Task 3.5 - ip ospf cost

Would somebody be able to tell me why it is nessesary to incude the "ip ospf cost 1" on Router 4 serial 0/1 if you want to ensure that traffic from Vlan 7 is to traverse the PPP link when traveling to Vlan 5? As soon as you create the Virtual Link between Router 4 and 5 traffic between these two Vlan's will automatically use this PPP link as OSPF will prefer to use intra-area routes as opposed to inter-area routes.

Any thoughts? 

Comments

  •  

    I had the same question.  Since they area both area 0 transit and ppp link is 64 and frame is 64 + 64 to get to R5, they must have just hard coded it to make sure in case a later QoS task has the bandwidth changed on the PPP interf.  I read ahead and it doesn't.  Interestingly, if I lower the bandwidth to raise the cost, it still choses the directly connected neighbor R5 for the route? 

    RSRack11R4#sh ip ospf int b
    Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
    VL0          1     0               164.11.45.4/29     64    P2P   1/1
    Se0/0        1     0               164.11.34.4/24     64    P2P   1/1
    Se0/1        1     1               164.11.45.4/29     64    P2P   1/1
    Lo0          1     1               150.11.4.4/24      1     P2P   0/0
    Fa0/0        1     38              164.11.47.4/24     1     BDR   1/1
    RSRack11R4#conf t
    Enter configuration commands, one per line.  End with CNTL/Z.
    RSRack11R4(config)#interf s0/1
    RSRack11R4(config-if)#band 128
    RSRack11R4(config-if)#end
    RSRack11R4#
    Mar 29 10:55:08.218: %SYS-5-CONFIG_I: Configured from console by console
    RSRack11R4#sh ip ospf int b
    Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
    VL0          1     0               164.11.45.4/29     781   P2P   1/1
    Se0/0        1     0               164.11.34.4/24     64    P2P   1/1
    Se0/1        1     1               164.11.45.4/29     781   P2P   1/1
    Lo0          1     1               150.11.4.4/24      1     P2P   0/0
    Fa0/0        1     38              164.11.47.4/24     1     BDR   1/1

    RSRack11R4#sh ip route | incl 164.11.5.0
    O       164.11.5.0/24 [110/782] via 164.11.45.5, 00:08:19, Serial0/1

     

  • Sorry for the double-post.  But the reason bandwidth 128 did not change the output is that R3 in IE's Rack11 has a different serial card.

    RSRack11R4#sh interf s0/0 | i Hardware|BW
      Hardware is PowerQUICC Serial
      MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
    RSRack11R4#sh interf s0/1 | i Hardware|BW
      Hardware is PowerQUICC Serial
      MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

    RSRack11R3#sh interf s1/0 | i Hardware|BW
      Hardware is CD2430 in sync mode
      MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,

    Therefore, the cost through R3 is 64 to R3, but R3 has a cost of 781 to get to R5.  So, you would have to set the bandwidth really low to break this.  Again, I think they were just making sure.  But I learned a good lesson.  Eventhough both these routers in my lab were 2611xm, don't assume they have the same cards!

     

Sign In or Register to comment.