Introduction to ospf features and operation
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3 Lab Exercise INTRODUCTION TO OSPF FEATURES AND OPERATION FA24 TELECOM SYSTEMS ENGINEER COURSE TRAINING OBJECTIVE: The Student will be familiarized with the operation and programming of the Cisco Router using the OSPF routing protocol. Conditions: EQUIPMENT: The student will be given applicable reference material and a Cisco router in an operational network and a student handbook. REFERENCES: OSPF Design Guide; Cisco IOS IP and IP Routing Configuration Guide: Configuring OSPF METHOD OF INSTRUCTION: Lecture with practical exercise. INTRODUCTION: Note: Show slide #1: Open Shortest Path First (OSPF) is a routing protocol developed for IP networks by the Interior Gateway Protocol (IGP) working group of the Internet Engineering Task Force (IETF). The group was formed in 1988 to design an IGP based on the shortest path first (SPF) algorithm for use in the Internet. OSPF was created because the Routing Information Protocol (RIP) was, in mid-1980, increasingly unable to serve large networks. As indicated by the acronym, OSPF has two primary characteristics. The first is that it is open, in that its specifications is in the public domain and was originally described in Request for Comments (RFC) 1131. The most recent version, known as OSPF2, is described in RFC 1583. The second principle characteristic is that it is based on the SPF algorithm, which is sometimes referred to as the Dijkstra algorithm, named for the person credited with its creation. BODY:
Static knowledge is administered manually: A network administrator enters it into the router’s configuration. The administrator must manually update this static route entry whenever an internetwork topology change occurs. Static knowledge can be private – by default it is not conveyed to other routers as part of an update process. You can, however, configure the routers to share this knowledge. Dynamic knowledge works differently. After the network administrator enters configuration commands to start dynamic routing, route knowledge is updated automatically by a routing process whenever new topology information is received from the internetwork. Changes in dynamic knowledge are exchanged between routers as part of the update process.
The IETF (Internet Engineering Task Force) was looking for a fast, scalable, efficient interior routing protocol that would replace RIP1. In 1987, work was began on OSPF, and in 1989, OSPF V1 was finalized as RFC 1131. OSPF V2 was defined in 1991, and the latest enhancements released in 1997. OSPF V2 is the standard that is addressed in this section. This standard is defined in RFC 2178. OSPF is a work in progress; features will be added and modified on an as-needed basis.
Show slide; provided for reference.
- Open, non-proprietary - Has no hop count limitation - Supports VLSM - Uses multicast addressing for updates - Has fast convergence - Allows for routing authentication
There are two primary elements in the OSPF hierarchy: Area – An area is a grouping of contiguous OSPF networks and hosts. OSPF areas are logical subdivisions of OSPF autonomous systems. The topology of each area is invisible to entities in other areas, and each area maintains its own topological database. Autonomous – OSPF autonomous systems are the largest entity within an OSPF internetwork. They consist of a collection of networks that are under a common administration and share a common routing strategy. An autonomous system, sometimes called a domain, is logically subdivided into multiple areas. The hierarchical topology of OSPF has several important benefits. Because the topology of an area is hidden from the rest of the autonomous system, routing update traffic can be reduced through route summarization, and the topological databases and SPF trees remain manageable and more efficient. Within each autonomous system, a central area must be defined as area 0. All others areas are connected off of the central, or backbone area. Area 0 is also called the transition area because all other areas communicate through it. The OSPF backbone also distributes routing information between OSPF areas. The OSPF backbone has all the properties of a normal OSPF area. Backbone routers maintain OSPF routing information using the same procedures and algorithms as internal routers. The backbone topology is invisible to routers in other areas, while the topologies of individual areas are invisible to backbone routers.
There are four network types defined for the OSPF routing protocol. Point-to-Point: Normally found on serial connections. Neighbor relationships are formed only with the other router on the point-to-point link. Both routers can independently communicate with all other OSPF routers. Broadcast Multi-access: Normally found on LAN connections. There is a potential for many neighbor relationships since several routers can be on the same segment. Through an election process, a Designated Router for the network is selected. The DR communicates with all other routers regarding the LAN network. Non-broadcast Multi-access: Routers setup in a hub spoke topology using non-broadcast media such as Frame Relay, x.25, and ATM. Special care must be taken when configuring this network. Neighbor relationships may have to be manually configured. Point-to-Multipoint: Defined as a numbered point to point interface having more than one neighbor. Occurs when there are sub-interfaces on one end of the point-to-point network.
