OSPF Basics

This chapter covers the following exam topics:

3.0 IP Connectivity

3.2 Determine how a router makes a forwarding decision by default

3.2.b Administrative distance

3.2.c Routing protocol metric

3.4 Configure and verify single area OSPFv2

3.4.a Neighbor adjacencies

3.4.b Point-to-point

3.4.c Broadcast (DR/BR selection)

3.4.d Router ID

Comparing Dynamic Routing Protocol Features

Routers add IP routes to their routing tables using three methods: connected routes, static routes, and routes learned by using dynamic routing protocols.

• Routing protocol: A set of messages, rules, and algorithms used by routers for the overall purpose of learning routes. This process includes the exchange and analysis of routing information. Each router chooses the best route to each subnet (path selection) and finally places those best routes in its IP routing table. Examples include RIP, EIGRP, OSPF, and BGP.

• Routed protocol and routable protocol: Both terms refer to a protocol that defines a packet structure and logical addressing, allowing routers to forward or route the packets. Routers forward packets defined by routed and routable protocols. Examples include IP Version 4 (IPv4) and IP Version 6 (IPv6).

path selection sometimes refers to part of the job of a routing protocol, in which the routing protocol chooses the best route.

Routing Protocol Functions

Learn routing information about IP subnets from neighboring routers.

Advertise routing information about IP subnets to neighboring routers.

If more than one possible route exists to reach one subnet, pick the best route based on a metric.

If the network topology changes—for example, a link fails—react by advertising that some routes have failed and pick a new currently best route. (This process is called convergence.)

Interior and Exterior Routing Protocols

IP routing protocols fall into one of two major categories: interior gateway protocols (IGP) or exterior gateway protocols (EGP).

IGP: A routing protocol that was designed and intended for use inside a single autonomous system (AS)

EGP: A routing protocol that was designed and intended for use between different autonomous systems

An AS is a network under the administrative control of a single organization.

routing protocols designed to exchange routes between routers in different autonomous systems are called EGPs. Today, Border Gateway Protocol (BGP) is the only EGP used.

IGP Routing Protocol Algorithms

Distance vector (sometimes called Bellman-Ford after its creators)

Advanced distance vector (sometimes called “balanced hybrid”)

Link-state (OSPF)

Routing Information Protocol (RIP) was the first popularly used IP distance vector protocol, with the Cisco-proprietary Interior Gateway Routing Protocol (IGRP) being introduced a little later.

 proprietary routing protocol called Enhanced Interior Gateway Routing Protocol (EIGRP)

it used more distance vector features than link-state, so it is more commonly classified as an advanced distance vector protocol.

Metrics

Routing protocols choose the best route to reach a subnet by choosing the route with the lowest metric

 RIP uses a counter of the number of routers (hops) between a router and the destination subnet

OSPF totals the cost associated with each interface in the end-to-end route, with the cost based on link bandwidth

The bandwidth interface subcommand does not change the actual physical speed of the interface. It just tells IOS what speed to assume the interface is using.)

Other IGP Comparisons

Classless routing protocols support variable-length subnet masks (VLSM) as well as manual route summarization (supernetting) by sending routing protocol messages that include the subnet masks in the message. The older RIPv1 and IGRP routing protocols—both classful routing protocols—do not.

Administrative Distance

If one company uses OSPF and the other uses EIGRP on at least one router, both OSPF and EIGRP must be used. Then that router can take routes learned by OSPF and advertise them into EIGRP, and vice versa, through a process called route redistribution.

When a single routing protocol learns multiple routes to the same subnet, the metric tells it which route is best. However, when two different routing protocols learn routes to the same subnet, because each routing protocol’s metric is based on different information, IOS cannot compare the metrics

When IOS must choose between routes learned using different routing protocols, IOS uses a concept called administrative distance. Administrative distance is a number that denotes how believable an entire routing protocol is on a single router. The lower the number, the better, or more believable, the routing protocol. For example, RIP has a default administrative distance of 120, OSPF uses a default of 110, and EIGRP defaults to 90. When using OSPF and EIGRP, the router will believe the EIGRP route instead of the OSPF route (at least by default). The administrative distance values are configured on a single router and are not exchanged with other routers

The show ip route command lists each route’s administrative distance as the first of the two numbers inside the brackets. The second number in brackets is the metric

IOS can be configured to change the administrative distance of a particular routing protocol, a particular route, or even a static route.

