Understanding BGP Routing and Troubleshooting Reachability Issues

-

 Basic Overview :-

  • BGP is a Path vector protocol.
  • It uses TCP port no. 179.
  • BGP use path vector attributes such as AS path length, local preference, origin type, and other optional attributes to determine the best path to reach a specific network prefix.
  • The AS path attribute helps prevent routing loops by specifying the sequence of ASNs (Autonomous System Numbers) through which the route has traversed.
  • BGP is also known as “AS-by-AS routing protocol”.
  • In short, internet means network of network. When user try to access towards internet packet traverse through different AS. 

How it’s happened?? Let’s check.  

 Illustration 1: PGCIL user wants to access cricbuzz website to see live cricket match score. How packet traverse from source to destination to access internet Via AS By AS is shown as below

  •                               Fig:1 IPV4 route propagation AS by AS routing protocol

    How can user access internet ??

     

    First of all which users who wants to access internet, need to advertise particular user prefix IP block in all upstream of ISP end.

    Same way this IP block need to be accepted at customer end for return traffic.

    Once IP block acceptance done need to check in looking glass in trace route log end to end IP reach ability is there or not.

     

    Illustration- 2 Kindly check reach-ability of IP block :102.220.198.0/24 towards internet. During trace-route log we observe request time out after 4th hop. How to investigate this case??

     

    If IP block: 102.220.198.0/24 is advertised properly towards all peering ISP then it’s reflected in Looking glass how packet traverse end to end. Also, it gives assurance that IP block advertisement done properly.

     

                                 Fig:2 trace route log of user IP block 


    If request time out occurs after 4th hop in trace route need to login in router and check route-table of 182.79.149.102 to investigate at which particular section issue occurs to reach ability towards internet for this IP block users.

    Generally, BGP takes shortest-AS path to traverse packet from source to destination. Nowadays users prefix block advertisement done at multiple peering location towards upstream. Hence, if any OFC cut happens, traffic automatically switches over 2nd shortest AS-path route. 

     


                                   Fig:3 Shortest-AS path entry in BGP route table


    Note:   

    1.BGP table always reflects best possible routes entry.

    2. By default local preference value is 100.  Local preference is used for path-manipulation of BGP and it generally used for exit path of an Autonomous system.

    3. MED is used for return traffic.


     

     

     

     

Comments

Post a Comment

Popular posts from this blog

AI in NOC: A New Era for Capacity Planning and Network Management

-
Image
   Introduction :- Predicting router interface traffic in modern network management is critical to achieve optimal network performance, resource allocation, and proactively eliminate bottlenecks. This case study suggests using machine learning on network datasets to develop a model that predicts an actual router’s interface traffic versus an ideal router’s traffic. By evaluating historical data of network traffic and utilizing well-established machine learning API-based algorithms like Neural Networks, Decision Trees, and Ensemble Methods, this analysis reviews the formation possibilities of a strong prediction model with grand accuracy. At the end of the study, this paper reviews multiple factors and features related to the traffic in the interface such as the hour in the day, the day’s sequence in the week, the network protocol, the application, and the structure of the network. Additionally, it explores the machine learning algorithms to compare the accuracy of the ML model...
Read more

Basics of Multiprotocol Label Switching

-
Image
  Objective := Fast switching of packet. Load-balancing traffic. Segregation of traffic is possible customer wise in MPLS. MPLS as a 2.5 layer protocol :       MPLS works on VPN type technology.     It works on layer 2.5 OSI model.   0       MPLS header added and inserted between layer 2 (Data link layer) and layer 3 (Network layer) It   is also known as “shim” Protocol. Label : 20 bits label value is used and it takes any values from 0 to 1048575. Experimental : 3 bits used for experimental and it generally used for Qos Service. QoS Policy :- Stack :  I t’s 1 bit long . Either 0 or 1. TTL :  It’s size 8 bit long. It generally defined as how packet reach from source to destination. its value is decreased by one at each hop to prevent packet to get stuck in  network. RTT (Round trip time)  = RTL (Time require to reach packet from source to destination) + RTA  (Time require to reach packe...
Read more

Traffic engineering: An attractive feature of MPLS

-
Image
  MPLS  technology: - n   MPLS is working on “Push—Swap—POP” methodology. n   PUSH : iLER--- (ingress Label Edge router) – add Label at the edge of router. it is done by PE router. n   SWAP : Label swapping – Label is swapped between P routers. LSP (Label Switched Path) is used to forward packet from Source to destination.   Labels are Swapped between these P routers and built between border PE routers. n   POP : eLER--- (Egress Label Edge router) – it is used to remove label.   But sometimes label removal process is done before the edge of last PE router, last P router which is called as PHP (Penultimate HOP Popping). M PLS Label  Mechanism: - -           Basically, two labels are used in  MPLS.   1.       Transport label 2.       Service label   1.       Transport Label:-   ...
Read more