OSPF Areas

An autonomous system can be divided into areas, with each area containing a group of networks. An area’s topology is concealed from the rest of the AS, which significantly reduces OSPF protocol traffic (LSA updates), simplifies the network topology, and simplifies the routing table by populating it with summarized routes rather than exact routes on each router. This decrease in LSA updates, link-state database size, and CPU time, all required for OSPF route calculations, results in a decrease in route calculation time.

All routers in an area have identical link-state databases for that area.

Areas within the same AS are linked to each other via area border routers (ABRs). An ABR is a router that belongs to more than one area. An ABR maintains a separate topological database for each area it is connected to.

Routing in the AS takes place on two levels, depending on whether the source and destination of a packet reside in the same area (intra-area routing) or different areas (inter-area routing). In intra-area routing, the packet is routed solely on information obtained within the area; that is, routing updates are only passed within the area. In inter-area routing, routing updates are passed between areas.

External routes refer to routing updates passed from another routing protocol into the OSPF domain.

Routers that pass information between an OSPF routing domain and a non-OSPF network are called autonomous system boundary routers (ASBRs).

Backbone Area

Every OSPF system requires a backbone area. The OSPF backbone area is uniquely identified as area 0 and uses the area ID 0.0.0.0. All other areas must be connected to the backbone area, either physically or logically. The backbone distributes routing information between areas. If it is not practical or possible to connect an area to the backbone (see area 0.0.0.5 in Figure 3), the ABRs (routers Y and Z in the figure) must be connected via a virtual link. The two ABRs form a point-to-point-like adjacency across the transit area (area 0.0.0.4).

Figure 3:  Backbone Area 

Virtual Links

The backbone area in an OSPF AS must be contiguous and all other areas must be directly connected to the backbone area via an ABR. If it is not practical or possible to physically connect an area to the backbone, virtual links can be used to connect to the backbone through a non-backbone area.

A virtual link functions as a point-to-point link that passes through a transit area. Figure 3 depicts routers Y and Z as the start and end points of the virtual link while area 0.0.0.4 is the transit area. In order to configure virtual links, the router must be an ABR. Virtual links are identified by the router ID of the other endpoint, which is another ABR.

These two endpoint routers must be attached to a common area, called the transit area. The area through which the virtual link passes must have full routing information.

Transit areas pass traffic from an area adjacent to the backbone or to another area. The traffic does not originate or terminate in the transit area. The transit area cannot be a stub area or an NSSA area.

Virtual links are part of the backbone and behave as if they were unnumbered point-to-point networks between the two routers. A virtual link uses the intra-area routing of its transit area to forward packets. Virtual links are brought up and down through the building of the shortest-path trees for the transit area.

Neighbors and Adjacencies

A router uses the OSPF Hello protocol to discover neighbors. Neighbors are routers that interface to a common network. In a broadcast-supported topology, one router sends Hello packets to a multicast address and receives Hello packets in return. Unicast Hello packets are used in non-broadcast topologies.

The neighbors then attempt to form adjacencies by exchanging link-state information with the goal of having identical link-state databases. When the link-state databases of two neighbors are synchronized, they are considered to be adjacent.

Designated Routers and Backup Designated Routers

In multi-access broadcast networks, such as Ethernet networks, with at least two attached routers, a designated router and a backup designated router can be elected. The concept of a designated router was developed in order to avoid the formation of adjacencies between all attached routers. Without a designated router, the area would be flooded with LSAs – a router would send LSAs to all its adjacent neighbors, and each in turn would send LSAs to all their neighbors, and so on. This would create multiple copies of the same LSA on the same link.

The designated router reduces the number of adjacencies required because each router forms an adjacency only with the designated router and backup designated router. Only the designated router sends LSAs in multicast format to the rest of the network, reducing the amount of routing protocol traffic and the size of the link-state database. If the designated router fails, the backup designated router becomes active.

