Neighbor relationship and authentication
The Hello and Dead timers must match for forming neighbour relationship.
The routers should be in the same Area to form neighbour relationship.
OSPF determines the router ID using the following criteria:
1. Use the address configured by the ospf router-id command
2. Use the highest numbered IP address of a loopback interface
3. Use the highest IP address of any physical interface
4. If no interface exists, set the router-ID to 0.0.0.0
If no OSPF router ID is explicitly configured, OSPF computes the router-ID based on the items 2, 3, and 4 and restarts OSPF (if the process is enabled and router-ID has changed).
A router with highest priority becomes the designated router and a router with priority 0 can never become designated router. If the priorities are the same, then the router with the highest router ID becomes the DR.
There is a mismatch in Hello, and Dead timers. It is important to configure the Hello and Dead timers to same value in neighboring routers. Otherwise, adjacencies will not take place. By default Dead timer is configured as 4 times the Hello timer.
The major advantages of hierarchical nature of OSPF are:
1.Reduced frequency of SPF calculations: This is because the packets are flooded only within an area, and not to the other areas.
2.Smaller routing tables.
3.Reduced LSU overhead.
1. A stub AS is a single-homed network with only one entry and exit point. This type of AS can be connected to the external world through the use of a statically configured route.
2. Transit AS: Data from one AS need to reach a remote AS, then it has to travel through intermediate AS. The AS or Autonomous Systems which carry the data from one AS to another AS is (are) called Transit AS (es).
3. eBGP: External BGP is used between two or more Autonomous Systems.
4. iBGP: Internal BGP is used within an AS.
Network types, area types, and router types
In an OSPF network, when a packet need to traverse from one area to another area to reach its destination, it is routed as below:
Source Area -> Source ABR -> Backbone Area -> Destination ABR -> Destination Area Routers
The sequence of steps followed in OSPF operation are as below:
1. Establish router adjacencies
2. Elect DR and BDR
3. Discover Routes
4. Choose appropriate routes for use
5. Maintain routing information.
The path cost in OSPF network is calculated using bandwidth. The formula used is [10^8 / Bandwidth]. For example, the cost of a 56kbps serial link is 1785. The default cost of a 10mbps Ethernet is 10. Higher the bandwidth, lower will be the path cost.
OSPF is a link state technology that uses Dijkstra algorithm to compute routing information. It has the following advantages over Distance Vector protocols such as RIP:
1. Faster convergence: OSPF network converges faster because routing changes are flooded immediately and computer in parallel.
2. Support for VLSM: OSPF supports VLSM. However, please note that RIP version2 also supports VLSM.
3. Network Reachability: RIP networks are limited to 15 hops. Therefore, networks with more than 15 hops can not be reached by RIP by normal means. On the other hand, OSPF has practically no reachability limitation.
4. Metric: RIP uses only hop count for making routing decisions. This may lead to severe problems in some cases, for example, that a route is nearer but is very slow compared to another route with plenty of bandwidth available. OSPF uses "cost" metric to choose best path. Cisco uses "bandwidth" as metric to choose best route.
5. Efficiency: RIP uses routing updates every 30 seconds. OSPF multicasts link-state updates and sends the updates only when there is a change in the network.
In an OSPF network, Type 2 LSAs are generated by a Designated Router (DR). Type 2 LSAs describe the set of routers attached to a particular network and are flooded within the area that contain the network only.
An OSPF area is a collection of networks and routers that has the same area identification.
OSPF process identifier is locally significant. Two neighboring router interfaces can have same or different process ids. It is required to identify a unique instance of OSPF database.
OSPF keeps up to six equal-cost route entries in the routing table for load balancing.
Further, OSPF uses Dijkstra algorithm to calculate lowest cost route. The algorithm adds up the total costs between the local router and each destination network. The lowest cost route is always preferred when there are multiple paths to a given destination.
The Hello packet contains the router ID and the hello and dead intervals and is sent to the multicast address 224.0.0.5.
Important features of stub area are:
1. A stub area reduces the size of the link-state database to be maintained in an area, which in turn result in less overhead in terms of memory capacity, computational power, and convergence time.
2. The routing in Stub and totally Stubby areas is based on default gateway. A default route (0.0.0.0) need to be configured to route traffic outside the area.
