Configure OSPF
Open Shortest Path First (OSPF) is an IGP developed by the OSPF working group of the Internet Engineering Task Force (IETF). Designed expressly for IP networks, OSPF supports IP subnetting and tagging of externally derived routing information. OSPF also allows packet authentication and uses IP multicast when sending/receiving packets.
We support RFC 1253, Open Shortest Path First (OSPF) MIB, August 1991. The OSPF MIB defines an IP routing protocol that provides management information related to OSPF and is supported by Cisco routers.
Cisco's OSPF Implementation
Cisco's implementation conforms to the OSPF Version 2 specifications detailed in the Internet RFC 1583. The list that follows outlines key features supported in Cisco's OSPF implementation:
- Stub areas---Definition of stub areas is supported.
- Route redistribution---Routes learned via any IP routing protocol can be redistributed into any other IP routing protocol. At the intradomain level, this means that OSPF can import routes learned via IGRP, RIP, and IS-IS. OSPF routes can also be exported into IGRP, RIP, and IS-IS. At the interdomain level, OSPF can import routes learned via EGP and BGP. OSPF routes can be exported into EGP and BGP.
- Authentication---Simple 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.
OSPF Configuration Task List
OSPF typically requires coordination among many internal routers, area border routers (routers connected to multiple areas), and autonomous system boundary routers. At a minimum, OSPF-based routers 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.
To configure OSPF, complete the tasks in the following sections. Enabling OSPF is mandatory; the other tasks are optional but might be required for your application.
- Enable OSPF
- Configure OSPF Interface Parameters
- Configure OSPF over Different Physical Networks
- Configure OSPF Area Parameters
- Configure Route Summarization between OSPF Areas
- Configure Route Summarization when Redistributing Routes into OSPF
- Create Virtual Links
- Generate a Default Route
- Configure Lookup of DNS Names
- Force the Router ID Choice with a Loopback Interface
- Disable Default OSPF Metric Calculation Based on Bandwidth
- Configure OSPF on Simplex Ethernet Interfaces
- Configure Route Calculation Timers
In addition, you can specify route redistribution; see the task "Redistribute Routing Information" later in this chapter for information on how to configure route redistribution.
Enable OSPF
As with other routing protocols, enabling OSPF requires that you create an OSPF routing process, specify the range of IP addresses to be associated with the routing process, and assign area IDs to be associated with that range of IP addresses. Perform the following tasks, starting in global configuration mode:
| Task | Command |
|---|---|
Step 1 Enable OSPF routing, which places you in router configuration mode. | router ospf process-id |
Step 2 Define an interface on which OSPF runs and define the area ID for that interface. |
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Configure OSPF Interface Parameters
Our OSPF implementation allows you to alter certain interface-specific OSPF parameters, as needed. You are not required to alter any of these parameters, but some interface parameters must be consistent across all routers in an attached network. Those are the parameters set by the ip ospf hello-interval, ip ospf dead-interval, and ip ospf authentication-key commands. Therefore, be sure that if you do configure any of these parameters, the configurations for all routers on your network have compatible values.
In interface configuration mode, specify any of the following interface parameters as needed for your network:
| Task | Command |
|---|---|
Explicitly specify the cost of sending a packet on an OSPF interface. | |
Specify the number of seconds between link state advertisement retransmissions for adjacencies belonging to an OSPF interface. | |
Set the estimated number of seconds it takes to transmit a link state update packet on an OSPF interface. | ip ospf transmit-delay seconds |
Set router priority to help determine the OSPF designated router for a network. | ip ospf priority number |
Specify the length of time, in seconds, between the hello packets that a router sends on an OSPF interface. | ip ospf hello-interval seconds |
Set the number of seconds that a router's hello packets must not have been seen before its neighbors declare the OSPF router down. | ip ospf dead-interval seconds |
ip ospf authentication-key key | |
Enable OSPF MD5 authentication. |
| |
Configure OSPF over Different Physical Networks
OSPF classifies different media into three types of networks by default:
- Broadcast networks (Ethernet, Token Ring, FDDI)
- Nonbroadcast multiaccess networks (SMDS, Frame Relay, X.25)
- Point-to-point networks (HDLC, PPP)
You can configure your network as either a broadcast or a nonbroadcast multiaccess network.
