Anjan Chandra

QoS Policing & Shaping

Shaping and Policing is a Traffic Condition Technique comes under QoS PHB. Both is used to rate limit traffic on a interface or sub-interface to adhere to a traffic contract or to restrict further congestion of traffic down to the remaining network devices. The way shaping and policing rate-limit traffic is different in a way that shaping buffer the the traffic exceeding a certain bit-rate unlike policing drop the exceeding bit-rate. Shaping A network device buffer packet in it’s output queue to slow down the rate of transmission and decrease the bit-rate. It does so by sending some packets then hold on for some moments and then send some packets again, so on and so fourth. Device uses it’s processor clock rate to achieve the goal. The purpose of shaping is to provide a CIR(committed information rate) without dropping packets. For example a serial link has a 128Kbps of physical […]

 burst phb policing qos shaping token-bucket 

QoS Classification & Marking

Classification & Marking are some technique to identify traffic types and segregate them by defining specific values in some additional fields of the IP/Frame Header. DSCP, Precedence(IP Header) DSCP and Precedence is a part of IP Header TOS field. There is a long story behind the development of DSCP and IP Precedence. So we will discuss long story in short. The TOS field of IP Header is a 8bits word. When using IP Precedence, we use first 3bits and while using DSCP, we use 6bits. In DSCP first 3 bytes called class-selector(CS), which is there to make DSCP compatible with IP Precedence and also to support PCP and MPLS Exp. Then the additional 3 fields in DSCP is called drop-precedence or drop-probability. Whether it’s a DSCP class-selector or Precedence, higher value always takes priority.IP Precedence has 8 values ranging from 0 to 7. Each of the values has special significance […]

 acl class-map policy-map qos 

Quality of Service(QOS) Fundamentals

QOS is a technique to prioritize important traffic over less important traffic based on the class and characteristics of the traffic. For an example a VoIP traffic adheres to low latency and guaranteed delivery with a low amount of bandwidth unlike a FTP traffic adheres more BW. By default a network device treat all kind of traffic in a same manner and use best effort in terms of delivering the packets to the destination. A network device place the traffic in a FIFO(First in First Out) Queue of the Output Buffer to accomplish it. But when there is a Congestion(Queue is full), the packets are likely to be dropped. In this situation the QOS comes into picture to handle the congestion and condition the packet depending on their characteristics. Path Characteristics A packed is originated from a source and to be delivered to a destination, travels many hops and links. […]

 cbwfq cos llq policing qos shaping tos WRED 

DHCP Operation

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 […]

 dhcp dhcp ack dhcp discover dhcp lease dhcp offer dhcp reuest dhcp transaction id dhcpdecline dhcpinform dhcpnak dhcprelease dora 


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 […]

 bgp bgp-fsm bgp-stuck-in-active bgp-stuck-in-idle 

BGP Message Types & FSM

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 […]

 bgp-fsm bgp-input-events bgp-message-types bgp-stuck-in-active 

BGP Path Selection Algorithm

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 […]

 bgp-pa bgp-path-attribute bgp-path-selection path-attribute 

Selection of iBGP & EBGP Paths

In a situation where a route coming from both ebgp and ibgp peer, the best path election will takes place in two different way. If the rote is locally originated then it will prefer ibgp path, otherwise it will chose ebgp path.

 BGP Distance bgp-path-selection iBGP over EBGP prefer iBGP over EBGP 

Remember OLD DR

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.

 dr ospf remember old dr 

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