Key Features Supported In The Cisco Ios Xr Software Ospf Implementation - Cisco NCS 6000 Series Configuration Manual

Implementing OSPF
All the OSPF neighbors, in turn, send the state information unchanged. This flooding process continues until
all devices in the area have the same link-state database.
To determine the best route to a destination, the software sums all of the costs of the links in a route to a
destination. After each router has received routing information from the other networking devices, it runs the
shortest path first (SPF) algorithm to calculate the best path to each destination network in the database.
The networking devices running OSPF detect topological changes in the network, flood link-state updates to
neighbors, and quickly converge on a new view of the topology. Each OSPF router in the network soon has
the same topological view again. OSPF allows multiple equal-cost paths to the same destination. Since all
link-state information is flooded and used in the SPF calculation, multiple equal cost paths can be computed
and used for routing.
On broadcast and nonbroadcast multiaccess (NBMA) networks, the designated router (DR) or backup DR
performs the LSA flooding. On point-to-point networks, flooding simply exits an interface directly to a
neighbor.
OSPF runs directly on top of IP; it does not use TCP or User Datagram Protocol (UDP). OSPF performs its
own error correction by means of checksums in its packet header and LSAs.
In OSPFv3, the fundamental concepts are the same as OSPF Version 2, except that support is added for the
increased address size of IPv6. New LSA types are created to carry IPv6 addresses and prefixes, and the
protocol runs on an individual link basis rather than on an individual IP-subnet basis.
OSPF typically requires coordination among many internal routers: Area Border Routers (ABRs), which are
routers attached to multiple areas, and Autonomous System Border Routers (ASBRs) that export reroutes
from other sources (for example, IS-IS, BGP, or static routes) into the OSPF topology. At a minimum,
OSPF-based routers or access servers can be configured with all default parameter values, no authentication,
and interfaces assigned to areas. If you intend to customize your environment, you must ensure coordinated
configurations of all routers.

Key Features Supported in the Cisco IOS XR Software OSPF Implementation

The Cisco IOS XR Software implementation of OSPF conforms to the OSPF Version 2 and OSPF Version
3 specifications detailed in the Internet RFC 2328 and RFC 2740, respectively.
The following key features are supported in the Cisco IOS XR Software implementation:
• Hierarchy—CLI hierarchy is supported.
• Inheritance—CLI inheritance is supported.
• Stub areas—Definition of stub areas is supported.
• NSF—Nonstop forwarding is supported.
• SPF throttling—Shortest path first throttling feature is supported.
• LSA throttling—LSA throttling feature is supported.
• Fast convergence—SPF and LSA throttle timers are set, configuring fast convergence. The OSPF LSA
• Route redistribution—Routes learned using any IP routing protocol can be redistributed into any other
throttling feature provides a dynamic mechanism to slow down LSA updates in OSPF during network
instability. LSA throttling also allows faster OSPF convergence by providing LSA rate limiting in
milliseconds.
IP routing protocol.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Key Features Supported in the Cisco IOS XR Software OSPF Implementation
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