Creating Command Aliases

Problem

You want to create aliases for commonly-used or complex commands.

Solution

You can create command aliases on your router with the alias command:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#alias exec rt show ip route
Router1(config)#alias exec on show ip ospf neighbor
Router1(config)#end
Router1#

Discussion

Unix system administrators have been using command aliases for many years to help reduce typing and save time. These shortcut commands allow you to reduce long or complex command sequences down to a few simple characters. This is most useful for extremely common commands, or for those that are complex or difficult to remember. You can create an alias for any command, including some or all of its associated keywords or variables.

Here we have created the alias rt for one of the most common commands that we use every day, show ip route:

Router1(config)#alias exec rt show ip route

We can now use this simple two-letter command to display the routing table, saving time and typing:

Router1#rt
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
   
Gateway of last resort is 172.25.1.1 to network 0.0.0.0
   
S    192.168.10.0/24 [1/0] via 172.22.1.4
     172.16.0.0/24 is subnetted, 1 subnets
C       172.16.2.0 is directly connected, FastEthernet0/0.2
     172.20.0.0/16 is variably subnetted, 3 subnets, 3 masks
O       172.20.10.0/24 [110/74] via 172.20.1.2, 00:52:55, Serial0/0.2
C       172.20.1.0/30 is directly connected, Serial0/0.2
O       172.20.100.1/32 [110/65] via 172.20.1.2, 00:52:55, Serial0/0.2
     172.22.0.0/16 is variably subnetted, 2 subnets, 2 masks
D       172.22.0.0/16 is a summary, 20:31:03, Null0
C       172.22.1.0/24 is directly connected, FastEthernet0/1
Router1#

The key to choosing a good alias command name is to pick something that is short and easy to remember. Of course, it is critical to select an alias that does not conflict with an existing command. In our example, we choose rt as a short and memorable mnemonic for "route table." This abbreviation does not conflict with any existing IOS command.

You can also use a command alias as part of a longer command. For example, we could use our rt alias to shorten the command show ip route 172.16.2.0.

Router1#rt 172.16.2.0
Routing entry for 172.16.2.0/24
  Known via "connected", distance 0, metric 0 (connected, via interface)
  Routing Descriptor Blocks:
  * directly connected, via FastEthernet0/0.2
      Route metric is 0, traffic share count is 1
Router1#

Command aliases are most effective if you use them consistently among all of the routers that you manage. Otherwise, you'll have to remember a different set of alias commands for each group of devices. If you want to use this feature, we recommend that the entire network management team work together to develop a standard set of aliases before implementing them. We also recommend keeping the aliases simple. And, above all, resist the urge to alias every possible command. Instead, create aliases for only the most common commands.

Command aliases are also useful for scripting. You can build a script to perform a task on a router that might be slightly different on each router. For example, suppose you want to clear the counters of a particular access list on a weekly basis. But, some of your routers use a different access list number. You can simply build an alias with the same name on each router, but make the actual commands represented by the alias appropriate to each individual router. Finally, you can build a script to issue the command alias and automate what would otherwise be an extremely onerous task.

The show aliases command displays all of the command aliases configured on the router:

Router1#show aliases
Exec mode aliases:
  h                     help
  lo                    logout
  p                     ping
  r                     resume
  s                     show
  u                     undebug
  un                    undebug
  w                     where
  rt                    show ip route
  on                    show ip ospf neighbor
   
Router1#

If you type this command on any router, you will see that Cisco implements several command aliases by default.

Redistributing Static Routes into RIP

Problem

You want RIP to redistribute static routes that you have configured on your router.

Solution

The redistribute static command tells RIP to forward static routes in addition to the directly connected routes and the routes that have been learned from other RIP routers, which it forwards by default:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip route 192.168.10.0 255.255.255.0 172.22.1.4
Router1(config)#router rip
Router1(config-router)#redistribute static
Router1(config-router)#end
Router1#

You can define how these routes look to other routers when they are redistributed:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#ip route 192.168.10.0 255.255.255.0 172.22.1.4
Router1(config)#router rip                  
Router1(config-router)#redistribute static metric 5
Router1(config-router)#distribute-list 7 out static
Router1(config-router)#exit
Router1(config)#access-list 7 permit 192.168.10.0
Router1(config)#end
Router1#

Discussion

The biggest potential problem that you will encounter with redistributing routes into RIP comes from breaking network class boundaries. RIP is classful, so you have to be rather careful about how you distribute routing information from other sources that may be classless. In this recipe, Router1 redistributes a static route for the Class C network 192.168.10.0. But if we tried instead to redistribute a larger range (such as 192.168.12.0/22), RIP would not generate any errors—the router would just quietly refuse to forward this route.