The designated router is automatically elected based on priority – the router with the highest priority becomes the designated router and the router with the second-highest priority becomes the backup. If two routers have the same priority, the one with the highest router ID wins.

A router with a priority set to 0 can never become a designated router.

After a designated router is elected, it begins sending Hello packets to all other attached routers in order to form adjacencies.

Note: In point-to-point networks, where a single pair of routers are connected, no designated or backup designated router is elected. An adjacency must be formed with the neighbor router. To significantly improve adjacency forming and network convergence, a network should be configured as point-to-point if only two routers are connected, even if the network is a broadcast media such as Ethernet.

Link-State Advertisements

Link-state advertisements (LSAs) describe the state of a router or network, including router interfaces and adjacency states. Each LSA is flooded throughout an area. The collection of LSAs from all routers and networks form the protocol’s link-state (or topological) database.

The distribution of topology database updates takes place along adjacencies. A router sends LSAs to advertise its state according to the configured interval and when the router’s state changes. These packets include information about the router's adjacencies, which allows detection of non-operational routes.

When a router discovers a routing table change or detects a change in the network, link-state information is advertised to other routers to maintain identical routing tables. Router adjacencies are reflected in the contents of its link-state advertisements. The relationship between adjacencies and the link states allow the protocol to detect non-operating routers. Link-state advertisements flood the area. The flooding mechanism ensures that all routers in an area have the same topological database. The database consists of the collection of LSAs received from each router belonging to the area.

OSPF sends only the changed information, not the whole topology information or whole link-state database, when a change takes place. From the topological database, each router constructs a tree of shortest paths with itself as root (that is, runs the Dijkstra algorithm). OSPF distributes routing information between routers belonging to a single AS.

Table 19 lists the types of LSAs generated by routers.

Metrics

In OSPF, all interfaces have a cost value or routing metric used in the OSPF link-state calculation. A metric value is configured based on hop count, bandwidth, or other parameters, to compare different paths through an AS. OSPF uses cost values to determine the best path to a particular destination – the lower the cost value, the more likely the interface will be used to forward data traffic.

Costs are also associated with externally derived routing data, such as those routes learned from an Exterior Gateway Protocol (EGP), for example, BGP, and are passed transparently throughout the AS. This data is kept separate from the OSPF protocol’s link-state data. Each external route can be tagged by the advertising router, enabling the passing of additional information between routers on the boundaries of the AS.

Authentication

All OSPF protocol exchanges can be authenticated. This guarantees that only trusted routers can participate in autonomous system routing. -Alcatel-Lucent’s implementation of OSPF supports plain text (simple password) and Message Digest 5 (MD5) authentication.

When authentication is enabled on a link, a text string password must be configured. Neighbor OSPF routers must supply the password in all OSPF packets they send to an interface.

Plain text authentication includes the password in each OSPF packet sent on a link.

MD5 authentication is more secure than plain text authentication. MD5 authentication uses the password as an encryption key. Routers in the same routing domain must be configured with the same key. When the MD5 hashing algorithm is used for authentication, MD5 is used to verify data integrity by creating a 128-bit message digest from the data input that is included in each packet. The packet is transmitted to the router neighbor and can only be decrypted if the neighbor has the correct password.

By default, authentication is not enabled on an interface.

Route Redistribution and Summarization

Route redistribution is the taking of routes from one protocol and sending them to another protocol. The 7705 SAR supports the redistribution of static routes into OSPF. These routes are advertised as type 5 or type 7 LSAs (external routes) and are included in each router’s link-state database.

Route redistribution involves the use of routing policies. For information on routing policies, refer to the 7705 SAR OS Router Configuration Guide, “Route Policies”.

Route summarization allows an ABR or ASBR to summarize routes with the same prefix into a single route and distribute it to other areas. Routes redistributed into OSPF from static routes can also be summarized.

Route summarization reduces the amount of routing information across areas and the size of routing tables on the routers, thus improving the calculation speed of the routers.

