Single Area and Multiarea OSPF in CCNA

Objectives

•Review OSPF Single Area•Multiarea OSPF Implementation
•Types of LSAs Exchanged Between Areas
•Configuring Multiarea OSPFv2 and OSPFv3
•Verifying an OSPFv2 and OSPFv3 Configuration
•Review OSPF Key Points

OSPF Single Area - Review

•Link State Routing Protocol

•Faster Convergence

•Cost Metric (Cisco – Bandwidth)

•Identical Link-State Databases (LSDBs)

•SPF – Dijkstra’s Algorithm

•Determine Neighbors on Directly-connected links

•Use Link-State Packets (LSP) for each directly-connected link

•Flood LSPs to neighbors

As a review, OSPF (Open Shortest Path First) is a Link State Routing Protocol with an Administrative distance (AD) of 110.

(AD = trustworthiness or preference of the routing protocol.) OSPF is classless; therefore, it supports VLSM and CIDR.

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OSPF quickly propagates network changes.  Routing changes trigger OSPF routing updates so it is more efficient than distant vector routing protocols such as RIPv2. (RIPv2 uses periodic updates of every 30 seconds.)

In OSPF, The cost of a link is based on bandwidth only.  Higher bandwidths will have a lower cost.

OSPF creates and maintains three databases:

•Adjacency database - Creates the neighbor table

•Link-state database (LSDB) - Creates the topology table

•Forwarding database - Creates the routing table

These tables contain a list of neighboring routers to exchange routing information with and are kept and maintained in RAM.

Once the network is converged all routers in an area will have identical link-state databases!

OSPF uses the Shortest Path First algorithm to choose the best path. The CPU processes the neighbor and topology tables using Dijkstra’s SPF algorithm This algorithm is based on the cumulative cost to reach a destination. The SPF algorithm creates an SPF tree by placing each router at the root of the tree and calculating the shortest path to each node. The SPF tree is then used to calculate the best routes. OSPF places the best routes into the forwarding database, which is used to make the routing table.

OSPF uses link-state packets (LSPs) to establish and maintain neighbor adjacencies and exchange routing updates. LSPs represent the state of a router and its links to the rest of the network. 

Single Area OSPF 

OSPF Packet Types

OSPF exchanges messages to convey routing information using five types of packets. These packets are:

•Hello packet

•Database description packet

•Link-state request packet

•Link-state update packet

•Link-state acknowledgment packet

These packets are used to discover neighboring routers and also to exchange routing information to maintain accurate information about the network.

OSPF Packet Type

OSPF – Hello Packet

Let’s look at the Hello Packet

The OSPF Type 1 packet is the Hello packet. Hello packets are used to:

•Discover OSPF neighbors and establish neighbor adjacencies.

•Advertise parameters on which two routers must agree to become neighbors.

•Hello Packets are also used to Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks like Ethernet and Frame Relay. Just a reminder…Point-to-point links do not require DR or BDR.

The figure displays the fields contained in the Type 1 Hello packet. Important fields shown in the figure include:

•Type - Identifies the type of packet. A one (1) indicates a Hello packet. A value 2 identifies a DBDescription packet, 3 an LSRequest packet, 4 an LSUpdate packet, and 5 an LSAck packet.

•Router ID - A 32-bit value expressed in dotted decimal notation used to uniquely identify the originating router. (an IPv4 address)

•Area ID - Area from which the packet originated.

•Network Mask - Subnet mask associated with the sending interface.

•Hello Interval - Specifies the frequency, in seconds, at which a router sends Hello packets. The default Hello interval on multiaccess networks is 10 seconds. This timer must be the same on neighboring routers; otherwise, an adjacency is not established.

•Router Priority - Used in a DR/BDR election. The default priority for all OSPF routers is 1, but can be manually altered from 0 to 255. The higher the value, the more likely the router becomes the DR on the link.

•Dead Interval - Is the time in seconds that a router waits to hear from a neighbor before declaring the neighboring router out of service. By default, the router Dead Interval is four times the Hello interval. This timer must be the same on neighboring routers; otherwise, an adjacency is not established.

•Designated Router (DR) - Router ID of the DR.

•Backup Designated Router (BDR) - Router ID of the BDR.

