Tag Archives: LSA

OSPF part 2

LSA Types

There are 6 common used (by Cisco) LSA types:

LSA type 1 updates describe the router itself: interfaces (in the area), list of neighbor routers and the router ID (RID). The RID is transmited by a linkstate-ID in this hello packet. The linkstate-ID is equal to the RID.

LSA type 2 updates represent a transit subnet for which a DR is elected. The LSID are the RID of the DR IP’s address on that particular subnet. I’ts only active on subnets with a active DR. For subnets without a DR (like a point-to-point), type 1 packets are enough for creating the topology database. Inside one area, only LSA types 1 and 2 are send out. All the routers can create a topology table with only these type 1 and 2 LSA’s. A “show ip ospf database” lists all the received LSA types.

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OSPF

OSPF is a link-state routing protoocol which uses factors such as speed or the link’s shortest path to decide which route is the best.

Link state protocol routers maintain a common picture of the network and exchange link informantion during discovery and/or network changes.

OSPF is designed for large and scalable networks, becauses of the following advantages:

  • Convergence speed
    • OSPF sends only routing changes instead of the entire routing table. Because of the small routing changes, updates are flooded rapidly across the network
  • Support for VLSM
  • Network size
  • Use of bandwidth
    • OSPF uses multicast to advertertise the updates: LSU’s (link state updates) are small in packetsize
  • path selection
    • OSPF selects optimal routes using cost instead of hopcount (RIP)
  • Member groupings
    • OSPF uses areas: every network segment is cut into smaller areas of routers: less LSU’s are sent out and more efficient routing takes place. eh Every router in a area does have the same topology table.

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