Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 586)

NOTE
By default, the Brocade device’s router ID is the IP address configured on the lowest numbered
loopback interface. If the device does not have a loopback interface, the default router ID is the lowest
numbered IP address configured on the device.
When multiple routers on the same network are declaring themselves DRs, then both the priority and
router ID are used to select the designated router and backup designated routers.
The DR and BDR election process is performed when one of the following events occurs:
• An interface is in a waiting state and the wait time expires.
• An interface is in a waiting state and receives a hello packet that addresses the BDR.
• A change in the neighbor state occurs, such as the following:
‐ A neighbor state transitions from ATTEMPT state to a higher state.
‐ Communication to a neighbor is lost.
‐ A neighbor declares itself to be the DR or BDR for the first time.
OSPF areas
Consider the topics discussed below when configuring OSPF areas.
Backbone area
The backbone area (also known as area 0 or area 0.0.0.0) forms the core of an OSPF network. All
other areas are connected to it, and inter-area routing happens via routers connected to the backbone
area and to their own associated areas. The backbone area is the logical and physical structure for the
OSPF domain and is attached to all nonzero areas in the OSPF domain.
The backbone area is responsible for distributing routing information between nonbackbone areas.
The backbone must be contiguous, but it does not need to be physically contiguous; backbone
connectivity can be established and maintained through the configuration of virtual links.
Area types
An area can be normal, a stub, a not-so-stubby area (NSSA), or a totally stubby area (TSA).
• Normal — OSPF routers within a normal area can send and receive external link state
advertisements (LSAs).
• Stub — OSPF routers within a stub area cannot send or receive external LSAs. In addition, OSPF
routers in a stub area must use a default route to the area’s Area Border Router (ABR) or
Autonomous System Boundary Router (ASBR) to send traffic out of the area.
• NSSA — The ASBR of an NSSA can import external route information into the area.
‐ ASBRs redistribute (import) external routes into the NSSA as type 7 LSAs. Type 7 External
LSAs are a special type of LSA generated only by ASBRs within an NSSA, and are
flooded to all the routers within only that NSSA.
‐ ABRs translate Type 7 LSAs into type-5 External LSAs, which can then be flooded
throughout the AS. You can configure summary-addresses on the ABR of an NSSA so that
the ABR converts multiple Type 7 external LSAs received from the NSSA into a single
Type 5 external LSA.
When an NSSA contains more than one ABR, OSPF elects one of the ABRs to perform the LSA
translation for NSSA. OSPF elects the ABR with the highest router ID. If the elected ABR becomes
OSPF areas
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