case R3 simply picks the lsa 5 carrying a prefix with a lesser cost and type E2.and installs the route in its routing table via a next hop neighbor that is used to reach the .particular advertiser of that lsa5

thanks