Specifications

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Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 3 Configuring OSPFv2
Information About OSPFv2
If there is a match, the following information is entered into the neighbor table:
• Neighbor ID—The router ID of the neighbor.
• Priority—Priority of the neighbor. The priority is used for designated router election (see the
“Designated Routers” section on page 3-3).
• State—Indication of whether the neighbor has just been heard from, is in the process of setting up
bidirectional communications, is sharing the link-state information, or has achieved full adjacency.
• Dead time—Indication of the time since the last Hello packet was received from this neighbor.
• IP Address—The IP address of the neighbor.
• Designated Router—Indication of whether the neighbor has been declared as the designated router
or as the backup designated router (see the “Designated Routers” section on page 3-3).
• Local interface—The local interface that received the Hello packet for this neighbor.
Adjacency
Not all neighbors establish adjacency. Depending on the network type and designated router
establishment, some neighbors become fully adjacent and share LSAs with all their neighbors, while
other neighbors do not. For more information, see the “Designated Routers” section on page 3-3.
Adjacency is established using Database Description packets, Link State Request packets, and Link
State Update packets in OSPF. The Database Description packet includes just the LSA headers from the
link-state database of the neighbor (see the “Link-State Database” section on page 3-7). The local router
compares these headers with its own link-state database and determines which LSAs are new or updated.
The local router sends a Link State Request packet for each LSA that it needs new or updated information
on. The neighbor responds with a Link State Update packet. This exchange continues until both routers
have the same link-state information.
Designated Routers
Networks with multiple routers present a unique situation for OSPF. If every router floods the network
with LSAs, the same link-state information will be sent from multiple sources. Depending on the type
of network, OSPFv2 might use a single router, the designated router (DR), to control the LSA floods
and represent the network to the rest of the OSPFv2 area (see the “Areas” section on page 3-4). If the
DR fails, OSPFv2 selects a backup designated router (BDR). If the DR fails, OSPFv2 uses the BDR.
Network types are as follows:
• Point-to-point—A network that exists only between two routers. All neighbors on a point-to-point
network establish adjacency and there is no DR.
• Broadcast—A network with multiple routers that can communicate over a shared medium that
allows broadcast traffic, such as Ethernet. OSPFv2 routers establish a DR and BDR that controls
LSA flooding on the network. OSPFv2 uses the well-known IPv4 multicast addresses 224.0.0.5 and
a MAC address of 0100.5300.0005 to communicate with neighbors.
The DR and BDR are selected based on the information in the Hello packet. When an interface sends a
Hello packet, it sets the priority field and the DR and BDR field if it knows who the DR and BDR are.
The routers follow an election procedure based on which routers declare themselves in the DR and BDR
fields and the priority field in the Hello packet. As a final tie breaker, OSPFv2 chooses the highest router
IDs as the DR and BDR.