Our initiative is to provide large scale knowledge-base on TCP/IP, BGP, OSPF, IS-IS, MPLS, Traffic Engineering. This a one of the very few places where you can find a integrated learning materials with hand on lab. Also the virtual appliances you can download from here to use in different network simulators like GNS-3 and EVE-NG.
DHCP is a client/server protocol used to assign configuration(ip, gateway, dns, options etc.) to hosts dynamically. It is an extension of BOOTP protocol and backward compatibility is maintained. DHCP essentially uses BOOTP message format added with some options filed using UDP(Port 67 & 68). Clients/Hosts send request to DHCP servers for IP address and Server respond with a free IP address from it’s IP pool. A IP Pool is a contiguous range of IPs allocated for DHCP use. Server/Client perform the sanity check for the IP offered/allocated by sending a ARP request. Server also inform the client about how much time it can use the allocated IP, it’s called lease duration. Once the lease duration is expired, a client can extend the lease by requesting a renew of that address from the server. DHCP Message Formats In a general scenario DHCP mainly uses Discover, Offer, Request and Acknowledge messages, it’s […] Tags dhcpdhcp ackdhcp discoverdhcp leasedhcp offerdhcp reuestdhcp transaction iddhcpdeclinedhcpinformdhcpnakdhcpreleasedora
Introduction - Preemption vs Remember OLD DR. Remembering old DR is an optimization of DR/BDR election process. It's a Cisco proprietary and use to enable fast DR recovery in a multi-access network. Tags drospfremember old dr
BGP Neighbor States – IDLE – CONNECT – ACTIVE From this explanation we might understand that a BGP process remains in ACTIVE state is due to TRANSPORT ERROR means the BGP process is unable to initiate a TCP Connection to the Neighbor. Refer the below flow chart where it explains the FSM transitions of three states IDLE, CONNECT and ACTIVE. In real-time BGP never remain in an particular state other than ESTABLISHED. Actually it flaps between IDLE, CONNECT and ACTIVE. BGP Stuck in IDLE Input Events 1 & 13 causes the IDLE state remain in IDLE state. IE1 is BGP START event and IE13 is BGP Notification Received. Also IE 2, 4, 6, 8, 9, 13 causes other BGP states to transit back to IDLE. So the possible reason for a BGP router remain in IDLE is mentioned below. BGP Stuck in ACTIVE Input Event 5 cause the ACTIVE state […] Tags bgpbgp-fsmbgp-stuck-in-activebgp-stuck-in-idle
In this tutorial we will discuss about different message types of BGP, BGP Peer Connection, BGP Finite State Machine. Before establishing BGP Peer Connection, two neighbor must perform TCP Three-Way-Handshake and open a connection to Port number 179. BGP is a Transport Layer Protocol, hence TCP provides fragmentation, acknowledgement, sequencing functions necessary for reliable connection. BGP Peer must be statically configured and a route must present in the routing table. All the BGP Messages are unicast over TCP Connection. BGP Uses five basic message. BGP Messages All BGP Messages have a common header and encapsulated into a TCP Fragment. Header consists of Marker, Length and Type of the BGP Messages. A Keepalive message is nothing but a BGP Header. Open Message consists of version number of bgp, autonomous system number of the originating router, hold time, bgp identifier and optional parameters for capabilities negotiation. A Update Message consists of withdrawn […] Tags bgp-fsmbgp-input-eventsbgp-message-typesbgp-stuck-in-active
In this discussion we will dive into the BGP Path Selection Algorithm. There are 3 (Rejection, Selection, Tie-Break) rounds of BGP Path Selection which involve 18 different steps. The first round is Rejection where any route matching certain condition will be rejected. The second Round is Selection where BGP Best Path Compiler will choose a path matching certain conditions. The last round is Tie-Break where the BGP Best Path Compiler will make the final decision based on certain criteria. Rejection Round Any roues matching the following conditions will be rejected by default. The above behavior can be changed with explicit configuration. We will discuss each with an example in another tutorial. Selection Round In this steps one route will be preferred over another in case it fulfill the any of the below conditions which include certain Path Attributes and other constraints. You can further remember the steps with the following […] Tags bgp-pabgp-path-attributebgp-path-selectionpath-attribute
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