3. The stub areas suited for Hub-Spoke topology.
4. Area 0 is not configured as Stubby or totally Stubby. This is because stub areas are configured mainly to avoid carrying external routes, whereas Area 0 carries external routes.
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 is used on the interface, it overrides this formulated cost.
LSA Type 1: Router link entry, generated by all routers for each area to which it belongs. These are flooded within a particular area.
LSA Type 2: Network link entry, generated by designated router (DRs). Type 2 LSAs are advertised only to routers that are in the area containing the specific network.
LSA Type 3 and Type 4: Summary link entry, these LSAs are generated by area border routers (ABRs). These are sent to all routers within an area. These entries describe the links between the ABR and the internal routers of an area. These entries are flooded throughout the backbone area and to the other ABRs.
LSA Type 5: Autonomous System External Link Entry, These are originated by ASBR. These entries describe routes to destinations external to the autonomous system. These LSAs are flooded throughout the OSPF autonomous system except for stubby and totally stubby areas.
Area backbone LSAs: The LSAs generated by Area Backbone Routers are LSA1, LSA2, LSA3, LSA4, and LSA5. Note that LSA6 is not supported by Cisco, and LSA7 is generated by NSSA router.
Stub area LSAs: The Stub area router generates LSA types 1, 2, and 3. i.e. Router LSA, Network LSA, and Summary LSA.
Totally Stubby LSAs:The Totally Stubby area routers generate LSA types 1 and 2
NSSA LSAs: A NSSA (Not So Stubby Area) router generates LSA types 1, 2, and 7. LSA 7 is translated into LSA 5 as it leaves the NSSA
Different LSA types are described below:
a. LSA 1 (Router LSA): Generated by all routers in an area to describe their directly attached links (Intra-area routes). These do not leave the area.
b. LSA 2 (Network LSA): Generated by the DR of a broadcast or Nonbroadcast segment to describe the neighbors connected to the segment. These do not leave the area.
c.LSA 3 (Summary LSA): Generated by the ABR to describe a route to neighbors outside the area. (Inter-area routes)
d. LSA 4 (Summary LSA): Generated by the ABR to describe a route to an ASBR to neighbors outside the area.
e. LSA 5 (External LSA): Generated by ASBR to describe routes redistributed into the area. These routes appear as E1 or E2 in the routing table. E2 (default) uses a static cost throughout the OSPF domain as it only takes the cost into account that is reported at redistribution. E1 uses a cumulative cost of the cost reported into the OSPF domain at redistribution plus the local cost to the ASBR.
f. LSA 6 (Multicast LSA): Not supported on Cisco routers.
g. LSA 7 (NSSA External LSA): Generated by an ASBR inside a NSSA to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes appear as N1 or N2 in the ip routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost upto the ASBR.
The cost of external route depends on the configuration of ASBR. There are two external packet types possible.
1. Type 1 (E1) - Here the metric is calculated by adding the external cost to the internal cost of each link that the packet crosses.
Type 2 (E2) - This type of packet will only have the external cost assigned, irrespective of where in the area it crosses. Type 2 packets are preferred over Type 1 packets unless there are two same cost routes existing to the destination.
Cost is a number from 1 to 65535 that indicates the metric assigned to the interface.
The cost of external route depends on the configuration of ASBR. There are two external packet types possible.
1.Type 1 (E1) - Here the metric is calculated by adding the external cost to the internal cost of each link that the packet crosses.
2.Type 2 (E2): E2 is the default route type for routes learned via redistribution.
The command, RouterD(config-router)#default-information originate
is used to instruct all the other OSPF routers to learn the default route.
OSPF process identifier is locally significant. Two neighboring router interfaces can have same or different process ids. It is required to identify a unique instance of OSPF database
When an area is configured as stub or totally stubby, a default route (0.0.0.0) is injected into the area.
"show ip ospf interface" can be used to check whether the interfaces have been configured properly. The command also gives the timer intervals, including hello intervals as well as neighbor adjacencies.
The following output provides a sample output of the command:
Each field in the output is explained below
Interface State: The first line of the output shows the Layer 1 and Layer 2 states of the interface. In this example, the interface Ethernet0 senses the carrier on line and shows Layer 1 as up. Line protocol on the Ethernet0 interface confirms that Layer 2 is up. For proper functioning, the interfaces should be in an up/up state.
