OSPF: Frequently Asked Questions

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OSPF: Frequently Asked Questions 我感觉这个也不错。：）
Why are loopbacks advertised as /32 host routes in OSPF?
How do I change the reference bandwidth in OSPF?
How does OSPF calculate its metric or cost?
Are OSPF routing protocol exchanges authenticated?
What is the link-state retransmit interval, and what is the command to set it?
What is the purpose of the variable IP-OPSF-Transmit-Delay?
Is it true that only the static option of the virtual link in OSPF allows discontiguous networks, regardless of the mask propagation properties?
Are the multicast IP addresses mapped to MAC-level multicast addresses?
Does the Cisco OSPF implementation support IP TOS-based routing?
Will the offset-list subcommand work for OSPF?
Can an OSPF default be originated into the system based on external information on a router that does not itself have a default?
Can I use the distribute-list in/out command with OSPF to filter routes?
How can I give preference to OSPF inter-area routes over intra-area routes?
Do we have to manually setup adjacencies for routers on the SMDS cloud with the OSPF neighbor subcommand?
When routes are redistributed between OSPF processes, are all SPF metrics preserved or is the default metric value used?
How does Cisco accommodate OSPF routing on partial-mesh Frame Relay networks?
Which address-wildmask pair should I use for assigning an unnumbered interface to an area?
Can I have one numbered side and leave the other side unnumbered in OSPF?
Why do I receive the "cannot allocate router id" error message when I configure Router OSPF One?
Why do I receive the "unknown routing protocol" error message when I configure Router OSPF One?
What do the states DR, BDR, and DR/OTHER mean in show is ospf interface output?
When I type the show ip ospf neighbor command, why do I only see FULL/DR and FULL/BDR, with all other neighbors showing 2-WAY/DROTHER?
Why don't I see OSPF neighbors as FULL/DR or FULL/BDR on my serial link?
Do I need any special commands to run OSPF over BRI/PRI links?
Do I need any special commands to run OSPF over asynchronous links?
Which Cisco IOS version began support for per-interface authentication type in OSPF?
Can I control the P-bit when importing external routes into an NSSA?
Why are OSPF show commands responding so slowly?   
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Q: Why are loopbacks advertised as /32 host routes in OSPF?

A: Loopbacks are considered host routes in OSPF, and they're advertised as /32. For more information, see section 9.1 of RFC 2328. In Cisco IOS &reg&#59; version 11.3T and 12.0, if the ip ospf network point-to-point command is configured under loopbacks, then OSPF advertises the loopback subnet as the actual subnet configured on loopbacks.



Q: How do I change the reference bandwidth in OSPF?

A: You can change the reference bandwith in Cisco IOS version 11.2 and higher, using the ospf auto-cost reference-bandwith command under router ospf. By default reference-bandwidth is 100 Mbps.



Q: How does OSPF calculate its metric or cost?

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's 100 Mbps/10 Mbps = 10.

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



Q: Are OSPF routing protocol exchanges authenticated?

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 enter the area <x>; authentication command to the OSPF router configuration to enable authentication, where x is the area number.



Q: What is the link-state retransmit interval, and what is the command to set it?

A: OSPF must send acknowledgment of each newly received link-state advertisement. It does this by sending link-state acknowledgment (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 five seconds.


Q: What is the purpose of the variable IP-OPSF-Transmit-Delay?

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 LSA propagates over the link is not considered. The default value is one second. This parameter has more significance on very low speed links.


Q: Is it true that only the static option of the virtual link in OSPF allows discontiguous networks, regardless of the mask propagation properties?

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


Q: Are the multicast IP addresses mapped to MAC-level multicast addresses?

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.


Q: Does the Cisco OSPF implementation support IP TOS-based routing?

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



Q: Will the offset-list subcommand work for OSPF?

A: The offset-list command doesn't work for OSPF&#59; it's used for distance vector protocols such as IGRP, RIP, RIPv2.

