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HSRP Hotspot

November 15th, 2018 in SWITCH 300-115 Go to comments

Question

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For your information, the “show running-config” commands are posted below for your reference but please notice in the exam you have to issue this command to get the output:

DSW1#show running-config

interface Vlan101
 ip address 192.168.101.1 255.255.255.0
 standby 1 ip 192.168.101.254
 standby 1 priority 200
 standby 1 track GigabitEthernet1/0/1 55
!
interface Vlan102
 ip address 192.168.102.1 255.255.255.0
 standby 2 ip 192.168.102.254
 standby 2 priority 200
 standby 2 preempt
 standby 2 track GigabitEthernet1/0/1 5
!
interface Vlan103
 ip address 192.168.103.1 255.255.255.0
 standby 3 ip 192.168.103.254
 standby 3 priority 200
 standby 3 preempt
 standby 3 track GigabitEthernet1/0/1
!
interface Vlan104
 ip address 192.168.104.1 255.255.255.0
 standby 4 ip 192.168.104.254
 standby 4 priority 150
 standby 4 preempt
 standby 4 track GigabitEthernet1/0/1 1
!
interface Vlan105
 ip address 192.168.105.1 255.255.255.0
 standby 5 ip 192.168.105.254
 standby 5 priority 150
 standby 5 preempt
 standby 5 track GigabitEthernet1/0/1 55
DSW2#show running-config

interface Vlan101
 ip address 192.168.101.2 255.255.255.0
 standby 1 ip 192.168.101.254
 standby 1 priority 150
 standby 1 preempt
 standby 1 track GigabitEthernet1/0/1
!
interface Vlan102
 ip address 192.168.102.2 255.255.255.0
 standby 2 ip 192.168.102.254
 standby 2 priority 190
 standby 2 preempt
 standby 2 track GigabitEthernet1/0/1
!
interface Vlan103
 ip address 192.168.103.2 255.255.255.0
 standby 3 ip 192.168.103.254
 standby 3 priority 190
 standby 3 preempt
 standby 3 track GigabitEthernet1/0/1 50
!
interface Vlan104
 ip address 192.168.104.2 255.255.255.0
 standby 4 ip 192.168.104.254
 standby 4 priority 200
 standby 4 preempt
 standby 4 track GigabitEthernet1/0/1 55
!
interface Vlan105
 ip address 192.168.105.2 255.255.255.0
 standby 5 ip 192.168.105.254
 standby 5 preempt
 standby 5 track GigabitEthernet1/0/1

Question 1

Explanation

The question clearly stated that there was an issue with the decrement value in VLAN 102 so we should check VLAN 102 on both DSW1 and DSW2 switches first. Click on the PC Console1 and PC Console2 to access these switches then use the “show running-config” command on both switches

DSW1>enable
DSW1#show running-config

DSW2>enable
DSW2#show running-config

HSRP_show_run

As shown in the outputs, the DSW1’s priority is 200 and is higher than that of DSW2 so DSW1 becomes active switch for the group. Notice that the interface Gig1/0/1 on DSW1 is being tracked so when this interface goes down, HSRP automatically reduces the router’s priority by a configurable amount, in this case 5. Therefore the priority of DSW1 goes down from 200 to 195. But this value is still higher than that of DSW2 (190) so DSW1 remains the active switch for the group. To make DSW2 takes over this role, we have to configure DSW1’s decrement value with a value equal or greater than 11 so that its result is smaller than that of DSW2 (200 – 11 < 190). Therefore C is the correct answer.

Question 2

Explanation

Continue to check VLAN 101 on both switches…

HSRP_show_run_2.jpg

We learned that DSW1 doesn’t have the “standby 1 preempt” command so it can’t take over the active role again even if its priority is the highest. So we need to enable this command on VLAN 101 of DSW1.

Question 3

Explanation:

HSRP_vlan103

The reason DSW2 has not become the active switch for Vlan103 is because the priority value of DSW1 is higher than that of DSW2. In order to make DSW2 become the active switch, we need to increase DSW2’s priority (to higher than 200) or decrease DSW1’s priority (to lower than 190) -> B and C are correct.

But there is another requirement from this question that “A failure on gig1/0/1 on primary device should cause the primary device to release its status as the primary device, unless GigabitEthernet 1/0/1 on backup device has also failed”. This requirement makes answer B incorrect. For example, we choose to decrease the priority value on DSW1 to 160 (according to answer B) then DSW2 will become active switch (that is good). When Gi1/0/1 on DSW2 goes down, the priority of DSW2 will be 190 – 50 = 140 < 160 -> DSW1 will become new active switch (it is good, too). But when Gi1/0/1 on DSW1 also goes down, the priority of DSW1 will be 160 – 10 = 150 and it is still greater than 140 of DSW2 -> DSW2 cannot retake the active role as the requirement of this question.

Question 4

Explanation

Below is the output of VLAN 105:

HSRP_vlan105.jpg

If G1/0/1 on DSW1 is shutdown, its priority will decrease 55 so, its value will be 150 – 55 = 95

Question 5

Explanation

Below is the output of VLAN 105 of DSW2:

HSRP_vlan105_DSW2.jpg

We don’t see the priority of DSW2 so it is using the default value (100).

Question 6

Explanation

HSRP_vlan104

The question asks us how to keep the active role of DSW2. From the outputs, we learned that if both interfaces G1/0/1 of DSW1 and DSW2 are shutdown, the priority of DSW1 will be 150 – 1 = 149 and that of DSW2 will be 200 – 55 = 145 -> DSW1 will become the active switch.

The main point here is that we have to configure so in such a way that when both interfaces G1/0/1 of DSW1 and DSW2 are shutdown, the priority of DSW2 is still greater than that of DSW1. Therefore the priority value of DSW1 should be smaller than 145, or we have to configure the decrement value of DSW1 to a value greater than 6 ( 6 = 150 – 144) -> C is the correct answer.

Notice: To keep the active role of DSW2, we can disable “preempt” on DSW1 (answer A) so that it will not take over the active role when DSW2 is downed but it also means that VLAN 104 will not have active switch -> VLAN104 will fail.

Comments
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  1. Anonymous
    January 16th, 2019

    May I know that for Q1, why the answer is C but not A?

  2. Silas
    January 23rd, 2019

    for question 1, why isn’t “greater than 190 but less than 200” not a valid answer?

    If the priority for group 102 is set to 200, and it drops 199, router B will take over with a priority of 190.

    what am I missing here?

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