IP Address and Area: The second line shows the IP address configured on this interface and the area in which this interface is placed. In the above example, the Ethernet0 has an IP address of 10.10.10.1/24 and is in OSPF area 0.
Process ID: The process ID is the ID of the OSPF process to which the interface belongs. The process ID is local to the router, and two OSPF neighboring routers can have different OSPF process IDs. (This is not true of Enhanced Interior Gateway Routing Protocol [EIGRP], in which the routers need to be in the same autonomous system). In this example, the process ID is 1.
Router ID: The OSPF router ID is a 32-bit IP address selected at the start of the OSPF process. The highest IP address configured on the router is the router ID. If a loopback address is configured, it is the router ID. In the case of multiple loopback addresses, the highest loopback address is the router ID. Once the router ID is elected, it does not change unless OSPF restarts or is manually changed with the router-id 32-bit-ip-address command under router ospf process-id . In this example, 192.168.45.1 is the OSPF router ID.
Network Type: In the example, the OSPF network type is BROADCAST, which uses OSPF multicasting capabilities. Under this network type, a designated router (DR) and backup designated router (BDR) are elected. For routers on an interface to become neighbors, the network type for all should match.
The possible OSPF network types are:
POINT-TO-POINT (for example, the interfaces of two routers connected through E1 or T1 links) ,NON-BROADCAST (such as X.25 and Frame Relay) ,POINT-TO-MULTIPOINT (such as Frame Relay)
Cost: This is an OSPF metric. Cost is calculated with this formula: 108 / bandwidth (in bits per second [bps])
In the formula, bandwidth refers to the bandwidth of the interface in bps, and 108 is the reference bandwidth. In the example, the bandwidth of Ethernet0 is 10 Mbps, which is equal to 107. The formula yields 108 /107, equaling a cost of 10.
Transmit Delay: The transmit delay is the amount of time OSPF waits before flooding a link-state advertisement (LSA) over the link. Before transmitting an LSA, the link-state age is incremented by this number. In this example, the transmit delay is 1 second, which is the default value.
State: This field defines the state of the link and can be any of these:
DR:The router is the DR on the network to which this interface is connected, and it establishes OSPF adjacencies with all other routers on this broadcast network. In this example, this router is the BDR on the Ethernet segment to which the Ethernet0 interface is connected.
BDR:The router is the BDR on the network to which this interface is connected, and it establishes adjacencies with all other routers on the broadcast network.
DROTHER:The router is neither the DR nor the BDR on the network to which this interface is connected, and it establishes adjacencies only with the DR and the BDR.
Waiting:The interface is waiting to declare the state of the link as DR. The amount of time the interface waits is determined by the wait timer. This state is normal in a nonbroadcast multiaccess (NBMA) environment.
Point-to-Point:This interface is point-to-point for OSPF. In this state, the interface is fully functional and starts exchanging hello packets with all of its neighbors.
Point-to-Multipoint:This interface is point-to-multipoint for OSPF.
Priority: This is the OSPF priority that helps determine the DR and BDR on the network to which this interface is connected. Priority is an 8-bit field based on which DRs and BDRs are elected. The router with the highest priority becomes the DR. If the priorities are the same, the router with the highest router ID becomes the DR. By default, priorities are set to 1.
Designated Router: This is the router ID of the DR for this broadcast network. In the example, it is 172.16.10.1.
Interface Address: This is the IP address of the DR interface on this broadcast network. In the example, the address is 10.10.10.2, which is Router 2.
Backup Designated Router: This is the router ID of the BDR for this broadcast network. In the example, it is 192.168.45.1.
Interface Address: This is the IP address of the BDR interface on this broadcast network. In the example, it is Router 1.
Timer Intervals: These are the values of the OSPF timers:
Hello-Interval: time in seconds that a router sends an OSPF hello packet. On broadcast and point-to-point links, the default is 10 seconds. On NBMA, the default is 30 seconds.
Dead:Time in seconds to wait before declaring a neighbor dead. By default, the dead timer interval is four times the hello timer interval.
Wait:Timer interval that causes the interface to exit out of the wait period and select a DR on the network. This timer is always equal to the dead timer interval.
Retransmit:Time to wait before retransmitting a database description (DBD) packet when it has not been acknowledged.
