The Open Shortest Path First (OSPF) Version 2 port

Revised for CPX 4.7.0.

Protocol overview
Terminology
Types of OSPF packets

Configuration of the OSPF port
Statistics of the OSPF port

Configuration of OSPF tables

Configuration examples

The Open Shortest Path First (OSPF) is classified as an Interior Gateway Protocol (IGP).
This means that it distributes routing information between routers belonging to a single Autonomous System.
The Open Shortest Path First (OSPF) protocol is based on link-state or SPF technology.

Protocol overview

The Open Shortest Path First (OSPF) protocol routes IP datagrams based solely on the destination IP address found in the IP datagram header. IP datagrams are routed "as they are", i.e. they are not encapsulated in any further protocol headers as they transit the Autonomous System (AS).
OSPF is a dynamic routing protocol. It quickly detects topological changes in the Autonomous System (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 describing the Autonomous System's topology.
This database is referred to as the link-state database. Each participating router has an identical database. Each individual piece of this database is a particular router's local state, e.g. the router's usable interfaces and reachable neighbors. The router distributes its local state throughout the Autonomous System by flooding.

All routers run exactly the same algorithm, in parallel. From the link-state database, each router constructs a tree of shortest paths with itself as root. This shortest-path tree gives the route to each destination in the Autonomous System. Externally derived routing information appears on the tree as leaves.

The Open Shortest Path First (OSPF) protocol allows sets of networks to be grouped together.
Such a grouping is called an area.
The topology of an area is hidden from the rest of the Autonomous System. This information hiding 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.

The Open Shortest Path First (OSPF) protocol enables the flexible configuration of IP subnets. Each route distributed by OSPF has a destination and mask. Two different subnets of the same IP network number may have different sizes (i.e. different masks). This is commonly referred to as variable length subnetting.
A datagram is routed to the best (i.e., longest or most specific) match. Host routes are considered to be subnets whose masks are "all ones" (0xFFFFFFFF).

All OSPF protocol exchanges are authenticated. This means that only trusted routers can participate in the Autonomous System's routing. A variety of authentication schemes can be used: in fact, separate authentication schemes can be configured for each IP subnet.

Externally derived routing data (e.g. routes learned from an Exterior Gateway Protocol such as BGP) is advertised throughout the Autonomous System. This externally derived data is kept separate from the OSPF protocol's link 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 Autonomous System.

Terminology

Router. A level three Internet Protocol packet switch. Formerly called a gateway in much of the IP literature.

Autonomous System (AS). A group of routers exchanging routing information via a common routing protocol. Abbreviated as AS.

Interior Gateway Protocol (IGP). The routing protocol spoken by the routers belonging to an Autonomous system. Abbreviated as IGP. Each Autonomous System has a single IGP. Separate Autonomous Systems may be running different IGPs.

Router ID. A 32-bit number assigned to each router running the OSPF protocol. This number uniquely identifies the router within an Autonomous System.

Network. In this documentation, an IP network/subnet/supernet. It is possible for one physical network to be assigned multiple IP network/subnet numbers. We consider these to be separate networks. Point-to-point physical networks are an exception - they are considered a single network no matter how many (if any at all) IP network/subnet numbers are assigned to them.

Network mask. A 32-bit number indicating the range of IP addresses residing on a single IP network/subnet/supernet. This specification displays network masks as hexadecimal numbers. For example, the network mask for a class C IP network is displayed as 0xFFFFFF00. Such a mask is often displayed elsewhere in the literature as 255.255.255.0.

Multi-access networks.Those physical networks that support the attachment of multiple (more than two) routers. Each pair of routers on such a network is assumed to be able to communicate directly (e.g., multi-drop networks are excluded).

Area. OSPF allows collections of contiguous networks and hosts to be grouped together. Such a group, together with the routers having interfaces to any one of the included networks, is called an area. Each area runs a separate copy of the basic link-state routing algorithm. This means that each area has its own topological database and corresponding graph. The topology of an area is invisible from the outside of the area. Conversely, routers internal to a given area know nothing of the detailed topology external to the area. This isolation of knowledge enables the protocol to effect a marked reduction in routing traffic as compared to treating the entire Autonomous System as a single link-state domain.

