OSPF is perhaps the most widely-used interior gateway protocol (IGP) in large enterprise networks; IS-IS, another link-state routing protocol, is more common in large service provider networks. The most widely-used exterior gateway protocol (EGP) is BGP.
Routers in the same broadcast domain or at each end of a point-to-point telecommunications link form adjacencies when they have detected each other. This detection occurs when a router "sees" itself in a hello packet. This is called a two way state and is the most basic relationship. The router in ethernet or frame relay select a designated router (DR) and a backup designated router (BDR) which act as a hub to reduce traffic between routers. OSPF uses both unicast and multicast to send "hello packets" and link state updates. Multicast addresses 22.214.171.124 (all SPF/link state routers, also known as AllSPFRouters) and 126.96.36.199 (all Designated Routers, AllDRouters) are reserved for OSPF (RFC 2328). In contrast to the Routing Information Protocol (RIP) or the Border Gateway Protocol (BGP), OSPF does not use TCP or UDP but uses IP directly, via IP protocol 89. OSPF handles its own error detection and correction, therefore negating the need for TCP or UDP functions.
The OSPF Protocol can operate securely between routers, optionally using a clear-text password or using MD5 to authenticate peers before forming adjacencies and before accepting link-state advertisements (LSA). A natural successor to the Routing Information Protocol (RIP), it was classless, or able to use Classless Inter-Domain Routing, from its inception. Multicast extensions to OSPF, the Multicast Open Shortest Path First (MOSPF) protocols, have been defined but these are not widely used at present.
These are logical groupings of routers whose information may be summarized towards the rest of the network. Several "special" area types are defined:
The backbone area is responsible for distributing routing information between nonbackbone areas. The backbone must be contiguous, but it does not need to be physically contiguous; backbone connectivity can be established and maintained through the configuration of virtual links.
All OSPF areas must connect to the backbone area. This connection, however, can be through a virtual link. For example, assume area 0.0.0.1 has a physical connection to area 0.0.0.0. Further assume that area 0.0.0.2 has no direct connection to the backbone, but this area does have a connection to area 0.0.0.1. Area 0.0.0.2 can use a virtual link through the transit area 0.0.0.1 to reach the backbone. To be a transit area, an area has to have the transit attribute, so it cannot be stubby in any way.
All routers in the area need to agree they are stub, so that they do not generate types of LSA not appropriate to a stub area. The Type 3 LSA for the default route is the only external that should enter the area, and none of its routers may generate externals.
Stub areas do receive inter-area (IA) routes, advertised with Type 3 and Type 4 LSAs. If the stub area has more than one area border router (ABR), the information on other non-backbone areas allows the routers in the stub area to pick the best route to another area.
Stub areas do not have the transit attribute and thus cannot be traversed by a virtual link.
Stub areas receive default routes as type 3 network summary LSAs.
Occasionally, it is said that a TSA can have only one ABR. This is not true. If there are multiple ABRs, as might be required for high availability, routers interior to the TSA will send non-intra-area traffic to the ABR with the lowest intra-area metric (the "closest" ABR).
An area can simultaneously be not-so-stubby and totally stubby. This is done when the practical place to put an ASBR, as, for example, with a newly acquired subsidiary, is on the edge of a totally stubby area. In such a case, the ASBR does send externals into the totally stubby area, and they are available to OSPF speakers within that area. In Cisco's implementation, the external routes can be summarized before injecting them into the totally stubby area. In general, the ASBR should not advertise default into the TSA-NSSA, although this can work with extremely careful design and operation, for the limited special cases in which such an advertisement makes sense.
By declaring the totally stubby area as NSSA, no external routes from the backbone, except the default route, enter the area being discussed. The externals do reach area 0.0.0.0 via the TSA-NSSA, but no routes other than the default route enter the TSA-NSSA. Routers in the TSA-NSSA send all traffic to the ABR, except to routes advertised by the ASBR.
Cisco also implements a proprietary version of a NSSA called a NSSA totally stubby area. It takes on the attributes of a TSA, meaning that type 3 and type 4 summary routes are not flooded into this type of area. It is also possible to declare an area both totally stubby and not-so-stubby, which means that the area will receive only the default route from area 0.0.0.0, but can also contain an autonomous system border router (ASBR) that accepts external routing information and injects it into the local area, and from the local area into area 0.0.0.0.
Redistribution into an NSSA area creates special type of LSA known as TYPE 7, which can exist only in an NSSA area. An NSSA ASBR generates this LSA, and an NSSA ABR router translates it into type 5 LSA which gets propagated into the OSPF domain.
Metrics, however, are only directly comparable when of the same type. There are four types of metrics, with the most preferred type listed in order below. An intra-area route is always preferred to an inter-area route regardless of metric, and so on for the other types.
These fields are distributed between network nodes via the TLV fields of an opaque LSA.
OSPF-TE is commonly used within MPLS and GMPLS networks, as a means to determine the topology over which MPLS paths can be established. MPLS then uses its own path setup and forwarding protocols, once it has the full IP routing map.
The router types are attributes of an OSPF process. A given physical router may have one or more OSPF processes. For example, a router that is connected to more than one area, and which receives routes from a BGP process connected to another AS, is both an ABR and an ASBR.
Each router has a router identifier, customarily written in the dotted decimal format (e.g.: 188.8.131.52) of an IP address. The way in which the router ID is determined is implementation-specific. The router ID, however, does not have to be a valid IP address or any IP address present in the routing domain, although it frequently will be advertised within the domain for troubleshooting purposes. Do not assume, until it is known how it is configured, that the router ID is anything more than a 32-bit number (e.g., 255.254.253.252 is legal as a router ID).
Do not confuse router types with designated router (DR), or backup designated router (BDR), which is an attribute of a router interface.
Note that: an area border router is always a backbone router, but a backbone router is not necessarily an area border router.
Do not confuse the DR with an OSPF router type. A given physical router can have some interfaces that are designated (DR), others that are backup designated (BDR), and others that are non-designated. If no router is DR or BDR on a given subnet, the DR is first elected, and then a second election is held if there is more than one BDR. The DR is elected based on the following default criteria:
DR's exist for the purpose of reducing network traffic by providing a source for routing updates, the DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in an area will form a slave/master relationship with the DR. They will form adjacencies with the DR and BDR only. Every time a router sends an update, it sends it to the DR and BDR on the multicast address 184.108.40.206. The DR will then send the update out to all other routers in the area, to the multicast address 220.127.116.11. This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load. DRs and BDRs are always setup/elected on Broadcast networks (Ethernet). DR's can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay or ATM. DRs or BDRs are not elected on point-to-point links (such as a point-to-point WAN connection) because the two routers on either sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized.
| width="4%"||Bits 0–7||8–15||16–18||19–31|
|224||Hello Interval||Options||Router Priority|
|256||Router Dead Interval|
|320||Backup Designated Router|
Cisco has defined the following three additional modes for OSPF in NBMA topologies:
As mentioned, OSPF can provide better load-sharing on external links than other IGPs. When the default route to an ISP is injected into OSPF from multiple ASBRS as a Type I external route and the same external cost specified, other routers will go to the ASBR with the least path cost from its location. This can be tuned further by adjusting the external cost.
In contrast, if the default route from different ISPs is injected with different external costs, as a Type II external route, the lower-cost default becomes the primary exit and the higher-cost becomes the backup only.
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