Showing posts with label CCNA. Show all posts
Showing posts with label CCNA. Show all posts
QoS - Bandwidth, Bandwidth Percent, Bandwidth Remaining Percent
Saturday, 12 June 2010
OK, I need to get this firmly lodged in my brain. What exactly are the differences in all of the bandwidth statements when using Modular QoS CLI (MQC)?
Let's start with two values:
The actual total bandwidth of the interface which we'll call int-bw.
The maximum bandwidth that can be reserved on an interface (default 75%) which we'll call max-resv-bw.
There are 3 bandwidth statements that can be used in MQC but note that you must only use one type of bandwidth statement per policy map (e.g. you cannot use bandwidth and bandwidth percent in the same policy map).
The first statement is simply bandwidth [kbps] which reserves the value specified from the actual total bandwidth, int-bw. Remember that the total value of all of the bandwidth statements cannot exceed the maximum bandwidth that can be reserved max-resv-bw.
The second statement is bandwidth percent [percent] which reserves the specified percentage of the actual total bandwidth, int-bw. Remember that the total value of all of the bandwidth percent statements cannot exceed the maximum bandwidth that can be reserved max-resv-bw.
The third statement is bandwidth remaining percent [percent] which reserves the specified percentage of the remaining maximum reservable bandwidth. Remember that the total value of all of the bandwidth remaining percent statements cannot exceed the maximum bandwidth that can be reserved max-resv-bw.
This is probably all better served with an example. Let's say that we have a policy-map with two classes in there, class1 and class2, applied to an interface whose bandwidth is 256kbps. We have the following two values:
int-bw = 256kbps
max-resv-bw = 256kbps * 0.75 = 192kbps
Note 0.75 in the max-resv-bw calculation as by default max-resv-bw is 75% of int-bw.
Let's see how the bandwidth statement affects the policy map:
class class1
bandwidth 64
class class2
bandwidth 32
Quite simply, class1 will be reserved a minimum of 64kbps and class2 will be reserved a minimum of 32kbps. However, if the total of all of the bandwidth statements exceeded the max-resv-bw of the interface (192kbps in this case) Cisco IOS would not allow the policy-map to be applied to the interface. In the example above the total of all of the bandwidth statements is 96kbps which is less than the max-resv-bw of 192kbps.
Let's see how the bandwidth percent statement affects the policy map:
class class1
bandwidth percent 20
class class2
bandwidth percent 10
In this case, class1 will be reserved a minimum of 20% of int-bw which is 52kbps in this example and class2 will be reserved a minimum of 10% of int-bw which is 25.6kbps. However, if the total of all of the bandwidth percent statements exceeded the max-resv-bw of the interface (192kbps in this case) Cisco IOS would not allow the policy-map to be applied to the interface. In the example above the total of all of the bandwidth statements is 77.6kbps which is less than the max-resv-bw of 192kbps.
Let's see how the bandwidth remaining percent statement affects the policy map:
class class1
bandwidth remaining percent 20
class class2
bandwidth remaining percent 10
In this case, class1 will be reserved a minimum of 20% of max-resv-bw which is 38.4kbps in this example and class2 will be reserved a minimum of 10% of max-resv-bw which is 19.2kbps. However, if the total of all of the bandwidth remaining percent statements exceeded the max-resv-bw of the interface (192kbps in this case) Cisco IOS would not allow the policy-map to be applied to the interface. In the example above the total of all of the bandwidth statements is 57.6kbps which is less than the max-resv-bw of 192kbps.
In essence the following formulas hold true:
Bandwidth - Reserves value specified. Total of all statements in same policy map cannot exceed max-resv-bw.
Bandwidth Percent - Reserves specified percentage of int-bw. Total of all statements in same policy-map cannot exceed max-resv-bw.
Bandwidth Remaining Percent - Reserves specified percentage of max-resv-bw. Total of all statements in same policy-map cannot exceed max-resv-bw.