Area ID. This is a 32-bit number that identifies the area. The Area ID of 0.0.0.0 is reserved for the backbone. If the area represents a subnetted network, the IP network number of the subnetted network may be used for the Area ID.

The backbone area of the Autonomous System.The backbone area consists of those networks not contained in any area, their attached routers, and those routers that belong to multiple areas. The backbone must be contiguous.

The Virtual Link. It is possible to define areas in such a way that the backbone is no longer contiguous. In this case the system administrator must restore backbone connectivity by configuring virtual links. Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra-area routing only.

Interface. The connection between a router and one of its attached networks. An interface has state information associated with it, which is obtained from the underlying lower level protocols and the routing protocol itself. An interface to a network has associated with it a single IP address and mask (unless the network is an unnumbered point-to-point network). An interface is sometimes also referred to as a link.

Neighboring routers. Two routers that have interfaces to a common network. On multi-access networks, neighbors are dynamically discovered by OSPF's Hello Protocol.

Adjacency. A relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers become adjacent.

Link state advertisement. Describes the local state of a router or network. This includes the state of the router's interfaces and adjacencies. Each link state advertisement is flooded throughout the routing domain. The collected link state advertisements of all routers and networks form the protocol's topological database.

Hello Protocol. The part of the OSPF protocol used to establish and maintain neighbor relationships. On multi-access networks the Hello Protocol can also dynamically discover neighboring routers.

Designated Router. Each multi-access network that has at least two attached routers has a Designated Router. The Designated Router generates a link state advertisement for the multi-access network and has other special responsibilities in the running of the protocol. The Designated Router is elected by the Hello Protocol. The Designated Router concept enables a reduction in the number of adjacencies required on a multi-access network. This in turn reduces the amount of routing protocol traffic and the size of the topological database.

Backup Designated Router. In order to make the transition to a new Designated Router smoother, there is a Backup Designated Router for each multi-access network. The Backup Designated Router is also adjacent to all routers on the network, and becomes Designated Router when the previous Designated Router fails. If there were no Backup Designated Router, when a new Designated Router became necessary, new adjacencies would have to be formed between the new Designated Router and all other routers attached to the network.

Point-to-point networks. A network that joins a single pair of routers. A 56Kb serial line is an example of a point-to-point network.

Broadcast networks. Networks supporting many (more than two) attached routers, together with the capability to address a single physical message to all of the attached routers (broadcast). Neighboring routers are discovered dynamically on these nets using OSPF's Hello Protocol. The Hello Protocol itself takes advantage of the broadcast capability. The protocol makes further use of multicast capabilities, if they exist. An ethernet is an example of a broadcast network.

Non-broadcast networks. Networks supporting many (more tan two) routers, but having no broadcast capability. Neighboring routers are also discovered on these nets using OSPF's Hello Protocol. However, due to the lack of broadcast capability, some configuration information is necessary for the correct operation of the Hello Protocol. On these networks, OSPF protocol packets that are normally multicast need to be sent to each neighboring router, in turn. An X.25 Public Data Network (PDN) is an example of a non-broadcast network.

Internal routers. A router with all directly connected networks belonging to the same area. Routers with only backbone interfaces also belong to this category. These routers run a single copy of the basic routing algorithm.

Area border routers (ABR). A router that is attacheed to multiple areas. Area border routers run multiple copies of the basic algorithm, one copy for each attached area and an additional copy for the backbone. Area border routers condense the topological information of their attached areas for distribution to the backbone. The backbone in turn distributes the information to the other areas.

Backbone routers. A router that has an interface to the backbone. This includes all routers that interface to more than one area (i.e., area border routers). However, backbone routers do not have to be area border routers. Routers with all interfaces connected to the backbone are considered to be internal routers.

AS boundary routers. (ASBR) A router that exchanges routing information with routers belonging to other Autonomous Systems. Such a router has AS external routes that are advertised throughout the Autonomous System. The path to each AS boundary router is known by every router in the AS. This classification is completely independent of the previous classifications: AS boundary routers may be internal or area border routers, and may or may not participate in the backbone.

Host. Hosts attached directly to routers (referred to as host routes) appear on the graph as stub networks. The network mask for a host route is always 0xFFFFFFFF, which indicates the presence of a single node.

