Brief Discrubtions OSPF

IQRA UNIVERSITY ISLAMABAD CAMPUS

Final project report

Open Shortest Path First

Computer Communication and Network

Mehboob Nazim Shehzad

051-08-101085

Instructor:Sir Raza-ur-Rehman Khan

Supervisor:

Sir Fawad Raza

Department of computing and science and technology

Iqra university Islamabad campus

h-9 islamabad

Computer Communication and Network MEHBOOB Page 1


OPEN SHORTEST PATH FIRST

The Open Shortest Path First (OSPF) protocol, defined in RFC 2328, is an Interior Gateway Protoc Used to distribute routing information within a single Autonomous System. This report examines how OSPF works and how it can be used to design and build large and complicated networks.

Background Information

OSPF protocol was developed due to a need in the internet community to introduce a high functionality non-proprietary Internal Gateway Protocol (IGP) for the TCP/IP protocol family. The discussion of the creation of a common interoperable IGP for the Internet started in 1988 and did not get formalized until 1991. At that time the OSPF Working Group requested that OSPF be considered for advancement to Draft Internet Standard. The OSPF protocol is based on link-state technology, which is a departure from the Bellman-Ford vector based algorithms used in traditional Internet routing protocols such as RIP. OSPF has introduced new concepts such as authentication of routing updates, Variable Length Subnet Masks (VLSM), route summarization, and so forth.

OSPF versus RIP

The rapid growth and expansion of today's networks has pushed RIP to its limits. RIP has certain limitations that can cause problems in large networks:

RIP has a limit of 15 hops. A RIP network that spans more than 15 hops (15 routers) is considered unreachable.

RIP cannot handle Variable Length Subnet Masks (VLSM). Given the shortage of IP addresses and the flexibility VLSM gives in the efficient assignment of IP addresses, this is considered a major flaw.

Periodic broadcasts of the full routing table consume a large amount ofbandwidth. This is a major problem with large networks especially on slow links and WAN clouds.

RIP converges slower than OSPF. In large networks convergence gets to be in the order of minutes. RIP routers go through a period of a hold-down and garbage collection and slowly time-out information that has not been received recently. This is inappropriate in large environments and could cause routing inconsistencies.

RIP has no concept of network delays and link costs. Routing decisions are based on hop counts. The path with the lowest hop count to the destination is always preferred even if the longer path has a better aggregate link bandwidth and less delays.

RIP networks are flat networks. There is no concept of areas or boundaries. With the introduction of classless routing and the intelligent use of aggregation and summarization, RIP networks seem to have fallen behind.



Some enhancements were introduced in a new version of RIP called RIP2. RIP2 addresses the issues of VLSM, authentication, and multicast routing updates. RIP2 is not a big improvement over RIP (now called RIP 1) because it still has the limitations of hop counts and slow convergence which are essential in today’s large networks.

OSPF

OSPF, on the other hand, addresses most of the issues previously presented:

With OSPF, there is no limitation on the hop count.

The intelligent use of VLSM is very useful in IP address allocation.

OSPF uses IP multicast to send link-state updates. This ensures less processing on routers that are not listening to OSPF packets. Also, updatesare only sent in case routing changes occur instead of periodically. This ensures a better use of bandwidth.

OSPF has better convergence than RIP. This is because routing changes are propagated instantaneously and not periodically.

OSPF allows for better load balancing.

OSPF allows for a logical definition of networks where routers can be divided into areas. This limits the explosion of link state updates over the whole network. This also provides a mechanism for aggregating routes and cutting down on the unnecessary propagation of subnet information.

OSPF allows for routing authentication by using different methods of password authentication.

OSPF allows for the transfer and tagging of external routes injected into an Autonomous System.

This keeps track of external routes injected by exterior protocols such as BGP.

This of course leads to more complexity in the configuration and troubleshooting of OSPF networks. Administrators that are used to the simplicity of RIP are challenged with the amount of new information they have to learn in order to keep up with OSPF networks. Also, this introduces more overhead in memory allocation and CPU utilization. Some of the routers running RIP might have to be upgraded in order to handle the overhead caused by OSPF.

What Do We Mean by Link-States?