•List of Neighbors - List that identifies the router IDs of all adjacent routers.

OSPF – Link State Updates (LSU)

•Type 4: Link-State Update (LSU) packet - Used to reply to LSRequests (type 3) and to announce new information. LSUs can be one of 11 different types of LSAs. LSUs are sometimes referred to as LSAs. Only the first 5 LSA types are covered on the CCNA.

Basic OSPF Configuration

R1(config)#int fa 0/0

R1(config-if)#ip address 172.16.1.17 255.255.255.240

R1(config)#int s 0/0/0

R1(config-if)#ip address 192.168.10.1 255.255.255.252

R1(config)#int s 0/0/1

R1(config-if)#ip address 192.168.10.5 255.255.255.252

R1(config-if)#router ospf 1

R1(config-router)#network 172.16.1.16 0.0.0.15 area 0

R1(config-router)#network 192.168.10.0 0.0.0.3 area 0

R1(config-router)#network 192.168.10.4 0.0.0.3 area 0

OSPF Diagram configuration

This is a basic single area OSPF configuration

Interfaces are configured and then networks are advertised.  The wildcard mask is used to identify which bits in the network address are significant.  The network 172.16.1.16/28 will be advertised as 172.16.1.16 0.0.0.15. This wildcard mask is the inverse of the subnet mask, 255.255.255.240.  Notice the subnet mask for /30 is 255.255.255.252 and the wildcard mask used to advertise this network is 0.0.0.3.  (255.255.255.255 – 255.255.255.252 = 0.0.0.3)

OSPF Router ID

1.Use the IP address configured with the OSPF router-id command.

2.If the router-id is not configured, the router chooses highest IP address of any of its loopback interfaces.

3.If no loopback interfaces are configured, the router chooses highest active IP address of any of its physical interfaces.

Every router requires a router ID to participate in an OSPF domain. The router ID can be defined by an administrator or automatically assigned by the router.

The router ID is used by the OSPF-enabled router to:

•Uniquely identify the router 

•Participate in the election of the DR and BDR

Let’s review how to determine the router ID? As illustrated in the figure, Cisco routers derive the router ID based on one of three criteria, in the following order:

1.The router ID is configured using the OSPF router-id rid router configuration mode command. The rid value is any 32-bit value expressed as an IPv4 address. This is the recommended method to assign a router ID. (Example: 1.1.1.1)

2.If the router ID is not configured, the router chooses the highest IPv4 address of any of configured loopback interfaces. This is the next best alternative to assigning a router ID.

3.If no loopback interfaces are configured, then the router chooses the highest active IPv4 address of any of its physical interfaces. This is the least recommended method because it makes it more difficult for administrators to distinguish between specific routers.

R1(config)#interface loopback 0

R1(config-if)#ip address 10.1.1.1 255.255.255.255

R1(config)#router ospf 1

R1(config-router)#router-id 10.1.1.1

Reload or use "clear ip ospf process" command, for this to take effect

OSPF Metric - Cost

•Cisco IOS uses the cumulative bandwidths of the outgoing interfaces from the router to the destination network as the cost value

•Cost for an interface is calculated as 10 to the 8th power divided by bandwidth in bps

•Results in interfaces with a bandwidth of 100 Mbps and higher having the same OSPF cost of 1

•Reference bandwidth can be modified to accommodate networks with links faster than 100 Mbps using the OSPF command auto-cost reference-bandwidth

•OR – Directly specify the cost for a link:

R1(config)#interface serial 0/0/0

R1(config-if)#ip ospf cost 1562

default Cisco OSPF Cost Value

OSPF uses cost as a metric. A lower cost indicates a better path than a higher cost. A 10-Mb/s Ethernet line has a higher cost than a 100-Mb/s Ethernet line.

The formula used to calculate the OSPF cost is:

•Cost = reference bandwidth /interface bandwidth

•The default reference bandwidth is 10^8 (100,000,000) as you can see in the graphic; therefore, the formula is:

•Cost = 100,000,000 bps / interface bandwidth in bps

•Refer to the table for a breakdown of the cost calculation. Notice that FastEthernet, Gigabit Ethernet, and 10 GigE interfaces share the same cost, because the OSPF cost value must be an integer. Consequently, because the default reference bandwidth is set to 100 Mb/s, all links that are faster than Fast Ethernet also have a cost of 1.