Stub areas. In some Autonomous Systems, the majority of the topological database may consist of AS external advertisements. An OSPF AS external advertisement is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". AS external advertisements are not flooded into/throughout stub areas; routing to AS external destinations in these areas is based on a (per-area) default only. This reduces the topological database size, and therefore the memory requirements, for a stub area's internal routers. The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been configured as a stub. This guarantees that no confusion will arise in the flooding of AS external advertisements. There are a couple of restrictions on the use of stub areas. Virtual links cannot be configured through stub areas. In addition, AS boundary routers cannot be placed internal to stub areas.

List of address ranges. An OSPF area is defined as a list of address ranges. Each address range consists of the following items:

[IP address, mask]. Describes the collection of IP addresses contained in the address range. Networks and hosts are assigned to an area depending on whether their addresses fall into one of the area's defining address ranges. Routers are viewed as belonging to multiple areas, depending on their attached networks' area membership.
Status. Set to either Advertise or DoNotAdvertise. Routing information is condensed at area boundaries. External to the area, at most a single route is advertised (via a summary link advertisement) for each address range. The route is advertised if and only if the address range's Status is set to Advertise. Unadvertised ranges allow the existence of certain networks to be intentionally hidden from other areas. Status is set to Advertise by default.

Types of OSPF packets

The Open Shortest Path First (OSPF) protocol runs directly over IP, using IP protocol 89. It does not provide any explicit fragmentation/reassembly support.
When fragmentation is necessary, IP fragmentation/reassembly is used. OSPF protocol packets have been designed so that large protocol packets can generally be split into several smaller protocol packets.
This practice is recommended; IP fragmentation should be avoided whenever possible.

OSPF protocol uses the types of packets that are described below:

HELLO	The Hello packet (Hello)	These packets are sent periodically on all interfaces (including virtual links) in order to establish and maintain neighbor relationships. In addition, Hello Packets are multicast on those physical networks having a multicast or broadcast capability, enabling dynamic discovery of neighboring routers.
DD	The Database Description packet	These packets are exchanged when an adjacency is being initialized. They describe the contents of the topological database. Multiple packets may be used to describe the database. For this purpose a poll-response procedure is used.
LSR	The Link State Request packet	After exchanging Database Description packets with a neighboring router, a router may find that parts of its topological database are out of date. The Link State Request packet is used to request the pieces of the neighbor's database that are more up to date. Multiple Link State Request packets may need to be used. The sending of Link State Request packets is the last step in bringing up an adjacency.
LSU	The Link State Update packet	These packets implement the flooding of link state advertisements. Each Link State Update packet carries a collection of link state advertisements one hop further from its origin. Several link state advertisements may be included in a single packet. Link State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the flooding procedure reliable, flooded advertisements are acknowledged in Link State Acknowledgment packets. If retransmission of certain advertisements is necessary, the retransmitted advertisements are always carried by unicast Link State Update packets.
LSA	The Link State Acknowledgment packet	To make the flooding of link state advertisements reliable, flooded advertisements are explicitly acknowledged. This acknowledgment is accomplished through the sending and receiving of Link State Acknowledgment packets. Multiple link state advertisements can be acknowledged in a single Link State Acknowledgment packet.

Configuration of the OSPF port

The Open Shortest Path First (OSPF) port is referred by the "OSPF" abbreviation and it has all the parameters described in this chapter.

Here there is an example of the OSPF port parameters. Displayed values are the default ones.

[00:50:16] ABILIS_CPX: D P PO:OSPF

PO:913 - Not Saved (SAVE CONF), Not Refreshed (INIT) --------------------------
OSPF   ------------------------------------------------------------------------
       LOG:DS       ACT:NO        max-lsa:200      max-routes:500     
       RFC1583COMP:YES            OSPF-RID:AUTO (192.168.000.060)   ASBR:YES    
       - ASBR section ---------------------------------------------------------
       LOCAL:NO            STATIC:NO            RIP:NO     
       LOCAL-METRIC:*      STATIC-METRIC:*      RIP-METRIC:*     
       DEF-METRIC:1

The parameters reported in the "ASBR section" are displayed only if the ASBR: parameter is set to "YES".

To activate changes made on the parameters displayed by low case characters, it is needed to restart the system; on the contrary for activating changes made on low case parameters it is enough to execute the initialization command INIT PO:.
The changes made on the LOG: parameter are immediately active.

