Chapter 11.OSPF

CCNA Exploration 2.

Routing protocols and Consepts.

RESUME. Chapter 11. OSPF

OSPF is a classless routing protocol that uses the concept of areas for scalability. 

The data portion of an OSPF message is encapsulated in a packet. 

Each packet serves a specific purpose in the OSPF routing process:

1. Hello - Hello packets are used to establish and maintain adjacency with other OSPF routers. The hello protocol is discussed in detail in the next topic; are used to:

           ~Discover OSPF neighbors and establish neighbor adjacencies.

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

           ~Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks       like Ethernet and Frame Relay.

2. DBD - The Database Description (DBD) packet contains an abbreviated list of the sending router's link-state database and is used by receiving routers to check against the local link-state database. 

3. LSR - Receiving routers can then request more information about any entry in the DBD by sending a Link-State Request (LSR). 

4. LSU - Link-State Update (LSU) packets are used to reply to LSRs as well as to announce new information. LSUs contain seven different types of Link-State Advertisements (LSAs). LSUs and LSAs are briefly discussed in a later topic.

5. LSAck - When an LSU is received, the router sends a Link-State Acknowledgement (LSAck) to confirm receipt of the LSU.

 

Before two routers can form an OSPF neighbor adjacency, they must agree on three values: Hello interval, Dead interval, and network type. The OSPF Hello interval indicates how often an OSPF router transmits its Hello packets. By default, OSPF Hello packets are sent every 10 seconds on multiaccess and point-to-point segments and every 30 seconds on non-broadcast multiaccess (NBMA) segments.

Each OSPF router maintains a link-state database containing the LSAs received from all other routers. Once a router has received all of LSAs and built its local link-state database, OSPF uses Dijkstra's shortest path first (SPF) algorithm to create an SPF tree. 

          OSPF can be configured for authentication. 

Note: Authentication does not encrypt the router's routing table. 

The cost of an OSPF route is the accumulated value from one router to the destination network. 

A multiaccess network is a network with more than two devices on the same shared media.

Multiaccess networks can create two challenges for OSPF regarding the flooding of LSAs:

         1. Creation of multiple adjacencies, one adjacency for every pair of routers.

         2. Extensive flooding of LSAs (Link-State Advertisements).

OSPF defines five network types:

          ~ Point-to-point 

          ~  Broadcast Multiaccess 

          ~  Nonbroadcast Multiaccess (NBMA) 

          ~  Point-to-multipoint 

          ~  Virtual links

The solution to managing the number of adjacencies and the flooding of LSAs on a multiaccess network is the Designated Router (DR). 

A Backup Designated Router (BDR) is also elected in case the Designated Router fails. 

All other routers become DROthers (this indicates a router that is neither the DR or the BDR).

Criteria of election DR/BDR:

     1. DR: Router with the highest OSPF interface priority.

     2. BDR: Router with the second highest OSPF interface priority. 

     3. If OSPF interface priorities are equal, the highest router ID is used to break the tie.

When the DR is elected, it remains the DR until one of the following conditions occurs:

             ~The DR fails.

             ~The OSPF process on the DR fails.

             ~The multiaccess interface on the DR fails.

The reference bandwidth can be modified to accommodate these faster links by using the OSPF command auto-cost reference-bandwidth. 

Настройка OSPF:

Сами прописываем cost  на интерфейсе:

R1(config-if)#ip ospf cost < >

Назначаем приоритет на интерфейсе:

R1(config-if)#ip ospf  priority <0-255 >

Router ID назначаем сами:

R1(config-router)#router-id <ip-address>

Если RID не был назначен нами, то он выбирается авт-ки, в зависимости от настроек роутера, по таким правилам:

1. Настроен один loopback-интерфейс и несколько интерфейсов с различными адресами:

     IP address loopback 0=Router ID.

2. Настроены Lo1, Lo2…Lo9  с IpAdd1,ApAdd2… 

    Наибольший IpAdd Lo = Router ID.

3. Настроены неск-ко интерфейсов с Ip Add:

    Наибольший Ip Add из всех акт.интерфейсов= RID.

Включить OSPF на интерфейсах в соответствующих сетях:

R1(config)# router ospf <process-id>

R1(config-router)# network <network> <wildcard mask> area <area-id>

Команда network  

1) включает OSPF на интерфейсе, IP-адрес которого совпадает с указанной сетью и маской,

2)  анонсирует сеть этого интерфейса через другие интерфейсы, на которых включен OSPF.

Если в route table есть default static route, то можно его распространить :

R1(config-router)# default-information originate

Включение OSPF на интерфейсах:

R1(config-if)# ip ospf <process-id> area <area-id>

Изменение hello-интервала:

R1(config-if)# ip ospf hello-interval <sec>

Изменение dead-интервала:

R1(config-if)# ip ospf dead-interval <sec>

Настройка аутентификации type 1 для зоны 1 (пароль надо задавать на интерфейсах):

R1(config-router)# area 1 authentication

TROUBLESHOOTING

R1# show ip route ospf

R1# show ip ospf interface

R1# show ip ospf interface brief

R1# show ip ospf database