Basic Concepts of OSPF

Router ID

A router ID is a 32-bit unsigned integer, which identifies a router in an autonomous system (AS). A router ID must exist before the router runs OSPF.

A router ID can be manually configured or automatically obtained.

If no router ID has been configured, the router automatically obtains a router ID using the following methods in descending order of priority.

1. The router preferentially selects the largest IP address from its loopback interface addresses as the router ID.

2. If no loopback interface has been configured, the router selects the largest IP address from its interface IP addresses as the router ID.

A router can obtain a router ID again only after a router ID is reconfigured for the router or an OSPF router ID is reconfigured and the OSPF process restarts.

Area

When a large number of routers run OSPF, link state databases (LSDBs) become very large and require a large amount of storage space. Large LSDBs also complicate shortest path first (SPF) computation and overload the routers. As the network grows, the network topology changes, which results in route flapping and frequent OSPF packet transmission. When a large number of OSPF packets are transmitted, bandwidth usage efficiency decreases, and each router on a network has to recalculate routes in case of any topology change.

OSPF resolves this problem by partitioning an AS into different areas. An area is regarded as a logical group, and each group is identified by an area ID. A router, not a link, resides at the border of an area. A network segment or link can belong only to one area. An area must be specified for each OSPF interface.

OSPF areas include common areas, stub areas, and not-so-stubby areas (NSSAs). Table 5-1 describes these OSPF areas.

Router Types

Routers are classified as internal routers, ABRs, backbone routers, or ASBRs by location in an AS. Figure 5-1 shows the four router types.

Figure 5-1 Router type layout

Table 5-2 describes the four router types.

LSA Types

OSPF encapsulates routing information into LSAs for transmission. Table 5-3 describes LSAs and their functions.

Table 5-4 describes whether a type of LSA is supported in an area.

Packet Types

OSPF encapsulates protocol packets directly with IP packets. The protocol number is 89. OSPF packets are classified as Hello, Database Description (DD), Link State Request (LSR), Link State Update (LSU), or Link State Acknowledgment (LSAck) packets. Table 5-5 describes OSPF packets and their functions.

Route Types

Route types are classified as intra-area, inter-area, Type 1 external, or Type 2 external routes. Intra-area and inter-area routes describe the network structure of an AS. Type 1 or Type 2 AS external routes describe how to select routes to destinations outside an AS.

Table 5-6 describes OSPF routes in descending order of priority.

Network Types

Networks are classified as broadcast, non-broadcast multiple access (NBMA), point-to-multipoint (P2MP), or point-to-point (P2P) networks by link layer protocol. Table 5-7 describes the network types.

OSPF Packet Authentication

OSPF supports packet authentication. Only the OSPF packets that have been authenticated can be received. If OSPF packets are not authenticated, a neighbor relationship cannot be established.

A router supports two authentication methods:

• Area-based authentication

• Interface-based authentication

When both area-based and interface-based authentication methods are configured, interface-based authentication takes effect.

DR and BDR

On broadcast or NBMA networks, any two routers need to exchange routing information. As shown in Figure 5-2, n routers are deployed on the network. n x (n - 1)/2 adjacencies must be established. Any route change on a router is transmitted to other routers, which wastes bandwidth resources. OSPF resolves this problem by defining a DR and a backup designated router (BDR). After a DR is elected, all routers send routing information only to the DR. Then the DR broadcasts LSAs. routers other than the DR and BDR are called DR others. The DR others establish only adjacencies with the DR and BDR and not with each other. This process reduces the number of adjacencies established between routers on broadcast or NBMA networks.

Figure 5-2 Network topologies before and after a DR election

If the original DR fails, routers must reelect a DR and the routers except the new DR must synchronize routing information to the new DR. This process is lengthy, which may cause incorrect route calculations. A BDR is used to shorten the process. The BDR is a backup for a DR. A BDR is elected together with a DR. The BDR establishes adjacencies with all routers on the network segment and exchanges routing information with them. When the DR fails, the BDR immediately becomes a new DR. The routers need to reelect a new BDR, but this process does not affect route calculations.

The DR priority of a router interface determines its qualification for DR and BDR elections. The router interfaces with their DR priorities greater than 0 are eligible. Each router adds the elected DR to a Hello packet and sends it to other routers on the network segment. When both router interfaces on the same network segment declare that they are DRs, the router interface with a higher DR priority is elected as a DR. If the two router interfaces have the same DR priority, the router interface with a larger router ID is elected as a DR.

OSPF Multi-Process

OSPF supports multi-process. Multiple OSPF processes can run on the same router, and they are independent from each other. Route exchanges between different OSPF processes are similar to route exchanges between different routing protocols.

Each interface on a router can belong to only one OSPF process.