Routers that share a common segment become neighbors on that segment using the Hello protocol. Hello packets are sent periodically out of each interface using IP multicast addresses. The Hello protocol serves the primary purposes of neighbor discovery, DR & BDR election, and link integrity verification. Two routers will become neighbors if they agree on the following: (1) must have the same area-id and be on the same subnet/mask; (2) they must both use the same type of authentication and password (if any), (3) the hello and dead intervals must be the same – hello is 10 sec by default and dead is 4 times the hello by default, (interface hello and dead intervals or timers can be manipulated under the interface configuration using the “ip ospf” command.), (4) must agree on the stub area flag – a bit in the hello packet that indicates whether the interface is a stub area.
Adjacency is the next step after the two routers from a neighbor relationship. Adjacent routers go beyond the hello exchange and proceed to the database exchange. This is a one-time swap of the entire OSPF topology database. Once completed, this is updated with only changes occurring to the database.
On a multi-access segment, two routers are elected, the designated router (DR) and the backup designated router (BDR). These routers act as the central point of contact for all information exchange on the network. The BDR maintains the same information as the DR and replaces it in the event it fails. Instead of each router on the network exchanging LSA’s with every other router, they simply exchange them with the DR/BDR. This significantly reduces the amount of router-related traffic on the segment. Election of the routers is done using the hello protocol. The router with the highest OSPF priority on a segment will become the DR and the process is then repeated for the BDR. OSPF priority must be set on an interface with a number from 0 to 255. The router with the highest priority is elected the DR. The priority default to 1 and in case of a tie, the highest router ID is used. A value of 0 indicates an interface that can’t be elected DR/BDR.
Each router maintains link-state records including information about each of its interfaces and reachable neighbors. Through flooding, each router distributes its state to all other routers in the area/autonomous system. As a result, each router possesses an identical database describing the area/autonomous system. All routers run the SPF algorithm in parallel. Using the link state database, each router then constructs a tree of the shortest paths with itself as the root. Each destination within the AS is contained within the SPF tree.
LSA’s are handled in a very efficient manner between the source router (attached to the link) and the nearest neighboring router. The incoming LSA is checked against existing entries in the topological database. Each database entry has a sequence number (also called a version number), and only the largest number (indicating the most recent record) is kept. If the entries are identical, then there is no need to forward the LSA to other routers. If the incoming LSA is different from the topological database, then the database is updated and the LSA is forwarded through the network until all databases are synchronized. Associating version numbers with LSA’s contributes to the efficiency of link-state routing technology.
External Routes (Type 5) fall into two categories, type 1 and type 2. The difference between the two is the way the cost is being calculated. A type 2 route is only the external cost; the internal is not added. A type 1 is the external plus the internal cost to reach a specific destination. Type 2 is the default.
Administrative distance is the first factor used to determine which routes are placed into the table. If routes have the same distance, the cost or metrics is then used.
Administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. Distance is an integer from 0 to 255. In general: the higher the value, the lower the trust rating. A distance of 255 means the routing information source cannot be trusted at all and should be ignored. Specifying distance values enables the router to discriminate between sources of routing information. The router always picks the route whose routing protocol has the lowest distance.
Administrative distance can be manually configured on the router to give certain routing protocols preference over others. Under the desired routing protocol configuration, use the “distance” command. Metrics (slide 22):
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. The default formula used to calculate the cost is {cost=108 / bandwidth in bps}. If no bandwidth statement is used, serial interfaces default to 1.544 mbs (T1) and Ethernet defaults to 10 mbs. The bandwidth statement has no actual affect on data transfer rate. It is simply used to calculate the cost of the link. The cost of an interface can be set manually which will override the bandwidth statement. Under the interface use the command “ip ospf cost”. Manipulating the cost of links can make them more or less preferential for use by the router. It is recommended cost be manipulated using the bandwidth statement.
