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Showing posts from June, 2018

Advantage of using System ID extension in Switch Bridge ID

The format of the original 802.1d bridge ID was redefined from two byte priority + MAC address to System ID extension mainly due to the advent of multiple spanning trees as supported by Per VLAN Spanning Tree Plus (PVST+) and IEEE 802.1s Multiple Spanning Trees (MST). With the old-style bridge ID format, a switch’s bridge ID for each STP instance (possibly one per VLAN) was identical if the switch used a single MAC address when building the bridge ID. Having multiple STP instances with the same bridge ID was confusing, so vendors such as Cisco Systems used a different Ethernet BIA for each VLAN when creating the old-style bridge IDs. This provided a different bridge ID per VLAN, but it consumed a large number of reserved BIAs in each switch.  The System ID Extension allows a network to use multiple instances of STP, even one per VLAN,  but without the need to consume a separate BIA on each switch for each STP instance. The System ID Extension field allows the VLAN ID to be plac

How to configure Default Gateway on Nexus 1000v

In case you are finding it hard to reach default gateway from your newly installed Nexus 1000v virtual machine, here is one quick thing to check and configure before you can reach to the default gateway and other allowed subnets from your Nexus 1000v VM. Configure the management IP Address and default gateway on Nexus 1000v as per below commands conf t interface mgmt 0 ip address exit vrf context management ip route exit copy run start Note: Change the IP address as per your subnet.

Why STP Bridge Priority is Configured in increment of 4096

Spanning-tree operation requires that each switch have a unique BID (Bridge ID). In the original 802.1D standard, the BID was composed of the bridge priority and the MAC address of the switch, and all VLANs were represented by a CST, Common Spanning Tree. Because Cisco started to use unique instance in PVST+ PVRST+ for each VLAN STP Process, there came need to provide Unique BID for each separate instance of STP per VLAN. So what Cisco Did! divided the Bridge priority field of 16 bits into two parts, 4 bits for priority and 12 bits for VLAN ID and named it as Extended VLAN ID. Now because only left most four bits are reserved for Bridge priority, you can only make the combinations of discrete values in increments of 4096 with those bits.

%Error opening tftp:// (Timed out)

After I reset my Cisco Router 1841, It started to give error messages like below;  %Error opening tftp:// (Timed out) %Error opening tftp:// (Timed out) %Error opening tftp:// out) %Error opening tftp:// (Timed out) Actually these error messages are caused due to default configurations in Cisco IOS Software, which attempts to access the service configuration files from a network Trivial File Transfer Protocol (TFTP) server, and if the router is unable to acces the TFTP Server then it start displaying these messages. Yo can stop displaying these error message by entering following commands at Cisco CLI  Router#config terminal  Enter configuration commands, one per line.  Router(config)#no service  (this is command to stop these error messages) config Router(config)#end Router#write

How to Convert Linux Ubuntu Partition into Windows Partition

You need to format the partition on which you want to install Linux Ubuntu and the type of Linux Format is completely different than those of Windows Format types. That's why hard disk partition on which Ubuntu is installed is not view able from Windows Operating System, because windows does not support Linux Ubuntu Formats.  This goes good until you want to both operating systems with dual boot system. When you do not want to use Linux Ubuntu you will have to convert its partition to NTFS or FAT32 in order to access the partition and store and access data from this partition. One method to convert Ubuntu partition to Windows is to convert and format the partition with Free MINITOOL Partition Program. Remember this procedure will only convert to Windows Supported partitions and will ask you to format the partition until you can use it. So you will loose any data on your Ubuntu partition.  See  How to access Ubuntu files from Windows   without formatting the Ubuntu partitio

Command rejected: An interface whose trunk encapsulation is "Auto" can not be co nfigured to "trunk" mode.

This error is returned by Cisco IOS, when the trunking encapsulation is not set on one side of the switch and the other hand switch is configured properly. One of the case may be, when trunking between the port of a switch on one side and port of Switch-type interface on the router on other side. As you know that Dynamic Trunking Protocol, DTP, is not run by the Router so the Trunking can not be negotiated at this situation. To mitigate the error at this situation do the following steps; networkpcworld(config-if)#switchport mode trunk   Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.   Go to the desired Interface where you are getting this error networkpcworld (config)#int fa0/0/1 Enable trunk ecnapsulation manually  networkpcworld (config-if)#switchport trunk encapsulation dot1q Note:-  After setting encapsulation, most probably the switch interface would be converted to trunk by switchport mode trunk