Hello Due In: An OSPF hello packet is sent on this interface after this time. In this example, a hello is sent three seconds from the time the show ip ospf interface is issued.
Neighbor Count: This is the number of OSPF neighbors discovered on this interface. In this example, this router has one neighbor on its Ethernet0 interface.
Adjacent Neighbor Count: This is the number of routers running OSPF that are fully adjacent with this router. Adjacent means that their databases are fully synchronized. In this example, this router has formed an OSPF adjacency with one neighbor on its Ethernet0 interface.
Suppress Hello: When IP OSPF demand circuits are created over ISDN links, the OSPF hello packets are suppressed to keep the link from continually staying up. In the above example, the output is shown for an Ethernet interface; therefore, hello packets are not suppressed for any neighbors.
Index: This is the index of the interface flood lists (area/autonomous system) used. In the example, the value is 1/1.
Flood Queue Length: This is the number of LSAs waiting to be flooded over an interface. From the example, the number of LSAs waiting to be flooded over the Ethernet interface is 0.
Next: This is the pointer to the next LSAs (index) to flood. It refers to the flood lists.
Last Flood Scan Length/Maximum: This is the size of the last list of LSAs flooded and the maximum size of the list. When using pacing, one LSA is transmitted at a time.
Last Flood Scan Time/Maximum: This is the time spent in the last flooding and the maximum time spent flooding.
The command that is used for configuring OSPF in NBMA mode is: "ip ospf network non-broadcast". However, note that NBMA mode is used by default.
OSPF determines the router ID using the following criteria:
1. Use the address configured by the ospf router-id command
2. Use the highest numbered IP address of a loopback interface
3. Use the highest IP address of any physical interface
4. If no interface exists, set the router-ID to 0.0.0.0
If no OSPF router ID is explicitly configured, OSPF computes the router-ID based on the items 2, 3, and 4 and restarts OSPF (if the process is enabled and router-ID has changed).
To modify router priority in an OSPF ip network, issue the command: "ip ospf priority <number>" where <number> is any number between 0 and 255. The default is 1.
A default route can be advertised into OSPF domain by an ASBR router in one of two ways:
"default-information originate" command: This command can be used when there is a default route (0.0.0.0/0) already existing. This command will advertise a default route into the OSPF domain.
"default-information originate always" command: This command can be used when there is a default route (0.0.0.0/0) is present or not. This command is particularly useful when the default route is not consistent. An inconsistent default route may result in flipping of the route advertised into the OSPF domain, resulting in instability of the OSPF domain routing information. Therefore, it is recommended to use "always" keyword.
In general, the path cost in OSPF network is calculated using bandwidth only. The formula used is [10^8 divided by Bandwidth]. For example, the cost of a 56kbps serial link is 1785. The default cost of a 10mbps Ethernet is 10.
The statements identify that the process-id of the OSPF is 100, and the statement "area 1 stub no-summary" signifies totally stubby area. The router is connecting two area, and hence not a backbone router.
The command that is used for configuring OSPF in NBMA mode is: "ip ospf network non-broadcast". However, note that NBMA mode is used by default.
Virtual link
The command "show ip ospf virtual-links" will show up the status of virtual links of a router.
Use the "area virtual-link" command to configure an OSPF virtual link between two routers
Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#router ospf 1
Router(config-router)#area 10 virtual-link 10.54.0.1
Router(config-router)#exit
Router(config)#end
Router#
This feature is commonly used when an area has become fragmented and two routers need to tunnel their OSPF neighbor relationship across multiple links. This is usually not a problem if the two routers and the intervening networks are all in the same area. However, it can be a serious problem in particular if you have an ABR(Area Border Router) that is buried inside a non-backbone area without a direct connection to area 0.
You can see the status of a virtual link with the "show ip ospf virtual-links" command: The following output provides a sample output of the command:
Path preference
Path cost is the total of the costs assigned to all interfaces that forward traffic along the path to the destination. External and summary routes are not injected into a totally stubby area in an OSPF network. The advantages of totally stubby areas are reduced routing tables, faster convergence, and stability.
The path cost in OSPF network is calculated using bandwidth only. The formula used is [10 ^8 divided by Bandwidth]. For example, the cost of a 56kbps serial link is 1785. The default cost of a 10mbps Ethernet is 10.