Use the router OSPF command to define an OSPF routing process. The process-id is an internally used identification number. A unique value is assigned for each OSPF routing process within a single router. The OSPF process-id does not have to match process-ids on other routers. It is possible to run multiple OSPF processes on the same router, but it is not recommended because it creates multiple databases, which add extra overhead to the router. The network command defines which interfaces will run OSPF. The command also assigns an interface to a certain area. The network command uses a “wildcard” mask, which is essentially the inverse of a traditional mask. The mask in the network command can be used as a shortcut for assigning a list of interfaces to the same area with one configuration line.
As stated above, the network command is used to define which interfaces will run OSPF. In addition to this, these will be the network addresses advertised to other routers. There may be cases where we want to advertise a network to other routers but do not necessarily want routing updates being sent from an interface. One case is an Ethernet interface with only hosts connected to it. The passive-interface command will keep updates from being sent from the interface even though there is a network statement relating to the address of the interface. Another instance where this command may be used is when interfacing to the Tactical Packet Network (TPN). OSPF Network Diagram (slide 25):
- config t, int loopback X, ip address xxx.xxx.xxx.xxx (a loopback address must be established for each area)
- config t, int sX, ip unnumbered loopback X 255.255.255.255, encapsulation ppp, clock rate 250000 (if needed), bw 256, no shut. (serial interfaces in different areas must reference different loopback addresses. - config t, int eX, ip address xxx.xxx.xxx.xxx 255.255.255.240, no shut.
- config t, router OSPF 1 - serial: network xxx.xxx.xxx.xxx 0.0.0.0 area X (statements must be made for each area/loopback address)
- config t, int loopback X, ip address xxx.xxx.xxx.xxx (a loopback address must be established for each area)
- config t, int sX, ip unnumbered loopback X 255.255.255.255, encapsulation ppp, clock rate 250000 (if needed), bw 256, no shut. (serial interfaces in different areas must reference different loopback addresses. - config t, int eX, ip address xxx.xxx.xxx.xxx 255.255.255.240, no shut.
- config t, router OSPF 1
The show IP protocol command provides information about all IP routing protocols configured. The routing protocol and process are identified along with information concerning routing filters, redistribution, and summarization. Routing network statements can be verified along with routing information sources. This is displayed using the sources router ID’s, the distance of the protocol, and when the last update was received. The show IP OSPF command can be used to verify your OSPF configuration and the overall configuration of the areas within the router. The router ID and process ID can be verified here. Information concerning frequency of updates and other timers are provided. Information is provided for each individual area the router is connected. The show IP OSPF neighbor command contains the following information:
2wy – neighbor discovered but adjacency not built Full – adjacency built, databases exchanged
declared dead
The show IP OSPF interface command provides an inventory of all the interfaces in your router and their status with respect to OSPF. The cost assigned to each interface along with the type of OSPF network it belongs to can be verified here. The show IP OSPF database command is used to view the OSPF link-state (topology) database. Each LSA gets an entry into this database and is organized by area and the type of LSA. The database contains six columns:
2) ADV Router – router ID of advertising router. 3) Age – age of LSA in seconds. 4) Seq# - sequence number of LSA, used to determine if LSA updates are newer, older, or duplicates. 5) Checksum – used for error detection. 6) Link count – the number of interfaces or links in an area, only available on Router Link States; OSPF adds a “stub link” for each point-to-point interface.
Summarizing is the consolidation of multiple routes into one single advertisement. Proper summarization requires contiguous addressing. Route summarization directly affects the amount of bandwidth, CPU, and memory resources consumed by the OSPF process. With summarization, if a network link fails, the topology change will not be propagated into the backbone (and other areas by way of the backbone). As such, flooding outside the area will not occur, so routers outside of the area with the topology change will not have to run the SPF algorithm (also called the Dijkstra algorithm after the computer scientist who invented it). Running the SPF algorithm is a CPU-intensive activity. There are two types of summarization:
Route summarization minimizes the number of entries in the routing table and database in the receiving routers. Summarization is done on ABRs and applies to routes within the autonomous system. Although summarization could be configured between any two areas, it is better to summarize in the direction of the backbone. This way, the backbone receives all the aggregate addresses and in turn injects them, already summarized, into other areas. In order to take advantage of summarization, network numbers in areas should be assigned in a contiguous way to be able to group these addresses into one range. Summary routes are advertised with a mask. The mask specifies the range of addresses to be summarized into one route. Because the mask 255.255.240.0 does not use the low-order four bits of the third octet, subnets 131.108.4.0 and 131.108.8.0 cannot be summarized using this mask. Neither can subnet 131.108.12.0 because it creates an invalid zero subnet (discussed on next slide). Even so, route summarization can represent the remaining four subnets with one advertisement.
