By : Akshay Mhatre, Technical Support Manager, TATA Communications
Activity: 0 comments 287 views last activity : 07 06 2010 20:18:04 +0000
Q. Why are loopbacks advertised as /32 host routes in OSPF?
A. Loopbacks are considered host routes in OSPF, and they are advertised as /32. For more information, see section 9.1 of RFC 2328 . In Cisco IOS® Software Release 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 Software Release 11.2 and later, 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 maybe 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 advertisement (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 efore transmission over a link, the time in which the LSA propagates over the link is not onsidered. The default value is one second. This parameter has more significance on very ow speed links.
Q. Is it true that only the static option of the virtual link in OSPF allows iscontiguous networks, regardless of the mask propagation roperties?
A. No. Virtual links in OSPF maintain connectivity to the backbone from non−backbone reas, but they are unnecessary for discontiguous addressing. OSPF provides support for iscontiguous networks, since every area has a collection of networks and OSPF attaches a ask to each advertisement.
Q. Are the multicast IP addresses mapped to MAC−level multicast ddresses?
A. OSPF sends all advertisements using multicast addressing. Except for Token Ring, the ulticast IP addresses are mapped to MAC−level multicast addresses. Cisco maps Token Rng to MAC−level broadcast addresses.
Q. Does the Cisco OSPF implementation support IP TOS−based Ruting?
A. Cisco OSPF only supports TOS 0. This means that routers route all packets on the TOS 0path, 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; it's used for distance vector protocolsch as IGRP, RIP, RIPv2.
Q. Can an OSPF default be originated into the system based on external nformation on a router that does not itself have a default?
A. OSPF generates a default only if it is configured using the command default−informationoriginate and if there is a default network in the box from a different process. The defaultroute in OSPF is 0.0.0.0. If you want an OSPF−enabled router to generate a default route evenif it doesn't have a default route itself, use the command default−information originatealways.
Q. Can I use the distribute−list in/out command with OSPF to filterroutes?A. OSPF routes can't be filtered from entering the OSPF database. The distribute−list incommand only filters routes from entering the routing table, but it doesn't prevent link−statepackets from being propagated.The command distribute−list out works only on the routes being redistributed by theautonomous system boundary routers (ASBRs) into OSPF. It can be applied to external type 2and 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 Routers?
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 SMDScloud with the OSPF neighbor subcommand?
A. Before Cisco IOS Software Release 10.0, you needed the neighbor command to establishadjacencies over NBMA networks (such as Frame Relay, X.25, and SMDS). As of Cisco IOSSoftware Release 10.0, you can use the ip ospf network broadcast command to define thenetwork as a broadcast network, eliminating the need for the neighbor command. If you'renot using a fully meshed SMDS cloud, you must use the ip ospf networkpoint−to−multipoint command.
Q. When routes are redistributed between OSPF processes, are all SPFmetrics preserved or is the default metric value used?
A. The SPF metrics are not preserved. The redistribution between them is like redistributionbetween any two IP routing processes.
Q. How does Cisco accommodate OSPF routing on partial−mesh FrameRelay networks?
A. You can configure OSPF to understand whether or not it should attempt to use multicastfacilities on a multi−access interface. Also, if multicast is available, OSPF uses it for itsnormal multicasts.Cisco IOS Software Release 10.0 includes a feature called subinterfaces. You can usesubinterfaces with Frame Relay to tie together a set of virtual circuits (VC) to form a virtualinterface, which acts as a single IP subnet. All systems within the subnet should be fullymeshed. As of Cisco IOS Software Releases 10.3 and 11.0, the ip ospf point−to−multipointcommand is also available.
Q. Which address−wildmask pair should I use for assigning anunnumbered 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 ofinterfaces to which the unnumbered interface is pointing.
Q. Can I have one numbered side and leave the other side unnumberedin OSPF?
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 inthe routing table.
Q. Why do I receive the "cannot allocate router id" error message when Iconfigure Router OSPF One?
A. OSPF picks up the highest IP address as a router ID. If there are no interfaces in up/upmode with an IP address, then it returns this error message. To correct the problem, configurea loopback interface.
Q. Why do I receive the "unknown routing protocol" error messagewhen I configure Router OSPF One?
A. Your software may not support OSPF. This error message occurs most frequently with theCisco 1600 series routers. If you are using a Cisco 1600 router, you will need a plus image torun OSPF.