Difference between Asynchronous and Synchronous Transmission

Asynchronous transmission uses start and stop bits to signify the beginning bit ASCII character would actually be transmitted using 10 bits e.g.: A "0100 0001" would become "1 0100 0001 0". The extra one (or zero depending on parity bit) at the start and end of the transmission tells the receiver first that a character is coming and secondly that the character has ended. This method of transmission is used when data is sent intermittently as opposed to in a solid stream. In the previous example the start and stop bits are in bold. The start and stop bits must be of opposite polarity. This allows the receiver to recognize when the second packet of information is being sent. Synchronous transmission uses no start and stop bits but instead synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signal(s) built into each component. A continual stream of data is then sent between the two nodes. Due to there being no st

Ubuntu Boot Error --- Missing modules (cat /proc/modules; ls /dev)

Boot from (hd0,0) ext3 5108701a-641d-43b182eb-aeb6da348d62 Starting up ... Loading, please wait... Gave up waiting for root device. Common problems: - Boot args (cat /proc/cmdline) - Check rootdelay= (did the system wait long enough ?) - Check root = (did the system wait for the right device ?) - Missing modules (cat /proc/modules; ls /dev) ALERT! /dev/disk/by-uuid/5108701a-641d-43b182eb-aeb6da348d62 does not exist . Dropping to a shell! Busybox v1.10.2 (Ubuntu 1:1.10.2-2ubuntu7) built-in shell (ash) Enter 'help' for a list of built-in commands. (initramfs) Just boot ubuntu by selecting another option of booting ubuntu from grub menu at startup, and upgrade ubuntu as per the steps given below Network Upgrade for Ubuntu Desktop You can easily upgrade over the network with the following procedure.  Start System/Administration/Update Manager Click the  Check  button to check for new updates.  If there are any updates to install, use the  Install Updates  button to in

What is Cisco Chassis, Backplane, Line Card

What does Chassis means? A Chassis is a frame/housing for mounting the circuit components for Cisco Switches and Routers or any type of devices that provides power and a high-speed backplane. The frame also protects all of the vital internal equipment from dust, moisture, and tampering.  Making the Chassis to define even easier is;  A chassis is an enclosure; a container that holds things together... i.e. an egg carton holds eggs together inside. A chassis does the same thing; it holds the important things inside like wiring, power supplies etc.  Cisco Chassis, Click to enlarge Cisco Backplane, Click to Enlarge What does Backplane means? Backplane is a circuit board with sockets that allows  Supervisor engines  Cards or modules to be inserted into these sockets and connect them to each other. Backplane is mounted on the Chassis.  Modules or line cards provide different types of interfaces, but the processing of packets is usually done in the Supervisor engine. Backplane is th

What is Cisco Supervisor Engine?

Supervisor Engine is a module that is installed in the Cisco Chassis-based Catalyst Switches or Routers. Supervisor engine contains nearly all the same components of a fixed Cisco Switches or Routers. These Supervisor engines come in a variety of different types with different functionalities and are installed in the Switches/Router Chassis as per requirements of the network types. Benefits of Supervisor Engines By installing Latest Supervisor Engines in your existing investments (Switches and Routers) you can scale system performance and integrate next-generation services into your Networks. Within a single multilayer switch chassis, two supervisor modules with integrated route processors can be used to provide hardware redundancy. If an entire supervisor module fails, the other module can pick up the pieces and continue operating the switch. The supervisor engine contains the following integrated daughter cards that perform forwarding and routing and provide the protocols suppo

Difference between In Band and Out of Band Protocols

In band In-band control is a characteristic of network protocols with which data control is regulated. In-band control passes control data on the same connection as main data.In Band Protocols Protocols that use in-band control include HTTP and SMTP. SMTP is in-band because the control messages, such as “HELO” and “MAIL FROM”, are sent in the same stream as the actual message content. Out of Band In computer networking, out-of-band data (called “urgent data” in TCP) looks — to the application — like a separate stream of data from the main data stream. This can be useful for separating two different kinds of data. Note that just because it is called “urgent data” does not mean that it will be delivered any faster or with higher priority than data in the in-band data stream. Also beware that unlike the main data stream, the out-of-band data may be lost if the application cannot keep up with it. “Urgent data” notifies the receiving connection that the separate stream is more important th

How to Connect Cisco Switches/Routers with Cisco Network Assistant

This post is about how to configure a Cisco standalone Device so that it may be connected with Cisco Network Assistant (CNA) Successfully. In production Networks, Cisco devices are often configured with basic required configuration for successful connection of CNA with the Cisco Devices and most of the time you do not need to configure them specially for CNA Connection. So for successful connectivity between a Cisco Device and a PC with CNA Installed is two part process.First of all we focus on how to configure a Cisco Switch then we will install CNA on a PC and Connect it with the switch. Configure a Switch with ip http server command in Global Configuration mode Switch(config)#ip http server Define a Vlan and SVI, assign an IP Address from a Private IP Address Range so that it can be connected with CNA. Here I am configuring VLAN 100, with SVI 100 and IP Address Subnet as Switch>en Switch#config t Enter configuration commands, one per line.  End with CNT

