Odom book WRONG? or am i wrong?

jovan88

I'm studying for the ICND2, and came across this STP question in Odom's book:

Which of the following bridge IDs would win election as root, assuming that the switches with these bridge IDs were in the same network?

a) 32768:0200.1111.1111
b) 32768:0200.2222.2222
c) 200:0200.1111.1111
d) 200:0200.2222.2222
e) 40,000:0200.1111.1111

It says the answer is C "The smallest numeric bridge ID wins the election"

But you can't set priority to 200 because its not in increments of 4096, so the correct answer would be A right?

Hope none of these dodgy questions come up in the ICND2!

MrNobody

Why do you think you can only set the priority in increments of 4096? Where did you hear/read this?

jovan88

CBT nuggets and Cisco Exploration

Switch(config)#spanning-tree vlan 1 priority ?
  <0-61440>  bridge priority in increments of 4096

stevi3

Just tested it in Packet Tracer 5.0 and got this:

Switch(config)#spanning-tree vlan 1 priority 200
% Bridge Priority must be in increments of 4096.
% Allowed values are:
  0     4096  8192  12288 16384 20480 24576 28672
  32768 36864 40960 45056 49152 53248 57344 61440

So looking at that, you're right. I guess its a good note for when people are studying STP in the future.

scheistermeister

Bridge ID and priority are two completely different things.

nel

scheistermeister wrote:

Bridge ID and priority are two completely different things.

Exactly. The bridge ID is made up of 2 things - the priority and the mac address. for a switch to win a root election it is defined by the priority field initially, so whoever has the lowest priority wins the election. if there is a tie then its defined by the lowest or oldest mac address.

wendell was refering to that 4096 increment as the increment used to create the default priority value's. So for example if you typed "spanning-tree vlan 10 root primary" the priority would be changed to a lower value than the default priority which would be an increment of 4096. but you can manually change the priority value yourself by typing a static value which does infact have to be an increment of the 4096 value.

I think the question in hand would have been for the learning process to allow student to analyse a BID and obviously work out who would win the election by identifying the lowest value's.

P.S. Odom is never wrong !!!

dtlokee

OK just to confuse things more... According to the IEEE spec you can have any value between 0 and 65535 for a priority, this was using all 16 bits. When Cisco implemented PVST they used 12 bits for the VLAN ID and the remaining 4 for the priority, this is why they are in increments of 4096.

PRI  <---- VLAN -->
0001 0000 0000 0000 = 4096
0010 0000 0000 0000 = 8192
0011 0000 0000 0000 = 12288
.
.
.
1000 0000 0000 0000 - 32768 (default priority)

Now this would assume the VLAN ID is 0, but that is not going to be the case. If the priority is set to 4096 for and the vlan ID is 200 then the priority would be advertised as 4296 (4096 + 200). The mac address is appended to the priority as a tie breaker if the priority is the same between bridges (switches)

Netwurk

The 4096 increment rule only applies if you turn on the STP extended system ID, which is done with the spanning-tree extend system-id command. You turn it off by using the "no" keyword.

So assuming it is off, the 200 value is permissable and the "c" answer is correct.

jovan88

dtlokee wrote:

If the priority is set to 4096 for and the vlan ID is 200 then the priority would be advertised as 4296 (4096 + 200).

so that means if you set the priority to 0, and have a VLAN ID of 200, the Bridge ID would be 200 (0 + 200)? so it IS possible

ChristianReed

Isnt there only RSTP on the ICND 2

nel

ChristianReed wrote:

Isnt there only RSTP on the ICND 2

No.

ChristianReed

nel wrote:

ChristianReed wrote:

Isnt there only RSTP on the ICND 2

No.

it only mentions RSTP in the exam topics..?

tech-airman

jovan88 wrote:

I'm studying for the ICND2, and came across this STP question in Odom's book:

Which of the following bridge IDs would win election as root, assuming that the switches with these bridge IDs were in the same network?

a) 32768:0200.1111.1111
b) 32768:0200.2222.2222
c) 200:0200.1111.1111
d) 200:0200.2222.2222
e) 40,000:0200.1111.1111

It says the answer is C "The smallest numeric bridge ID wins the election"

But you can't set priority to 200 because its not in increments of 4096, so the correct answer would be A right?

Hope none of these dodgy questions come up in the ICND2!

jovan88,

Unfortunately, you made a common error that even I make sometimes which is called "overthinking the problem." The point that he was trying to test is your understanding of:

1. The bridge with the lowest priority wins.
2. IN THE EVENT THERE IS A TIE OF LOWEST PRIORITY, the tie-breaker is the bridge with the lowest MAC address wins.

So since the priorities for c) and d) are the same, then it goes to the tie-breaker of the lowest MAC which in the case of this question, is the MAC address of 0200.1111.1111 therefore the correct answer is c). That's all the question was asking and nothing more.

Another general overall thing to understand is that for as long as I can remember, the CCNA certificate is based on material that is NOT based on a specific Cisco platform or platforms. In other words, it's designed to be "platform neutral." That means you could be configuring a Cisco Catalyst 2960 up to a Cisco Catalyst 6500 switch and the commands you configure should be the same due to the commonality of the Cisco IOS.

Netwurk

tech-airman has the right idea

Switches can run all sorts of flavors of STP - what you you need to understand is the bridge priority/lowest mac addy rule

Worrying about the 4096 increment stuff will only sidetrack you in this case - not all switches will follow the increment rule and even those that do can be changed by turning spanning-tree extend system-id on or off

CCNA is all about the basics - don't sweat the details until you get to CCNP