Network Learners

The purpose of this document is to bring together certain points about Domain Name System (DNS) use by Cisco routers. Prerequisites Requirements Readers of this document should have knowledge of these topics: Cisco IOS ®  Command Line Interface (CLI) General DNS behavior Components Used The information in this document is based on these software and hardware versions: Cisco 2500 series routers Cisco IOS software 12.2(24a) The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command. Conventions For more information on document conventions, refer to the  Cisco Technical Tips Conventions . Setting Up a Router to Use DNS Lookups Your router can be configured to use DNS lookups if you wish to use the  ping  or  tracerout...

Connection Monitoring for WAN Failover Table of contents Connection Monitor Overview Connectivity Tests SD-WAN Monitoring Connection Monitor Overview When the primary uplink goes down on an MX Security Appliance, events will appear under  Network-wide >  Monitor > Event log  indicating a change in the primary uplink status. In the example below "uplink: 0" indicates that Internet 1 is being used, while "uplink: 1" indicates that Internet 2 is being used .  In Dashboard the  preferred  primary uplink can be configured, but that only matters when both are functioning. The MX will use the non-preferred uplink as the primary if it is the only one available. The MX monitors all uplinks and when it decides an uplink has no connectivity, will discontinue use of that link. Note : If the MX is using the non-preferred uplink as the primary and the preferred uplink comes back online, the MX will wait about 15 seconds be...

Introduction This document describes the configuration of device access with Telnet or Secure Shell (SSH) across a Virtual Routing and Forwarding (VRF). Background Information In IP-based computer networks, VRF is a technology that allows multiple instances of a routing table to co-exist within the same router at the same time. Because the routing instances are independent, the same or overlapping IP addresses can be used without any conflict with each other. Network functionality is improved because network paths can be segmented without the requirement of multiple routers. VRF might be implemented in a network device by distinct routing tables known as Forwarding Information Bases (FIBs), one per routing instance. Alternatively, a network device may have the ability to configure different virtual routers, where each one has its own FIB that is not accessible to any other virtual router instance on the same device. Telnet is an application layer pro...

Advertising a default route in BGP There are four ways to distribute a default route in BGP. Three of them, the network 0.0.0.0 , the default-information originate and redistribution from another routing protocol, are all similar in the resulting effect: they will inject the default route into BGP RIB and it will be advertised to all BGP neighbors.  The difference is in the origin of the default route that is injected into BGP. Specifically:  network 0.0.0.0 will inject the default route into BGP only if the default route is currently present in the routing table.  redistribution will inject the default route into BGP only if the default route is currently present in the routing table and if it has been learned by a specific source protocol we are redistributing from.  default-information originate causes the default route to be artificially generated and injected into the BGP RIB, regardlessly of whether it is present in the routing table. The newly inj...

Let's do it,,,, When working with MPLs Layer 3 VPN a lot of people get stuck with the verification, simply because they don’t know the bgp vpnv4 troubleshooting commands. This post will step through some of the verification you can use to verify the routes end to end through a simple MPLS Layer 3 vpn topology. The topology I will be using is below and is a very simple 3 router core with one vrf  and ospf being the CE to PE protocol on each side. I have created vrf ONE on Router 3 and on Router 5, OSPF 1 Area 0 is running between R3,R4,R5 and R3 is peering with MP-BGP to R5 All your troubleshooting commands will now need to include vpnv4 and the commands do get quite long. Here are my top bgp vpnv4 troubleshooting commands. BGP VPNv4 troubleshooting commands Before we start with the troubleshooting commands lets just verify everything is working, I will jump onto R1 and ping R6 loopback 1 2 3 4 5 6 7 R1 #ping 6.6.6.6 Type escape seq...