Different OSPF Control Packets

There are five distinct OSPF packet types.

Type Description
1 Hello
2 Database Description
3 Link State Request
4 Link state Update
5 Link State Acknowledgement
  1. The Hello packet
    Hello packets are OSPF packet type 1. These packets are sent periodically on all interfaces in order to establish and maintain neighbor relationships. In addition, Hello Packets are multicast on those physical networks having a multicast or broadcast capability, enabling dynamic discovery of neighboring routers. All routers connected to a common network must agree on certain parameters (Network mask, Hello Interval and Router Dead Interval). These parameters are included in Hello packets, so that differences can inhibit the forming of neighbor relationships.
  2. The Database Description packet
    Database Description packets are OSPF packet type 2. These packets are exchanged when an adjacency is being initialized. They describe the contents of the link-state database. Multiple packets may be used to describe the database. For this purpose, a poll-responseprocedure is used. One of the routers is designated to be the master, the other the slave. The master sends Database Description packets (polls) which are acknowledged by Database Description packets sent by the slave (responses). The responses are linked to the polls via the packets DD sequence numbers.
  3. The Link State Request packet
    Link State Request packets are OSPF packet type 3. After exchanging Database Description packets with a neighboring router, a router may find that parts of its link-state database are out-of-date. The Link State Request packet is used to request the pieces of the neighbor’s database that are more up to date. Multiple Link State Request packets may need to be used.A router that sends a Link State Request packet has in mind the precise instance of the database pieces it is requesting. Each instance is defined by its LS sequence number, LS checksum, and LS age, although these fields are not specified in the Link State Request Packet itself. The router may receive even more recent instances in response.
  4. The Link State Update packet
    Link State Update packets are OSPF packet type 4. These packets implement the flooding of LSAs. Each Link State Update packet carries a collection of LSAs one hop further from their origin. Several LSAs may be included in a single packet. Link State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the flooding procedure reliable, flooded LSAs are acknowledged in Link State Acknowledgment packets. If retransmission of certain LSAs is necessary, the retransmitted LSAs are always sent directly to the neighbor.
  5. The Link State Acknowledgment packet
    Link State Acknowledgment Packets are OSPF packet type 5. To make the flooding of LSAs reliable, flooded LSAs are explicitly acknowledged. This acknowledgment is accomplished through the sending and receiving of Link State Acknowledgment packets. Multiple LSAs can be acknowledged in a single Link State Acknowledgment packet.

Open NetSim, Select Examples->Internetworks->Different OSPF Control Packets then click on the tile in the middle panel to load the example as shown in below Figure.

Figure4-39

The following network diagram illustrates what the NetSim UI displays when you open the example configuration file for Different-OSPF-Control-Packets in NetSim as shown Figure

Figure4-40

Network Settings

  1. Set OSPF Routing protocol under Application_Layer properties of a router.
  2. Configured CBR application with default properties and set application Start Time(s) to 30Sec.
  3. Enabled Packet Trace and Plot.
  4. Simulate for 100 sec.

Results and Discussion#

Open Packet animation window and click on play button. Users can observe all the OSPF packets. OSPF neighbors are dynamically discovered by sending Hello packets out each OSPF-enabled interface on a router. Then Database description packets are exchanged when an adjacency is being initialized. They describe the contents of the topological database. After exchanging Database Description packets with a neighboring router, a router may find that parts of its topological database are out of date. The Link State Request packet is used to request the pieces of the neighbor's database that are more up to date. The sending of Link State Request packets is the last step in bringing up an adjacency. A packet that contains fully detailed LSAs, typically sent in response to an LSR message. LSAck is sent to confirm receipt of an LSU message

Figure4-41

The same can be observed in Packet trace by filtering CONTROL_PACKET_TYPE/ APP_NAME to OSPF_HELLO, OSPF_DD, OSPF_LSACK, OSPF_LSUPDATE and OSPF_LSREQ packets as shown below Figure

Figure4-42 Figure4-42