Finally, you can change the value of max-resv-bw at the interface level. However, Cisco does not recommend that you do this as to allow for control traffic. To do this use the max-reserved-bandwidth [percent] command. For example, if I wanted to be able to reserve 85% of Serial 0/0 bandwidth I would do the following:
interface Serial0/0
max-reserved-bandwidth 85
I hope this has helped you as much as it has helped me bt typing it out.
Good luck with your studies!
Posted byChris Bloomfield at 09:34 0 comments
Labels: bandwidth, bandwidth percent, bandwidth remaining percent, cbwfq, CCIE, ccip, CCNA, ccnp, ccvp, Cisco, llq, modular qos cli, mqc, police, policer, qos, shape, shaper
Subnetting Made Easy - Critically Acclaimed!
Wednesday, 28 November 2007
We need to start with the fundamentals of IP addressing. An IP address is made up of 32 bits, split into 4 octets (oct = 8, yes?). Some bits are reserved for identifying the network and the other bits are left to identify the host.
There are 3 main classes of IP address that we are concerned with.
| Class A | Range 0 - 127 in the first octet (0 and 127 are reserved) |
| Class B | Range 128 - 191 in the first octet |
| Class C | Range 192 - 223 in the first octet |
Below shows you how, for each class, the address is split in terms of network (N) and host (H) portions.
| NNNNNNNN | . | HHHHHHHH | . | HHHHHHHH | . | HHHHHHHH | Class A Address |
| NNNNNNNN | . | NNNNNNNN | . | HHHHHHHH | . | HHHHHHHH | Class B Address |
| NNNNNNNN | . | NNNNNNNN | . | NNNNNNNN | . | HHHHHHHH | Class C Address |
At each dot I like to think that there is a boundary, therefore there are boundaries after bits 8, 16, 24, and 32. This is an important concept to remember.
We will now look at typical questions that you may see on subnetting. More often than not they ask what a host range is for a specific address or which subnet a certain address is located on. I shall run through examples of each, for each class of IP address.
What subnet does 192.168.12.78/29 belong to?
You may wonder where to begin. Well to start with let's find the next boundary of this address.
Our mask is a /29. The next boundary is 32. So 32 - 29 = 3. Now 23 = 8 which gives us our block size.
We have borrowed from the last octet as the 29th bit is in the last octet. We start from zero and count up in our block size. Therefore it follows that the subnets are:-
192.168.12.0
192.168.12.8
192.168.12.16
192.168.12.24
192.168.12.32
192.168.12.40
192.168.12.48
192.168.12.56
192.168.12.64
192.168.12.72
192.168.12.80
.............etc
Our address is 192.168.12.78 so it must sit on the 192.168.12.72 subnet.
What subnet does 172.16.116.4/19 sit on?
Our mask is /19 and our next boundary is 24. Therefore 24 - 19 = 5. The block size is 25 = 32.
We have borrowed into the third octet as bit 19 is in the third octet so we count up our block size in that octet. The subnets are:-
172.16.0.0
172.16.32.0
172.16.64.0
172.16.96.0
172.16.128.0
172.16.160.0
.............etc
Our address is 172.16.116.4 so it must sit on the 172.16.96.0 subnet. Easy eh?
What subnet does 10.34.67.234/12 sit on?
Our mask is 12. Our next boundary is 16. Therefore 16 - 12 = 4. 24 = 16 which gives us our block size.
We have borrowed from the second octet as bit 12 sits in the second octet so we count up the block size in that octet. The subnets are:-
10.0.0.0
10.16.0.0
10.32.0.0
10.48.0.0
.............etc
Our address is 10.34.67.234 which must sit on the 10.32.0.0 subnet.
Hopefully the penny is starting to drop and you are slapping the side of your head realising that you were a fool to think it was hard. We will now change the type of question so that we have to give a particular host range of a subnet.
What is the valid host range of of the 4th subnet of 192.168.10.0/28?
