Cisco Cisco Packet Data Gateway (PDG)

 

This section gives an overview of Open Shortest Path First (OPSF) routing and its implementation in the system. It also 
describes how to enable the base OSPF functionality and lists the commands that are available for more complex 
configurations. 

You must purchase and install a license key before you can use this feature. Contact your Cisco account representative 
for more information on licenses. 

  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. 

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.