How to Configure Microsoft Windows Server 2008 R2 as an Internal NTP Server for Cisco Devices

Here we are Configuring a Windows Server 2008 R2 as an NTP Server, I am configuring my Domain Controller as an NTP Server in my Production Environment. So you must be easy while changing some registry values as stated below to make the Windows Server as an NTP Server. (but only do what is directed here). Go to you Domain Controller Server,Click  Start , In  Run , Type  Regedit  and press  Enter Navigate to the default Registry Value of NTP Server in Windows Server HKLM > System > CurrentControlSet > Services > W32Time > TimeProviders > NtpServer A default installation of Windows Server 2008 R2 can be seen in below picture Change the Registry Value of  Enabled  from 0 to  1 . Close the Registry Window Go to Command Line Window and Type this Command in it  w32tm /config /update  and you are done. Now go to your Cisco Device (switch/Router), and configure it to get time from this newly configured ntp server Switch(Config-t)#ntp server 10.

What is DHCP Protocol and How does it Work

DHCP, Dynamic Host Control Protocol, is a Protocol that operates at Application layer and Automatically assigns IP Addresses to requesting Hosts. DHCP eliminates the manual task by a network Administrator. It also provides a central database of devices that are connected to the network and eliminate duplicate resource assingments. DHCP uses UDP (User Datagram Protocol) to send its request messages to the DHCP Server on Port number 67. A DHCP Server can provide to a host alot of information when the host is requesting an IP address from a DHCP Server. Here’s a list of the information a DHCP Server can provide: IP Address Subnet Mask Domain Name Default Gateway (routers) DNS WINS information How DHCP Server is Discovered by Client to get IP address? The client broadcasts messages on the physical subnet to discover available DHCP servers. Network administrators can configure a local router to forward DHCP packets to a DHCP server from a different subnet. This client-i

How the Traceroute Works, Understanding Traceroute Output and Troubleshooting

How the TRACEROUTE Command Works The traceroute command is used to discover the routes that packets actually take when traveling to their destination. The device (for example, a router or a PC) sends out a sequence of User Datagram Protocol (UDP) datagrams to an invalid port address at the remote host. Three datagrams are sent, each with a Time-To-Live (TTL) field value set to one. The TTL value of 1 causes the datagram to “timeout” as soon as it hits the first router in the path; this router then responds with an ICMP Time Exceeded Message (TEM) indicating that the datagram has expired. Another three UDP messages are now sent, each with the TTL value set to 2, which causes the second router to return ICMP TEMs. This process continues until the packets actually reach the other destination. Since these datagrams are trying to access an invalid port at the destination host, ICMP Port Unreachable Messages are returned, indicating an unreachable port; this event signals the Tra

Multi-layer Switching Exceptions – Packets that Need further Processing

There are some packets that are not forwarded directly by Multi layer switches, but are required further processing. To forward packets using the simultaneous decision processes, the packet must be “Multi Layer Switch-ready” and must require no additional decisions. For example, CEF can directly forward most IP packets between hosts. This occurs when the source and destination addresses (both MAC and IP) are known already and no other IP parameters must be manipulated. Other packets cannot be directly forwarded by CEF and must be handled in more detail. This is done by a quick inspection during the forwarding decisions.  If a packet meets criteria such as the following, it is flagged for further processing and sent to the switch CPU for process switching: ARP requests and replies IP packets requiring a response from a router (TTL has expired, MTU is exceeded, fragmentation is needed, and so on) IP broadcasts that will be relayed as unicast (DHCP requests, IP helper-address funct

Some Hot Tips on OSPF Filtering and OSPF Area Types

Here is the list of some of the tricky concepts of Types of OSPF areas and OSPF Filtering; OSPF routers do not advertise routes, instead they advertise LSAs. Any filtering applied to OSPF messages would need to filter the transmission of LSAs. However, inside one area, all routers must know all LSAs, or the whole SPF concept fails, and routing loops could occur. As a result, OSPF cannot and does not allow the filtering of LSAs inside and area, specifically the type-1 and type-2 LSAs that describe the intra-area topology. OSPF is a link state protocol that populates the Link State Database, LSD, to give routers the same area and identical perspective of the OSPF routing domain that perspective is tempered by the type of area the routers are in. An ABR can also be an ASBR. When an external Route is defined as an E1, ABRs generate a type 4 (ASBR Summary) LSA into non-backbone, non-stub areas. The type 4 LSA reflects the cost from that area’s ABR to the ASBR (itself) that redistributed th

How to send emails to undisclosed recipients?

