CCSG Chap 6 OSPF

OSPF

OSPF (Open Shortest Path First)

an open standard Link State, classless Routing protocol. It uses the Djikstra algorithm to initially construct a SPF tree and follows that by populating the routing table with the best paths. Supports IPv4/v6. Quick convergence (not as fast as EIGRP). Supports multiple, equal cost routes to the same destination.

Metric is based on BW. Updates are event triggered. It allows for the creation of areas and autonomous numbers. Must have a Area 0.

What are the benefits to OSPFs hierarchial design?

Decrease routing overhead

Speed up convergence

confine ntwk instability to a single area of the ntwk

ABR

Area Border Router -a router that connects other areas to the backbone area (area 0).

ASBR

Autonomous System Border Router - Router that connects Autonomous Systems.

OSPF Link

network or router interface assigned to any given network. When the interface is added to the OSPF process it is considered a link. This link will have an up/down status associated with it as well as one or more IPs.

RID (Router ID)

How OSPF IDs an OSPF router. Router IDs can be set with the router-id command. It can also be an IP addresses that OSPF uses to identify a router. Cisco uses the highest loopback IP as RID. If no loopback then the highest IP configed on a active physical interface on that router.

What’s an OSPF neighbor?

____ are 2 or more routers that have an interface on a common network. OSPF requires a number of common configuration options to establish a ____ relationship.

To form an OSPF neighbor relationship, two routers must be on the same network segment, have matching Hello and Dead interval timers, share the same Area ID, and have consistent network mask and authentication settings on the connecting interfaces; essentially, they need to be able to exchange Hello packets with compatible parameters to establish adjacency. 

What are the common configuration options needed to establish an OSPF relationship?

AREA ID

Stub area flag

Authentication Password (if using one)

Hello and Dead Intervals

What is an OSPF Adjacency?

a relationship between 2 OSPF routers that permits the exchange of route updates. Not all neighbors will become adjacent.

In multi access networks, routers form adjacencies with DR and BDR.

In point to point and point to multipoint routers form adjacencies with the router on the opposite side of the connection.

Designated Router

____is elected whenever a OSPF routers are connected to the same broadcast network to minimize the # of adjacencies and to publicize received routing info.

DRs are determined by elections.

Elections are won by Routers priority level with the highest # wining. If the priorities are tied then its based on the highest RID. All routers on the shared network will establish adjacencies with the DR and BDR, which ensures the topology table are synchronized.

Backup Designated Router

the hot standby for the DR on broadcast, multi-access links. The ____ receives all routing updates from OSPF adjacent routers but does not disperse LSA updates.

Hello protocol

This protocol is provides dynamic neighbor discovery and is used to discover neighbors, establish adjacencies and maintain relationships with other OSPF routers. These packets are addressed to multicast address 224.0.0.5. Hello packets and LSA build and maintain the topological database.

Hello packets are sent out every 10 seconds for point-to- point and broadcast connections, but non broadcast and point-to-multipoint networks send them every 30 seconds

Link State Advertisement

an OSPF data packet containing link state and routing info that’s shared among OSPF routers. there are different types of LSA. OSPF will only exchange LSA with routers it has established an adjacency with.

OSPF Areas

a grouping of contiguous routers and networks. All routers in the same area share the same area ID. The area ID is associated with the interface on a router so a router may be a member in more then one area.

All routers in the same area have the same topology table.

In OSPF there must be an area 0 also called the backbone area.

Broadcast Multi-access Network

____, like ethernet, allows multiple devices to connect to the same network, enabling broadcast ability. In OSPF, a DR and BDR must be elected for each broadcast multi-access network.

NBMA

NonBroadcast Multi-Access Network

_____ like Frame relay, X.25 and ATM allow for multi access without broadcast abilities. NBMA requires special OSPF configuations.

point to point

a type of network topology made up of a direct connection between 2 routers that provides a single commo path. The _____ connection can be physical or logical. Either way, _____ configurations eliminate the need for DRs and BDRs in OSPF.

point to multipoint

a network topology made up of a series of connections tween a single interface on a router and multiple destination routers. All interfaces on all routers share the ____ connections and belong to the same network. ____ can be further classified by if they support broadcasts or not which determines which OSPF configuration to deploy.

Link State flooding

method where OSPF uses to share routing info by sharing LSU (Link State Update)

How is the topology table created?

This table is created by link state flooding of LSU packets.

In OSPF what is the Multicast address for Point-to-point networks and what does it represent?

224.0.0.5 AllSPFRouters

In OSPF what is the Multicast address for broadcast networks and what does it represent?

224.0.0.6 All DR routers

Cisco bases link cost on BW. Other vendors may use other metrics to calculate a given links cost. When connecting links between routers from different vendors, you’ll probably have to adjust the cost to match another vendors router because both routers must assign the same cost to the link for OSPF to work well.

