Network PC World

The term recursive in the context of recursive static routes refers to the process where the router has to resolve the next-hop IP address by performing multiple lookups in its routing table. Here’s how it works: When a router receives a packet destined for a particular network (e.g., 10.22.22.0/24), it checks its static route configuration. In the case of a recursive static route, the route specifies a next-hop IP address (e.g., 192.168.1.1) rather than an interface. The router then needs to look up where that next-hop IP address (192.168.1.1) is located in its routing table to find the outgoing interface . If the next-hop IP itself requires further resolution (e.g., through another lookup to figure out its outgoing interface), the router has to perform recursive lookups until it resolves the final interface. In simple terms, recursive means the router must go through multiple steps or lookups (like peeling back layers) to finally determine how to forward the packet.

  VLSM (Variable Length Subnet Mask) and CIDR (Classless Inter-Domain Routing) are both techniques for efficient IP address allocation, but they serve different purposes: VLSM (Variable Length Subnet Mask) : VLSM allows different subnets within the same network to use different subnet masks, making it possible to allocate IP addresses more efficiently based on need (i.e., smaller subnets for smaller networks, larger subnets for larger ones). Used mainly within internal networks (intra-domain) to maximize the usage of available IP space. Requires routers that support classless routing protocols (e.g., OSPF, EIGRP, or RIPv2). CIDR (Classless Inter-Domain Routing) : CIDR is a method of assigning IP addresses without adhering to the traditional class-based system (A, B, C), allowing for more flexible and hierarchical IP address allocation. CIDR is primarily used for routing between networks (inter-domain), particularly on the Internet, to reduce routing table sizes and prevent IP exh...

 Classful Routing: Classful routing refers to a method where routing decisions are made based on the fixed subnet mask of IP address classes (A, B, C). It doesn’t transmit subnet mask information in routing updates, assuming default subnet masks based on IP address class. This approach was commonly used in older protocols like RIPv1 and IGRP . Key Characteristics : No subnet information is shared between routers. IP addresses are divided strictly into classes (A, B, C, etc.). It doesn’t support Variable Length Subnet Masking (VLSM). Less efficient use of IP address space due to fixed class boundaries. Example : If a router sees an IP address in the range 192.168.1.0 , it assumes the default subnet mask of /24 (255.255.255.0), as per Class C rules. Classless Routing: Classless routing allows for the use of Variable Length Subnet Masking (VLSM) and sends routing updates with subnet mask information. This allows for more flexible and efficient use of IP address space. Classless rou...

Multicast Overview Multicast is a network communication method that delivers a single stream from a source to multiple destinations. It optimizes network bandwidth usage, especially in applications like video conferencing, IPTV, and stock tickers. The Internet Group Management Protocol (IGMP) is used in Layer 2 networks, while Protocol Independent Multicast (PIM) handles Layer 3 communication. Technical Tip : Multicast is UDP-based, which lacks the reliability mechanisms of TCP, such as windowing and error correction, potentially leading to duplicate packets and out-of-order delivery. Multicast Addressing Multicast addresses are in the Class D range (224.0.0.0 to 239.255.255.255) . These addresses are not assigned to individual devices but represent groups. Well-known Multicast Address Types : Local network control block (224.0.0.0/24) : Protocol control traffic within a broadcast domain. Internetwork control block (224.0.1.0...

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...

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 clie...

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...

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...

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 ...

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...

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 ...

LocalHost Localhost is the standard hostname given to the address of the loopback network interface (usually 127.0.0.1). Simply LocalHost means this computer, the computer on which you are working.Localhost as a hostname translates to an IPv4 address in the 127.0.0.0/8 (loopback), usually 127.0.0.1, or ::1 in IPv6. LoopBack Loopback (loop-back) describes ways of routing electronic signals, digital data streams, or flows of items from their originating facility back to the source without intentional processing or modification. This is primarily a means of testing the transmission or transportation infrastructure. In TCP/IP a loopback device is a virtual network interface implemented in software only and not connected to any hardware, but which is fully integrated into the computer system’s internal network infrastructure. Any traffic that a computer program sends to the loopback interface is immediately received on the same interface. Correspondingly, the Internet Protocol (IP) specifi...

