Network Topology

Device Configuration

[Router-A] ospf 1 router-id 10.0.0.1
[Router-A-ospf-1] area 0
[Router-A-ospf-1-area-0.0.0.0] network 10.0.0.1 0.0.0.0
[Router-A-ospf-1-area-0.0.0.0] network 192.168.10.1 0.0.0.0
[Router-A] interface LoopBack 0
[Router-A-LoopBack0] ip address 10.0.0.1 255.255.255.255
[Router-A-LoopBack0] interface GigabitEthernet 0/0/0
[Router-A-GigabitEthernet0/0/0] ip address 192.168.10.1 255.255.255.0

[Router-B] ospf 1 router-id 10.0.0.2
[Router-B-ospf-1] area 0
[Router-B-ospf-1-area-0.0.0.0] network 10.0.0.2 0.0.0.0
[Router-B-ospf-1-area-0.0.0.0] network 192.168.10.2 0.0.0.0
[Router-B-ospf-1-area-0.0.0.0] network 192.168.20.1 0.0.0.0
[Router-B] interface LoopBack 0
[Router-B-LoopBack0] ip address 10.0.0.2 255.255.255.255
[Router-B-LoopBack0] interface GigabitEthernet 0/0/0
[Router-B-GigabitEthernet0/0/0] ip address 192.168.10.2 255.255.255.0
[Router-B-GigabitEthernet0/0/0] interface GigabitEthernet 0/0/1
[Router-B-GigabitEthernet0/0/1] ip address 192.168.20.1 255.255.255.0

[Router-C] ospf 1 router-id 10.0.0.3
[Router-C-ospf-1] area 0
[Router-C-ospf-1-area-0.0.0.0] network 10.0.0.3 0.0.0.0
[Router-C-ospf-1-area-0.0.0.0] network 192.168.20.2 0.0.0.0
[Router-C] interface LoopBack 0
[Router-C-LoopBack0] ip address 10.0.0.3 255.255.255.255
[Router-C] interface GigabitEthernet 0/0/2
[Router-C-GigabitEthernet0/0/2] ip address 192.168.20.2 255.255.255.0

Adjacency Formation Analysis (Router-A & Router-B)

[Router-A] display ospf peer

 OSPF Process 1 with Router ID 10.0.0.1
	 Neighbors 

 Area 0.0.0.0 interface 192.168.10.1(GigabitEthernet0/0/0)'s neighbors
 Router ID: 10.0.0.2          Address: 192.168.10.2
   State: Full  Mode:Nbr is Master  Priority: 1
   DR: 192.168.10.1  BDR: 192.168.10.2  MTU: 0
   Dead timer due in 38 sec 
   Retrans timer interval: 5 
   Neighbor is up for 27:09:11     
   Authentication Sequence: [ 0 ] 

OSPF Neighbor States:

• Down: Initial state. No Hello packets received. In NBMA networks, Hellos can still be sent at the Poll Interval.
• Attempt: Exists only in NBMA networks. Indicates active attempts to form an adjacency.
• Init: A Hello packet has been received from the neighbor.
• 2-Way: The received Hello packet contains the local Router ID. Communication is bidirectional.
• ExStart: First step of adjacency. Masterslave relationship and DD sequence numbers are negotiated.
• Exchange: LSDB synchronization begins. DD, LSR, and LSU packets are exchanged.
• Loading: LSDB synchronization is in progress. LSR and LSU packets are exchanged.
• Full: LSDBs are fully synchronized. Full adjacency is established.

[Router-A] display ospf lsdb

 OSPF Process 1 with Router ID 10.0.0.1
	 Link State Database 

	         Area: 0.0.0.0
 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric
 Router    10.0.0.2        10.0.0.2           336  60    8000003F       1
 Router    10.0.0.1        10.0.0.1           565  48    8000003C       0
 Router    10.0.0.3        10.0.0.3           344  48    8000003A       1
 Network   192.168.20.1    10.0.0.2           336  32    80000038       0
 Network   192.168.10.1    10.0.0.1           565  32    80000038       0

[Router-A] display ospf routing

 OSPF Process 1 with Router ID 10.0.0.1
	  Routing Tables 

 Routing for Network 
 Destination        Cost  Type       NextHop         AdvRouter       Area
 10.0.0.1/32        0     Stub       10.0.0.1        10.0.0.1        0.0.0.0
 192.168.10.0/24    1     Transit    192.168.10.1    10.0.0.1        0.0.0.0
 10.0.0.2/32        1     Stub       192.168.10.2    10.0.0.2        0.0.0.0
 10.0.0.3/32        2     Stub       192.168.10.2    10.0.0.3        0.0.0.0
 192.168.20.0/24    2     Transit    192.168.10.2    10.0.0.2        0.0.0.0

 Total Nets: 5  
 Intra Area: 5  Inter Area: 0  ASE: 0  NSSA: 0 

Packet Analysis

• Hello Packet: Used for neighbor discovery and keepalive.
• DD (Database Description) Packet: Contains summaries of LSAs in the LSDB.
• LSR (Link State Request) Packet: Requests specific LSAs from a neighbor.

Adjacency Handshake Sequence

Capturing traffic on Router-A's GE0/0/0 interface reveals the following exchange:

1. Discovery: Router-A sends a Hello packet. The neighbor table is currently empty.
2. Init State: Router-A receives a Hello from Router-B. Router-B is added to the neighbor table with state Init. Router-B simultaneously adds Router-A to its table as Init.
3. 2-Way Transition: Router-A sends a Hello packet listing Router-B (10.0.0.2) in its neighbor list. Upon receipt, Router-B transitions Router-A's state to 2-Way.
4. Mutual 2-Way: Router-B sends a Hello packet listing Router-A (10.0.0.1). Router-A transitions Router-B's state to 2-Way.
5. ExStart Negotiation: Both routers transition to ExStart. Router-A sends an empty DD packet (no LSA headers) with sequence number 63959 to negotiate master/slave.
6. Slave Response: Router-B sends an empty DD packet with sequence number 63962.
7. Master Election: Since 10.0.0.2 > 10.0.0.1, Router-B becomes the Master. Router-A transitions Router-B to Exchange and sends a DD packet containing its LSA headers, using the Master's sequence number (63962).
8. Exchange: Router-B acknowledges by moving Router-A to Exchange and sends its own DD packet with LSA headers, incrementing the sequence number to 63963.
9. Acknowledgment: Router-A sends a DD packet acknowledging the sequence number 63963.
10. Loading: After DD exchanges, both transition to Loading. Router-A identifies missing LSAs and sends LSR packets to Router-B.
11. Udpate: Router-B replies with an LSU packet containing the requested LSAs.
12. Acknowledgment: Router-A sends an LSAck packet. Similar LSR/LSU/LSAck exchanges occur until databases match, transitioning both nieghbors to Full.