Network Troubleshooting Methods

Troubleshooting a network connectivity issue is quite challenging in a complex Data Center Network with live production traffic.

Below are few basic methods to identify a node in the traffic path that drops packets which would cause network connectivity issues. This procedure is in general and not related to any specific vendor as troubleshooting options varies with each vendor.

Let’s consider below simple topology for troubleshooting purpose:

R1(IP:— R2 — R3 — R4(IP:

Problem statement#1: 100% ping loss between R1 and R4In this case, ping from R1 to R4’s interface IP fails 100%. Continue reading

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NPG (NPIV Proxy Gateway) Mode Vs Full Fabric Mode in FCoE

In this article, I will be discussing about two different modes that an FCF switch can operate in FCoE topology.

FCF(FCoE Forwarder) is a switch that supports both Ethernet(FCoE) functionality like FIP, DCBx, LAN protocols and FC(Fiber Channel) functionality like fabric login, zoning and name services. FCF  connects the CNA on the server end to any storage array(Native FC), by bridging the gap between Ethernet LAN and storage . Dell S5000 is an example of FCF switch which can be configured in two different modes- NPG or Full Fabric mode. Continue reading

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Port names in FC/FCoE Topology

Following are most common port names used in FC/FCoE Technology:

N_port(Node port)/VN_port:

  • Ports on end host(i:e HBA) and storage array(target port) are called as N_port in FC topology.
  • Called as VN_port in FCoE terminology.
  • HBA’s that handles IP traffic are called as CNA in FCoE terminology.
  • Each N-port will have a unique WWPN (64 bit) assigned by vendor. We use this WWPN while configuring Zoning on FC switch.
  • Fabric Provided Mac Address (FPMA) is a 48-bit mac-address that uniquely identifies a single VN_port(CNA) on an Enode. FPMA is assigned to an VN_port by FC switch during FLOGI process.
  • FPMA (48-bit mac) = FC-map (24 bits) + FC_ID (24 bits)
    • FC-MAP: 24 bit value that identifies the FC switch. Default value: 0efc00
    • FC_ID:  Also called as N_port ID. A 24-bit value assigned to a VN_Port by the FC switch to uniquely identify it in the local fabric network.
  • So, there will be one-one mapping between physical N-port WWPN and FPMA/N_port ID in a fabric network.

Continue reading

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Configuring Qlogic CNA for FCoE on an ESXi Host

This article is about how to configure FCoE functionality on Qlogic CNA card-QL8262 installed on a Dell PowerEdge R720 server running ESXi 5.5.

QL8262 is dual port CNA card that supports LAN, iSCSI and FCoE. Configuring Qlogic FCoE card on an ESXi host is a two-step process;

  1. Install driver on ESXi so that it can detect Qlogic cards.
  2. Enable FCoE functionality on Qlogic CNA.

Continue reading

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Installing ESXi on a Bare-metal Dell Server

  • Quick way to install ESXi on a Dell server is via iDRAC(a management console pre-built with all Dell Poweredge servers).
  • Download ESXi hypervisor iso image from vmware download page to your laptop.
  • On a Bare-metal Dell server, after power-on, configure IP address of iDRAC via server LCD panel

Continue reading

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Vmware Virtualization Basics

  • Below are the needs for a Data Center Virtualization(DCV):
    • Availability – DCV helps to achieve zero down-time for apps & VMs.
    • Scalability – DCV helps to scale underlying hardware without any impact to applications.
    • Optimization – DCV helps to utilize available hardware resource more effectively.
    • Management – DCV helps to manage VMs more efficiently.
  • Each Virtual Machine (VM) has its own OS, dedicated virtual hardware(CD-Drive, Hard disc, Network card) and real applications running on each VM.
  • Hypervisor – Resource management for VMs.
    • Type-1: Bare-metal: Installing Vmware ESXi (a hypervisor) directly on a server(ESXi Host).
    • Type-2: Hosted: Installed as an application on top of an OS(like Windows, Linux). Ex: Vmware Workstation.
  • Features:
    • Snapshots : Take a snap of a VM along with its all data. Useful for recovery in case of crash after a patch update.
    • Image backup : VM Image backup similar to snapshots.
    • “Hot Add” : Adds more CPU and memory to servers without any downtime.
    • “Thin Provisioning” : VMs are allowed to use disk on demand.
    • “Thick provisioning” : VMs are hard-allocated the configured memory size.
    • “VM storage profiles” : an Automatic way to map servers to a storage class so that storage is used efficiently and effectively.

Continue reading

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Issue#2: Packets were not seen at the egress interface in a snake test


Snake test is usually performed to check whether a device can handle traffic Tx/Rx to/from all active interfaces at line-rate. The snake test is usually performed as below;

Traffic SENDER—- 0/1(Router)0/2 —- 0/3(same router)0/4—-0/5(same router)0/6— Traffic RECEIVER

ports 0/1-2 are untagged(access ports) in a vlan; say vlan2
ports 0/3-4 are untagged(access ports) in a vlan; say vlan3
ports 0/5-6 are untagged(access ports) in a vlan; say vlan4

So, packets sent by the SENDER at line-rate will be received at the RECEIVER. Same can be done vice-versa. ie: line-rate traffic from RECEIVE to SENDER. Continue reading

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Issue#1: Firewall not receiving packets from a router


When a router receives a packet, it would check the routing table to forward the packet to the egress(outgoing) interface based on the destination IP address. If we need to forward packet based on other field, like source IP etc, we need to use PBR rules to override the routing table entry. The issue is, after configuration PBR, the packets are not routed to the firewall as in the PBR rule rather forwarded based on the routing table.

Troubleshooting steps:

  • PBR was configured to route packets to a firewall instead of taking the normal route as in Routing table. Continue reading
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VNC-server on Linux

VNC is used to display X windows session running on a remote server. For network engineers, it is really useful to have Konsole/xterm session running on a Linux server connecting to network devices. Whenever we need access to the switches/routers, we can just login to VNC server using a VNC client. All sessions will be active (unless idle timeout is configured on switches/routers) and ready to use.

Steps to install VNCserver in Linux: Continue reading

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  • Tool Command Language (Tcl) – interpreted programming language.
  • Tk is a library of procedures written to create graphical user interfaces (GUI) – Not available in Cisco IOS.
  • Another offshoot of Tcl is “Expect”. Expect is highly specialized to match output strings. Used for telnet,ssh,ftp. Continue reading
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