Looking at the RIP database on a router with IOS level 12.0(6)T or higher shows the redistributed static route:

Router1#show ip rip database 192.168.10.0 255.255.255.0
192.168.10.0/24    redistributed
    [5] via 0.0.0.0, 
Router1#

After configuring the second example, the output of show ip protocols includes information about the filtering. This command also tells you what protocols RIP is distributing:

Router1#show ip protocols 
Routing Protocol is "rip"
  Sending updates every 30 seconds, next due in 5 seconds
  Invalid after 180 seconds, hold down 180, flushed after 240
  Outgoing update filter list for all interfaces is not set
    Redistributed static filtered by 7
  Incoming update filter list for all interfaces is not set
  Redistributing: static, rip
  Default version control: send version 2, receive version 2
    Interface             Send  Recv  Triggered RIP  Key-chain
    FastEthernet0/0.1     2     2 
    Serial0/0.2           2     2                                    
    FastEthernet0/1       2     2                                    
   Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    172.22.0.0
    172.25.0.0
  Routing Information Sources:
    Gateway         Distance      Last Update
    172.25.1.7           120      00:00:03
    172.25.2.2           120      00:00:06
    172.22.1.4           120      00:00:08
  Distance: (default is 120)
Router1#

In addition to static routes, you can distribute information from other dynamic routing protocols with RIP simply by specifying which protocol's routes you want RIP to use. For example, if you have an EIGRP network that uses process number 65530 on the same router, you would redistribute the EIGRP routes into RIP like this:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router eigrp 65530
Router1(config-router)#network 192.168.1.0
Router1(config-router)#exit
Router1(config)#router rip
Router1(config-router)#redistribute eigrp 65530
Router1(config-router)#end
Router1#

If you look at the show ip protocols command, you can see that RIP redistributes routes it learns from EIGRP, but EIGRP does not redistribute routes learned from RIP. If you also want EIGRP to redistribute RIP routes, you must explicitly configure it to do so. 

Router1#show ip protocols   
Routing Protocol is "rip"
  Sending updates every 30 seconds, next due in 0 seconds
  Invalid after 180 seconds, hold down 180, flushed after 240
  Outgoing update filter list for all interfaces is 
  Incoming update filter list for all interfaces is 
  Redistributing: static, rip, eigrp 65530
   

  Default version control: send version 1, receive any version
    Interface        Send  Recv   Key-chain
    FastEthernet0/0.1     2     2 
    Serial0/0.2           2     2                                    
    FastEthernet0/1       2     2                                    
   Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    172.22.0.0
    172.25.0.0
  Routing Information Sources:
    Gateway         Distance      Last Update
    172.25.1.7           120      00:00:03
    172.25.2.2           120      00:00:06
    172.22.1.4           120      00:00:08
  Distance: (default is 120)
   
Routing Protocol is "eigrp 65530"
  Outgoing update filter list for all interfaces is 
  Incoming update filter list for all interfaces is 
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  EIGRP maximum hopcount 100
  EIGRP maximum metric variance 1
  Redistributing: eigrp 65530
  Automatic network summarization is in effect
  Routing for Networks:
    192.168.1.0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: internal 90 external 170
   

Frame Relay Switch Configuration By Chris Bryant, CCIE #12933

Many CCNA certification candidates want to add a frame relay switch to their Cisco router lab, but aren't quite sure how to configure one.

Other candidates aren't quite sure what a frame relay switch is, or what Cisco routers can serve as such a switch. This CCNA case study will examine how to add one of these pivotal devices to a Cisco lab.

A Cisco lab's frame relay switch is not a switch at all; it's a Cisco router. Almost any Cisco router can serve as your frame switch, but you will need multiple serial interfaces to make a router particularly effective in this role.

I recommend you get a Cisco router with at least four serial interfaces. Cisco 2520s make excellent frame switches, and by doing a search on eBay for "frame relay switch", you'll quickly find several others that can as well.

The frame relay switch is going to play the role of the frame provider in your lab. In essence, you've got a one-switch frame relay cloud, which gives you a tremendous opportunity to practice frame relay scenarios.

In this example, I've got three Cisco routers that are going to be the production routers in my lab - R1, R2, and R3. I have a fourth router that will serve as the frame relay switch. The DLCI assignments I've come up with are as follows:

• R1 is the hub and will use DLCI 122 to reach R2, DLCI 123 to reach R3.
• R2 is a spoke router and will use DLCI 221 to reach both R1 and R3.
• R3 is a spoke router and will use DLCI 321 to reach both R1 and R2.

