Now next we need to talk about the addresses that are going to be used by HSRP.
And we're not just talking about the virtual address because we're going to
configure
our own virtual address and each subnet and inside of each group.
But just like with a routing protocol, when HSRP wants to send messages back
and forth
to each other, it's going to do so via multicast.
And so we're going to need a multicast address to use.
And this is going to differ whether we're talking about HSRP version 1 or WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And this is going to differ whether we're talking about HSRP version 1 or
version 2.
Version 1 is going to use 224.0.0.2, which is known as the all routers multic
ast address.
This means that if I send a multicast packet destined for this, like an HSRP
version
1 node would do, that all layer 3 devices on the network are going to get that.
Now for the most part, we usually only have a couple of these on a user subnet
anyways,
the two of them being the default gateway, for example, or the pair of default WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the two of them being the default gateway, for example, or the pair of default
gateways.
And so maybe this isn't that bad, but from an efficiency perspective, we'd
rather not
send it to all layer 3 devices because there might actually be a lot more on
the network
segment.
So with version 2, Cisco actually used a Cisco owned multicast address that
they registered,
which is 224.0.0.0.102.
And this again is owned by Cisco.
And it's used exclusively for HSRP with a bit of a caveat to it. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And it's used exclusively for HSRP with a bit of a caveat to it.
So not just HSRP, but also GLBP, and we'll see that coming up when we talk
about that particular
protocol.
But it is used by Cisco devices.
So unless I'm running multiple protocols, which usually I'm not, we're talking
about
HSRP now when I send an HSRP message out into the layer 2 network, it's only
going to be
delivered to the other HSRP node or nodes.
For that matter, if we have a third or fourth node out there, those nodes will
receive it
and process it. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and process it.
And so the goal has been accomplished there.
Now we're not quite done yet because this is great from a layer 3 perspective,
but we do
still need to worry about layer 2.
So we're going to need that virtual MAC address that gets mapped to each or to
the virtual
IP address that we're also using on the network.
And so this is going to require that virtual MAC address configuration.
And what we'll find is that this group ID, for example, group 10, is going to
help us determine
what the virtual MAC is going to be. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
what the virtual MAC is going to be.
So once again, we have a version 1 and version 2 version, if I can say it that
way, version
1, virtual MAC address is going to be 0, 0, 0, 0, 0, c.
And then we have 0, 7, AC.
And then these last two characters can be defined by the group ID.
So when we take a group ID of let's say 10, we can translate 10 into hexadec
imal, which
would be 0a.
And 0a would insert itself at the end of the MAC address. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And 0a would insert itself at the end of the MAC address.
That would be the virtual MAC address we use for group 10.
So 20 would have a hexadecimal equivalent of 1, 4.
And so we would use the group ID to determine what the V MAC is.
But that raises an interesting point here.
Each hex character is 4 bits, which means we have 8 total bits to use for group
IDs.
Now, 2 to the 8th power means that I get 256 group IDs when I use HSRP version WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now, 2 to the 8th power means that I get 256 group IDs when I use HSRP version
1.
But next with version 2, we're going to find we have a few extra bits.
Because the V MAC that we use for version 2 is 0, 0, 0, 0, 0.0 c.
So the same 6 characters.
And then it's 9 F F. And then 3 hex characters reserved for the group ID.
Well 3 characters gives us 4 extra bits.
So that's 12 bits in total.
2 to the 12th power tells us that we get 4,096 group IDs. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
2 to the 12th power tells us that we get 4,096 group IDs.
And so we get a whole lot more group IDs with HSRP version 2.
And that might lead us to one to use HSRP version 2 over version 1.
Now that said, might be looking at this and saying, but when in the real world
would we
ever need more than 256 group IDs?
And the answer for the most part is we're not going to.
I'm very rarely will we have 256 VLAN interfaces or more, I guess, on a
particular layer 3 or
a switch.
So why do we need more group IDs? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So why do we need more group IDs?
Well it's really not about needing more than 256.
It's having access to group IDs that exceed 255.
So for example, let's say that I have VLAN 50 and I want to associate an HSRP
group with
that.
Well I would choose group 50 not from it because it's required, but because
from a human
perspective it's easier for me to look at HSRP group 50 and know that that ties
to VLAN
50. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
50.
So it's a human thing.
I could map this to group ID 51 and HSRP configuration has no problem with that
, but
from a human perspective we wouldn't really enjoy that mismatch.
But what if I have VLAN 500?
Well now with HSRP version 1 I have a problem because I can only go as high as
255.
So maybe I would choose group 50 or if I already have a VLAN 50 maybe I'd have
to go with group
150 maybe group 18.
I mean it doesn't matter at this point it's not going to match. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I mean it doesn't matter at this point it's not going to match.
And so that gets to be very difficult when I only have 256 group IDs.
Well with HSRP version 2 I have 4,096.
So I guess what that perfectly aligns with.
That perfectly aligns with the IDs that I can use for my VLANs because VLANs
have 12 bits
as well.
So now when I have VLAN 500 using HSRP version 2 I can match this up with group
500 using WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
500 using
again HSRP version 2.
And it doesn't matter what VLAN ID I choose I have a group ID that I can match
with it.
So 4,096 group IDs with HSRP version 2, 256 with HSRP version 1.
Again in a production modern network we should absolutely be using HSRP version
2.
And the number of group IDs that we get is just another reason for that.