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.

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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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