For example, the command ip route 10.1.3.0 255.255.255.0 10.1.130.253 defines a static route with a default administrative distance of 1, but the command ip route 10.1.3.0 255.255.255.0 10.1.130.253 210 defines the same static route with an administrative distance of 210

Topology Information and LSAs

Each LSA is a data structure with some specific information about the network topology; the LSDB is simply the collection of all the LSAs known to a router.

before sending an LSA to yet another neighbor, routers communicate, asking “Do you already have this LSA?,” and then sending the LSA to the next neighbor only if the neighbor has not yet learned about the LSA.

Routers reflood an LSA when some information changes (for example, when a link goes up or comes down). They also reflood each LSA based on each LSA’s separate aging timer (default 30 minutes).

Applying Dijkstra SPF Math to Find the Best Routes

Becoming neighbors: A relationship between two routers that connect to the same data link, created so that the neighboring routers have a means to exchange their LSDBs.

Exchanging databases: The process of sending LSAs to neighbors so that all routers learn the same LSAs.

Adding the best routes: The process of each router independently running SPF, on their local copy of the LSDB, calculating the best routes, and adding those to the IPv4 routing table.

The Basics of OSPF Neighbors

OSPF neighbors are routers that both use OSPF and both sit on the same data link. Two routers can become OSPF neighbors if connected to the same VLAN, or same serial link, or same Ethernet WAN link.

they must send OSPF messages and agree to become OSPF neighbors. To do so, the routers send OSPF Hello messages, introducing themselves to the potential neighbor. Assuming the two potential neighbors have compatible OSPF parameters, the two form an OSPF neighbor relationship, and would be displayed in the output of the show ip ospf neighbor command.

allows new routers to be dynamically discovered. That means new routers can be added to a network without requiring every router to be reconfigured. Instead, OSPF routers listen for OSPF Hello messages from new routers and react to those messages, attempting to become neighbors and exchange LSDBs.

Meeting Neighbors and Learning Their Router ID

process starts with messages called OSPF Hello messages. The Hellos in turn list each router’s router ID (RID), which serves as each router’s unique name or identifier for OSPF. Finally, OSPF does several checks of the information in the Hello messages to ensure that the two routers should become neighbors.

OSPF RIDs are 32-bit numbers.

By default, IOS chooses one of the router’s interface IPv4 addresses to use as its OSPF RID. However, the OSPF RID can be directly configured

As soon as a router has chosen its OSPF RID and some interfaces come up, the router is ready to meet its OSPF neighbors. OSPF routers can become neighbors if they are connected to the same subnet. To discover other OSPF-speaking routers, a router sends multicast OSPF Hello packets to each interface and hopes to receive OSPF Hello packets from other routers connected to those interfaces.

They continue to send Hellos at a regular interval based on their Hello timer settings. The Hello messages themselves have the following features:

The Hello message follows the IP packet header, with IP protocol type 89.

Hello packets are sent to multicast IP address 224.0.0.5, a multicast IP address intended for all OSPF-speaking routers.

OSPF routers listen for packets sent to IP multicast address 224.0.0.5, in part hoping to receive Hello packets and learn about new neighbors.

Each router keeps an OSPF state variable for how it views the neighbor.

Hello. This message tells R1 that R2 exists, and it allows R1 to move through the init state and quickly to a 2-way state.

The 2-way state is a particularly important OSPF state. At that point, the following major facts are true:

2-way means that the router is available to exchange its LSDB with the neighbor. In other words, it is ready to begin a 2-way exchange of the LSDB.

First, they tell each other a list of LSAs in their respective databases—not all the details of the LSAs, just a list. (Think of these lists as checklists.) Then each router can check which LSAs it already has and then ask the other router for only the LSAs that are not known yet.

the OSPF messages that actually send the LSAs between neighbors are called Link-State Update (LSU) packets. That is, the LSU packet holds data structures called link-state advertisements (LSA). The LSAs are not packets, but rather data structures that sit inside the LSDB and describe the topology.

