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BCMSN - HSRP & Stacked Switches

Hello,

I am preparing for the BCMSN exam, and am refering to the official cisco courseware v2.2.

In Module 6, (6-88 through 6-90) there is a section on HSRP and stacked switches, with the suggestion that you implement a loopback cable between stacked switches, incase of a L2 link failure.

This is all well and good.

However, it mentions that this is to maintain HSRP communication.

However, in the diagram, there is a L3 link between the 2 distribution layer switches (R1 & R2), that are in the HSRP group.

Now I would assume, that HSRP would rely on that L3 link to exchange hello messages on UDP port 1985 (i.e at Layer 3).

Thus the role of active and standby would not be compromized if there is a L2 failure at the access layer.

So am I right in assuming that the diagram is mis-leading ??

S1----------------R1

+.................|

S2............... | (L3)

+.................|

S3----------------R2

2 REPLIES

Re: BCMSN - HSRP & Stacked Switches

This is what I think is the reason

Your understanding is perfect regarding operation of HSRP. I guess the importance of loopback link is from the perspective of that stack. In case stack connection from AccessSwitch 1 to AccessSwitch 2 fails and there is no loopback cable connected. In this case, there are two possibilities

1. If we have connected only AccessSwitch 1 to both the L3 switches, then the stack will be effectively out of network.

2. If we have connected AccessSwitch 1 to L3 1 and AccessSwitch 3 to L3 2, then since the link from AccessSwitch 1 to L3 1 will no more be accessible to AccessSwitch 2 and 3, STP will get recalculated and these switches will start using link from AccessSwitch 3 to L3 2. This may cause some downtime for this stack (unless Uplinkfast is configured).

But, if we connect loopback link then since there will be redundancy at L2 connections within the stack, all this will not happen.

New Member

Re: BCMSN - HSRP & Stacked Switches

just a question regarding this? With the loopback cable not connected; if PVST is running, and VLAN 2 is primary for the "top" switch and VLAN3 is primary for the "bottom" switch, and they are standby for each other. in the case of a link failing, will the "top" switch be active for VLAN 3?

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