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 cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidth indicates a lower cost. The formula used to calculate the cost is:
cost= 10000 0000/bandwith in bps
By default, the cost of an interface is calculated based on the bandwidth; you can force the cost of an interface.
OSPF uses flooding to exchange link-state updates between routers. Any change in routing information is flooded to all routers in the network. Areas are introduced to put a boundary on the explosion of link-state updates. Flooding and calculation of the Dijkstra algorithm on a router is limited to changes within an area. All routers within an area have the exact link-state database.
A router that has all of its interfaces within the same area is called an internal router (IR). A router that has interfaces in multiple areas is called an area border router (ABR). Routers that act as gateways (redistribution)between OSPF and other routing protocols (IGRP, EIGRP, IS-IS, RIP, BGP, Static) or other instances of the OSPF routing process are called autonomous system boundary router (ASBR). Any router can be an ABR or an ASBR.
There are different types of Link State Packets. The different types are illustrated in the following diagram:
the router links are an indication of the state of the interfaces on a router belonging to a certain area. Each router will generate a router link for all of its interfaces.
Summary links are generated by ABRs; this is how network reachability information is disseminated between areas. Normally, all information is injected into the backbone (area 0) and in turn the backbone will pass it on to other areas. ABRs also have the task of propagating the reachability of the ASBR. This is how routers know how to get to external routes in other ASs.
Network Links are generated by a Designated Router (DR) on a segment (DRs will be discussed later). This information is an indication of all routers connected to a particular multi-access segment such as Ethernet, Token Ring and FDDI (NBMA also).
External Links are an indication of networks outside of the AS. These networks are injected into OSPF via redistribution. The ASBR has the task of injecting these routes into an autonomous system.
It is possible to authenticate the OSPF packets such that routers can participate in routing domains based on predefined passwords. By default, a router uses a Null authentication which means that routing exchanges over a network are not authenticated. Two other authentication methods exist: Simple password authentication and Message Digest authentication (MD-5).
OSPF has special restrictions when multiple areas are involved. If more than one area is configured, one of these areas has be to be area 0. This is called the backbone.
all areas have to be directly connected to the backbone. In the rare situations where a new area is introduced that cannot have a direct physical access to the backbone, a virtual link will have to be configured. Routes that are generated from within an area (the destination belongs to the area) are called intra-area routes. These routes are normally represented by the letter O in the IP routing table. Routes that originate from other areas are called inter-area or Summary routes. The notation for these routes is O IA in the IP routing table. Routes that originate from other routing protocols (or different OSPF processes) and that are injected into OSPF via redistribution are called external routes. These routes are represented by O E2 or O E1 in the IP routing table. Multiple routes to the same destination are preferred in the following order: intra-area, inter-area, external E1, external E2.
The OSPF router-id is usually the highest IP address on the box, or the highest loopback address if one exists.
Virtual links are used for two purposes:
.Linking an area that does not have a physical connection to the backbone.
.Patching the backbone in case discontinuity of area 0 occurs.
Neighbors
Routers that share a common segment become neighbors on that segment. Neighbors are elected via the Hello protocol. Hello packets are sent periodically out of each interface using IP multicast (Appendix B). Routers become neighbors as soon as they see themselves listed in the neighbor's Hello packet. This way, a two way communication is guaranteed. Neighbor negotiation applies to the primary address only. Secondary addresses can be configured on an interface with a restriction that they have to belong to the same area as the primary address.
Two routers will not become neighbors unless they agree on the following:
Area-id: Two routers having a common segment; their interfaces have to belong to the same area on that segment. Of course, the interfaces should belong to the same subnet and have a similar mask.
Authentication: OSPF allows for the configuration of a password for a specific area. Routers that want to become neighbors have to exchange the same password on a particular segment.
Hello and Dead Intervals: OSPF exchanges Hello packets on each segment. This is a form of keepalive used by routers in order to acknowledge their existence on a segment and in order to elect a designated router (DR) on multiaccess segments.The Hello interval specifies the length of time, in seconds, between the hello packets that a router sends on an OSPF interface. The dead interval is the number of seconds that a router's Hello packets have not been seen before its neighbors declare the OSPF router down.
Stub area flag: Two routers have to also agree on the stub area flag in the Hello packets in order to become neighbors. Stub areas will be discussed in a later section. Keep in mind for now that defining stub areas will affect the neighbor election process.
Adjacencies
Adjacency is the next step after the neighboring process. Adjacent routers are routers that go beyond the simple Hello exchange and proceed into the database exchange process. In order to minimize the amount of information exchange on a particular segment, OSPF elects one router to be a designated router (DR), and one router to be a backup designated router (BDR), on each multi-access segment. The BDR is elected as a backup mechanism in case the DR goes down. The idea behind this is that routers have a central point of contact for information exchange. Instead of each router exchanging updates with every other router on the segment, every router exchanges information with the DR and BDR. The DR and BDR relay the information to everybody else. In mathematical terms, this cuts the information exchange from O(n*n) to O(n) where n is the number of routers on a multi-access segment.
DR and BDR election is done via the Hello protocol. Hello packets are exchanged via IP multicast packets on each segment. The router with the highest OSPF priority on a segment will become the DR for that segment. The same process is repeated for the BDR. In case of a tie, the router with the highest RID will win. The default for the interface OSPF priority is one. Remember that the DR and BDR concepts are per multiaccess segment. A priority value of zero indicates an interface which is not to be elected as DR or BDR. The state of the interface with priority zero will be DROTHER.
