The document addresses the most frequently asked questions (FAQ) associated with Open Shortest Path First (OSPF). The document covers OSPF version 2 only. OSPF version 3, introduced in Cisco IOS^® Software Releases 12.0(24)S, 12.2(18)S, and 12.2(15)T, is used for distributing IP version 6 routing information; it is not explicitly covered in this document. In the scope of this document, "OSPF" refers to OSPF version 2 and "IP" refers to IP version 4.

A. You can change the reference bandwidth in Cisco IOS Software Release 11.2 and later using the ospf auto-cost reference-bandwidth command under router ospf. By default, reference bandwidth is 100 Mbps. The ospf link-cost is a 16-bit number. Therefore, the maximum value supported is 65,535.

A. OSPF uses a reference bandwidth of 100 Mbps for cost calculation. The formula to calculate the cost is reference bandwidth divided by interface bandwidth. For example, in the case of Ethernet, it is 100 Mbps / 10 Mbps = 10.

Note: If ip ospf cost cost is used on the interface, it overrides this formulated cost.

A. Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication may be through simple passwords or through MD5 cryptographic checksums. To configure simple password authentication for an area, use the command ip ospf authentication-key to assign a password of up to eight octets to each interface attached to the area. Then, issue the area x authentication command to the OSPF router configuration to enable authentication. (In the command, x is the area number.)

Cisco IOS Software Release 12.x also supports the enabling of authentication on a per-interface basis. If you want to enable authentication on some interfaces only, or if you want different authentication methods on different interfaces that belong to the same area, use the ip ospf authentication interface mode command.

A. OSPF must send acknowledgment of each newly received link-state advertisement (LSA). It does this by sending LSA packets. LSAs are retransmitted until they are acknowledged. The link-state retransmit interval defines the time between retransmissions. You can use the command ip ospf retransmit-interval to set the retransmit interval. The default value is 5 seconds.

A. This variable adds a specified time to the age field of an update. If the delay is not added before transmission over a link, the time in which the link-state advertisement (LSA) propagates over the link is not considered. The default value is 1 second. This parameter has more significance on very low-speed links.

A. No, virtual links in OSPF maintain connectivity to the backbone from nonbackbone areas, but they are unnecessary for discontiguous addressing. OSPF provides support for discontiguous networks because every area has a collection of networks, and OSPF attaches a mask to each advertisement.

A. OSPF sends all advertisements using multicast addressing. Except for Token Ring, the multicast IP addresses are mapped to MAC-level multicast addresses. Cisco maps Token Ring to MAC-level broadcast addresses.

A. Cisco OSPF only supports TOS 0. This means that routers route all packets on the TOS 0 path, eliminating the need to calculate nonzero TOS paths.

A. The offset-list command does not work for OSPF. It is used for distance vector protocols such as Interior Gateway Routing Protocol (IGRP), Routing Information Protocol (RIP), and RIP version 2.

A. OSPF generates a default only if it is configured using the command default-information originate and if there is a default network in the box from a different process. The default route in OSPF is 0.0.0.0. If you want an OSPF-enabled router to generate a default route even if it does not have a default route itself, use the command default-information originate always.

A. In Cisco IOS Software releases earlier than Cisco IOS Software Release 10.0, the neighbor command was required to establish adjacencies over nonbroadcast multiaccess (NBMA) networks (such as Frame Relay, X.25, and SMDS). With Cisco IOS Software Release 10.0 and later, you can use the ip ospf network broadcast command to define the network as a broadcast network, eliminating the need for the neighbor command. If you are not using a fully meshed SMDS cloud, you must use the ip ospf network point-to-multipoint command.

A. The SPF metrics are preserved. The redistribution between them is like redistribution between any two IP routing processes.

A. You can configure OSPF to understand whether it should attempt to use multicast facilities on a multi-access interface. Also, if multicast is available, OSPF uses it for its normal multicasts.

Cisco IOS Software Release 10.0 includes a feature called subinterfaces. You can use subinterfaces with Frame Relay to tie together a set of virtual circuits (VCs) to form a virtual interface, which acts as a single IP subnet. All systems within the subnet should be fully meshed. With Cisco IOS Software Releases 10.3, 11.0 and later, the ip ospf point-to-multipoint command is also available.

A. When an unnumbered interface is configured, it references another interface on the router. When enabling OSPF on the unnumbered interface, use the address-wild-mask pair of interfaces to which the unnumbered interface is pointing.

A. No, OSPF does not work if you have one side numbered and the other side unnumbered. This creates a discrepancy in the OSPF database that prevents routes from being installed in the routing table.

A. OSPF picks up the highest IP address as a router ID. If there are no interfaces in up/up mode with an IP address, it returns this error message. To correct the problem, configure a loopback interface.

A. Your software may not support OSPF. This error message occurs most frequently with the Cisco 1600 series routers. If you are using a 1600 router, you need a Plus image to run OSPF.

A. DR means designated router. BDR means backup designated router. DROTHER indicates a router that is neither the DR or the BDR. The DR generates a Network Link-State Advertisement, which lists all the routers on that network.

A. To reduce the amount of flooding on broadcast media, such as Ethernet, FDDI, and Token Ring, the router becomes full with only designated router (DR) and backup designated router (BDR), and it shows 2-WAY for all other routers.

