OSPF Router Startup and Link State Update - …

1 Router Startup and Link State Update OSPF (Open Shortest Path First) is a routing protocol that is used as an interior gateway protocol in large enterprises. This flow shows the message exchange that takes place when a new OSPF Router comes online. You can click on any message in the flow tosee full message contents. Covered sequence: (1)OSPF hello message exchange (2) Master-slave resolution (3) Router database synchronization (4) Link State Update This sequence diagram was generated from a PCAP file with VisualEther ( ) and then modified withEventStudio (( ) to add further design details. OSPF periodically multicast Hello messages on the network. Routers establish neighborrelationships by listening for OSPF Hello messages from other routers. starts listening for multicasts Start listening for multicasts starts This is the first OSPF neighbor State .)

2 It meansthat no hello packets have been received fromthis neighbor, but hello packets can still be sentto the neighbor in this HELLO PACKETS ource OSPF Router : ,Network Mask : ,Hello Interval [sec] : 10,Designated Router : ,Backup Designated Router : Router is the only Router on thenetwork. It assumes that is it the designatedrouter. The Router multicasts an OSPF Hellomessage on the multicast the hello/dead intervals,netmask, area id and authenticationpassword The received hello is processed only if allparameters This State specifies that the Router has receiveda hello packet from its neighbor, but thereceiving Router 's ID was not included in thehello HELLO PACKETS ource OSPF Router : ,Network Mask : ,Hello Interval [sec] : 10,Designated Router : ,Backup Designated Router : The Router starts up and sends out an OSPFH ello.

3 The Router is yet to learn about thedesignated as a neighbor OSPF HELLO PACKETS ource OSPF Router : ,Network Mask : ,Hello Interval [sec] : 10,Designated Router : ,Backup Designated Router : ,Active Neighbor : The DR sends out a periodic multicast, as a Hello Message exchange a neighbor setup as a neighbor 2-Way This State is entered when bi-directionalcommunication has been established betweentwo routers. Bi-directional means that eachrouter has seen the other's hello packet ( thehello packet received from another Router liststhis Router 's ID as an active HELLO PACKETS ource OSPF Router : ,Network Mask : ,Hello Interval [sec] : 10,Designated Router : ,Backup Designated Router : ,Active Neighbor : also reports that a the link establishment, the neighboring routers exchange their link status database.)

4 Exstart At this point the Designated Router (DR) hasbeen finalized and the routing informationexchange can routers first establish a master-slave relationship. This is followed by the exchange ofdatabase information. Note that only link State headers will be exchanged during thisphase. OSPF DB DESCRIPTIONS ource OSPF Router : ,DB Description : 0x07 (I, M, MS),DD Sequence : 1098361214 Router asserts that is the master by setting theMS option bit. The message also sets the I bitsignifying that this is the initial message of theDatabase synchronization handshake. The morebit (M) signals that more data is going DB DESCRIPTIONS ource OSPF Router : ,DB Description : 0x07 (I, M, MS),DD Sequence : 1098361214 Router asserts that is the master by setting theMS option bit.

5 The message also sets the I bitsignifying that this is the initial message of theDatabase synchronization handshake. The morebit (M) signals that more data is going resolution: 'I amslave' is less than so thisrouter assumes that it is a DB DESCRIPTIONS ource OSPF Router : ,DB Description : 0x02 (M),DD Sequence : 1098361214,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router .

6 Router removes the master assertion byresetting the MS bit. The Router also startssending its database. The more bit(M)signalsthat their are more database Update need to resolution for DB synchronization Database synchronization resolution: 'I ammaster' is greater than ,so the Router asserts that it is the DB DESCRIPTIONS ource OSPF Router : ,DB Description : 0x03 (M, MS),DD Sequence : 1098361215,LS Type : Router -LSA,Link State ID : ,Advertising Router : The Router asserts that it is the master bysetting the MS bit. The master also incrementsthe DD Master-slave is now settled. The routers initiatethe exchange of database DB DESCRIPTIONS ource OSPF Router : ,DB Description : 0x00,DD Sequence : 1098361215 The slave acknowledges the DD DB DESCRIPTIONS ource OSPF Router : ,DB Description : 0x01 (MS),DD Sequence : 1098361216 The master again updates the DD DB DESCRIPTIONS ource OSPF Router : ,DB Description : 0x00,DD Sequence : 1098361216 The slave acknowledges the changed database Update has synchronized the link State headers between the routers.

7 Routersnow scan the synchronized link State headers to determine if they are missing any link stateupdates. This is followed by requesting the link State updates to completely sychronize thelink states. Loading OSPF LINK State REQUESTS ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : After exchanging Database Description packetswith a neighboring Router , " " Router finds that parts of its topologicaldatabase are out of date. The Link StateRequest packet is used to request the pieces ofthe neighbor's database that are more up LINK State REQUESTS ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : " " also requests Update for its outof date link State Update LINK State UPDATES ource OSPF Router : ,LS Type.

8 Router -LSA,Link State ID : ,Advertising Router : ,Link ID : - IP network /subnet number,Link Type : 3 - Connection to a stub network,0 Metric : 10 The routers then send out their link stateupdates as a multicast flood. The Link StateUpdate packets contain a collection of link stateadvertisements that are one hop away from LINK State UPDATES ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,Link ID : - IP network /subnet number,Link Type : 3 - Connection to a stub network,0 Metric : 10,Link ID : - IP network /subnet number,Link Type : 3 - Connection to a stub network,0 Metric : 10,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID.

9 ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ( ),Advertising Router : OSPF LINK State UPDATES ource OSPF Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : OSPF LINK State UPDATES ource OSPF Router : ,LS Type : Network-LSA,Link State ID : ,Advertising Router : OSPF LINK State UPDATES ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,Sequence Number : 0x80000dc4,Link ID : - IP address of Designated Router ,Link Type : 2 - Connection to a transit network,0 Metric : 10 LINK State ACKNOWLEDGES ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type.

10 AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : ,LS Type : AS-External-LSA (ASBR),Link State ID : ,Advertising Router : All routers acknowledge the link State acknowledgements contain link stateheaders to identify the link State updates thatare being LINK State UPDATES ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,Sequence Number : 0x80000002,Link ID : - IP network /subnet number,Link Type : 3 - Connection to a stub network,0 Metric : 10,Link ID : - IP network /subnet number,Link Type : 3 - Connection to a stub network,0 Metric : 10 OSPF LINK State ACKNOWLEDGES ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : OSPF LINK State UPDATES ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,Link ID : - IP network /subnet number,Link Type : 3 - Connection to a stub network,0 Metric : 10,Link ID : - IP network /subnet number,Link Type : 3 - Connection to a stub network,0 Metric : 10 OSPF LINK State ACKNOWLEDGES ource OSPF Router : ,LS Type : Router -LSA,Link State ID : ,Advertising Router : ,Sequence Number : 0x80000001 LINK State UPDATES ource OSPF Router : ,LS Type.