Noh Ironogi Spotting Scopes

Scope Node

OSPF Simulation: DR LSDB Synchronization

Q1: How is a subnet's Router LSA and Network LSA used?

Answer: When a router outside of the subnet (call it outside router) wants to calculate shortest paths to destinations inside the subnet, it needs to know the complete subnet information: How many routers in the subnet? What's their Router ID? What are their interface IP addresses? Router LSA specifies a router's interface IP addresses. It tells outside routers destinations in the subnet.   Network LSA lists routers attached to subnet. Outside routers use it to reach destinations in the subnet.

Q2.  Why LSDB synchronization uses DD, Request, and Update?

Answer: The short answer is scalability.  It would work if routers just dump their entire collection of LSAs stored in LSDB. But the side effect is big: Network will be filled with LSAs and routers CPU are busy to parse large number of LSA received. To optimize resources, OSPF uses DD - Request - Update to synchronize their LSA on demand. Now routers only send LSAs that are requested by others.

Q3. Why non-DR/BDR flood Update differently than DR/BDR flood Update?

Answer: Their flooding scopes are different between these two groups.  Non-DR/BDR is called DROther. A DROther node has 2-way relationships with DR and BDR, not with other DROther. Therefore, DROther nodes flood their LSA to both DR and BDR, not other DROther. For DR, it needs to synchronize its LSAs to all other routers, including DROther and BDR. So its flooding covers all OSPF routers. BDR does the same.

Q4. How does OSPF handle two types of Update flooding?

Answer: OSPF uses two IP multicast addresses as follows:

 

1. 224.0.0.6 (OSPF AllDRouters):  DROTHER nodes have 2-way relationships with DR and BDR only, not with each other. When they have LSA to transmit to DR and BDR, they want to send one Update and use 224.0.0.6 as destination IP address.

2. 224.0.0.5 (AllSPFRouters ):  When DR has LSAs to send, it wants to send one Update to all other OSPF nodes. It uses 224.0.0.5 for Update's destination IP address.

Q5. How does OSPF flooding work physically?

 

Answer: LAN flooding involves several aspects:

 

1. DR is in a subnet with broadcast capability (E.g., Ethernet.) When a packet is transmitted, it reaches allinterfaces attached to the LAN.

2. Flooding effect is achieved by MAC filtering. There are three types of MAC:

    

    

    

    

   • Unicast. When receiving a packet, compare the packet's destination MAC with the interface unicastMAC. Accept if match. Otherwise, discard. This MAC is assigned by manufacture. It is also calledhardware address.

   • Multicast MAC.  When receiving a packet, compare its destination MAC with multicast MAC on the

     interface. Accept if match. Otherwise discard. Not all routers have the same multicast MAC. 

   • Broadcast MAC. (0xfffffff) Accept an arriving frame if its destination MAC is 0xffffff.

    

    

Q6. How does OSPF create multicast MAC?

 

Answer: Multicast-224.0.0.5.MAC is added to a router interface when OSPF is started at the interface. OSPF converts 224.0.0.5 (AllSPFRouters) to a multicast MAC and adds it to the interface. All OSPF interfaces have 2 MAC: The assigned unicast MAC, and Multicast-224.0.0.5. 

Multicast-224.0.0.6.MAC is created for DR/BDR. When a router is elected as DR or BDR, OSPF converts 224.0.0.6 (OSPF AllDRouters) and adds it to DR/BDR's interface. Now the DR/BDR's  interface have 3 MACs: unicast, Multicast-224.0.0.5, and Multicast-224.0.0.6.

About the Author

External links for OSPF network simulation: OSPF simulation: DR LSDB Synchronization (This article): http://pre.visualland.net/view.php?cid=929 OSPF simulation list (Table of Content): http://pre.visualland.net/protocol.overview.php?p=OSPF OSPF Simulation: Basic http://pre.visualland.net/view.php?cid=767

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