Cisco Cisco Packet Data Gateway (PDG) Fehlerbehebungsanleitung
Routing
OSPF Routing ▀
Cisco ASR 5000 Series Enhanced Feature Configuration Guide ▄
OL-22983-01
OSPF Routing
This section gives an overview of OSPF (Open Shortest Path First) routing and its implementation in the system. It also
provides the procedure for enabling the base OSPF functionality, and lists the commands that are available for more
complex configuration.
provides the procedure for enabling the base OSPF functionality, and lists the commands that are available for more
complex configuration.
OSPF routing is included with the IPV4 Routing Protocols feature. You must purchase and install a license key before
you can use this feature.
you can use this feature.
Important:
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.
dynamic routing protocol during the recovery.
OSPF Version 2 Overview
OSPF is a link-state routing protocol, an interior gateway protocol (IGP) that routes IP packets using the shortest path
first based solely on the destination IP address in the IP packet header. IP packets are routed are not encapsulated in any
further 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.
first based solely on the destination IP address in the IP packet header. IP packets are routed are not encapsulated in any
further 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.
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 individual piece of 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 Autonomous System by flooding.
Autonomous System's topology.Each participating router has an identical database. Each individual piece of 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 Autonomous System 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 Autonomous System. Externally derived routing information appears
on the tree as leaves. The cost of a route is described by a single dimensionless metric.
paths with itself as root to each destination in the Autonomous System. 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.
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).
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 Autonomous System‘s routing. You
can specify a variety of authentication schemes and, in fact, you can configure separate authentication schemes for each
IP subnet.
MD5-based passwords. This means that only trusted routers can participate in the Autonomous System‘s routing. You
can specify a variety of authentication schemes and, in fact, you can configure separate authentication schemes for each
IP subnet.