Test

= Preparation Points =


 * OSPF CBT Nuggets Videos + LAB
 * BGP CBT Nuggets Videos + LAB
 * OSPF, BGP Concept Recap & R&S Quick Notes
 * Interview Questions Sarabjit
 * Interview Questions Round 1
 * Packet Capture - OSPF, BGP Study
 * TCP/IP
 * Linux top 100 Commands
 * Python Interview Questions
 * Scripts - Case Alert, VS Top 10 list Study
 * VoIP Repro, WanEm Lab Study, Juniper NSM Repro Study

= ARP vs MAC Table =

= Fragmentation =


 * Before fragmentation:


 * After fragmentation:

= Headers =

Hardware type Protocol type Hardware address length Protocol address length Operation Source MAC Source IP Dest MAC Dest IP
 * ARP Header

Code Checksum Rest of Header
 * ICMP Header

= TCP =

MSS WSF SACK Permitted
 * Parameters determined during Handshake:


 * MTU vs MSS



- Sender starts with cwnd = 1 MSS, Size increases 1 MSS each time one Ack arrives, Increases the rate exponentially(1,2,4,8....) until a threshold is reached
 * Congestion Control
 * Slow Start - Exponential Increase


 * Congestion Avoidance - Additive Increase

- Increases the cwnd Additively, When a “window” is Ack cwnd is increased by 1, Window = No of segments transmitted during RTT - The increase is based on RTT, not on the number of arrived ACKs, Congestion window increases additively until congestion is detected


 * Congestion Detection - Multiplicative Decrease

- If congestion occurs, Window size must be decreased, Sender knows about congestion via RTO or 3 Dup Acks received, Size of Threshold is dropped to half

- If RTO occured, TCP Reacts Strongly - Reduces cwnd back to 1 Segment, starts the slow start phase again
 * Tahoe

- If 3 Duplicate ACKs are received, TCP has a Weaker Reaction - Starts the Congestion Avoidance phase - This is called fast transmission and fast recovery
 * Reno


 * Silly Window Syndrome: Sender creates data slowly or Receiver consumes slowly or both.

Syndrome due to Sender: - Nagle’s Algorithm: Send data initially, accumulate data in output buffer, Wait for Ack or till 1 MSS Data in Buffer

Syndrome due to Receiver: - Clark’s Solution: Announce window size 0 till 1) enough space for 1 MSS in Buffer or Half Receive buffer is empty - Delayed Acknowledgment: Segment not acknowledged immediately, Sender TCP does not slide its window, reduces traffic, sender may unnecessarily retransmit, Not delay more than 500 ms.

- If RTO has a larger value - If sender receives four acknowledgments with same value (three duplicates) - Segment expected by all of these Ack is resent immediately
 * Fast Retransmission


 * Persistence Timer

- Issue of Deadlock created by Lost Ack, used to reset Window size 0 advertized earlier, is resolved by this timer - Sending TCP sends a special segment(1 byte of new data) called Probe, causes the receiving TCP to resend Ack - If no reply, another probe is sent and value of persistence timer is doubled and reset - Sender continues sending probes, doubling, resetting value of persistence timer until it reaches a threshold(generally 60s) - After that the sender sends one probe segment every 60s until the window is reopened

= OSPF = Down Attempt Init 2-Way ExStart (Master/Slave; highest Router ID is master) Exchange (DBD exchanged) Loading (LSR, LSU, LSAck sent) Full
 * States

Type 1 - Router LSAs Type 2 - Network LSAs Type 3 - Network Summary LSA Type 4 - ASBR summary LSA Type 5 - AS external LSA Type 7 - NSSA External LSA Type 1 - Hello Type 2 - Database Description (DBD) Type 3 - Link-State request (LSR) Type 4 - LSU Type 5 - LSAck Same area Aame authentication config Same subnet Same hello/dead interval Matching stub flags
 * LSA Type
 * Packet Types
 * Neighbor Requirements:



= BGP =


 * Route Selection Criteria

Idle Active        Attempting to connect Connect       TCP session established OpenSent      Open message sent OpenConfirm   Response received Established   Adjacency established
 * BGP States

Open Update Keepalive      Sent every 60 seconds Notification   Always indicate something is wrong
 * BGP Messages