8. Protocol Packet Processing

This section discusses the general processing of OSPF routing protocol packets. It is very important that the router topological databases remain synchronized. For this reason, routing protocol packets should get preferential treatment over ordinary data packets, both in sending and receiving.

          +---+            +---+
          |RT1|------------|RT2|            o---------------o
          +---+    N1      +---+           RT1             RT2

                                                 RT7
                                                  o---------+
            +---+   +---+   +---+                /|\        |
            |RT7|   |RT3|   |RT4|               / | \       |
            +---+   +---+   +---+              /  |  \      |
              |       |       |               /   |   \     |
         +-----------------------+        RT5o RT6o    oRT4 |
                  |       |     N2            *   *   *     |
                +---+   +---+                  *  *  *      |
                |RT5|   |RT6|                   * * *       |
                +---+   +---+                    ***        |
                                                  o---------+
                                                 RT3

                  Figure 10: The graph of adjacencies

Routing protocol packets are sent along adjacencies only (with the exception of Hello packets, which are used to discover the adjacencies). This means that all routing protocol packets travel a single IP hop, except those sent over virtual links.

All routing protocol packets begin with a standard header. The sections below give the details on how to fill in and verify this standard header. Then, for each packet type, the section is listed that gives more details on that particular packet type's processing.

• 8.1. Sending protocol packets
• 8.2. Receiving protocol packets