Easy as pie! The block size is 16 since 32 - 28 = 4 and 24 = 16. We need to count up in the block size in the last octet as bit 28 is in the last octet.
192.168.10.0
192.168.10.16
192.168.10.32
192.168.10.48
192.168.10.64
.................etc
Therefore the 4th subnet is 192.168.10.48 and the host range must be 192.168.10.49 to 192.168.10.62, remembering that the subnet and broadcast address cannot be used.
What is the valid host range of the 1st subnet of 172.16.0.0/17?
/17 tells us that the block size is 224-17 = 27 = 128. We are borrowing in the 3rd octet as bit 17 is in the 3rd octet. Our subnets are:-
172.16.0.0
172.16.128.0
The first subnet is 172.16.0.0 and the valid host range is 172.16.0.1 to 172.16.127.254. You must remember not to include the subnet address (172.16.0.0) and the broadcast address (172.16.127.255).
What is the valid host range of the 7th subnet of address 10.0.0.0/14?
The block size is 4, from 16 - 14 = 2 then 22 = 4. We are borrowing in the second octet so count in the block size from 0 seven times to get the seventh subnet.
The seventh subnet is 10.24.0.0. Our valid host range must be 10.24.0.1 to 10.27.255.254 again remebering not to include our subnet (10.24.0.0) and the broadcast address (10.27.255.255).
What if they give me the subnet mask in dotted decimal?
If you're lucky and they give you a mask in dotted decimal format then you should have an even easier time. All you need again is your block size.
Let's say they have given a mask of 255.255.255.248 and you wish to know the block size. Here's the technique:
1. Starting from the left of the mask find which is the first octet to NOT have 255 in it.
2. Subtract the number in that octet from 256 to get your block size (e.g. above it is 256 - 248 = block size of 8).
3. Count up from zero in your block size in the octet identified in step 1 as you have learned above (the example above would be in the last octet).
Another example is a mask of 255.255.192.0 - you would simply count up in 256 - 192 = 64 in the third octet.
One more example is 255.224.0.0 - block size is 256 - 224 = 32 in the second octet.
What other questions may they ask?
You may find they ask for how many bits you need to borrow for a certain amount of subnets, the subnet mask needed for a certain number of hosts, or the number of hosts per subnet. THESE ARE ALL EASY TO CALCULATE! All you need to remember is that you borrow bits for subnets and reserve bits for hosts.
There are two simple formulas:
Number of subnets = 2n where n is the number of bits borrowed
Number of hosts = 2(32 - n) - 2 where n is the number of bits in your subnet mask
Let's think of some questions. How many bits do you need to borrow to accommodate 6 subnets? No matter what address you are given the maths is still the same. The formula is 6 = 2n so you must find n which in this case is 3 as n = 2 gives only 4 subnets and n = 3 gives 8 subnets. Simply add n to your mask for your new subnet mask. For example, if you had a /24 address and you wanted 8 subnets then your mask will be 24 + 3 = /27.
What subnet mask should you use if you wanted 60 hosts per subnet? The formula is 60 = 2(32 - n) - 2 so you must find n which is 26. This is easy to find as you know that 26 - 2 = 62 so simply subtract 6 from 32 to get the 26. Therefore your mask is /26.
Lastly the number of hosts per subnet. How many hosts per subnet in the address 172.16.0.0/23? You have a /23 address therefore you formula is x = 2(32 - 23) - 2 = 29 - 2 = 510.
Another typical question they may ask will be giving you an IP address and mask and asking how many subnets and hosts there are from that address, for example:
Question: How many subnets and hosts per subnet can you get from the network 172.30.0.0/28?
From this you only need two pieces of information:
1. The default subnet mask of the address class.
2. The subnet mask in the question
Using the example above we know that:
1. The default subnet mask is /16 as the address given is a class B address
2. The subnet mask in the question is /28
The number of subnets = 2 ^ (subnet_mask_in_question - default_subnet_mask)
The number of hosts = (2 ^ (32 - subnet_mask_in_question)) - 2
For our example question the number of subnets = 2 ^ (28 - 16) = 2 ^ 12 = 4096 subnets.
