Troubleshooting in a network can be required for multiple reasons. To troubleshoot a network it is best to start with the problems that are easily traceable such as interface with the wrong IP address or interface still in shutdown mode, etc. After overcoming these small errors, we move to major issues like an error in the static routing configuration. For errors like a missing connection in end to end connectivity, we first make sure that you can ping your own interface and the other devices connected directly to your interface. When the verification for this connectivity has been done, then check the connectivity with remote networks of different devices. Networks are often subjected to circumstances that can cause their status to change. Some of these are: An interface failing, a service provider dropping a connection, links becoming oversaturated or an administrator may enter the wrong configuration and many more. Connectivity may be lost in all of these scenarios since there is a change in the network. It lies in the hands of the network administrators to isolate and solve these issues. To figure out and resolve these matters, a network administrator has to be familiar to the various concepts and tools that are used for the following task.

A few common troubleshooting commands include: ping, traceroute, show ip route, show ip interface brief, show cdp neighbour details. Now to find these connectivity errors and solving them is a piece of cake if the right methodology is used in the right manner and order. The troubleshooting for static routes can be learnt by troubleshooting an IPv4and IPv6 network configured to the basic settings.

There are 3 alternative ways a routing table is built:

  • Statically
  • By default
  • Dynamically

you'll be getting to observe the primary two-static and default routing tables that aredesigned by network directors.

Remember that associate degree internetwork is outlined as 2 or additional networks connected with a router or routers. conjointly recall that routers do not keep track of, or care even the slightest bit concerning, hosts, however they're extraordinarily involved concerning networks and therefore the best path to access each. Logical addressing (IP, for example) is what is accustomed establish every host on the internetwork. Routers scan the network portion of associate degree informatics address to workout wherever within the internet world a number is then use a routing table to see the simplest path to the network that the destination host is found on.

Once that network is found, the packet is distributed to the destination network by forwarding the packet, hop-to-hop, till it reaches the particular router that is directly connected to the destination host's network. From there, the destination host's distinctive hardware address is employed to induce the packet to the host that is presupposed to receive it.

Routers have maps, or a minimum of a sort of them. they have to have a map of the complete internetwork to clarify to them wherever every logical network is found, furthermore on guide their call in selecting the fastest, most effective thanks to get there. This map is named a routing table, and every routed protocol you utilize needs to have its own map. as an example, if you are running informatics, IPX, and AppleTalk on your network, every of your routers can have 3 maps-one for every routing protocol-all describing an equivalent physical networks in a very completely different means.

Static routes are routes organized and entered into the routing table by the administrator. Typically, in larger networks, making nothing however static routes would be an ugly task (if not impossible), thus dynamic routing is typically used (examples are RIP and OSPF). However, in smaller networks, static routing will work well; it saves overhead on the router central processor associate degreed information measure on the serial links that dynamic routing protocols avariciously consume as quick as my retriever can steal and swallow an unattended turkey sandwich!

If we want to feature a default route to the routing table of the 2500B router: This is often a bit like adding a static route, except wildcards of all zeros (0s) are used rather than a network and mask.

Routers want current, up-to-date "maps." If a routing table does not have a route to every network that it's getting to be needed to send packets to, then packets are going to be born.

  • Troubleshooting a routing table downside is hard. however by victimization the ping program and Traceroute command, you'll be able to notice precisely wherever a packet is failing inassociate degree internetwork.


To do...

Use the command...

Add a static route

ip route-static ip-address { mask | mask-length } { interface-type interface-number } [ nexthop-ip-address ] [ preferencepreference-value | tag tag-value ] * [ reject | blackhole ] [description text ]
ip route-static [ vpn-instance vpn-instance-name-list ip-address { mask | mask-length } { interface-type interface-number | vpn-instance vpn-instance-name nexthop-ip-address } [ public ] [ preference preference-value | tag tag-value | public ] * [ reject | blackhole ]  [ description text ]

Delete a static route

undo ip route-static ip-address mask | mask-length } {interface-type interface-number } nexthop-ip-address ] [preference preference-value ]
undo ip route-static vpn-instance vpn-instance-name-list destination-ip-address mask | mask-length } interface-name | vpn-instance vpn-nexthop-name ] nexthop-ip-address [ public ] [ preference preference-value ]


To do...

Use the command...

Configure a default route

ip route-static { | 0 } { interface-typeinterface-number | gateway-address } [ preferencevalue ] [ reject | blackhole ]

Delete a default route

undo ip route-static { | 0 } [ interface-type interface-number | gateway-address ] [preference value ]

The parameters are explained as follows:

The information processing address and mask are in an exceedingly dotted decimal format. As 1s within the 32-bit mask are needed to be consecutive, the dotted decimal mask can even get replaced by the mask-length (which refers to the digits of the consecutive 1s within the mask).

When configuring a static route, you'll specify successive hop-ip-address to make your mind up the next hop address, betting on the particular conditions.

In fact, for all the routing entries, successive hop address should be given. once information processing layer transmits AN information processing packet, it'll initial search the matching route within the routing table in line with the destination address of the packet. only if successive hop address of the route is given will the link layer realize the corresponding link layer address, so forward the packet in line with this address.

