This topic covers HDLC and Point-to-Point Protocol (PPP) as Layer 2 WAN protocols, explaining how they differ from Ethernet and how they operate over T1 serial connections between routers.
HDLC & PPP on Cisco Router
I have a router here that I'm going to connect into my network. This is a Cisco 1841. Now, the Cisco 1841 has two Ethernet ports. This is a layer 3 device, meaning that it is designed to route traffic on layer 3 using IP addresses. It will route traffic to where it needs to go. But since I have two Ethernet ports on here, it provides layer 1 and 2 Ethernet services. Ethernet operates on layer 1 and layer 2, so that is what I'm mainly going to be using on my networks.
However, that's not the only thing that this is capable of providing. I also have a card in here, so this gives it extra functionality. There's two spaces here where I can slip cards in here to provide extra functionality, and what I have is a card slipped into here which is a T1. Key one is a layer 1 protocol call, and so it is that CSU/DSU, or that data communication equipment, that is integrated into here. So that is what I'm going to be making connection between my routers with. So I'll use my Ethernet to connect into my local area networks, my LANs, for my business, but I'm going to use this T1 to connect from router to router.
Now that's the layer 1 technology, and this is a layer 3 device. That still leaves layer 2 that we need to configure on here. And the two things that I'm going to configure on these Cisco devices is going to be HDLC and PPP, point-to-point protocol. What we're going to do is we're going to configure this T1 interface, and we're going to configure our point-to-point protocol and HDLC protocols at layer 2.
We're actually going to compare it to Ethernet, because we've talked a lot about Ethernet, so let's just compare and contrast the difference between these protocols and Ethernet.
Up until now we've been configuring a router, a few switches, some pies and a demo laptop, so this is what we currently have right here. Although what we also will see is that we have these different connections here that we need to establish across our network. And what I'm going to do, so we can understand that there are different protocols out there, is we're going to set up our T1 lines across here, and we're going to set up HDLC across one of them and point-to-point protocol across the other.
So what will happen is the data that gets sent, let's say it's going from this demo laptop to the switch over here, it's going to carry up to this layer 3 switch right here. From this layer 3 switch the traffic is going to be routed to this router that's right here, and then from this router it's going to be routed across this HDLC line and then across this point-to-point protocol line, and then into this network right here, which is going to be another Ethernet network. And so across here we have this LAN connection and then it will switch. And once again, if we recall, we actually tear down that layer 1 and layer 2 and have to reform it, and this is going to show us how that gets reformed into these different protocols to be sent across to a whole other network.
You don't really need to know HDLC and point-to-point protocol real in depth. Both of those are very simplistic and we don't really need to cover it to any huge degree, but I thought let's just take a glance at some of the differences.
One of the differences we see is within the frame and what the frame looks like, and it answers a lot of our questions when we look at the frame. So here I have an Ethernet frame right here, a layer 2 Ethernet frame, because remember Ethernet operates on both layer 1 and layer 2, so this is the layer 2 part of this Ethernet frame, versus HDLC and point-to-point protocol, which are just layer 2. And then it goes across what we're going to be going across as a T1. You could have some sort of serial connection, or there are other types of connections as well.
So the Ethernet frame has a destination MAC address, which is six bytes, and a source MAC address, which is six bytes. You can see that with HDLC and point-to-point protocol, there's just an address field. It's just a destination address, there is no source address, and it's just one byte. HDLC and point-to-point protocol have a few minor variances, but they're very similar protocols.
So we can see that the header on this Ethernet frame is much larger, and the trailer can be a little bit smaller on these HDLC and point-to-point protocol frames. So there's some differences that we see within there. And then we see a little variation between this point-to-point protocol and HDLC: they both have control fields, and this one has a protocol field for point-to-point protocol.
And that address, as we saw in those frames, is much simpler. HDLC is just all ones, point-to-point protocol is all ones, versus Ethernet uses the MAC address, which can vary depending on what the equipment is that it's being sent on. So there you have it, that's some of the differences between Ethernet, HDLC and point-to-point protocol.
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