Types of Areas (slide 31):
OSPF allows certain areas to be configured as stub areas. Configuring a stub area reduces the size of the topological database inside an area and as a result reduces the memory requirements of routers inside that area. External networks, such as those redistributed from other protocols into OSPF, are not allowed to be flooded into a stub area. Routing from these areas to the outside world is based on a a default route (0.0.0.0). This allows routers within the stub to reduce the size of their routing tables because of single default route replaces the many external routes. If your network has no external routes, there is no need to configure a stub area.
An area could be qualified as a stub when there is a single exit point from that area or if routing to outside of the area does not have to take an optimal path. The latter description is just an indication that a stub area with multiple exit points will have one or more ABRs injecting a default into that area. Routing to the outside world could take a sub-optimal path in reaching the destination by going out of the area via an exit point that is farther to the destination than other exit points. Other stub area restrictions are that a stub area cannot be used as a transit area for virtual links. Also, an ASBR cannot be internal to a stub area. These restrictions are made because a stub area is mainly configured not to carry external routes, and any of the situations described cause external links to be injected in that area. The backbone, of course, cannot be configured as a stub.
A totally stubby area is a stub area that blocks external routes and summary routes (interarea routes) from going into the area. This way, intra-area routes and the default of 0.0.0.0 are the only routes known to the stub area. ABRs inject the default summary link of 0.0.0.0 into the totally stubby area. Each router picks the closest ABR as a gateway to everything outside the area. The totally stubby area is a Cisco-specific feature.
In this example, area 2 is defined as the stub area. No external routes from the external autonomous system will be forwarded into the stub. The last line in each configuration, area 2 stub, defines the stub area. The area stub default-cost has not been configured on R3, so this router will advertise 0.0.0.0 (the default route) with a default cost metric of 1 plus any internal costs. Each router in the stub must be configured with the area stub command. The only routes that will appear in R4’s routing table are intra-area routes (designated with an O in the routing table), the default route, and interarea rotes (both designated with an IA in the routing table; the default route will also be denoted with an asterisk).
In this example, the keyword no-summary has been added to the area stub command on R3. This keyword causes summary routes (interarea) to also be blocked from the stub. Each router in the stub picks the closest ABR as a gateway to everything outside the area. The only routes that will appear in R4’s routing table are intra-area routes (designated with an O in the routing table) and the default route. No interarea routes (designated with an IA in the routing table) will be included. With the area stub default-cost command, R3 adds 20 to the internal cost when it injects the default route into the stub area. It is only necessary to configure the no-summary keyword on the stub border routers. This is because the area is already configured as a stub.
OSPF has certain restrictions when multiple areas are configured. One area must be defined as area 0. Area 0 is also called the backbone because all communication must go through it. In addition, all areas should be physically connected to area 0. All other areas must be logically connected to area 0. This is because all other areas inject routing information into area 0, which in turn disseminates that information to other areas. In special cases where a new area is added after the OSPF network has been designed and Configured, it is not always possible to provide that new area with direct access to the backbone. In These cases, a virtual link will have to be defined to provide the needed connectivity to the backbone. The virtual link provides the disconnected area a logical path to the backbone. The virtual link must be established between two routers that share a common area, and one of these routers must be connected to the backbone. Virtual Links continued (slide 39):
Virtual links serve two purposes:
This slide illustrates the second purpose. Discontinuity of the backbone might occur if, for example, two companies, each running OSPF, are trying to merge the two separate networks into one with a common area 0. The alternative would be to redesign the entire OSPF network and create a unified backbone. Another reason for creating a virtual link is to add redundancy in cases where a router failure causes the backbone to be split into two. In the graphic, the disconnected area 0s are linked via a virtual link through the common area 3. If a common area does not already exist, one can be created to become the transit area.
In this example, area 3 does not have a direct physical connection to the backbone (area 0). This is an OSPF requirement because the backbone is a collection point for LSAs.s forward summary LSAs to the backbone, which in turn forwards the traffic to all areas. All interarea traffic transits the backbone.