The number of hosts = (2 ^ (32 - 28)) - 2 = (2 ^ 4) - 2 = 14 hosts per subnet
Let's use another address: 192.168.1.0/29
We know that:
1. The default subnet mask is /24 as the address given is a class C address
2. The subnet mask in the question is /29
The number of subnets = 2 ^ (29 - 24) = 2 ^ 5 = 32 subnets.
The number of hosts = (2 ^ (32 - 29)) - 2 = (2 ^ 3) - 2 = 6 hosts per subnet
Finally, let's use another address: 10.1.1.0/24
We know that:
1. The default subnet mask is /8 as the address given is a class A address
2. The subnet mask in the question is /24
The number of subnets = 2 ^ (24 - 8) = 2 ^ 16 = 65536 subnets.
The number of hosts = (2 ^ (32 - 24)) - 2 = (2 ^ 8) - 2 = 254 hosts per subnet
Easy isn't it?
What now?
Now it's time to go and pick up those books again and go straight to the practice questions, completely by-passing any of their techniques. Use my method and you will be laughing!
If you are unsure that you have the correct answers why not download a subnet calculator to double-check your answers? There is a great one by 3Com and can be downloaded from here.
Happy subnetting!
Posted byChris Bloomfield at 16:20 78 comments
Labels: best subnetting guide, block size, ccda, CCNA, ccnp, cidr, Cisco, easy, guide, ip, made, mask, Subnetting, VLSM
CCNA - Connecting devices
Wednesday, 21 November 2007
Don't worry about remembering which type of cable should be used when connecting alike or different devices. Use the simple diagram to the left and you won't go wrong!
Posted byChris Bloomfield at 14:50 6 comments
Labels: cable, cabling, CCNA, crossover, host, hub, layout, rollover, router, straight-through, switch
NAT in a Nutshell
Network Address Translation (NAT) and Port Address Translation (PAT)
NAT operates on a Cisco router, usually connecting two networks together, and translates the private (inside local) addresses in the internal network to public addresses (inside global) before packets are forwarded to another network.
As part of this functionality, you can configure NAT to advertise only one address for the entire network to the outside world. This effectively hides the internal network from the world, thus providing additional security.
In NAT terminology, the inside network is the set of networks that are subject to translation. The outside network refers to all other addresses. Usually these are valid addresses located on the Internet.
Cisco defines the following NAT terms:
- Inside local address: The IP address assigned to a host on the inside network. The address is likely not an IP address assigned by the NIC or service provider.
- Inside global address: A legitimate IP address assigned by the NIC or service provider that represents one or more inside local IP addresses to the outside world.
- Outside local address: The IP address of an outside host as it appears to the inside network. Not necessarily a legitimate address, it is allocated from an address space routable on the inside.
- Outside global address: The IP address assigned to a host on the outside network by the host owner. The address is allocated from a globally routable address or network space.
NAT has many different forms and can work in the following ways:
- Static NAT: Maps an unregistered IP address to a registered IP address on a one-to-one basis. Static NAT is particularly useful when a device needs to be accessible from outside the network.
- Dynamic NAT: Maps an unregistered IP address to a registered IP address from a group of registered IP addresses.
- Overloading: is a form of dynamic NAT that maps multiple unregistered IP addresses to a single registered IP address (many-to-one) by using different ports, known also as PAT.
PAT uses unique source port numbers on the inside global IP address to distinguish between translations. Because the port number is encoded in 16 bits, the total number of internal addresses that can be translated using NAT to one external address could theoretically be as many as 65,536 per IP address.
Configuring Static Translation
The ip nat inside source static command establishes static translation between an inside local address and an inside global address.
Router(config)#ip nat inside source static local-ip global-ip
To remove static translation use no ip nat inside source static
The ip nat inside command marks the interface as connected to the inside.