A. This is normal. On point-to-point and point-to-multipoint networks, there are no designated routers (DRs) or backup designated routers (BDRs).

A. In addition to the normal OSPF configuration commands, you should use the dialer map command. When using the dialer map command, use the broadcast keyword to indicate that broadcasts should be forwarded to the protocol address.

A. In addition to the normal OSPF configuration commands, you should use the async default routing command on the asynchronous interface. This command enables the router to pass routing updates to other routers over the asynchronous interface. Also, when using the dialer map command, use the broadcast keyword to indicate that broadcasts should be forwarded to the protocol address.

A. When external routing information is imported into an NSSA in a type 7 link-state advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The P-bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated. Only those LSAs with the P-bit set are translated. When you redistribute information into the NSSA, the P-bit is automatically set. A possible workaround applies when the Autonomous System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR can then summarize with the not-advertise keyword, which results in not advertising the translated type 7 LSAs.

A. You may experience a slow response when issuing OSPF show commands, but not with other commands. The most common reason for this delay is that you have the ip ospf name-lookup configuration command configured on the router. This command causes the router to look up the device Domain Name System (DNS) names for all OSPF show commands, making it easier to identify devices, but resulting in a slowed response time for the commands. If you are experiencing slow response on commands other than just OSPF show commands, you may want to start looking at other possible causes, such as the CPU utilization.

A. The clear ip ospf redistribution command flushes all the type 5 and type 7 link-state advertisements (LSAs) and scans the routing table for the redistributed routes. This causes a partial shortest path first algorithm (SPF) in all the routers on the network that receive the flushed/renewed LSAs. When the expected redistributed route is not in OSPF, this command may help to renew the LSA and get the route into OSPF.

A. The only time that OSPF forms adjacencies between neighbors that are not on the same subnet is when the neighbors are connected through point-to-point links. This may be desired when using the ip unnumbered command, but in all other cases, the neighbors must be on the same subnet.

A. OSPF sends out its self-originated LSAs when the LSA age reaches the link-state refresh time, which is 1800 seconds.

A. When you have two type 5 LSAs for the same external network in the OSPF database, prefer the external LSA that has the shortest path to the Autonomous System Boundary Router (ASBR) and install that into the IP routing table. Use the show ip ospf border-routers command to check the cost to the ASBR.

A. OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing Protocol (EIGRP), does not check the process number (or autonomous system number) when adjacencies are formed between neighboring routers and routing information is exchanged. The only case in which the OSPF process number is taken into account is when OSPF is used as the routing protocol on a Provider Edge to Customer Edge (PE-CE) link in a Multiprotocol Label Switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute derived from the OSPF process number to indicate whether the route originated within the same OSPF domain or from outside it. If the OSPF process numbering is inconsistent on PE routers in the MPLS VPN, the domain-id OSPF mode command should be used to mark that the OSPF processes with different numbers belong to the same OSPF domain.

This means that, in many practical cases, you can use different autonomous system numbers for the same OSPF domain in your network. However, it is best to use consistent OSPF-process numbering as much as possible. This consistency simplifies network maintenance and complies with the network designer intention to keep routers in the same OSPF domain.

A. OSPF uses the metric aCost, which is related to the bandwidth. If there are equal cost paths (the same bandwidth on both multilinks), OSPF installs both routes in the routing table. The routing table tries to use both links equally, regardless of the interface utilization. If one of the links in the first multilink fails, OSPF does not send all the traffic down the second multilink. If the first multilink peaks 100%, OSPF does not send any traffic down the second multilink because OSPF tries to use both links equally, regardless of the interface utilization. The second is used fully only when the first multilink goes down.

A. In order to have a rapid fault detection of topology changes, the hello timer value needs to be set to 1 second. The hold timer value, which is is four times that of the hello timer, also needs to be configured. There is a possibility of more routing traffic if the hello and hold timer values are reduced from their default values.

A. Yes, the 3800 Series Router that runs Advanced IPServices image supports the OSPF Stub feature.

A. The error message is due to the some router that is flushing the network LSA because the network LSA received by the router whose LSA ID conflicts with the IP address of one of the router's interfaces and flushes the LSA out of the network. For OSPF to function correctly the IP addresses of transit networks must be unique. If it is not unique the conflicting routers reports this error message. In the error message the router with the OSPF router ID reported as adv-rtr reports this message.

A. Type 3 LSA is originated by the Area Border Router (ABR) as a summary route. Manipulating the summary route is not possible in an ABR router.

A. When the NSSA ABR is configured to move from nssa no-summary to nssa, the OSPF neighborship does not flap.

A. The OSPF neighbor was changed state from FULL to EXSTART because of the receipt of a Database Description (DBD) packet from the neighbor with an unexpected sequence number.

SeqNumberMismatch means that a DBD packet during OSPF neighborship negotiation has been received that either:

• has an unexpected DBD sequence number

• unexpectedly has the Init bit set

• has an Options field differing from the last Options field received in a Database Description packet.

A. Cisco IOS has a limit of 32 routing processes. Two of these are saved for static and directly connected routes. The Cisco 7600 router supports 28 OSPF processes per VRF.