OSPF-TE Extensions

OSPF traffic engineering (TE) extensions enable the 7705 SAR to include traffic engineering information in the algorithm in order to calculate the best route to a destination. The traffic information includes:

IP Subnets

OSPF enables the flexible configuration of IP subnets. Each distributed OSPF route has a destination and mask. A network mask is a 32-bit number that indicates the range of IP addresses residing on a single IP network/subnet. This specification displays network masks as hexadecimal numbers; for example, the network mask for a class C IP network is displayed as 0xffffff00. This mask is often displayed as 255.255.255.0.

Two different subnets with the same IP network number might have different masks, called variable-length subnets. A packet is routed to the longest or most specific match. Host routes are considered to be subnets whose masks are all ones (0xffffffff).

For example, for a packet destined for IP address 10.1.1.1, 10.1.1.0/24 is a longer (better) match than 10.1.1.0/16. If both entries are in the routing table, the route designated by 10.1.1.0/24 will be used.

OSPF Instances

A routing instance is a routing entity for a router. The 7705 SAR supports the default routing instance only; it does not support multiple instances. The default routing instance is associated with the global routing table.

Bidirectional Forwarding Detection (BFD) for OSPF

BFD is a simple protocol for detecting failures in a network. BFD uses a “hello” mechanism that sends control messages periodically to the far end and receives periodic control messages from the far end. BFD can detect device, link, and protocol failures.

BFD can be enabled using OSPFv2 (for IPv4) or OSPFv3 (for IPv6). Additionally, a network can be configured to use both OSPFv2 and OSPFv3.

When BFD is enabled on an OSPF interface, the state of the interface is tied to the state of the BFD session between the local node and remote (far-end) node. BFD is implemented in asynchronous mode only, meaning that neither end responds to control messages; rather, the messages are sent in the time period configured at each end.

If the configured number of consecutive BFD missed messages is reached, the link is declared down and OSPF takes the appropriate action (for example, generates an LSA update against the failed link or reroutes around the failed link).

Due to the lightweight nature of BFD, it can detect failures faster than other detection protocols, making it ideal for use in applications such as mobile transport.

Graceful Restart Helper

Graceful Restart and non-stop routing (NSR) both provide mechanisms that allow neighbor routers to handle a service interruption due to a CSM switchover. Data packets continue to be forwarded along known routes while the OSPF information is being restored or refreshed following the switchover.

With Graceful Restart, a router undergoing a switchover informs its adjacent neighbors and requests a grace period whereby traffic is still forwarded based on the last known good FIB while the router restarts. The neighbor must cooperate with the requesting router in order for the traffic to be forwarded. After the switchover, the neighbor relationships must be re-established.

With NSR (or high-availability service), routing neighbors are unaware of any event on the router performing a switchover. All activity switches to the inactive CSM, which maintains up-to-date routing information, so that routing topology and reachability are not affected. NSR is a more reliable and robust way of handling service interruptions than Graceful Restart.

The 7705 SAR supports NSR; therefore, Graceful Restart is not implemented on the router. However, to support neighbor routers that do not have high-availability service, the 7705 SAR supports Graceful Restart Helper. In Graceful Restart Helper mode, the 7705 SAR never requests graceful restart support. However, if a grace LSA is received from an OSPF neighbor, the 7705 SAR keeps the link toward that neighbor up and operational until the specified grace period in the grace LSA expires or the graceful restart is successful, whichever comes first.

Preconfiguration Requirements

The router ID must be available before OSPF can be configured. The router ID is a 32-bit IP address assigned to each router running OSPF. This number uniquely identifies the router within an AS. OSPF routers use the router IDs of the neighbor routers to establish adjacencies. Neighbor IDs are learned when Hello packets are received from the neighbor.

Before configuring OSPF parameters, ensure that the router ID is derived by one of the following methods:

General

Reference Sources

For information on supported IETF drafts and standards, as well as standard and proprietary MIBs, refer to Standards and Protocol Support.