The physical connections are as follows. All connections are using DTE/DCE cables with the DCE end of the cable connected to the frame relay switch.

• R1 is connected to the frame switch's Serial1 port.
• R2 is connected to the frame switch's Serial2 port.
• R3 is connected to the frame switch's Serial3 port.

On the frame switch, the global command frame-relay switching is required to make the router act as a frame relay switch. Let's take a look at the commands we'll need on the frame switch's Serial1 port, which is connected to R1.

interface Serial1
 no ip address
 encapsulation frame-relay
 logging event subif-link-status
 logging event dlci-status-change
 clockrate 56000
 no frame-relay inverse-arp
 frame-relay intf-type dce
 frame-relay route 122 interface Serial2 221
 frame-relay route 123 interface Serial3 321

Note that there is no IP address on the port, and frame relay encapsulation is enabled. The clockrate command is necessary on the DCE end of the connection, so you see it here.

You also see that the interface is hard-coded as a DCE with theframe-relay intf-type dce command. Not all current IOS versions require this; just make sure you have the DCE end of the cable attached to the frame switch and verify that with show controller serial x.

Finally, we come to the frame-relay route command. The syntax seems a little tricky, but once you break it down it's pretty simple.

• frame-relay route 122 = the incoming DLCI
• interface serial2 = data coming in on DLCI 122 is sent out this port
• 221 = data sent out interface serial2 will use this DLCI

Getting those statements correct is the hardest part of configuring your frame relay switch. The good part is that once you have your frame switch configured and working properly, you can just leave the configuration there.

We'll take a look at how to verify your frame switch configuration in just a moment, but first, let's review the important section of the frame switch configuration we've talked about here.

hostname FRAME_SWITCH
!
!
ip subnet-zero
no ip domain-lookup
frame-relay switching
!
!
!
interface Ethernet0
 no ip address
 no ip directed-broadcast
 shutdown
!
interface Serial0
 no ip address

interface Serial1
 no ip address
 no ip directed-broadcast
 encapsulation frame-relay
 logging event subif-link-status
 logging event dlci-status-change
 clockrate 56000
 no frame-relay inverse-arp
 frame-relay intf-type dce
 frame-relay route 122 interface Serial2 221
 frame-relay route 123 interface Serial3 321
!
interface Serial2
 no ip address
 no ip directed-broadcast
 encapsulation frame-relay
 logging event subif-link-status
 logging event dlci-status-change
 clockrate 56000
 no frame-relay inverse-arp
 frame-relay intf-type dce
 frame-relay route 221 interface Serial1 122

interface Serial3
 no ip address
 no ip directed-broadcast
 encapsulation frame-relay
 logging event subif-link-status
 logging event dlci-status-change
 clockrate 56000
 no frame-relay inverse-arp
 frame-relay intf-type dce
 frame-relay route 321 interface Serial1 123

To verify that your frame relay configuration is functioning correctly, run the global command show frame route on the frame relay switch. If you see active next to all frame routes as shown below, you're in good shape.

If you see anything else - say, the word "inactive" - then there is a problem.

Troubleshooting A Frame Relay Switch

The key to troubleshooting your frame switch is that you cannotconcentrate on the frame switch's config. You can get that part perfect, but if you're using other DLCIs on your routers or there's a physical issue - perhaps you forgot to open some interfaces - you're not going to get the active frame routes you want.

Just make sure you're got all the appropriate interfaces open, don't forget the clockrate and frame-relay route commands on the frame switch, and you'll successfully add this important device to your Cisco lab!

Scroll down for two CCNA training videos on frame relay switching - and take advantage of this exclusive CCNA training opportunity!

A subnet question with answer

Question:


" On the Network 131.1.123.0/27, what is the last IP address that can be assigned to a host?"

1) 131.1.123.30
2) 131.1.123.31
3) 131.1.123.32
4) 131.1.123.33


Answer:


Your network address is 131.1.123.0 /27= 10000011.00000001.0111 1011.00000000

Always the first host is one greater that the network address.

So “First Host Address” = 131.1.123.1
 
The subnet is /27 which means first 27 bits represents network portion and last 5 bits represents host portion.

You find the broadcast by putting 1 for all your host portion of the network address.

So 10000011.00000001.01111011.000 00000   becomes
     10000011.00000001.01111011.000 11111 = 131.1.123.31

Which is your broadcast address.