Maintaining Neighbors and the LSDB

First, routers monitor each neighbor relationship using Hello messages and two related timers: the Hello Interval and the Dead Interval. Routers send Hellos every Hello Interval to each neighbor. Each router expects to receive a Hello from each neighbor based on the Hello Interval, so if a neighbor is silent for the length of the Dead Interval (by default, four times as long as the Hello Interval), the loss of Hellos means that the neighbor has failed.

Next, routers must react when the topology changes as well,

Each router’s LSDB now reflects the fact that the original router’s G0/0 interface failed, so each router will then use SPF to recalculate any routes affected by the failed interface.

A third maintenance task done by neighbors is to reflood each LSA occasionally, even when the network is completely stable. By default, each router that creates an LSA also has the responsibility to reflood the LSA every 30 minutes (the default), even if no changes occur. (Note that each LSA has a separate timer, based on when the LSA was created, so there is no single big event where the network is overloaded with flooding LSAs.)

Using Designated Routers on Ethernet Links

On Ethernet links, OSPF defaults to use a network type of broadcast, which causes OSPF to elect one of the routers on the same subnet to act as the designated router (DR).

These five OSPF routers elect one router to act as the DR and one router to be a backup DR (BDR).

The database exchange process on an Ethernet link does not happen between every pair of routers on the same VLAN/subnet. Instead, it happens between the DR and each of the other routers, with the DR making sure that all the other routers get a copy of each LSA.

 The BDR watches the status of the DR and takes over for the DR if it fails. (When the DR fails, the BDR takes over, and then a new BDR is elected.)

The DR can send a packet to all OSPF routers in the subnet by using multicast IP address 224.0.0.5

The DR can send one set of messages to all the OSPF routers rather than sending one message to each router.

any OSPF router needing to send a message to the DR and also to the BDR (so it remains ready to take over for the DR) can send those messages to the “All SPF DRs” multicast address 224.0.0.6.

routers that are neither a DR nor a BDR—called DROthers by OSPF—never reach a full state because they do not exchange LSDBs directly with each other.

full state (called fully adjacent neighbors

2-way state (called neighbors)

all OSPF routers on the same link that reach the 2-way state—that is, they send Hello messages and the parameters match—are called neighbors. The subset of neighbors for which the neighbor relationship continues on and reaches the full state are called adjacent neighbors.

OSPF Areas and LSAs

A larger topology database requires more memory on each router.

The SPF algorithm requires processing power that grows exponentially compared to the size of the topology database.

A single interface status change anywhere in the internetwork (up to down, or down to up) forces every router to run SPF again!

OSPF Areas

Put all interfaces connected to the same subnet inside the same area.

An area should be contiguous.

Some routers may be internal to an area, with all interfaces assigned to that single area.

Some routers may be Area Border Routers (ABR) because some interfaces connect to the backbone area, and some connect to nonbackbone areas.

All nonbackbone areas must have a path to reach the backbone area (area 0) by having at least one ABR connected to both the backbone area and the nonbackbone area.

How Areas Reduce SPF Calculation Time

Routers require fewer CPU cycles to process the smaller per-area LSDB with the SPF algorithm, reducing CPU overhead and improving convergence time.

The smaller per-area LSDB requires less memory.

Changes in the network (for example, links failing and recovering) require SPF calculations only on routers in the area where the link changed state, reducing the number of routers that must rerun SPF.

Less information must be advertised between areas, reducing the bandwidth required to send LSAs.

(OSPFv2) Link-State Advertisements

One router LSA for each router in the area

One network LSA for each network that has a DR plus one neighbor of the DR

One summary LSA for each subnet ID that exists in a different area Router LSAs Build Most of the Intra-Area Topology

Router LSAs, also known as Type 1 LSAs, describe the router in detail. Each lists a router’s RID, its interfaces, its IPv4 addresses and masks, its interface state, and notes about what neighbors the router knows about via each of its interfaces.