Read an Excerpt
Chapter 4: OSPF AreasThis chapter is the introduction to Open Shortest Path First (OSPF) areas. It will introduce the term OSPF areas and discuss their role in OSPF routing. It is very important that you take the time to learn the terminology used in OSPF. Without this knowledge, the remaining sections of the chapter will be difficult to follow.
Open Shortest Path FirstOpen Shortest Path First (OSPF) is an open standards routing protocol. It is important to recognize that Cisco's implementation of OSPF is a standards-based version. This means that Cisco based its version of OSPF on the open standards. While doing so, Cisco also has added features to its version of OSPF that may not be found in other implementations of OSPF. This becomes important when interoperability is needed.
John Moy heads up the working group of OSPF. Two RFCs define OSPF: Version 1 is defined by RFC 1131, and Version 2 is defined by RFC 2328. Version 2 is the only version to make it to an operational status. However, many vendors modify OSPF. OSPF is known as a link-state routing protocol (link-state routing protocols were discussed in Chapter 2, "Routing Principles"). The Dijkstra algorithm is used to calculate the shortest path through the network. Within OSPF, links become synonymous with interfaces.
OSPF is a robust protocol, and due to the robustness, you must learn many terms in order to understand the operation of OSPF. The next section covers the terminology necessary to enable you to understand the many operations and procedures performed by the OSPF process.
OSPF TerminologyThe most basic of terms that are related to OSPF are related to many routing protocols. We begin by defining relationships among routers. From there, we will move on to defining terms relating to OSPF operations.
Neighbor A neighbor refers to a connected (adjacent) router that is running an OSPF process with the adjacent interface assigned to the same area. Neighbors are found via Hello packets. No routing information is exchanged with neighbors unless adjacencies are formed.
Adjacency An adjacency refers to the logical connection between a router and its corresponding designated routers and backup designated routers. The formation of this type of relationship depends heavily on the type of network that connects the OSPF routers.
Link In OSPF, a link refers to a network or router interface assigned to any given network. Within OSPF, link is synonymous with interface. Interface An interface is the physical interface on a router. When an interface is added to the OSPF process, it is considered by OSPF as a link. If the interface is up, then the link is up. OSPF uses this association to build its link database.
Link State Advertisement Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information that is shared among OSPF routers.
Designated router A designated router (DR) is used only when the OSPF router is connected to a broadcast (multi-access) network. To minimize the number of adjacencies formed, a DR is chosen to disseminate/ receive routing information to/from the remaining routers on the broadcast network or link.
Backup designated router A backup designated router (BDR) is a hot standby for the DR on broadcast (multi-access) links. The BDR receives all routing updates from OSPF adjacent routers but does not flood LSA updates.
OSPF areas OSPF areas are similar to EIGRP Autonomous Systems. Areas are used to establish a hierarchical network. OSPF uses four types of areas, all of which will be discussed later in this chapter.
Area border router An area border router (ABR) is a router that has multiple area assignments. An interface may belong to only one area. If a router has multiple interfaces and if any of these interfaces belong to different areas, the router is considered an ABR.
Autonomous system boundary router An autonomous system boundary router (ASBR) is a router with an interface connected to an external network or a different AS. An external network or autonomous system refers to an interface belonging to a different routing protocol, such as EIGRP. An ASBR is responsible for injecting route information learned by other Interior Gateway Protocols (IGPs) into OSPF.
Non-broadcast multi-access Non-broadcast multi-access (NMBA) networks are networks such as Frame Relay, X.2S, and ATM. This type of network allows for multi-access but has no broadcast ability like Ethernet. NBMA networks require special OSPF configuration to function properly.
Broadcast (multi-access) Networks such as Ethernet allow multiple access as well as provide broadcast ability. A DR and BDR must be elected for multi-access broadcast networks.
Point-to-point This type of network connection consists of a unique NMBA configuration. The network can be configured using Frame Relay and ATM to allow point-to-point connectivity. This configuration eliminates the need for DRs or BDRs.
Router ID The Router ID is an IP address that is used to identify the router. Cisco chooses the Router ID by using the highest IP address of all configured loopback interfaces. If no loopback addresses are configured, OSPF will choose the highest IP address of the functional physical interfaces.
All of these terms play an important part in understanding the operation of OSPF. You must come to know and understand each of these terms. As you read through the chapter, you will be able to place the terms in their proper context.
OSPF OperationOSPF operation can be divided into three categories:
- Neighbor and adjacency initialization
- LSA flooding
- SPF tree calculation We will discuss each in the following sections.
We begin with neighbor/adjacency formation. This is a very big part of OSPF operation. These relationships are often easily formed over point-to-point connections, but much more complex procedures are required when multiple OSPF routers are connected via a broadcast multi-access media.
The Hello protocol is used to discover neighbors and establish adjacencies. Hello packets contain a great deal of information regarding the originating router. Hello packets are multicast out every interface on a 10-second interval by default. The data contained in the Hello packet can be seen in Table 4.1. It is important to remember that the Router ID, Area ID, and authentication information are carried in the common OSPF header. The Hello packet uses the common OSPF header....