Do you want to send emails to multiple recipients and want to keep their identities confidential from each other? Have you ever received an email that was addressed to “undisclosed recipient”? This is a way to send to a group of people without exposing their identities to others. When you need to send to a group of people and want to keep their identity confidential, you can utilize the “BCC” function in your e-mail system. Bcc recipients get a copy of the email, but their email address is automatically deleted at delivery. Nobody except you and the Bcc: recipient will know that they got a copy, and their email address will not be exposed. Whether you use Yahoo, MSN, Gmail, or any other email service, do the followings: • in the “To:” field, put: Undisclosed Recipients <> then replace youruserlogin and with your own e-mail address • put all your email recipients in the “BCC” box, separate by comma. For example:,

runapp.shtml error while connecting SDM with cisco device

re you getting this error of runapp.shtml while connecting Cisco Secure Device Manager, SDM, with Cisco Devices? The error looks like the snap shot below Without going into the details of the error, we just find out how to avoid this error and connect SDM to the Router Successfully.  I hope you have configured your Router correctly. To avoid this runapp.shtml error do the following steps 1. Open Internet Explorer 2. Click the  Tools Menu  of IE and the Select  Internet Options 3. From Internet Options Window, Click the  Advanced Tab 4. Scroll down Setting Options up to Security Settings and Check the “ Allow Active Content to run in files on My Computer” . (As shown in Snap Below) 5. Click  OK  6. Restart SDM and Connect it with the IP Address of the Router 7. Click the Bar in Internet Explorer Saying Block scrip or Active X Control, and Click  Allow Blocked Content , As Shown Below.  Your are done, your Secure Device Manager is Running Normally

EIGRP PDM, Protocol Dependent Module

EIGRP supports different Network Layer Protocols, i.e IPv4, IPv6, IPX and AppleTalk (Though Last two are useless now). To support all of the protocols at a time EIGRP uses PDM for each of the protocols separately. Each PDM will maintain a separate series of tables containing the routing information that applies to a specific protocol, means there be separate IPv4/EIGRP table and IPv6/EIGRP table in the router. This way EIGRP can add PDMs to easily adapt to new or revised routed protocols such as IPv6. Each PDM is responsible for all functions related to its specific routed protocol. The EIGRP module sends and receives packets but passes received information to DUAL, which makes routing decision, i.e chooses the best route from Local Topology Tables and put into the Routing Table. The main function of IPv4/EIGRP module are: Send and receive EIGRP packets that bear IP data. Notify DUAL (Diffusing Update Algorithm) of new IP routing information that is received. Maintain the results o

Why MAC address is used and not only IP Address for Networking

Though A very basic Question but asked by CCNA Beginners!MAC address(Identifier) or Hardware Address that is burnt on the Network Interface Cards by its manufacturing companies is used for communication because devices on a LAN must also be uniquely and individually identified or they, like humans sharing the same name, will receive data not intended for them. When data is to be delivered on a LAN, it is encapsulated within an entity called a  Frame , a kind of binary envelope. Think of data encapsulation as being the digital equivalent of placing a letter inside an envelope. A destination address and a return (source) address are written on the outside of the envelope. Without a destination address, the postal service would have no idea where to deliver the letter. Likewise, when a frame is placed on a data link, all devices attached to the link “see” the frame; therefore, some mechanism must indicate which device should pick up the frame and read the enclosed data, and rest of

What is Difference Between Hardware and Software Switching

If I summarize the answer of this question in one line then I would say Hardware Switching is performed by ASICs and Software Switching is performed by CPUs. But let’s have some explanation of both to clearly understand the difference. The term  hardware-switching  refers to the act of processing packets at any Layers 2 through 7, via specialized hardware components referred to as Application-Specific Integrated Circuits (ASIC). ASICs can generally reach throughput at wire speed without performance degradation for advanced features such as QoS marking, ACL processing, or IP rewriting. Other terms used to describe hardware-switching are in-hardware, using ASICs, and hardware-based. Multilayer switching (MLS) is another term commonly used to describe hardware-switching.  MLS describes the capability to route and switch frames at line-rate (the speed of all ports sending traffic at the same time, full-duplex, at the maximum speed of the interface) with advanced features such as N

What is Difference between Contiguous and Discontiguous Networks With Example

I am trying to Explain both terms in some details with example, and trying to keep them as simple as I can so that you can easily understand the meaning of the terms and differences between them. Contiguous Network The Synonyms of the word  Contiguous  is  Continuous  that means “connected together so as to form an unbroken sequence in time”. Now come to the Technical meaning of the Contiguous Network, is that Series of Classfull Subnets that lie under the same Major Classfull Network where the packets can pass between the Subnets crossing the subnet that also lies in the same Major Classfull Network. Or Define in another Way A classful network in which packets sent between every pair of subnets can pass only through subnets of that same classful network, without having to pass through subnets of any other classful network. Still Not got my Point, Ok lets have and Example of Contiguous Network with IP Subnet Addresses. Suppose you have a Classfull network with Subnets as follo