Got stuck with this error on Cisco Cloud Services Router, CSR Version 03.16.04a.S. When configuring static NAT rule as per below CSR(config)#ip nat inside source static tcp 10.0.1.21 443 interface GigabitEthernet1 443 %Port 443 is being used by system Found some Work around while Googling and also a Cisco Bug with same error in Cisco Advanced Services Router. But nothing worked for me. Here is how i resolved the issue by my self disable the ip http secure service on router by following command CSR(Config)#no ip http secure-server Save Configurations and Reload the router After successful reload, enter you static NAT rule.

On a Cisco Router, you don’t need to use telnet command; you can just type in an IP address from a command prompt and the router will assume that you want to telnet to the device having IP address you just typed.

IP Packet In general, the term packet applies to any message formatted as a packet for reliable service, i.e When a message is delivered with IP (Network Layer) + TCP (Transport Layer) Protocols then the Protocol data unit at Layer 3 is called an IP PACKET. Note:- A “reliable” service is one that notifies the user if delivery fails. IP Datagram The term IP datagram is reserved for packets of an “unreliable” service on Layer 3 of OSI reference model. When a message is delivered with IP (Network Layer) + UDP (Transport Layer) protocols then the Protocol Data Unit at Layer 3 is called an IP Datagram.Note:- “unreliable” Service or Protocol does not notify the user if delivery fails. Simply Said IP + TCP = IP Packet IP + UDP = IP Datagram

access-class access-class  command is used to bind Access Control Lists ACLs to the Router/Switches’ management Lines, e.g. telnet, SSH, Aux etc. There management lines are connections that terminate on Routers or Switches Example of using access-class command ! line vty 0 4 ip access-class 10 in ! access-group access-group command is used to bind Access Control Lists ACLs to the Routers/Switches’ interfaces Example of using access-class command ! interface fa0/1 access-group 10 in !

Differences between layer 2, 3, 4 Switching, Multilayer switching and Routing has been given below according to the function/operation of devices Layer 2 Switching  Devices that forward frames at Layer 2 involve the following functions: MAC addresses are learned from the incoming frames’ source addresses. A table of MAC addresses and their associated bridge and switch ports is built and maintained. Broadcast and multicast frames are flooded out to all ports (except the one that received the frame). Frames destined for unknown locations are flooded out to all ports (except the one that received the frame). Bridges and switches communicate with each other using the Spanning Tree Protocol to eliminate bridging loops.  A Layer 2 switch performs essentially the same function as a transparent bridge; however, a switch can have many ports and can perform hardware-based bridging. Frames are forwarded using specialized hardware, called application-specific integrated circui...

EIGRP is a classless Routing Protocol, but like RIP and RIPv2 it auto summarizes subnets to their classfull boundaries. So in the environment where you have Discontiguous Networks, EIGRP is not going to work until you prevent EIGRP manually from Auto Summarization. Configuration is very simple, in fact only one command after you configure EIGRP Network i.e  Router(config-router)#no auto-summary  Router#Config t Router(config)#router eigrp 50 Router(config-router)#network 10.0.0.0 Router(config-router)#no auto-summary

I placed few questions on Cisco forum to discuss the differences between the usage of a Router and Multilayer switch. There were my questions;Today’s Multilayer switches are versatile; they can do all the things a router can do; Will Router be eliminated by Multilayer Switches? Are there any limitations to a Multi-layer switch that it can not completely replace a router? If both functions can be done by a multilayer switches then why a router is still produced? Will in future be only one device in the market that will do Routing+switching, and will there be no separate Switch or Separate router? These types of question are usually in the mind of Networking (Cisco) guys who are only studying and have no experience of networking hardware in any firm. Here are the answers in the reply of above questions that will certainly answer to most of your questions. Reply 1  Generally router would have better processing power to perform routing more efficiently than ...

RTP is a Cisco proprietry protocol, used in EIGRP to manage the communication of messages between EIGRP-Speaking Routers. Reliable Transport or Delivery of EIGRP packets means Acknowledgement is required from the receiving Router and the packet should be delivered in order. Ordered delivery is ensured by including two sequence numbers in the packet. One sequence Number is assigned by the sending router for the packet order Number and that is incremented by one each time the sending Router sends a packet. Another Number is the Last Acknowledgement number received from destination or Neighbor Router. Why ordered delivery of EIGRP Packets is important? Let me give you a simplest example to explain question. Suppose an EIGRP Router’s Link went down and up after some time. But the neighbor Router receives a message of  Link-up first  and then  Link-Down after ! What will happen? Out of order execution of packets, can result in corruption of th...