OSPF is a link-state protocol. We could think of a link as being an interface on the router. The state of the link is a description of that interface and of its relationship to its neighboring routers. A description of the interface would include, for example, the IP address of the interface, the mask, the type of network it is connected to, the routers connected to that network and so on. The collection of all these link-states would form a link-state database.

Shortest Path First Algorithm :



OSPF uses a shorted path first algorithm in order to build and calculate the shortest path to all known destinations.The shortest path is calculated with the use of the Dijkstra algorithm. The algorithm by itself is quite complicated. This is a very high level, simplified way of looking at the various steps of the algorithm:

1.Upon initialization or due to any change in routing information, a router generates a link-state advertisement. This advertisement represents the collection of all link-states on that router.

2.All routers exchange link-states by means of flooding. Each router that receives a link-state update should store a copy in its link-state database and then propagate the update to other routers.

3.After the database of each router is completed, the router calculates a Shortest Path Tree to all destinations. The router uses the Dijkstra algorithm in order to calculate the shortest path tree. The destinations, the associated cost and the next hop to reach those destinations form the IP routing

table.

4.In case no changes in the OSPF network occur, such as cost of a link or a network being added or deleted, OSPF should be very quiet. Any changes that occur are communicated through link-state packets, and the Dijkstra algorithm is recalculated in order to find the shortest path.

5. The algorithm places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost required to reachthat destination. Each router will have its own view of the topology even though all the routers will build a shortest path tree using the same link-state database.

The following sections indicate what is involved in building a shortest path tree.

OSPF Cost

The cost (also called metric) of an interface in OSPF is an indication of the overhead required to send packets across a certain interface. The cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidth indicates a lower cost. There is more overhead (higher cost) and time delays involved in crossing a 56k serial line than crossing a 10M ethernet line. The formula used to calculate the cost is:

cost= 10000 0000/bandwith in bps Shortest Path Tree

Assume we have the following network diagram with the indicated interface costs. In order to build the shortest path tree for RTA, we would have to make RTA the root of the tree and calculate the smallest cost for each destination.



Broadcast neighbors

Procedure:

In this report, I will set up a network that utilizes OSPF as its routing protocol. You will analyze the routing tables generated in the routers and will observe how the resulting routes are affected by assigning areas and enabling load balancing.

Create a New Project:

1. Start OPNET IT Guru Academic Edition ⇒ Choose New from the File menu.

2. Select Project and click OK ⇒ Name the project <your initials>_OSPF, and the scenario No_Areas ⇒ Click OK.

3. In the Startup Wizard: Initial Topology dialog box, make sure that Create Empty Scenario is selected ⇒ Click Next ⇒ Select Campus from the Network Scale

list ⇒ Click Next three times ⇒ Click OK.

Create and Configure the Network:

Initialize the Network:

1. The Object Palette dialog box should now be on top of your project workspace. If it is not there, open it by clicking . Select the routers item from the pull-down menu on the object palette.

a. Add to the project workspace eight routers of type slip8_gtwy. To add an object from a palette, click its icon in the object palette ⇒ Move your mouse to the workspace and click to place the object ⇒ You



can keep on left-clicking to create additional objects. Right-click when you are finished placing the last object.

2. Switch the palette configuration so it contains the internet_toolbox. Use bidirectional PPP_DS3 links to connect the routers. Rename the routers as shown below.

3. Close the Object Palette and then save your project.

Configure the Link Costs

1. We need to assign link costs to match the following graph:

2. Like many popular commercial routers, OPNET router models support a parameter called a reference bandwidth to calculate the actual cost, as follows:

Cost = (Reference bandwidth) / (Link bandwidth) where the default value of the reference bandwidth is 1,000,000 Kbps.

3. For example, to assign a cost of 5 to a link, assign a bandwidth of 200,000 Kbps to that link. Note that this is not the actual bandwidth of the link in the sense of transmission speed, but merely a parameter used to configure link costs.

4. To assign the costs to the links of our network, do the following:



i. Select all links in your network that correspond to the links with a cost of 5 in the above graph by shift-clicking on them.

ii. Select the Protocols menu ⇒ IP ⇒ Routing ⇒ Configure Interface Metric Information.

iii. Assign 200000 to the Bandwidth (Kbps) field ⇒ Check the Interfaces across selected links radio button, as shown ⇒ Click OK.

