Welcome

First of all, may I welcome you to my site. My name is Chris and I'm from the UK and work as a Systems Engineer for Cisco. This blog was initially created to post up my subnetting technique but has now got more stuff to do with attaining Cisco certifications. Either way I really hope that the content is sufficent for your needs and I look forward to hearing your feedback. If you find that the content really helps you please feel free to donate using the PayPal link on the right.

To view the index of all my articles please click here.

CCIE Study - Written v4 Chapters 1 to 3 - 22/02/10
Tuesday, 22 February 2011

Well I've learned lots actually, or more accurately put, I remembered lots of stuff that I had forgotten. Stupid stuff that I should have known right off the bat but slipped the mind.

Etherchannel
Cisco recommends for PAgP that both ends of the link be configured as Desirable.

Ethernet Basics
802.3ab defines GigabitEthernet over UTP whereas 802.3z defines GigabitEthernet over Fibre. 802.3u defines FastEthernet.

MAC addresses are in canonical format which means that the most significant bit is on the right. Take the first two hexidecimal values from a MAC address and convert them to binary to give you an 8-bit string. The Individual/Group bit (I/G) is the right-most bit (i.e. the most signficant bit). If that is set to 0 then the MAC address is a unicast. If it is set to 1 then the MAC address is a broadcast or multicast. The second right-most bit is the Universal/Local bit (U/L). If this is set to 0 then the MAC address has been assigned by the vendor. If it is set to 1 then the MAC address has been administratively assigned.

Q-in-Q
VPLS and EoMPLS offer alternatives to Q-in-Q.

Spanning Tree
If a switch does not have any trunks configured at boot time but has the "spanning-tree root primary" command issued the priority of the switch will go to 24576 which is 8192 less than the default priority of 32768. If a trunk link is then formed and a switch has a higher priority then it will become the root and not the one with the root primary macro.

If you want to configure BPDUGuard at interface level you must take off any interface-level PortFast configuration first.

If running 802.1D and the root port does not receive any BPDUs the switch will wait for the Max Age timer to expire (default 20 seconds) before using another port.

Port priority and port number when used as a tiebreaker are those on the advertising switch and not on the switch that receives the BPDU.

If a root port fails then switchover to an alternative port is almost immediate.

SPAN/RSPAN
Destination ports do not forward Layer 2 protocols such as CDP, DTP, VTP, and STP.

Up to 64 destination ports may be configured.

The monitor session number can range between 1 and 66.

VLANs
The only VLANs that can be pruned are VLANs 2-1001. VLANs 1, and 1002-1005 are not prune eligible and can never be deleted.

Posted byChris Bloomfield at 18:37 0 comments  

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Frame Relay - BECN/FECN
Saturday, 19 February 2011

BECN - Backward Explicit Congestion Notification is a bit in the Frame Relay header that is set by the destination and sent BACK to the originator indicating congestion in the path and to slow down transmission of data.

FECN - Forward Explicit Congestion Notification is a bit in the Frame Relay header that is set by the sender and is FORWARDED to the destination to indicate congestion in the path and to slow down requests for data.

Note that these are set by a Frame Relay switch in general so are received by a router rather than sent by a router.

Posted byChris Bloomfield at 14:27 0 comments  

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How to calculate multicast MAC address
Thursday, 17 February 2011

Hi,

It's been a while but here's a quick post on how to calculate a multicast MAC address from an IP address.

The first half of a multicast MAC address is 01-00-5E so we need to work out the second half.

To do this we need to convert the last 23 bits of the IP address in question. If you think about this we are not using the high order bit in the second octet which carries a value of 128. Therefore it must follow that a value of 6 in the second octet must be the same as 134 in the second octet as the high-order bit (i.e. a value of 128) is ignored.

So this leads to a simple method. Let us try and convert 192.168.35.1 to a multicast MAC address

1. Start with a half-filled multicast MAC address of 01-00-5E-XX-YY-ZZ

2. To calculate the value of XX take the second octet. If the value of the second octet is greater than 128 then subtract 128 from the second octet. In this example, the value of 168 is greater than 128 so we subtract 128 from 168 to give us a value of 40. Convert this value to hexadecimal. Decimal 40 = 0x28. Our multicast MAC address is now 01-00-5E-28-YY-ZZ

3. To calculate the value of YY take the third octet and convert it to hex. In this example the value is 35 which equals 0x23. Our multicast MAC address is now 01-00-5E-28-23-ZZ

4. To calculate the value of ZZ take the fourth octet and convert it to hex. In this example the value is 1 which equals 0x01. Our multicast MAC address is now 01-00-5E-28-23-01

So 192.168.35.1 has a multicast MAC address of 01-00-5E-28-23-01.

Can you spot an issue here? Hopefully you can. Basically any IP address with 40.35.1 or 168.35.1 as the last three octets carry the same multicast MAC address so you have potentially 32 addresses with the same multicast MAC address!

Back to the books for me!

Good luck with your studies.

Posted byChris Bloomfield at 20:48 2 comments  

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CCNP Training in the UK with Networks Inc
Sunday, 21 November 2010

Guys,


As I obviously have oodles of time on my hands I have become a Senior Instructor at Networks Inc here in the UK. We offer weekend CCNP courses from the new v6 track with the emphasis very much on hands-on practice. There is no equipment sharing - you will have your own kit to work on exclusively!

And do you know what really rocks? You'll get to meet me of course! :-B

For further details please see http://www.networksinc.co.uk/CCNP_boot_camp.htm

I look forward to meeting some of you!

Chris

Posted byChris Bloomfield at 13:11 0 comments  

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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  

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MPLS Command Memorizer Review
Tuesday, 27 April 2010

Some time ago I purchased the CCIE Command Memorizer from http://www.configureterminal.com which bundled in the MPLS Command Memorizer. For those of you that give a damn, I am currently taking my CCIP due to the CCIE v4 blueprint change which focuses more on MPLS. I'm halfway there with MPLS slated for 18th May leaving QoS which I aim to complete by the Summer.

UPDATE: Passed with 987 in no small part to the MPLS Command Memorizer :-)

I've played a lot with the MPLS Command Memoriser and have found it to be a wonderful tool to exercise fingers and mind and a couple of typos in the correct answers actually showed me that I knew more about the commands than I thought. Perhaps this has deliberately been put in by David ;-)

It covers a wide range of topics including basic setting up of an MPLS network through to the various IGPs that can be used between CE and PE. There were also some useful exercises on those commands that may slip out of your mind such as "no ip mpls propagate-ttl forwarded" and conditional label advertisement. You have to type the commands in full as well so for me that is a great way of remembering the commands.

My only recommendation for this product would be to include an AToM link and most definitely for me a section on MPLS Traffic Engineering - that would be real boon.

Overall though I cannot recommend the Command Memorisers highly enough and would like to thank David Bombal for a terrific application which is not only a great standalone product but perfectly complements all other training materials such as books and labbing. I would also like to thank David for his support while I have been going through PCs like no tomorrow :-)

Posted byChris Bloomfield at 10:23 0 comments  

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Frame Relay - CIR, Bc, Tc
Tuesday, 18 August 2009

Very quick post. For whatever reason this formula does not stick in my head although it should be fairly straightforward to second guess.

CIR = Bc/Tc

Tc can never be smaller than 10ms!

Posted byChris Bloomfield at 15:33 0 comments  

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