OSPF Demand Circuit

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Pic. 1 - Topology Diagram.

Task 1

Remove OSPF filtering applied in the previous lab.

Task2

Since the point-to-point connection between R4 and R5 is slow, prevernt OSPF on this link from sending LSA refresh advertisements. Routers should not even use hello packets on this point-to-point link.

Questions

Try to answer the following questions:

1. What is the LSA refresh time used by Cisco?
2. Which types of connection the solution is particularly useful on?

Lab Solution

Solution configuration can be accessed below (if you want to save it, click the link, then go to File-->Download):
https://drive.google.com/file/d/0BwE5C95tpjZOWWNNVlVKZmp6Y0k/edit?usp=sharing

Task 1

Remove OSPF filtering applied in the previous lab.

R4 Config:

!

interface Serial0/1

 ip address 172.16.45.4 255.255.255.0

 no ip ospf database-filter all out

!

R5 Config:

!

no ip route 0.0.0.0 0.0.0.0 s0/1

!

Task2

Since the point-to-point connection between R4 and R5 is slow, prevernt OSPF on this link from sending LSA refresh advertisements. Routers should not even use hello packets on this point-to-point link.

Pic. 2- OSPF Interface Information without Demand Circuit Feature.

R4 Config:
!
interface Serial0/1
 ip address 172.16.45.4 255.255.255.0
 ip ospf demand-circuit

!

Pic. 3 - OSPF Interface Information with Demand Circuit Feature.

Pic. 4 - R4-R5 Neighbor Dead Timer.

Study Drill

RFC 2328 stipulates that the self originated LSAs must be refreshed using LSRefreshTime (every 30 minutes):

LSRefreshTime
        The maximum time between distinct originations of any particular
        LSA.  If the LS age field of one of the router's self-originated
        LSAs reaches the value LSRefreshTime, a new instance of the LSA
        is originated, even though the contents of the LSA (apart from
        the LSA header) will be the same.  The value of LSRefreshTime is
        set to 30 minutes.

However, RFC 1793, describes OSPF operation to support Demand Circuits (Dial-up, ISDN etc.). Hellos and LSA refresh mechanisms are being suppressed on those types of OSPF connections. This feature will be negotiated between OSPF routers.

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