Cisco bases link cost on BW. Other vendors may use other metrics to calculate a given links cost. When connecting links between routers from different vendors, you’ll probably have to adjust the cost to match another vendors router because both routers must assign the same cost to the link for OSPF to work well.

a command used to change the cost on each interface and ranges from 1-65,535.

ip ospf cost

The 2 factors that are the basis of OSPF configuration is enabling OSPF and Configing OSPF areas. To enable OSPF form global mode.

router ospf <process-id 1-65535>

conf t

Router(config)#router ospf 1

The process ID is locally significant, The router can run multiple processes of OSPF but each process will have separate topology tables.

To config OSPF areas you will deal with wildcard masks

Router(conf-router) network 10.0.0.0 0.255.255.255 area 0

The 2 factors that are the basis of OSPF configuration is enabling OSPF and Configing OSPF areas. To enable OSPF form global mode.

router ospf <process-id 1-65535>

conf t

Router(config)#router ospf 1

The process ID is locally significant, The router can run multiple processes of OSPF but each process will have separate topology tables.

To config OSPF areas you will deal with wildcard masks

Router(conf-router) network 10.0.0.0 0.255.255.255 area 0

When dealing with wildcard masks remember it is 1 less then the subnet block size.

When dealing with wildcard masks remember it is 1 less then the subnet block size.

LA(config)# router ospf 100

LA(config-router)#passive-interface fastethernet 0/1

What is this output displaying?

For router LA this instance (or process100) of OSPF has been activated for the router but has been disabled on interface fa0/1.

it is recommended to issue show ip interface brief before configing and verifying dynamic routing protocols

it is recommended to issue show ip interface brief before configing and verifying dynamic routing protocols

Corp#ip route 0.0.0.0 0.0.0.0 fa0/0

Corp(config)#router ospf 1

Corp(config-router)default-information originate

What is going on here?

A default route has been set up stating if you don’t know where to go go to interface fa0/0.

OSPF was then enabled on the router with the process-id of 1

Router Corp is advertising the default route to all router’s that are part of that AS.

In this example in the book the router advertising the default route has not been configed with a priority nor does it have the highest IP set on it. It is not the DR.

The output of show ip router in the neighbor routers will show that default route was learned by O.

Corp(config)# interface loopback 0

Corp(config-if)# ip address 172.31.1.1. 255.255.255.255

**make sure the loopback address is unique in your AS.

The /32 is often used for loopback interfaces it does not waste a subnet.

The loopback wont be considered for OSPF elections if elections were already performed. The quickest way to resolve this without disabling OSPF is to reboot the router.

config a loopback address with the ip 172.31.1.

set the router ID of 223.225.255.254 for ospf 1

Corp#conf t

Corp(config)#router ospf 1

Corp(config-router)#router-id 223.255.255.254

Corp(config)#do clear ip ospf process

What is the hierarchy for determining DR/BDR.

1. Highest Active interface by default.

2. Highest logical interface overrides a physical interface.

3. the router-id overrides the interface and loopback interface.

What are the 4 OSPF verification commands?

show ip ospf

show ip ospf database

show ip ospf interface

show ip ospf neighbors

show ip OSPF

An OSPF verification command used to display information for one or all of the OSPF processes running on the router. Info displayed would be Router ID, Area info, SPF statistics and LSA timer information.

Show IP OSPF Database

An OSPF verification command that gives info about the number of routers on the internetwork (AS) plus the neighboring routers RID.

Show IP OSPF Interface

An OSPF verification command that reveals all interface-related OSPF information. Displayed info includes Configed IP, Area info,Process ID, RID, Network Type and the cost, priority. DR and BDR info and Hello/ Dead timers and Adjacent neighbor info

Show IP OSPF Neighbor

An OSPF verification command that summarizes OSPF info regarding neighbors and their adjacencies. It also displays if the DR and BDR have been elected.

What to look for if multi vendor router are unable to establish OSPF adjacencies.

Verify that the hello and dead timers are matching. Also check that the COSTs match. Different vendors can treat cost differently.

By default OSPF elections don’t happen on Point to Point links and will show FULL on SHOW IP OSPF Neighbor

By default OSPF elections don’t happen on Point to Point links and will show FULL on SHOW IP OSPF Neighbor

show ip protocols

shows an overview of the actual operation of all currently running protocols.

What are the default Hello and Dead timers in OSPF and what command do you issue to see these settings?

Hello is 10 seconds.

Dead Timer is 40 seconds

in OSPF and can be seen with show ip ospf interface interfacenumber

Updates addressed to 224.0.0.6 are destined to which type of OSPF router?

A DR

B ASBR

C ABR

D All OSPF Routers

A. 224.0.0.6 is used on broadcast networks to reach the DR and BDR.

When OSPF learns multiple paths to a network, how does it select a route?

It divides a reference bandwidth of 100 Mbps by the actual bandwidth of the existing interface to calculate the router with the lowest cost.