5. Repeat step 4 for all links with a cost of 10 but assign 100,000 Kbps to the Bandwidth (Kbps) field.

6. Repeat step 4 for all links with a cost of 20 but assign 50,000 Kbps to the Bandwidth (Kbps) field.

7. Save your project.

Configure the Traffic Demands:

1. Select both Router A and Router C by shift-clicking on them.

i. Select the Protocols menu ⇒ IP ⇒ Demands ⇒ Create Traffic Demands

⇒ Check the From Router A radio button as shown ⇒ Keep the color as

Blue ⇒ Click Create. Now you should see a blue-dotted line representing the

Traffic demand between Router A and Router C.



2. Select both Router B and Router H by shift-clicking on them.

i. Select the Protocols menu ⇒ IP ⇒ Demands ⇒ Create Traffic Demands

⇒ Check the From Router B radio button ⇒ Change the color to red ⇒ Click

OK ⇒ Click Create.

Now you can see the lines representing the traffic demands as shown.

3. To hide these lines: Select the View menu ⇒ Select Demand Objects ⇒ Select Hide All.


Configure the Routing Protocol and Addresses:

1. Select the Protocols menu ⇒ IP ⇒ Routing ⇒ Configure Routing Protocols.

2. Check the OSPF check box ⇒ Uncheck the RIP check box ⇒ Uncheck the



Visualize Routing Domains check box, as shown:

3. Click OK.

4. Select Router and Router only ⇒ Select the Protocols menu ⇒ IP ⇒

Routing ⇒ Select Export Routing Table for Selected Routers ⇒ Click OK on the Status Confirm dialog box.

5. Select the Protocols menu ⇒ IP ⇒ Addressing ⇒ Select Auto-Assign IP Addresses.


Configure the Simulation

Here we need to configure some of the simulation parameters:

1. Click on and the Configure Simulation window should appear.

2. Set the duration to be 10.0 minutes.

3. Click OK and then save your project.

Duplicate the Scenario



In the network we just created, all routers belong to one level of hierarchy (i.e., one area). Also, we didn’t enforce load balancing for any routes. Two new scenarios will be created.

The first new scenario will define two new areas in addition to the backbone area. The second one will be configured to balance the load for the traffic demands between .RouterB and RouterH.

The Areas Scenario:

1. Select Duplicate Scenario from the Scenarios menu and give it the name

Areas ⇒ Click OK.

2. Area 0.0.0.1:

i. Select the three links that connect RouterA, RouterB, and RouterC by shift-clicking on them ⇒ Select the Protocols menu ⇒ OSPF ⇒ Configure Areas ⇒ Assign the value 0.0.0.1 to the Area Identifier, as shown ⇒ Click OK.

ii. Right-click on RouterC ⇒ Edit Attributes ⇒ Expand the OSPF Parameters hierarchy ⇒ Expand the Loopback Interfaces hierarchy ⇒ Expand the row0 hierarchy ⇒ Assign 0.0.0.1 to the value of the Area ID attribute ⇒ Click OK.

3. Area 0.0.0.2:

i. Click somewhere in the project workspace to disable the selected links and then repeat step 2-i for the three links that connect RouterF, RouterG, and RouterH but assign the value 0.0.0.2 to their Area Identifier.

4. To visualize the areas we just created, select the Protocols menu ⇒ OSPF ⇒ Visualize Areas ⇒ Click OK. The network should look like the following one with different colors assigned to each area (you may get different colors though).



Note:

- The area you did not configure is the backbone area and its Area Identifier = 0.0.0.0.

- The figure shows the links with a thickness of 3.

The Balanced Scenario:

1. Under the Scenarios menu, Switch to Scenario ⇒ Select No_Areas.

2. Select Duplicate Scenario from the Scenarios menu, and give it the name Balanced ⇒ Click OK.

3. In the new scenario, select both RouterB and RouterH by shift-clicking on them.

4. Select the Protocols menu ⇒ IP ⇒ Routing ⇒ Configure Load Balancing Options ⇒ Make sure that the option is Packet-Based and the radio button Selected Routers is selected as shown ⇒ Click OK.




