Abstract of CCNA study guide-27 - OSPF 1

Open Shortest Path First (OSPF) Basics

Open Shortest Path First (OSPF) is an open standard routing protocol that’s been implemented by a wide variety of network vendors, including Cisco. If you have multiple routers and not all of them are Cisco (what!), then you can’t use EIGRP, can you? So your remaining CCNA objective options are basically RIP, RIPv2, and OSPF. If it’s a large network, then, really, your only options are OSPF.

OSPF works by using the Dijkstra algorithm. First, a shortest path tree is constructed, and then the routing table is populated with the resulting best paths. OSPF converges quickly, although perhaps not as quickly as EIGRP, and it supports multiple, equal-cost routes to the same destination. Like EIGRP, it does support both IP and IPv6 routed protocols.

OSPF provides the following features:

-        Consists of areas and autonomous systems

-        Minimizes routing update traffic

-        Allows scalability

-        Supports VLSM/CIDR

-        Has unlimited hop count

-        Allows multi-vendor deployment (open standard)

OSPF is the first link-state routing protocol that most people are introduced to.

TABLE 7.3 OSPF and RIP Comparison

Characteristic	OSPF	RIPv2	RIPv1
Type of protocol	Link state	Distance vector	Distance vector
Classless support	Yes	Yes	No
VLSM support	Yes	Yes	No
Auto-summarization	No	Yes	Yes
Manual summarization	Yes	No	No
Discontiguous support	Yes	Yes	No
Route propagation	Multicast on change	Periodic multicast	Periodic broadcast
Path metric	Bandwidth	Hops	Hops
Hop count limit	None	15	15
Convergence	Fast	Slow	Slow
Peer authentication	Yes	Yes	No
Hierarchical network	Yes (using areas)	No (flat only)	No (flat only)
Updates	Event triggered	Route table updates	Route table updates
Route computation	Dijkstra	Bellman-Ford	Bellman-Ford

OSPF is designed in a hierarchical fashion, which basically means that you can separate the larger internetwork into smaller internetworks called areas. This is the best design for OSPF.

The following are reasons for creating OSPF in a hierarchical design:

-        To decrease routing overhead

-        To speed up convergence

-        To confine network instability to single areas of the network

Figure 7.4 shows a typical OSPF simple design.

-        Notice how each router connects to the backbone called area 0, or the backbone area. OSPF must have an area 0, and all other areas should connect to this area.

-        Routers that connect other areas to the backbone area within an AS are called Area Border Routers (ABRs). Still, at least one interface of the ABR must be in area 0.

-        OSPF runs inside an autonomous system, but it can also connect multiple autonomous systems together. The router that connects these ASes is called an Autonomous System Boundary Router (ASBR).

OSPF Terminology

Link A link is a network or router interface assigned to any given network.

Router ID (RID) is an IP address used to identify the router.  the Router ID assigned using the highest IP address of all configured loopback interfaces. If no loopback interfaces are configured, OSPF will choose the highest IP address of all active physical interfaces.

Neighbor : Neighbors are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link.

Adjacency : is a relationship between two OSPF routers that permits the direct exchange of route updates.

Hello protocol : provides dynamic neighbor discovery and maintains neighbor relationships.

Neighborship database: is a list of all OSPF routers for which Hello packets have been seen.

Topological database :contains information from all of the Link State Advertisement packets that have been received for an area.

Link State Advertisement (LSA) is an OSPF data packet containing link-state and routing information that’s shared among OSPF routers.

Designated router (DR) is elected whenever OSPF routers are connected to the same multi-access network. Cisco likes to call these “broadcast” networks

Backup designated router (BDR) is standby for the DR on multi-access link. The BDR receives all routing updates from OSPF adjacent routers but doesn’t flood LSA updates.

OSPF areas is a grouping of contiguous networks and routers. All routers in the same area share a common Area ID

Broadcast (multi-access) such as Ethernet allow multiple devices to connect to (or access) the same network as well as provide a broadcast ability in which a single packet is delivered to all nodes on the network. In OSPF, a DR and a BDR must be elected for each broadcast multi-access network.

Non-broadcast multi-access: (NBMA) networks are types such as Frame Relay, X.25, and Asynchronous Transfer Mode (ATM). These networks allow for multi-access but have no broadcast ability like Ethernet. So, NBMA networks require special OSPF configuration to function properly and neighbor relationships must be defined.

Point-to-point : refers to a type of network topology consisting of a direct connection between two routers that provides a single communication path. This type of configuration eliminates the need for DRs or

BDRs—but neighbors are discovered automatically.

Point-to-multipoint : refers to a type of network topology consisting of a series of connections between a single interface on one router and multiple destination routers.

As with point-to-point, no DRs or BDRs are needed.

SPF Tree Calculation

Within an area, each router calculates the best/shortest path to every network in that same area. This calculation is based upon the information collected in the topology database and an algorithm called shortest path first (SPF).

each router in an area constructing a tree where the router is the root and all other networks are arranged

along the branches and leaves. This is the shortest path tree used by the router to insert routes into the routing table.

It’s important to understand that this tree contains only networks that exist in the same area as the router itself does. If a router has interfaces in multiple areas, then separate trees will be constructed for each area.

OSPF uses a metric referred to as cost. The cost of the entire path is the sum of the costs of the outgoing interfaces along the path.

Cisco uses a simple equation of    10^8  / bandwidth. The bandwidth is the configured bandwidth for the interface.

Using this rule, a 100Mbps Fast Ethernet interface would have a default OSPF cost of 1 and a 10Mbps Ethernet interface would have a cost of 10.An interface set with a bandwidth of 64,000 would have a default cost of 1,563.

This value may be overridden by using the ip ospf cost command. The cost is manipulated by changing the value to a number within the range of 1 to 65,535. Because the cost is assigned to each link, the value must be changed on the interface that you want to change the cost.