Q: Can an OSPF default be originated into the system based on external information on a router that does not itself have a default?

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 doesn't have a default route itself, use the command default-information originate always.


Q: Can I use the distribute-list in/out command with OSPF to filter routes?

A: OSPF routes can't be filtered from entering the OSPF database. The distribute-list in command only filters routes from entering the routing table, but it doesn't prevent link-state packets from being propagated.

The command distribute-list out works only on the routes being redistributed by the autonomous system boundary routers (ASBRs) into OSPF. It can be applied to external type 2 and external type 1 routes, but not to intra-area and inter-area routes.


Q: How can I give preference to OSPF inter-area routes over intra-area routes?

A. According to RFC, the order of preference for OSPF routes are as follows:

intra-area routes, O
inter-area routes O IA
external routes type 1, O E1
external routes type 2, O E2
Q: Do I have to manually setup adjacencies for routers on the SMDS cloud with the OSPF neighbor subcommand?

A: Before Cisco IOS 10.0, you needed the neighbor command to establish adjacencies over NBMA networks (such as Frame Relay, X.25, and SMDS). As of IOS 10.0, 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're not using a fully meshed SMDS cloud, you must use the ip ospf network point-to-multipoint command.


Q: When routes are redistributed between OSPF processes, are all SPF metrics preserved or is the default metric value used?

A: The SPF metrics are not preserved. The redistribution between them is like redistribution between any two IP routing processes.



Q: How does Cisco accommodate OSPF routing on partial-mesh Frame Relay networks?

A: You can configure OSPF to understand whether or not 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 10.0 includes a feature called subinterfaces. You can use subinterfaces with Frame Relay to tie together a set of virtual circuits (VC) to form a virtual interface, which acts as a single IP subnet. All systems within the subnet should be fully meshed. As of 10.3 and 11.0, the ip ospf point-to-multipoint command is also available.


Q: Which address-wildmask pair should I use for assigning an unnumbered interface to an area?

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

Q: Can I have one numbered side and leave the other side unnumbered in OSPF?

A: No, OSPF doesn't 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.

Q: Why do I receive the &quot;cannot allocate router id&quot; error message when I configure Router OSPF One?

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, then it returns this error message. To correct the problem, configure a loopback interface.

Q: Why do I receive the &quot;unknown routing protocol&quot; error message when I configure Router OSPF One?

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

Q: What do the states DR, BDR, and DROTHER mean in show is ospf interface output?

A: DR means designated router, BDR means backup designated router, and DROTHER means 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.

Q: When I type the show ip ospf neighbor command, why do I only see FULL/DR and FULL/BDR, with all other neighbors showing 2-WAY/DROTHER?

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

Q: Why don't I see OSPF neighbors as FULL/DR or FULL/BDR on my serial link?

A: This is normal. On point-to-point and point-to-multipoint networks there are no DRs or BDRs.

Q: Do I need any special commands to run OSPF over BRI/PRI links?

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.

Q: Do I need any special commands to run OSPF over asynchronous links?

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.

Q: Which Cisco IOS version began support for per-interface authentication type in OSPF?

A: Per-interface authentication type, as described in RFC 2178 , was added to Cisco IOS in version 12.0(.

Q: Can I control the P-bit when importing external routes into an NSSA?

A: When external routing information is imported into a Not-So-Stubby Area (NSSA) in a Type-7 LSA, the Type-7 LSA has only area flooding scope. In order to futher 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 wether 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 ASBR is also an area border router (ABR). The NSSA ASBR can then summarize using the not-adverise keyword, which results in not advertising the translated Type-7 LSAs.

Q: Why are OSPF show commands responding so slowly?

A: You might experience a slow response when issuing OSPF show commands, but not with other commands. The most common reason for this delay is because 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 more than just OSPF show commands, you may want to start looking at other possible causes, such as the CPU utilization.