Run the Simulation

To run the simulation for the three scenarios simultaneously:

1. Go to the Scenarios menu ⇒ Select Manage Scenarios.

2. Click on the row of each scenario and click the Collect Results button. This should change the values under the Results column to <collect> as shown.

3. Click OK to run the three simulations. Depending on the speed of your processor, this may take several seconds to complete.

4. After the three simulation runs complete, one for each scenario, click Close and then save your project.

View the Results

The No_Areas Scenario:

1. Go back to the No_Areas scenario.

2. To display the route for the traffic demand between RouterA and RouterC: Select the Protocols menu ⇒ IP ⇒ Demands ⇒ Display Routes for Configured Demands ⇒ Expand the hierarchies as shown and select Router A, Router C ⇒ Go to the Display column and pick Yes ⇒



Click Close.

3. The resulting route will appear on the network as shown:

4. Repeat step 2 to show the route for the traffic demand between RouterB and RouterH. The route is as shown below. (Note: Depending on the order in which you created the network topology, the other “equal-cost” path can be used, that is, the RouterB-RouterA-RouterD-RouterF-RouterH path).



The Areas Scenario:

1. Go to scenario Areas.

2. Display the route for the traffic demand between Router A and Router C. The route is as shown:


The Balanced Scenario:

1. Go to scenario Balanced.

2. Display the route for the traffic demand between RouterB and RouterH. The route is as shown:




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, refer to section 9.1 of RFC 2328 . In Cisco IOS Software Releases 11.3T and 12.0, if the ip ospf network point-to-pointcommand is configured under loopbacks, OSPF advertises the loopback subnet as the actual subnet configured on loopbacks. ISDN dialer interface advertises /32 subnet instead of its configured subnet mask. This is an expected behavior if ip ospf network point-to-multipoint is configured.

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

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.

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 is 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, 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.

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 (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.

Q. What is the purpose of the variable IP-OSPF-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 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.

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


http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc2328.html


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.

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 nonzero TOS paths.

Q. Does the offset-list subcommand work for OSPF?

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.

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 does not 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. The distribute-list commands are supported in OSPF but work differently than distance-vector routing protocols such as Routing Information Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). OSPF routes cannot be filtered from entering the OSPF database. The distribute-list in command only filters routes from entering the routing table; it does not prevent link-state packets from being propagated. Therefore, this command does nothelp conserve router memory, and it does not prohibit a router from propagating filtered routes to other routers.

Caution: Use of the distribute-list in command in OSPF may lead to routing loops in the network if not implemented carefully.

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 interarea routes.

Q. How can I give preference to OSPF interarea routes over intra-area routes?

A. According to section 11 of RFC 2328 , the order of preference for OSPF routes is:

intra-area routes, O interarea routes, O IA


http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc2328.html


external routes type 1, O E1

external routes type 2, O E2

This rule of preference cannot be changed. However, it applies only within a single OSPF process. If a router is running more than one OSPF process, route comparison occurs. With route comparison, the metrics and administrative distances (if they have been changed) of the OSPF processes are compared. Route types are disregarded when routes supplied by two different OSPF processes are compared.

Q. Do I need to manually set up adjacencies for routers on the Switched Multimegabit Data Service (SMDS) cloud with the OSPF neighbor subcommand?

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.

Q. When routes are redistributed between OSPF processes, are all shortest path first algorithm (SPF) metrics preserved, or is the default metric value used?

A. The SPF metrics are 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 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.

Q. Which address-wild-mask 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-wild-mask 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 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.

Q. Why do I receive the "cannot allocate router id" 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, it returns this error message. To correct the problem, configure a loopback interface.



Q. Why do I receive the "unknown routing protocol" 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 are using a 1600 router, you need a Plus image to run OSPF.

Q. What do the states DR, BDR, and DROTHER mean in show ip ospf interface command output?

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.

Q. When I issue 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 becomes full with only designated router (DR) and backup designated router (BDR), and it shows 2-WAY for all other routers.

Q. Why do I not 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 designated routers (DRs) or backup designated routers (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 Software release began support for per-interface authentication type in OSPF?