Router(config-if)#ip nat inside
The ip nat outside command marks the interface as connected to the outside.
Router(config-if)#ip nat outside
The complete process is below (variables in italics):
Router(config)#ip nat inside source static local-ip global-ip
Router(config)#interface type number
Router(config-if)#ip_address subnetmask
Router(config-if)#ip nat inside
Router(config-if)#interface type number
Router(config-if)#ip_address subnetmask
Router(config-if)#ip nat outside
Configuring Dynamic Translation
The ip nat pool command defines a pool of global addresses to be allocated as needed.
Router(config)#ip nat pool name start-ip end-ip {netmask netmask prefix-length prefix-length}
To remove dynamic translation use the no ip nat pool command.
The access-list access-list-number command defines a standard IP access list permitting those inside local addresses that are to be translated.
Router(config)#access-list access-list number permit source [source-wildcard]
The ip nat inside source command establishes dynamic source translation, specifying the access list defined in the prior step.
The complete process is shown below:
Router(config)#ip nat pool name start-ip end-ip {netmask netmask prefix-length prefix length}
Router(config)#access-list access-list-number permit source [source-wildcard]
Router(config)#ip nat inside source list access-list-number pool pool-name
Router(config)#interface type number
Router(config-if)#ip nat inside
Router(config-if)#interface type number
Router(config-if)#ip nat outside
Configuring Overloading (PAT)
The access-list access-list-number command defines a standard IP access list permitting those inside local addresses that are to be translated.
Router(config)#access-list access-list-number permit source source-wildcard
Enter the global no access-list access-list-number command to remove the access list.
The ip nat inside source list command establishes dynamic source translation, specifying the access list defined in the prior step.
Router(config)#ip nat inside source list access-list-number interface interface_type/number overload
It is the “overload” keyword that enables PAT.
The complete process is shown below:
Router(config)#access-list access-list-number permit source [source-wildcard]
Router(config)#ip nat inside source list access-list-number interface interface_type/number overload
Router(config)#interface type number
Router(config-if)#ip nat inside
Router(config-if)#interface type number
Router(config-if)#ip nat outside
Example:
Router(config)#access-list 1 permit 192.168.3.0 0.0.0.255
Router(config)#access-list 1 permit 192.168.4.0 0.0.0.255
Router(config)#ip inside nat source list 1 interface Serial0 overload
Router(config)#interface Ethernet0
Router(config-if)#ip address 192.168.3.1 255.255.255.0
Router(config-if)#ip nat inside
Router(config)#interface Ethernet1
Router(config-if)#ip address 192.168.4.1 255.255.255.0
Router(config-if)#ip nat inside
Router(config)#interface Serial0
Router(config-if)#ip address 172.17.38.1 255.255.255.0
Router(config-if)#ip nat outside
Clearing the NAT translation table
After you have configured NAT, verify that it is operating as expected. You can do this by using the clear and show commands.
Router#clear ip nat translation *
This clears all dynamic address translation entries from the NAT translation table, the * is a wildcard meaning "all".
Router#clear ip nat translation inside global-ip local-ip [outside local-ip global-ip]
This clears a simple dynamic translation entry containing an inside translation, or both inside and outside translation.
Router#clear ip nat translation outside local-ip global-ip
This clears a simple dynamic translation entry containing an outside translation.
Router#clear ip nat translation protocol inside global-ip global-port local-ip local-port [outside local-ip local-port, global-ip global-port]
This clears an extended dynamic translation entry.
Displaying NAT information
You can display translation information by performing one of the following tasks in EXEC mode.
Router#show ip nat translations – displays active translations
Router#show ip nat statistics – displays translation statistics.
Troubleshooting the NAT and PAT Configuration
Use Router#debug ip nat
Posted byChris Bloomfield at 13:29 5 comments
Labels: CCNA, Cisco, Dynamic, Inside Global, Inside Local, NAT, Outside Global, Outside Local, PAT, Static, Subnetting
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