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
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.
• Route redistribution—Routes learned using any IP routing protocol can be redistributed into any other
IP routing protocol.
• Authentication—Plain text and MD5 authentication among neighboring routers within an area is
supported.
• Routing interface parameters—Configurable parameters supported include interface output cost,
retransmission interval, interface transmit delay, router priority, router “dead” and hello intervals, and
authentication key.
• Virtual links—Virtual links are supported.
• Not-so-stubby area (NSSA)—RFC 1587 is supported.
• OSPF over demand circuit—RFC 1793 is supported.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
334 OL-30423-03
Implementing OSPF
Key Features Supported in the Cisco IOS XR Software OSPF Implementation
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