A. Per-interface authentication type, as described in RFC 2178 , was added in Cisco IOS Software Release 12.0(8).

Q. Can I control the P-bit when importing external routes into a not-so-stubby area (NSSA)?

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


http://www.ietf.org/rfc/rfc2178.txt


ASBR can then summarize with the not-advertise keyword, which results in not advertising the translated type 7 LSAs.

Q. Why are OSPF show commands responding so slowly?

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 OSPFshow 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.

Q. What does the clear ip ospf redistribution command do?

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.

Q. Does OSPF form adjacencies with neighbors that are not on the same subnet?

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 unnumberedcommand, but in all other cases, the neighbors must be on the same subnet.

Q. How often does OSPF send out link-state advertisements (LSAs)?

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

Q. How do I stop individual interfaces from developing adjacencies in an OSPF network?

A. To stop routers from becoming OSPF neighbors on a particular interface, issue the passive-interface command at the interface.

In Internet service provider (ISP) and large enterprise networks, many of the distribution routers have more than 200 interfaces. 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 a single passive-interface default command. Then, configure individual interfaces where adjacencies are desired using the no passive-interface command. For more information, refer to Default Passive Interface Feature.

There are some known problems with the passive-interface default command. Workarounds are listed in Cisco bug ID CSCdr09263 ( registered customers only) .

Q. When I have two type 5 link-state advertisements (LSAs) for the same external network in the OSPF 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 Autonomous System Boundary Router (ASBR) and install


http://tools.cisco.com/RPF/register/register.do

http://www.cisco.com/pcgi-bin/Support/Bugtool/onebug.pl?bugid=CSCdr09263

http://www.cisco.com/en/US/docs/ios/12_0t/12_0t2/feature/guide/defint.html

http://www.cisco.com/en/US/docs/ios/12_0t/12_0t2/feature/guide/defint.html


that into the IP routing table. Use the show ip ospf border-routers command to check the cost to the ASBR.

Q. Why is it that my Cisco 1600 router does not recognize the OSPF protocol?

A. Cisco 1600 routers require the Plus feature set image of Cisco IOS Software to run OSPF. Refer to Table 3: Cisco 1600 Series Routers Feature Sets in the Release Notes for Cisco IOS Release 11.2(11) Software Feature Packs for Cisco 1600 Series Routers for more information.

Q. Why is it that my Cisco 800 router does not run OSPF?

A. Cisco 800 routers do not support OSPF. However, they do support Routing Information Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). You can use the Software Advisor ( registered customers only) tool for more information on feature support.

Q. Should I use the same process number while configuring OSPF on multiple routers within the same network?

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.

Q. I have a router that runs Cisco Express Forwarding (CEF) and OSPF, who does load-balancing when there are multiple links to a destination?

A. CEF works by performing the switching of the packet based on the routing table which is populated by the routing protocols such as OSPF. CEF does the load-balancing once the routing protocol table has been calculated. For more details on load balancing, refer to How does load-balancing work?

Q. How does OSPF use two Multilink paths to transfer packets?

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.

Q. How can you detect the topological changes rapidly?

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.


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094820.shtml

http://tools.cisco.com/RPF/register/register.do

http://www.cisco.com/pcgi-bin/Support/CompNav/Index.pl

http://www.cisco.com/pcgi-bin/Support/CompNav/Index.pl

http://www.cisco.com/en/US/products/sw/iosswrel/ps1824/prod_release_note09186a008007fba6.html

http://www.cisco.com/en/US/products/sw/iosswrel/ps1824/prod_release_note09186a008007fba6.html

http://www.cisco.com/en/US/products/sw/iosswrel/ps1824/prod_release_note09186a008007fba6.html#25447


There is a possibility of more routing traffic if the hello and hold timer values are reduced from their default values.

Q. Does the 3825 Series Router support the OSPF Stub feature?

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

Q. What does the error message %OSPF-4-FLOOD_WAR: Process process-id re-originates LSA ID ip addresstype-2 adv-rtr ip address in area area id means?

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.

Q. Can we have OSPF run over a GRE tunnel?

A. Yes, refer to Configuring a GRE Tunnel over IPSec with OSPF.

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