1 will be the transit area and R1 will be the entry point into area 0. R2 will have a logical connection to the backbone through the transit area.
area and the router ID of the other side of the virtual link is configured.
area and the router ID of the other side of the virtual link is configured. Stub Area Restrictions Chart (slide42):
Not-so-stubby areas (NSSAs) are an extension of OSPF stub areas. Like stub areas, they prevent the flooding of AS-external link-state advertisements (LSAs) into NSSAs, relying instead on default routing to external destinations. As a result, NSSAs (like stub areas) must be placed at the edge of an OSPF routing domain. NSSAs are more flexible than stub areas in that an NSSA can import external routes into the OSPF routing domain, thereby providing transit service to small routing domains that are not part of the OSPF routing domain.
To define an NSSA stub area, use the OSPF router configuration command area X nssa. To define an NSSA totally stub area, use the OSPF router configuration command area X nssa no-summary.
1) Configure routers for IP unnumbered network or TFTP config files if available. 2) Router 7 add six sequential loopback interfaces with IP addresses for each. - Loopback 21 – 26 - IP 150.150.150.1 – 6 - add network statements under OSPF 3) All routers do sho ip route; loopback addresses from router 7 should be in routing table 4) Routers 1,3,4, & 6 are ABR’s. Summarize loopback addresses there. 5) Routers 2 & 5 do sho ip route; shows two summarized routes, one from each ABR. 6) One of the ABR’s in each area change the bandwidth on a serial interface. 7) Routers 2 & 5 should only show one summarized route now.
1) Configure routers for IP unnumbered network or TFTP config files if available. 2) Everyone telnet to router 2 or 5 and examine routing table; note “IA” routes. 3) Routers 2 & 5 configure for stub area. 4) Routers 1,3,4,& 6 configure for totally stub area. 5) Router 2 & 5 should have two default routes, one from each ABR. 6) On one of the serial interfaces on routers 2 & 5, change the bandwidth. 7) Routers 2 & 5 should only show one default route now.
Reference slide 47 network diagram. Do not disconnect cables; just turn off interfaces not used in diagram. 1) Configure routers as per diagram or TFTP config files if available. 2) Install ethernet link between routers 2 & 5. 3) Review all router routing tables; area 1 & 0 should not see routes in area 2 and vise versa. 4) Configure a virtual link between routers 1 & 5. 5) All routes should have connectivity to the entire network. Grading: 40 points: Print out final router configurations and routing tables for one each routers 1, 3, 4, or 6; router 2 or 5; and router 7. 60 points: Completed questions (typed) Total: 100 points OSPF Questions 1. Which of the following are the benefits of OSPF routing over RIP v1? [Choose all that apply and explain your answer]. A. No hop count limitation B. Faster convergence C. Best path selection D. Support VLSM 15 pts 2. The OSPF path cost in Cisco routers is calculated using which parameters? Explain A. Bandwidth, Number of Hops B. Bandwidth only C. Ticks D. Bandwidth, MTU, Reliability, Delay, and Load. 5 pts 3. Match the following in the context of an OSPF area. Explain your choice: 1. LSU A. This packet is sent by slave router if the DDP has more up-to-date link-state entry. 2. LSA B. This packet is sent when a router notices a change in a link-state 3. LSR C. These are contained in LSPs and have the information about neighbors and path costs. A. 1->A; 2->B; 3->C B. 1->B; 2->A; 3->C C. 1->B; 2->C; 3->A D. 1->C; 2->A; 3->B 15 pts
B. 4 C. 6 D. 16 10 pts 5. Match the following with regard to OSPF operation: Explain your choice A. DDP 1. A router that resides within an area. B. Hello Packet 2. This includes summary information about link-state entries C. Internal router 3. This packet includes information that enables routers to establish themselves as neighbors. A. A->3; B->2; C->1 B. A->2; B->3; C->1 C. A->1; B->2; C->3 D. A->3; B->1; C->2 5 pts 6. Which of the following statements is true in multiple OSPF areas environment? A. ASBR is responsible for redistribution (import) of routing information in an OSPF network. B. ABRs have all interfaces in the same area. C. Internal routers are responsible for routing information redistribution (import). D. In a multi-area OSPF network, Area 0 may or may not be present. 10 pts Download 107.45 Kb. Share with your friends:
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