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10. The Neighbor Data Structure Connected: An Internet Encyclopedia
10. The Neighbor Data Structure

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10. The Neighbor Data Structure

10. The Neighbor Data Structure An OSPF router converses with its neighboring routers. Each separate conversation is described by a "neighbor data structure". Each conversation is bound to a particular OSPF router interface, and is identified either by the neighboring router's OSPF Router ID or by its Neighbor IP address (see below). Thus if the OSPF router and another router have multiple attached networks in common, multiple conversations ensue, each described by a unique neighbor data structure. Each separate conversation is loosely referred to in the text as being a separate "neighbor".

The neighbor data structure contains all information pertinent to the forming or formed adjacency between the two neighbors. (However, remember that not all neighbors become adjacent.) An adjacency can be viewed as a highly developed conversation between two routers.

State

The functional level of the neighbor conversation. This is described in more detail in Section 10.1.

Inactivity Timer

A single shot timer whose firing indicates that no Hello Packet has been seen from this neighbor recently. The length of the timer is RouterDeadInterval seconds.

Master/Slave

When the two neighbors are exchanging databases, they form a master/slave relationship. The master sends the first Database Description Packet, and is the only part that is allowed to retransmit. The slave can only respond to the master's Database Description Packets. The master/slave relationship is negotiated in state ExStart.

DD Sequence Number

A 32-bit number identifying individual Database Description packets. When the neighbor state ExStart is entered, the DD sequence number should be set to a value not previously seen by the neighboring router. One possible scheme is to use the machine's time of day counter. The DD sequence number is then incremented by the master with each new Database Description packet sent. The slave's DD sequence number indicates the last packet received from the master. Only one packet is allowed outstanding at a time.

Neighbor ID

The OSPF Router ID of the neighboring router. The Neighbor ID is learned when Hello packets are received from the neighbor, or is configured if this is a virtual adjacency (see Section C.4).

Neighbor Priority

The Router Priority of the neighboring router. Contained in the neighbor's Hello packets, this item is used when selecting the Designated Router for the attached network.

Neighbor IP address

The IP address of the neighboring router's interface to the attached network. Used as the Destination IP address when protocol packets are sent as unicasts along this adjacency. Also used in router links advertisements as the Link ID for the attached network if the neighboring router is selected to be Designated Router (see Section 12.4.1). The Neighbor IP address is learned when Hello packets are received from the neighbor. For virtual links, the Neighbor IP address is learned during the routing table build process (see Section 15).

Neighbor Options

The optional OSPF capabilities supported by the neighbor. Learned during the Database Exchange process (see Section 10.6). The neighbor's optional OSPF capabilities are also listed in its Hello packets. This enables received Hello Packets to be rejected (i.e., neighbor relationships will not even start to form) if there is a mismatch in certain crucial OSPF capabilities (see Section 10.5). The optional OSPF capabilities are documented in Section 4.5.

Neighbor's Designated Router

The neighbor's idea of the Designated Router. If this is the neighbor itself, this is important in the local calculation of the Designated Router. Defined only on multi-access networks.

Neighbor's Backup Designated Router

The neighbor's idea of the Backup Designated Router. If this is the neighbor itself, this is important in the local calculation of the Backup Designated Router. Defined only on multi-access networks.

The next set of variables are lists of link state advertisements. These lists describe subsets of the area topological database. There can be five distinct types of link state advertisements in an area topological database: router links, network links, and Type 3 and 4 summary links (all stored in the area data structure), and AS external links (stored in the global data structure).

Link state retransmission list

The list of link state advertisements that have been flooded but not acknowledged on this adjacency. These will be retransmitted at intervals until they are acknowledged, or until the adjacency is destroyed.

Database summary list

The complete list of link state advertisements that make up the area topological database, at the moment the neighbor goes into Database Exchange state. This list is sent to the neighbor in Database Description packets.

Link state request list

The list of link state advertisements that need to be received from this neighbor in order to synchronize the two neighbors' topological databases. This list is created as Database Description packets are received, and is then sent to the neighbor in Link State Request packets. The list is depleted as appropriate Link State Update packets are received.


Next: 10.1. Neighbor states

Connected: An Internet Encyclopedia
10. The Neighbor Data Structure

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