Reference bandwidth can be modified to accommodate networks with links faster than 100 Mbps using the OSPF command auto-cost reference-bandwidth

The “auto-cost reference-bandwidth” command must be configured on every router in the OSPF domain. The value is expressed in Mb/s; therefore, to adjust the costs for:

•Gigabit Ethernet - auto-cost reference-bandwidth 1,000

•10 Gigabit Ethernet - auto-cost reference-bandwidth 10,000

•To return to the default reference bandwidth, use the auto-cost reference-bandwidth 100 command.

As seen in the last bullet, you do have the choice to define the cost that will be used in OSPF calculations with the interface command, ip ospf cost.

OSPF and Multiaccess Networks

•Link-state routers flood their link-state packets when OSPF is initialized or when there is a change in the topology.

•In a multiaccess network this flooding can become excessive.

•On multiaccess networks, OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR) in case the Designated Router fails.

•All other routers become DROthers

•DROthers only form full adjacencies with the DR and BDR in the network, and send their LSAs to the DR and BDR using the multicast address 224.0.0.6 (IPv6 FF02::06)

•The solution to managing the number of adjacencies and the flooding of LSAs on a multiaccess network is the DR. On multiaccess networks such as ethernet or frame relay, OSPF elects a DR to be the collection and distribution point for LSAs sent and received. A BDR is also elected in case the DR fails. All other routers become DROTHERs. A DROTHER is a router that is neither the DR nor the BDR.

•DROthers only form full adjacencies with the DR and BDR in the network, and send their LSAs to the DR and BDR using the OSPF multicast address 224.0.0.6  (IPv6 FF02::06)

Why Multiarea OSPF?

Here we have an implementation of Multi-Area OSPF with 3 areas, area 1, area 0, and area 51.  The result is smaller routing tables and less LSAs.  SPF is only run within an area if there is a change in the network.

OSPF Two-Layer Area Hierarchy

Multiarea OSPF is implemented in a two-layer area hierarchy:

Backbone (Transit) area -

•Area whose primary function is the fast and efficient movement of IP packets.

•Interconnect with other OSPF area types

•Called OSPF area 0 which all other areas directly connect

Regular (Non-backbone) area -

•Connects users and resources

•A regular area does not allow traffic from another area to use its links to reach other areas

Multiarea OSPF is implemented in a two-layer area hierarchy:

•Backbone (Transit) area - Hierarchical networking defines the backbone area or area 0 as the core to which all other areas directly connect. Backbone areas interconnect with other OSPF area types. An OSPF backbone area’s primary function is the fast and efficient movement of IP packets. Generally, end users are not found within a backbone area.

•Regular (Non -backbone) area -Connects users and resources. Regular areas are usually set up along functional or geographical groupings. By default, a regular area does not allow traffic from another area to use its links to reach other areas. All traffic from other areas cross area 0.

Types of OSPF Routers

There are four different types of OSPF routers:

•Internal router – This is a router that has all of its interfaces in the same area. All internal routers in an area have identical LSDBs.

•Backbone router – This is a router in the backbone area. Generally, the backbone area is set to area 0.

•Area Border Router (ABR) – This is a router that has interfaces attached to multiple areas. It must maintain separate LSDBs for each area it is connected to, and can route between areas. ABRs are exit points for the area, which means that routing information destined for another area can get there only via the ABR of the local area. ABRs can be configured to summarize the routing information from the LSDBs of their attached areas. ABRs distribute the routing information into the backbone. The backbone routers then forward the information to the other ABRs. In a multiarea network, an area can have one or more ABRs.

•Autonomous System Boundary Router (ASBR) – This is a router that has at least one interface attached to an external internetwork (another autonomous system), such as a non-OSPF network. An ASBR can import non-OSPF network information to the OSPF network, and vice versa, using a process called route redistribution.

•Redistribution in multiarea OSPF occurs when an ASBR connects different routing domains (e.g., EIGRP and OSPF) and configures them to exchange and advertise routing information between those routing domains.

•A router can be classified as more than one router type. For example, if a router connects to area 0 and area 1, it falls under two different classifications: a backbone router, and an ABR.

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