Now the last host of your network address is one less that the broadcast address so it is 131.1.123.30
----------------
So Network address = 131.1.123.0
First host = 131.1.123.1
Last host = 131.1.123.30
Broadcast address = 131.1.123.31
-------------------

So I think the correct answer is 131.1.123.30

Ps:

The /27 means that the first 27 bits are used for the "network portion".
Total bits of your network ip are 32 so you left with 5 bits for yours host(s ) portion (2^5-1).
Your subnet mask is 1111 1111. 1111 1111.1111 1111.1110 0000 = 255.255.255.224.

Hope it was helpful !

Collision Domain and Broadcast Domain

A collision domain is a physical network segment where data packets can collide with one another when being sent on a shared medium, in particular, when using the Ethernet networking protocol. A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. Collisions are resolved using carrier sense multiple access or a variant thereof in which the competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in the network.[1]

This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Collision domains are also found in wireless networking such as Wi-Fi. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network in order to avoid data collisions. Because only one device may be transmitting at any one time, total network bandwidth is shared among all devices. Collisions also decrease network efficiency on a collision domain; if two devices transmit simultaneously, a collision occurs, and both devices must retransmit at a later time.
To relieve the network of collision domains, it is recommended to use a network switch which increases the number of collision domains while decreasing each collision domain's size. This is because each port on a switch is its own collision domain.A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments.

In terms of current popular technologies: Any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain. Further, any computer connected to the same set of inter-connected switches/repeaters is a member of the same broadcast domain. Routers and other higher-layer devices form boundaries between broadcast domains.

This is as compared to a collision domain, which would be all nodes on the same set of inter-connected repeaters, divided by switches and learning bridges. Collision domains are generally smaller than, and contained within, broadcast domains.

While some layer two network devices are able to divide the collision domains, broadcast domains are only divided by layer 3 network devices such as routers or layer 3 switches.

CCNA subnetting questions

Question 1

Given a subnet mask of 255.255.255.224, which of the following addresses can be assigned to network hosts? (Choose three)

A – 15.234.118.63
B – 92.11.178.93
C – 134.178.18.56
D – 192.168.16.87
E – 201.45.116.159
F – 217.63.12.192



Answer: B C D

Explanation

A subnet mask of 255.255.255.224 has an increment of 32 (the binary form of the last octet is 1110 0000) so we can’t use numbers which are the multiples of 32 because they are sub-network addresses. Besides, we can’t use broadcast addresses of these sub-networks (the broadcast address of the previous subnet is calculated by subtracting 1 from the network address). For example the network address of the 2nd subnet is x.x.x.32 then the broadcast address of the 1st subnet is 32 – 1 = 31 (means x.x.x.31).

By this method we can calculate the unusable addresses, which are (notice that these are the 4th octets of the IP addresses only):

+ Network addresses: 0, 32, 64, 96, 128, 160, 192, 224.
+ Broadcast addresses: 31, 63, 95, 127,159, 191, 223.

Question 2

Which of the following host addresses are members of networks that can be routed across the public Internet? (Choose three)

A – 10.172.13.65
B – 172.16.223.125
C – 172.64.12.29
D – 192.168.23.252
E – 198.234.12.95
F – 212.193.48.254



Answer: C E F

Explanation

Addresses that can be routed accross the public Internet are called public IP addresses. These addresses belong to class A, B or C only and are not private addresses.

Note:

Private class A IP addresses: 10.0.0.0 to 10.255.255.255
Private class B IP addresses: 172.16.0.0 to 172.31.255.255
Private class C IP addresses: 192.168.0.0 to 192.168.255.255

Class D addresses are reserved for IP multicast addresses and can’t be routed across the Internet (their addresses begin with 224.0.0.0 address).

Also we can’t use 127.x.x.x address because the number 127 is reserved for loopback and is used for internal testing on the local machine.

Question 3

A national retail chain needs to design an IP addressing scheme to support a nationwide network. The company needs a minimum of 300 sub-networks and a maximum of 50 host addresses per subnet. Working with only one Class B address, which of the following subnet masks will support an appropriate addressing scheme? (Choose two)

A – 255.255.255.0
B – 255.255.255.128
C – 255.255.252.0
D – 255.255.255.224
E – 255.255.255.192
F – 255.255.248.0



Answer: B E

Explanation

We need to remember the default subnet mask of class B is 255.255.0.0. Next, the company requires a minimum of 300 sub-networks so we have to use at least 512 sub-networks (because 512 is the minimum power of 2 and greater than 300). Therefore we need to get 9 bits for network mask (29=512), leaving 7 bits for hosts which is 27= 128 > 50 hosts per subnet.This scheme satisfies the requirement -> B is correct.

