Adjacency; Designated Routers - Cisco Nexus 7000 Series Configuration Manual

Chapter 6 Configuring OSPFv2
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .
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If there is a match, the following information is entered into the neighbor table:
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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
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
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
represent the network to the rest of the OSPFv2 area (see the
fails, OSPFv2 selects a
Network types are as follows:
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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.
OL-20002-02
Optional capabilities
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
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
Local interface—The local interface that received the Hello packet for this neighbor.
backup designated router
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.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
6-3).
"Designated Routers" section on page
"Designated Routers" section on page
"Link-State Database" section on page
designated router (DR), to control the LSA floods and
"Areas" section on page
(BDR). If the DR fails, OSPFv2 uses the BDR.
Information About OSPFv2
6-3).
6-3.
6-7). The local router
6-4). If the DR
6-3