A typical application of OSPF multi-process is that OSPF runs between PEs and CEs in a VPN, whereas OSPF is used as an IGP on the backbone of the VPN. Two OSPF processes on the same PE are independent from each other.

OSPF Default Route

A default route is the route whose destination address and mask are both all 0s. When no matching route is discovered, a router uses a default route to forward packets.

A default route generally applies to the following scenarios:

• An ABR in an area advertises Type 3 LSAs carrying a default route within the area. The routers in the area use the received default route to forward inter-area packets.

• An ASBR in an AS advertises Type 5 or Type 7 LSAs carrying a default route within the AS. The routers in the AS use the received default route to forward AS external packets.

OSPF routes are hierarchically managed. The priority of the default route carried in Type 3 LSAs is higher than the priority of the default route carried in Type 5 or Type 7 LSAs.

A router advertises LSAs carrying a default route by adhering to the following principles:

• A router in an area can advertise LSAs carrying a default route only when the router has an interface connected to a device outside the area.

• If a router has advertised LSAs carrying a default route, the router no longer learns the same type of LSA advertised by other routers, which carry a default route. That is, the router uses only the LSAs advertised by itself to calculate routes. The LSAs advertised by other routers are still saved in the LSDB.

• If a router must use a route to advertise LSAs carrying an external default route, the route cannot be a route learned by the local OSPF process. A router in an area uses an external default route to forward packets outside the area. If the next hops of routes in the area are routers in the area, packets cannot be forwarded outside the area.

• Before a router advertises a default route, it checks whether a neighbor in the full state is present in area 0. The router advertises a default route only when a neighbor in the full state is present in area 0. If no such a neighbor exists, the backbone area cannot forward packets and advertising a default route is meaningless.

Table 5-8 describes the principles for advertising default routes in different areas.

OSPF Route Summarization

Route summarization means that an ABR in an area summarizes the routes with the same prefix into one route and advertises the summarized route to other areas.

Route summarization between areas reduces the amount of routing information to be transmitted, reducing the size of routing tables and improving device performance.

Route summarization can be carried out by an ABR or an ASBR:

• Route summarization on an ABR:

  When an ABR in an area advertises routing information to other areas, it generates Type 3 LSAs by network segment. If this area contains consecutive network segments, you can run a command to summarize these network segments into one network segment. The ABR only sends one summarized LSA, and will not send the LSAs that belong to the summarized network segment specified in the command.

• Route summarization on an ASBR:

  If the local device is an ASBR and route summarization is configured, the ASBR will summarize the imported Type 5 LSAs within the aggregated address range. After an NSSA is configured, the ASBR also summarizes the imported Type 7 LSAs within the aggregated address range.

  If the local device functions both as an ASBR and an ABR, the device summarizes the Type 5 LSAs that are translated from Type 7 LSAs.

OSPF Route Filtering

OSPF supports route filtering using routing policies. By default, OSPF does not filter routes.

Routing policies used by OSPF include route-policy, access-list, and prefix-list.

OSPF route filtering can be used for:

• Importing routes

  OSPF can import routes learned by other routing protocols. You can configure routing policies to filter the imported routes so that OSPF can import only the routes that match specific conditions.

• Advertising imported routes

  OSPF advertises the imported routes to its neighbors.

  You can configure filtering rules to filter the routes to be advertised. The filtering rules can be configured only on ASBRs.

  If OSPF imports a large number of external routes and advertises them to a device with a smaller routing table capacity, the device may restart unexpectedly. To address this problem, you can configure a limit on the number of LSAs that can be generated for imported external routes in an OSPF process.

• Learning routes

  Filtering rules can be configured to allow OSPF to filter the received intra-area, inter-area, and AS external routes.

  After receiving routes, an OSPF device adds only the routes that match the filtering rules to the local routing table, but can still advertise all routes from the OSPF routing table.

• Learning inter-area LSAs

  You can run a command to configure an ABR to filter the Summary LSAs entering the local area. This configuration takes effect only on ABRs because only ABRs can advertise Summary LSAs.

  Table 5-9 Differences between filtering for inter-area LSA learning and filtering for route learning

  Filtering for Inter-area LSA Learning	Filtering for Route Learning
  Directly filters the LSAs entering the local area.	Filters the routes that are calculated based on LSAs, but does not filter LSAs. This means that all incoming LSAs are learned, but only routes matching filtering conditions are added to the local routing table.

• Advertising inter-area LSAs

  You can run commands to configure an ABR to filter outgoing Summary LSAs. This configuration takes effect only on ABRs.

OSPF RFC 1583 Compatibility

RFC 1583 is an earlier version of OSPFv2.

When OSPF calculates external routes, routing loops may occur due to differences of route selection rules between RFC 2328 and RFC 1583. To prevent routing loops, both communication ends must use the same route selection rules.

By default, OSPF uses the route selection rules defined in RFC 1583.