Building the Adjacency
The adjacency building process takes effect after multiple stages have been fulfilled. Routers that become adjacent will have the exact link-state database.
Down: No information has been received from anybody on the segment.
Attempt: On non-broadcast multi-access clouds such as Frame Relay and X.25, this state indicates that no recent information has been received from the neighbor. An effort should be made to contact the neighbor by sending Hello packets at the reduced rate PollInterval.
Init: The interface has detected a Hello packet coming from a neighbor but bi-directional communication has not yet been established.
Two-way: There is bi-directional communication with a neighbor. The router has seen itself in the Hello packets coming from a neighbor. At the end of this stage the DR and BDR election would have been done. At the end of the 2way stage, routers will decide whether to proceed in building an adjacency or not. The decision is based on whether one of the routers is a DR or BDR or the link is a point-to-point or a virtual link.
Exstart: Routers are trying to establish the initial sequence number that is going to be used in the information exchange packets. The sequence number insures that routers always get the most recent information. One router will become the primary and the other will become secondary master/slave). The primary router will poll the secondary for information.
Exchange: Routers will describe their entire link-state database by sending database description packets. At this state, packets could be flooded to other interfaces on the router.
Loading: At this state, routers are finalizing the information exchange. Routers have built a link-state request list and a link-state retransmission list. Any information that looks incomplete or outdated will be put on the request list. Any update that is sent will be put on the retransmission list until it gets acknowledged.
Full: At this state, the adjacency is complete. The neighboring routers are fully adjacent. Adjacent routers will have a similar link-state database.
OSPF will always form an adjacency with the neighbor on the other side of a point-to-point interface such as point-to-point serial lines. There is no concept of DR or BDR. The state of the serial interfaces is point to point.
Special care should be taken when configuring OSPF over multi-access non-broadcast medias such as Frame Relay, X.25, ATM. The protocol considers these media like any other broadcast media such as Ethernet. NBMA clouds are usually built in a hub and spoke topology.
OSPF and Route Summarization
Summarizing is the consolidation of multiple routes into one single advertisement. This is normally done at the boundaries of Area Border Routers (ABRs). 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 will injects them, already summarized, into other areas. There are two types of summarization:
.Inter-area route summarization
.External route summarization
Inter-area route summarization is done on ABRs and it applies to routes from within the AS. It does not apply to external routes injected into OSPF via redistribution. In order to take advantage of summarization, network numbers in areas should be assigned in a contiguous way to be able to lump these addresses into one range.
External route summarization is specific to external routes that are injected into OSPF via redistribution. Also, make sure that external ranges that are being summarized are contiguous.
OSPF allows certain areas to be configured as stub areas. 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 default route.
An area could be qualified 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.
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.
All OSPF routers inside a stub area have to be configured as stub routers.
An extension to stub areas is what is called "totally stubby areas". Cisco indicates this by adding a "no-summary" keyword to the stub area configuration. A totally stubby area is one that blocks external routes and summary routes (inter-area routes) from going into the area. This way, intra-area routes and the default of 0.0.0.0 are the only routes injected into that area.
External routes fall under two categories, external type 1 and external type 2. The difference between the two is in the way the cost (metric) of the route is being calculated. The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. A type 1 cost is the addition of the external cost and the internal cost used to reach that route. A type 1 route is always preferred over a type 2 route for the same destination.
Injecting Defaults into OSPF
An autonomous system boundary router (ASBR) can be forced to generate a default route into the OSPF domain. As discussed earlier, a router becomes an ASBR whenever routes are redistributed into an OSPF domain. However, an ASBR does not, by default, generate a default route into the OSPF routing domain.
To have OSPF generate a default route use the following:
default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]
There are two ways to generate a default. The first is to advertise 0.0.0.0 inside the domain, but only if the ASBR itself already has a default route. The second is to advertise 0.0.0.0 regardless whether the ASBR has a default route. The latter can be set by adding the keyword always. You should be careful when using the always keyword. If your router advertises a default (0.0.0.0) inside the domain and does not have a default itself or a path to reach the destinations, routing will be broken.
Lsa type:
1 Router Link advertisements. Generated by each router for each area it belongs to. They describe the states of the router's link to the area. These are only flooded within a particular area.
2 Network Link advertisements. Generated by Designated Routers. They describe the set of routers attached to a particular network. Flooded in the area that contains the network.
3 or 4 Summary Link advertisements. Generated by Area Border routers. They describe inter-area (between areas) routes. Type 3 describes routes to networks, also used for aggregating routes. Type 4 describes routes to ASBR.
5 AS external link advertisements. Originated by ASBR. They describe routes to destinations external to the AS. Flooded all over except stub areas.
The OSPF not-so-stubby area (NSSA) feature is described by RFC 1587.
Redistribution into an NSSA area creates a special type of link-state advertisement (LSA) known as type 7, which can only exist in an NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, which gets propagated into the OSPF domain.
In order to make a stub area into an NSSA, issue this command under the OSPF configuration,This command must be configured on every single router in Area 1.
router ospf 1
Area 1 nssa
In order to configure an NSSA totally stub area, issue this command under the OSPF configuration,Configure this command on NSSA ABRs only
router ospf 1
Area 1 nssa no-summary
Original link:
http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml
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