We can increase the sub-networks to 1024 ( 1024 = 2¹⁰), leaving 6 bits for hosts that is 2⁶= 64 > 50 hosts. This scheme satisfies the requirement, too -> E is correct.

Notice: The question asks “The company needs a minimum of 300 sub-networks and a maximum of 50 host addresses per subnet” but this is a typo, you should understand it as “”The company needs a minimum of 300 sub-networks and a minimum of 50 host addresses per subnet”.

Question 4

Which of the following IP addresses fall into the CIDR block of 115.64.4.0/22? (Choose three)

A – 115.64.8.32
B – 115.64.7.64
C – 115.64.6.255
D – 115.64.3.255
E – 115.64.5.128
F – 115.64.12.128

Answer: B C E

Explanation

CIDR stands for Classless In4ter-Domain Routing, the difference between CIDR and VLSM is slim and those terms are interchangeable at CCNA level.

To specify which IP addresses fall into the CIDR block of 115.64.4.0/22 we need to write this IP address and its subnet mask in binary form, but we only care 3rd octet of this address because its subnet mask is /22.

Question 5

Refer to the diagram. All hosts have connectivity with one another. Which statements describe the addressing scheme that is in use in the network? (Choose three)

A – The subnet mask in use is 255.255.255.192.
B – The subnet mask in use is 255.255.255.128.
C – The IP address 172.16.1.25 can be assigned to hosts in VLAN1.
D – The IP address 172.16.1.205 can be assigned to hosts in VLAN1.
E – The LAN interface of the router is configured with one IP address.
F – The LAN interface of the router is configured with multiple IP addresses.

Answer: B C F

Explanation

VLAN 2 has 114 hosts so we need to leave 7 bits 0 for the host addresses (2⁷ – 2 = 126 > 114). Notice that we are working with class B (both Host A and Host B belong to class B) and the default subnet mask of class B is /16 so we need to use 16 – 7 = 9 bits 1 for the sub-network mask, that means the subnet mask should be 255.255.255.128 -> B is correct.

By using above scheme, C is correct because the IP 172.16.1.25 belongs to the sub-network of VLAN 1 (172.16.1.0/25) and can be assigned to hosts in VLAN 1.

For communication between VLAN 1 and VLAN 2, the LAN interface of the router should be divided into multiple sub-interfaces with multiple IP addresses -> F is correct.

Question 6

The network 172.25.0.0 has been divided into eight equal subnets. Which of the following IP addresses can be assigned to hosts in the third subnet if the ip subnet-zero command is configured on the router? (Choose three)

A – 172.25.78.243
B – 172.25.98.16
C – 172.25.72.0
D – 172.25.94.255
E – 172.25.96.17
F. 172.25.100.16

Answer: A C D

Explanation

If the “ip subnet-zero” command is configured then the first subnet is 172.25.0.0. Otherwise the first subnet will be 172.25.32.0 (we will learn how to get 32 below).

The question stated that the network 172.25.0.0 is divided into eight equal subnets therefore the increment is 256 / 8 = 32 and its corresponding subnet mask is /19 (1111 1111.1111 1111.1110 0000).

First subnet: 172.25.0.0/19
Second subnet: 172.25.32.0/19
Third subnet: 172.25.64.0/19
4th subnet: 172.25.96.0/19
5th subnet: 172.25.128.0/19
6th subnet: 172.25.160.0/19
7th subnet: 172.25.192.0/19
8th subnet: 172.25.224.0/19

In fact, we only need to specify the third subnet as the question requested. The third subnet ranges from 172.25.64.0/19 to 172.25.95.255/19 so A C D are the correct answers.

Getting Started with CCENT/CCNA

Hello welcome to the world of networking.In case of CCNA we can do all most all labs in your home with the help of simulators.Both Packet Tracer and GNS3 is good in aspect of CCNA.But best is follow these steps..

a) For learners new to CCENT / CCNA - Packet Tracer's advantage is that it restricts the number of commands that can be viewed at any mode.  Therefore, the student should  feel less overwhelmed (or distracted) as he/she progresses through the CCENT / CCNA curriculum.

b) After the learner reaches a good confidence level and have done several labs using PT. Then, she/he can consider using GNS3 for a more realistic IOS feel.  GNS3 runs the actual IOS so that the learner will be exposed the all the commands on the actual router.

c) Please note though that GNS3 requires much more hardware resources (CPU, RAM etc) to do the more complex labs.  For example :  A simple lab with 3 routers that are running OSPF may cause GNS3 to give "mixed results -- errors" if the hardware requirement (Processor and RAM) are not sufficient.