H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – IPv4 Routing
H3C S7500E Series Ethernet Switches Chapter 3 OSPF Configuration
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III. DD packet
Two routers exchange database description (DD) packets describing their LSDBs for
database synchronization, contents in DD packets including the header of each LSA
(uniquely representing a LSA). The LSA header occupies small part of an LSA to
reduce traffic between routers. The recipient checks whether the LSA is available using
the LSA header.
The DD packet format:
Figure 3-11 DD packet format
Major fields:
z Interface MTU: Size in bytes of the largest IP datagram that can be sent out the
associated interface, without fragmentation.
z I (Initial) The Init bit, which is set to 1 if the packet is the first packet of database
description packets, and set to 0 if not.
z M (More): The More bit, which is set to 0 if the packet is the last packet of DD
packets, and set to 1 if more DD Packets are to follow.
z MS (Master/Slave): The Master/Slave bit. When set to 1, it indicates that the router
is the master during the database exchange process. Otherwise, the router is the
slave.
z DD Sequence Number: Used to sequence the collection of database description
packets for ensuring reliability and intactness of DD packets between the master
and slave. The initial value is set by the master. The DD sequence number then
increments until the complete database description has been sent.
IV. LSR packet
After exchanging DD packets, any two routers know which LSAs of the peer routers are
missing from the local LSDBs. In this case, they send LSR (link state request) packets,