The "Not Saved (SAVE CONF)" message is displayed every time the port configuration is modified but not saved with the SAVE CONF command.

The "Not Refreshed (INIT)" message is displayed every time the port configuration is modified but not refreshed with the INIT PO: command.

Details of the OSPF port parameters

LOG:	Events logging activation and generation of alarm signals
DS	NO, D, S, A, L, T, ALL, +E

Usually this parameter makes possible to activate/deactivate logging functionalities of meaningful events of the port as well as the detection and signalling of alarms in case of critical events.

The following table shows the available options and the related functionalities usable by the parameter:

Option	Meaning
D	Recording of the driver state changes and/or the meaningful events in Debug Log
S	Recording of the driver state changes and/or the meaningful events in the System Log
A	Periodic detection of possible alarms. The detected alarms can be displayed the command ALARM VIEW or by the analogous command available on the UTILITY of the LCD display on the front panel
L	On alarm detection, acoustic signal generation plus a message on the LCD display. This function depends on activation of alarms detection by the "A" option
T	Generation by the Agent SNMP of Abilis CPX of SNMP traps corresponding to any change of the driver state and/or occurring of meaningful events

Beside the already described options the following values are also allowed:

Option	Meaning
NO	It means that all the logging functionalities, alarms detection and generation, above mentioned, are disabled.
ALL	It means that all the logging functionalities, alarms detection and generation, above mentioned, are enabled.
+E	This option added to one or more of the previous ones, extends its (their) set of meaningful events. The value "ALL+E" activates all the options and extends the set of meaningful events. The value "NO+E" is meaningless so it is ignored.

Options can be combined together.

Some examples:

setting "LOG:DS+E", activates the extended logging functions for Events Log and System Log
setting "LOG:STA", activates the extended logging functions for System Log, SNMP traps generation and periodic detection of alarm states;

By using the characters "+" and "-" as prefix of one or more options is possible to add or delete one or more functionalities without setting from the scratch the value of the parameters.

Some examples:

Suppose the current value of the parameter is "LOG:DSTA", by setting "LOG:-A", the periodic detection of eventual alarm states is removed, leaving unchanged all the remaining options; in such way the final value of the parameter will be "LOG:DST";
Suppose the current value of the parameter is "LOG:ST", by setting "LOG:+DA", the logging function of the events on the Events Log and the periodic alarm detection are added to the already activated options; in such way the final value of the parameter will be "LOG:DSTA".

The changes made on this parameter are immediately activated, without the need of initialization commands.

OSPF_ACT:	OSPF runtime activation/deactivation
NO	NO, YES

NO: OSPF is disabled.

YES: OSPF is enabled.

max-lsa:	The size of LSA's database
200	1 - 1500

The router use LSA's database for detecting state of links between routers this AS. A router generate some LSA that describe a state of its interfaces. All routers belong to one area have copys of all this LSAs in their LSA's database. That is why the size of LSA's database can be large. To calculate a precision size of LSA database not simply. It depends of many other paramaters (count of areas, count of neighbors, etc).

max-routes:	The size of the route table
500	1 - 10000

The route table of each OSPF route is contain routes to points of destination. This table is contain as routes that was detected during work OSPF protocol as routes that was received from other routing protocols.

RFC1583COMP:	Enable/disable compatibility with RFC1583
YES	NO, YES

This parameter permit OSPF compatibility with RFC 1583. In order to minimize the chance of routing loops, all OSPF routers in an OSPF routing domain should have RFC1583COMP set identically.

OSPF-RID:	OSPF Router ID
AUTO	AUTO, 0.0.0.0 - 255.255.255.255

This parameter allows to set the OSPF Router ID.

ASBR:	Defines this router as Autonomous System Boundary Router (ASBR)
YES	YES, NO

ASBR router locates in point where OSPF AS connects with other AS that does not use OSPF routing protocol. ASBR router can redistribute routes between different AS.
Usually this parameter will set to YES, because only in this case OSPF can import external routing. In fact, in case that ASBR is set to NO, OSPF table will be filled only by routings obtained by other OSPF routers in the network.

LOCAL:	ASBR LOCAL activation
YES	YES, NO

It enables/disables the use of LOCAL (connected) routes present in the routes table of IPRTR for redistribution into OSPF.