The packets sent to NULL interface, a sort of virtual interface, are discarded right away. this may decrease the system load. Depending on the configuration of preference, you'll win totally different route management policies. for instance, to implement load sharing, you'll specify identical preference for multiple routes to identical destination network. To implement route backup, you'll specify totally different preferences for them. Tag tag-value: Specifies a tag price for the static route. The default tag price is zero. The attributes reject and black hole severally indicate the unreached route and therefore the black hole route.

Description: Specifies an outline for the configuration.


The metric structures and algorithms of most routing protocols are not compatible with other protocols. Hence for a network having multiple routing protocols, it is critical for the network to have the ability to find the best path to exchange route information. Administrative distance is that feature which a router uses to select the best path from different paths which are to the same destination from two different routing protocols. It defines how reliable a routing protocol is. Each if these is prioritized in the order of most to least valuable.

The smaller the executive distance worth, the additional and more reliable is the protocol. For instance, if a router receives a route to an exact network from each Open Shortest Path initial (OSPF) (default body distance - 110) and Interior entryway Routing Protocol (IGRP) (default body distance - 100), the router chooses IGRP as a result of IGRP is additional reliable. this suggests the router adds the IGRP version of the route to the routing table. If you lose the supply of the GRP-derived info (for example, owing to an influence shutdown), the software system uses the OSPF-derived info till the IGRP-derived info reappears.

If the executive distance is 255, the router doesn't believe the supply of that route and doesn't install the route within the routing table.

When you use route distribution, sometimes you would like to switch the executive distance of a protocol so it takes precedence. for instance, if you would like the router to pick out RIP-learned routes (default price 120) instead of IGRP-learned routes (default price 100) to an equivalent destination, you want to increase the executive distance for IGRP to 120+, or decrease the executive distance of RIP to a worth but one hundred.

You can modify the executive distance of a protocol through the space command within the routing method sub configuration mode. This command specifies that the executive distance is allotted to the routes learned from a specific routing protocol. You would like to use this procedure usually once you migrate the network from one routing protocol to a different, and therefore the latter includes a higher body distance. However, an amendment within the body distance will cause routing loops and black holes. So, use caution if you modify the executive distance.

The default values that CISCO supports for Administrative Distance:

Route Source

Default Distance Values

Connected interface


Static route


Enhanced Interior Gateway Routing Protocol (EIGRP) summary route


External Border Gateway Protocol (BGP)


Internal EIGRP






Intermediate System-to-Intermediate System (IS-IS)


Routing Information Protocol (RIP)


Exterior Gateway Protocol (EGP)


On Demand Routing (ODR)


External EIGRP


Internal BGP




Common Applications for Administrative distance can be shown when we use Static Routes for backing up an existing IGP route.


You can use the passive-interface command so as to regulate the promotional material of routing info. The command allows the suppression of routing updates over some interfaces whereas it permits updates to be changed usually over different interfaces.

With most routing protocols, the passive-interface command restricts outgoing advertisements solely. But, once used with increased Interior entree Routing Protocol (EIGRP), the result is slightly completely different. This document demonstrates that use of the passive-interface command in EIGRP suppresses the exchange of salutation packets between 2 routers, which ends up within the loss of their neighbour relationship. This stops not solely routing updates from being publicized, however it conjointly suppresses incoming routing updates. This document conjointly discusses the configuration needed so as to permit the suppression of outgoing routing updates, whereas it conjointly permits incoming routing updates to be learned usually from the neighbour.

The software and hardware used in these are:

  • Cisco IOS Software Release 12.2(10b)
  • Cisco 2600 series routers

The information during this document was created from the devices in a very specific science laboratory atmosphere. All of the devices utilized in this document started with a cleared (default) configuration. If your network is live, ensure that you simply perceive the potential impact of any command.

In giant enterprise networks, several distribution devices have over two hundred interfaces. Before the introduction of the Default Passive Interfaces feature, routing data can be obtained from these interfaces in these ways:

Tack a routing protocol like Open Shortest Path initial (OSPF) on the backbone interfaces and distribute connected interfaces. Tack a routing protocol on all interfaces and manually set most of them as passive.

Network operators won't invariably be able to summarize kind five link-state advertisements (LSAs) at the device level wherever distribution happens, as within the initial chance. Thus, an outsized variety of kind five LSAs will be flooded over the domain.

In the second chance, giant kind one LSAs may well be flooded over the domain. The realm Border Router (ABR) creates kind three LSAs, one for every kind one LSA, and floods them to the backbone. You can, however, have distinctive account at the ABR level, that injects just one outline route into the backbone, thereby reducing the process overhead.

Before the introduction of the Default Passive Interfaces feature, you'll tack the routing protocol on all interfaces and manually set the passive-interface router configuration command on interfaces wherever adjacencies weren't desired. however in some networks, this resolution meant configuring two hundred or additional passive interfaces. The Default Passive Interfaces feature resolved this drawback by permitting all interfaces to be set as passive by default. you'll set all interfaces as passive by default by victimization the passive-interface default command and so tack individual interfaces wherever adjacencies area unit desired victimization the no passive-interface command.

The Default Passive Interfaces feature simplifies the configuration of distribution devices and permits the network administrator to get routing data from interfaces in ISPs and enormous enterprise networks.


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