Ospf Version 2 Overview - Cisco ASR 5000 Series Administration Manual

Routing
Important
Important

OSPF Version 2 Overview

OSPF is a link-state routing protocol that employs an interior gateway protocol (IGP) to route IP packets using
the shortest path first based solely on the destination IP address in the IP packet header. OSPF routed IP
packets are not encapsulated in any additional protocol headers as they transit the network.
An Autonomous System (AS), or Domain, is defined as a group of networks within a common routing
infrastructure.
OSPF is a dynamic routing protocol that quickly detects topological changes in the AS (such as router interface
failures) and calculates new loop-free routes after a period of convergence. This period of convergence is
short and involves a minimum of routing traffic.
In a link-state routing protocol, each router maintains a database, referred to as the link-state database, that
describes the Autonomous System's topology. Each participating router has an identical database. Each entry
in this database is a particular router's local state (for example, the router's usable interfaces and reachable
neighbors). The router distributes its local state throughout the AS by flooding.
All routers run the same algorithm in parallel. From the link-state database, each router constructs a tree of
shortest paths with itself as root to each destination in the AS. Externally derived routing information appears
on the tree as leaves. The cost of a route is described by a single dimensionless metric.
OSPF allows sets of networks to be grouped together. Such a grouping is called an area. The topology of this
area is hidden from the rest of the AS, which enables a significant reduction in routing traffic. Also, routing
within the area is determined only by the area's own topology, lending the area protection from bad routing
data. An area is a generalization of an IP subnetted network.
OSPF enables the flexible configuration of IP subnets so that each route distributed by OSPF has a destination
and mask. Two different subnets of the same IP network number may have different sizes (that is, different
masks). This is commonly referred to as variable-length subnetting. A packet is routed to the best (longest or
most specific) match. Host routes are considered to be subnets whose masks are "all ones" (0xffffffff).
OSPF traffic can be authenticated or non-authenticated, or can use no authentication, simple/clear text
passwords, or MD5-based passwords. This means that only trusted routers can participate in the AS routing.
You can specify a variety of authentication schemes and, in fact, you can configure separate authentication
schemes for each IP subnet.
Externally derived routing data (for example, routes learned from an exterior protocol such as BGP) is
advertised throughout the AS. This externally derived data is kept separate from the OSPF ink state data.
Each external route can also be tagged by the advertising router, enabling the passing of additional information
between routers on the boundary of the AS.
OSPF uses a link-state algorithm to build and calculate the shortest path to all known destinations.
During system task recovery, it is possible for a dynamically-learned forwarding entry to incorrectly
remain in the system forwarding table if that forwarding entry has been removed from the dynamic routing
protocol during the recovery.
On the ASR 5500, OSPF routes with IPv6 prefix lengths less than /12 and between the range of /64 and
/128 are not supported.
ASR 5500 System Administration Guide, StarOS Release 21.4
OSPF Version 2 Overview
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