X.25 and Frame Relay provide an optional broadcast capability that can be configured in the map to allow OSPF to run as a broadcast network. See the x25 map and frame-relay map command descriptions in the Router Products Command Reference publication for more detail.
Configure Your OSPF Network Type
You have the choice of configuring your OSPF network type as either broadcast or nonbroadcast multiaccess, regardless of the default media type. Using this feature, you can configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure nonbroadcast multiaccess networks, such as X.25, Frame Relay, and SMDS, as broadcast networks. This feature saves you from having to configure neighbors, as described in the section "Configure OSPF for Nonbroadcast Networks."
Configuring nonbroadcast, multiaccess networks as either broadcast or nonbroadcast assumes that there are virtual circuits from every router to every router or fully-meshed network. This is not true for some cases, for example, due to cost constraints or when you have only a partially-meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network. Routing between two routers that are not directly connected will go through the router that has virtual circuits to both routers. Note that you do not need to configure neighbors when using this feature.
An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. It creates multiple host routes. An OSPF point-to-multipoint network has the following benefits compared to nonbroadcast multiaccess and point-to-point networks:
- Point-to-multipoint is easier to configure because it requires no configuration of neighbor commands, it consumes only one IP subnet, and it requires no designated router election.
- It costs less because it does not require a fully-meshed topology.
- It is more reliable because it maintains connectivity in the event of virtual circuit failure.
To configure your OSPF network type, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
Configure the OSPF network type for a specified interface. | ip ospf network {broadcast | non-broadcast | point-to-multipoint} |
| |
For an example of an OSPF point-to-multipoint network, see the section "OSPF Point-to-Multipoint Example" at the end of this chapter.
Configure OSPF for Nonbroadcast Networks
Because there might be many routers attached to an OSPF network, a designated router is selected for the network. It is necessary to use special configuration parameters in the designated router selection if broadcast capability is not configured.
These parameters need only be configured in those routers that are themselves eligible to become the designated router or backup designated router (in other words, routers with a nonzero router priority value).
To configure routers that interconnect to nonbroadcast networks, perform the following task in router configuration mode
| Task | Command |
|---|---|
Configure routers interconnecting to nonbroadcast networks. | neighbor ip-address [priority number] [poll-interval seconds] |
| |
You can specify the following neighbor parameters, as required:
- Priority for a neighboring router
- Nonbroadcast poll interval
- Interface through which the neighbor is reachable
Configure OSPF Area Parameters
Our OSPF software allows you to configure several area parameters. These area parameters, shown in the following table, include authentication, defining stub areas, and assigning specific costs to the default summary route. Authentication allows password-based protection against unauthorized access to an area.
Stub areas are areas into which information on external routes is not sent. Instead, there is a default external route generated by the area border router into the stub area for destinations outside the autonomous system. To further reduce the number of link state advertisements (LSA) sent into a stub area, you can configure no-summary on the Area Border Router (ABR) to prevent it from sending summary link advertisement (LSA type 3) into the stub area.
| Task | Command |
|---|---|
Enable authentication for an OSPF area. | |
Define an area to be a stub area. | |
Assign a specific cost to the default summary route used for the stub area. | area area-id default-cost cost |
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Configure Route Summarization between OSPF Areas
To specify an address range, perform the following task in router configuration mode:
| Task | Command |
|---|---|
Specify an address range for which a single route will be advertised. |
| |
Configure Route Summarization when Redistributing Routes into OSPF
When redistributing routes from other protocols into OSPF (as described in the section "Configure Routing Protocol-Independent Features" later in this chapter, each route is advertised individually in an external link state advertisement (LSA). However, you can configure the router to advertise a single route for all the redistributed routes that are covered by a specified network address and mask. Doing so helps decrease the size of the OSPF link state database.