Q. What do the states DR, BDR, and DROTHER mean in show is ospfinterface output?
A. DR means designated router, BDR means backup designated router, and DROTHERmeans a router that is neither the DR or the BDR. The DR generates a Network Link StateAdvertisement, which lists all the routers on that network.
Q. When I type the show ip ospf neighbor command, why do I only seeFULL/DR and FULL/BDR, with all other neighbors showing2−WAY/DROTHER?
A. To reduce the amount of flooding on broadcast media, such as Ethernet, FDDI, and TokenRing, the router gets full with only DR and BDR, and shows 2−WAY for all other routers.
Q. Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on myserial link?
A. This is normal. On point−to−point and point−to−multipoint networks there are no DRs orBDRs.
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 mapcommand. When using the dialer map command, use the broadcast keyword to indicate thatbroadcasts should be forwarded to the protocol address.
Q. Do I need any special commands to run OSPF over asynchronouslinks?
A. In addition to the normal OSPF configuration commands, you should use the asyncdefault routing command on the asynchronous interface. This command enables the router topass routing updates to other routers over the asynchronous interface. Also, when using thedialer map command, use the broadcast keyword to indicate that broadcasts should beforwarded to the protocol address.
Q. Which Cisco IOS version began support for per−interfaceauthentication type in OSPF?
A. Per−interface authentication type, as described in RFC 2178 , was added in Cisco IOSSoftware Release 12.0(8).
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 aType−7 LSA, the Type−7 LSA has only area flooding scope. In order to futher distribute theexternal 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 betranslated. Only those LSAs with the P−bit set are translated. When you redistributeinformation into the NSSA, the P−bit is automatically set. A possible workaround applieswhen the ASBR is also an area border router (ABR). The NSSA ASBR can then summarizeusing 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 withother commands. The most common reason for this delay is because you have the ip ospfname−lookup configuration command configured on the router. This command causes therouter to look up the device Domain Name System (DNS) names for all OSPF showcommands, making it easier to identify devices, but resulting in a slowed response time forthe commands. If you are experiencing slow response on more than just OSPF showcommands, you may want to start looking at other possible causes, such as the CPUutilization.
Q. What does the clear ip ospf redistribution command do?
A. The clear ip ospf redistribution command flushes all the type 5 and 7 LSAs and scans therouting table for the redistributed routes. This will cause a partial shortest path first (SPF) inall the routers on the network which receive the flushed/renewed LSAs. When the expectedredistributed route is not in OSPF, this command may help to renew the LSA and get theroute into OSPF.
Q. Will OSPF form adjacencies with neighbors that are not on the samesubnet?
A. The only time that OSPF will form adjacencies between neighbors that are not on the samesubnet is when the neighbors are connected through point−to−point links. This may bedesired when using the ip unnumbered command, but in all other cases the neighbors mustbe on the same subnet.
Q. How often will OSPF send out LSAs?
A. OSPF sends out its self−originated LSAs when the LSA age reaches the link state refreshtime, which is 1800 seconds.
Q. How do I stop individual interfaces from developing adjacencies in anOSPF network?
A. To stop routers from becoming OSPF neighbors on a particular interface, configure thepassive−interface command at the interface.In ISP and large enterprise networks, many of the distribution routers have more than 200interfaces, and configuring passive−interface on each of the 200 interfaces can be difficult.The solution in such situations is to configure all the interfaces as passive by default using asingle passive−interface default command. Then configure individual interfaces whereadjacencies are desired using the no passive−interface command. For more information referto the Default Passive Interface Feature.There are some known problems with the passive−interface default command. Workaroundsare listed in Cisco bug ID CSCdr09263 ( registered customers only)
Q. When I have two Type−5 LSAs for the same external network in theOSPF database, which path should be installed in the IP routing table?
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 ASBR and install that into the IProuting table. Use the show ip ospf border−routers command to check the cost to theASBR.
Q. Why doesn't my Cisco 1600 router recognize the OSPF protocol?
A. Cisco 1600 routers require the Plus feature set image of Cisco IOS to run OSPF. Refer toRelease Notes for Cisco IOS Release 11.2(11) Software Feature Packs for Cisco 1600 SeriesRouters to see the Cisco 1600 Series Routers Feature Sets matrix for more information.
Q. Why won't my Cisco 800 router run OSPF?
A. Cisco 800 routers do not support OSPF; however, they do support RIP and EIGRPdynamic routing protocols. You can use the Software Advisor ( registered customers only) tool formore information on feature support.
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