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Now next we need to talk about the addresses that are going to be used by HSRP.

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And we're not just talking about the virtual address because we're going to

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configure

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our own virtual address and each subnet and inside of each group.

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But just like with a routing protocol, when HSRP wants to send messages back

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and forth

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to each other, it's going to do so via multicast.

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And so we're going to need a multicast address to use.

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And this is going to differ whether we're talking about HSRP version 1 or
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And this is going to differ whether we're talking about HSRP version 1 or

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version 2.

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Version 1 is going to use 224.0.0.2, which is known as the all routers multic

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ast address.

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This means that if I send a multicast packet destined for this, like an HSRP

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version

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1 node would do, that all layer 3 devices on the network are going to get that.

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Now for the most part, we usually only have a couple of these on a user subnet

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anyways,

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the two of them being the default gateway, for example, or the pair of default
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the two of them being the default gateway, for example, or the pair of default

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gateways.

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And so maybe this isn't that bad, but from an efficiency perspective, we'd

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rather not

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send it to all layer 3 devices because there might actually be a lot more on

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the network

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segment.

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So with version 2, Cisco actually used a Cisco owned multicast address that

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they registered,

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which is 224.0.0.0.102.

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And this again is owned by Cisco.

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And it's used exclusively for HSRP with a bit of a caveat to it.
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And it's used exclusively for HSRP with a bit of a caveat to it.

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So not just HSRP, but also GLBP, and we'll see that coming up when we talk

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about that particular

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protocol.

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But it is used by Cisco devices.

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So unless I'm running multiple protocols, which usually I'm not, we're talking

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about

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HSRP now when I send an HSRP message out into the layer 2 network, it's only

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going to be

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delivered to the other HSRP node or nodes.

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For that matter, if we have a third or fourth node out there, those nodes will

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receive it

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and process it.
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and process it.

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And so the goal has been accomplished there.

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Now we're not quite done yet because this is great from a layer 3 perspective,

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but we do

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still need to worry about layer 2.

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So we're going to need that virtual MAC address that gets mapped to each or to

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the virtual

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IP address that we're also using on the network.

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And so this is going to require that virtual MAC address configuration.

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And what we'll find is that this group ID, for example, group 10, is going to

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help us determine

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what the virtual MAC is going to be.
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what the virtual MAC is going to be.

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So once again, we have a version 1 and version 2 version, if I can say it that

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way, version

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1, virtual MAC address is going to be 0, 0, 0, 0, 0, c.

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And then we have 0, 7, AC.

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And then these last two characters can be defined by the group ID.

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So when we take a group ID of let's say 10, we can translate 10 into hexadec

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imal, which

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would be 0a.

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And 0a would insert itself at the end of the MAC address.
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And 0a would insert itself at the end of the MAC address.

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That would be the virtual MAC address we use for group 10.

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So 20 would have a hexadecimal equivalent of 1, 4.

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And so we would use the group ID to determine what the V MAC is.

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But that raises an interesting point here.

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Each hex character is 4 bits, which means we have 8 total bits to use for group

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IDs.

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Now, 2 to the 8th power means that I get 256 group IDs when I use HSRP version
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Now, 2 to the 8th power means that I get 256 group IDs when I use HSRP version

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1.

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But next with version 2, we're going to find we have a few extra bits.

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Because the V MAC that we use for version 2 is 0, 0, 0, 0, 0.0 c.

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So the same 6 characters.

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And then it's 9 F F. And then 3 hex characters reserved for the group ID.

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Well 3 characters gives us 4 extra bits.

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So that's 12 bits in total.

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2 to the 12th power tells us that we get 4,096 group IDs.
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2 to the 12th power tells us that we get 4,096 group IDs.

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And so we get a whole lot more group IDs with HSRP version 2.

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And that might lead us to one to use HSRP version 2 over version 1.

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Now that said, might be looking at this and saying, but when in the real world

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would we

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ever need more than 256 group IDs?

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And the answer for the most part is we're not going to.

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I'm very rarely will we have 256 VLAN interfaces or more, I guess, on a

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particular layer 3 or

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a switch.

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So why do we need more group IDs?
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So why do we need more group IDs?

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Well it's really not about needing more than 256.

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It's having access to group IDs that exceed 255.

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So for example, let's say that I have VLAN 50 and I want to associate an HSRP

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group with

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that.

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Well I would choose group 50 not from it because it's required, but because

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from a human

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perspective it's easier for me to look at HSRP group 50 and know that that ties

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to VLAN

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50.
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50.

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So it's a human thing.

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I could map this to group ID 51 and HSRP configuration has no problem with that

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, but

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from a human perspective we wouldn't really enjoy that mismatch.

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But what if I have VLAN 500?

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Well now with HSRP version 1 I have a problem because I can only go as high as

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255.

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So maybe I would choose group 50 or if I already have a VLAN 50 maybe I'd have

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to go with group

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150 maybe group 18.

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I mean it doesn't matter at this point it's not going to match.
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I mean it doesn't matter at this point it's not going to match.

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And so that gets to be very difficult when I only have 256 group IDs.

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Well with HSRP version 2 I have 4,096.

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So I guess what that perfectly aligns with.

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That perfectly aligns with the IDs that I can use for my VLANs because VLANs

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have 12 bits

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as well.

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So now when I have VLAN 500 using HSRP version 2 I can match this up with group

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500 using
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500 using

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again HSRP version 2.

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And it doesn't matter what VLAN ID I choose I have a group ID that I can match

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with it.

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So 4,096 group IDs with HSRP version 2, 256 with HSRP version 1.

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Again in a production modern network we should absolutely be using HSRP version

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2.

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And the number of group IDs that we get is just another reason for that.