To have the router advertise one summary route for all redistributed routes covered by a network address and mask, perform the following task in router configuration mode:
| Task | Command |
|---|---|
Specify an address and mask that covers redistributed routes, so only one summary route is advertised. |
| |
Create Virtual Links
In OSPF, all areas must be connected to a backbone area. If there is a break in backbone continuity, or the backbone is purposefully partitioned, you can establish a virtual link. The two end points of a virtual link are Area Border Routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other Area Border Router), and the nonbackbone area that the two routers have in common (called the transit area). Note that virtual links cannot be configured through stub areas.
To establish a virtual link, perform the following task in router configuration mode:
| Task | Command |
|---|---|
area area-id virtual-link router-id [hello-interval seconds] [retransmit-interval seconds] [transmit-delay seconds] [dead-interval seconds] [[authentication-key key] | [message-digest-key keyid md5 key]] |
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Generate a Default Route
You can force an autonomous system boundary router to generate a default route into an OSPF routing domain. Whenever you specifically configure redistribution of routes into an OSPF routing domain, the router automatically becomes an autonomous system boundary router. However, an autonomous system boundary router does not, by default, generate a default route into the OSPF routing domain.
| Task | Command |
|---|---|
Force the autonomous system boundary router to generate a default route into the OSPF routing domain. | default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name] |
| |
See also the discussion of redistribution of routes in the "Configure Routing Protocol-Independent Features" section later in this chapter.
Configure Lookup of DNS Names
You can configure OSPF to look up Domain Name System (DNS) names for use in all OSPF show command displays. This feature makes it easier to identify a router, because it is displayed by name rather than by its router ID or neighbor ID.
To configure DNS name lookup, perform the following task in global configuration mode:
| Task | Command |
|---|---|
Configure DNS name lookup. | ip ospf name-lookup |
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Force the Router ID Choice with a Loopback Interface
OSPF uses the largest IP address configured on the router's interfaces as its router ID. If the interface associated with this IP address is ever brought down, or if the address is removed, the OSPF process must recalculate a new router ID and resend all of its routing information out its interfaces.
If a loopback interface is configured with an IP address, the router will use this IP address as its router ID, even if other interfaces have larger IP addresses. Since loopback interfaces never go down, greater stability in the routing table is achieved.
OSPF automatically prefers a loopback interface over any other kind, and it chooses the highest IP address among all loopback interfaces in the router. If no loopback interfaces are present, the highest IP address in the router is chosen. You cannot tell OSPF to use any particular interface.
To configure an IP address on a loopback interface, perform the following tasks, starting in global configuration mode:
| Task | Command |
|---|---|
Step 1 Create a loopback interface, which places you in interface configuration mode. | interface loopback 01 |
Step 2 Assign an IP address to this interface. | ip address address mask2 |
Disable Default OSPF Metric Calculation Based on Bandwidth
In Cisco IOS Release 10.2 and earlier, OSPF assigned default OSPF metrics to interfaces regardless of the interface bandwidth. It gave both 64K and T1 links the same metric (1562), and thus required an explicit ip ospf cost command in order to take advantage of the faster link.
In Cisco IOS Release 10.3, by default, OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. For example, a 64K link gets a metric of 1562, while a T1 link gets a metric of 64. To disable this feature, perform the following task in router configuration mode:
| Task | Command |
|---|---|
Disable default OSPF metric calculations based on interface bandwidth, resulting in a fixed default metric assignment. |
| |
Configure OSPF on Simplex Ethernet Interfaces
Because simplex interfaces between two routers on an Ethernet represent only one network segment, for OSPF you have to configure the transmitting interface to be a passive interface. This prevents OSPF from sending hello packets for the transmitting interface. Both routers are able to see each other via the hello packet generated for the receiving interface.
| Task | Command |
|---|---|
Suppress the sending of hello packets through the specified interface. |
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Configure Route Calculation Timers
| Task | Command |
|---|---|
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