Comments for Network Ninja

Comment on Layer 2 Network by shouli

shouli — Thu, 27 May 2010 16:46:47 +0000

diagram of how to create seperate collision domain

Comment on 802.1D Spanning Tree Protocol by Armando Garcia

Armando Garcia — Wed, 26 May 2010 14:38:44 +0000

Hi Jeff and Deon..

Here it goes a numeric explanation of the process.

A BID with extended system ID have 12 bits for Priority and VID (4 bits for priority and 12 bits for VID). In this complete 16 bits field (4 bits of Priority + 12 bits VID) the priority bits are the 4 upper most; therefore, each 1 bit increment is actually being analyzed as an increment of upper most bits, and the 12 lower most bits are not analyzed. Remember that I am describing the process to JUST calculate the priority.

So the increments would go like this, (remember to see it as a complete 12 bit field, and the VID is not analyzed)

Priority-VID

0000-xxxxxxxxxxxx = 2^12 = 4096 (equivalent)
0001-xxxxxxxxxxxx = 2^13 = 8192
0010-xxxxxxxxxxxx = 2^14 = 16384
0011-xxxxxxxxxxxx = 2^14 + 2^13 = 16384 + 8192 = 24576
0100-xxxxxxxxxxxx = 2^15 = 32768

and it keeps going until Priority = 1111,

Hope it helps,

Armando

Comment on Open Shortest Path First – OSPF Fundamentals – DR and BDR by wakatana

wakatana — Mon, 26 Apr 2010 15:31:38 +0000

Nice , but dont you have mistake in table ?
- Number of adjacencies with DR and BDR is 2n-3
- So if you have 5 routers you have 7 adjacencies (not 9) (also according your picture)

Comment on Open Shortest Path First – OSPF Fundamentals – DR and BDR by killus

killus — Sun, 21 Feb 2010 05:25:02 +0000

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Comment on 802.1D Spanning Tree Protocol by Deon Botha

Deon Botha — Fri, 19 Feb 2010 06:46:51 +0000

Hi Jeff,

After some searching I’ve uncovered this gem.

“Cisco’s implementation of spanning-tree is done, by default, at the VLAN level. Each VLAN runs its own instance of spanning-tree. This is called PVST (per-vlan spanning-tree). To support this, Cisco modified the bridge ID to include a 12-bit VLAN ID field in the bridge ID which is called the Extended System ID. The rationale for this was that the priority field did not need the granularity provided by a 16-bit field, but instead could get by with the top four bits of the field, leaving 12 bits to express a VLAN ID. This is consistent with the VLAN ID field in a dot1q frame, which also has a 12-bit VLAN ID field. This means then, that the priority bits have decimal values of 4096, 8192, 16384, and 32768. Using this form of the bridge ID means that priorities are always expressed as some multiple of 4096.”

This hopefully answers your question.

Deon

Comment on 802.1D Spanning Tree Protocol by Jeff Harper

Jeff Harper — Fri, 19 Feb 2010 04:39:50 +0000

Great info, I’ve always wondered and have tried to research it, but why must the bridge priority be in increments of 4096?

Comment on Hot Standy Router Protocol (HSRP) by Telecom Testing Training Bangalore

Telecom Testing Training Bangalore — Fri, 05 Feb 2010 06:40:09 +0000

Thank you for the great web site – a true resource, and one many people clearly enjoy thanks for sharing the info, keep up the good work going….

Comment on Open Shortest Path First – OSPF Fundamentals – DR and BDR by Satya

Satya — Tue, 26 Jan 2010 16:19:36 +0000

Superb explanation…good work…my best luck to u..

Comment on Open Shortest Path First – OSPF Fundamentals – DR and BDR by MOS

MOS — Mon, 23 Nov 2009 11:40:07 +0000

excellent explanation mate

cheers

Comment on Gotta go see Wall-E by Kenny Spenny

Kenny Spenny — Sat, 14 Nov 2009 23:39:26 +0000

Good stuff, bookmarked for further reading.