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You need to know GRE for the CCNA exam.
Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol network.
Now in packet tracer we can see what actual packets looked like.
On PC 1, PC on the left I’m going start it ping to PC 2. So I’m going to ping PC 2 from PC 1; that works. But what I’ll do now is go into simulation mode and then I can do the ping again. Notice the packet is queued on PC 1 to be sent out of the router, that’s what the packet looks like. It’s got a source IP address of 10.1.1.2 destination of 10.1.2.2 that’s correct. The local IP address of this PC is 10.1.1.2 and we’re sending traffic to the remote PC 10.1.2.2
I’m going to click capture forward packet is sent to the router and this is what the packet looks like on the router, inbound PDU it looks like this or inbound Protocol Data Unit looks like this. Traffic is coming from PC 1 going to PC 2 but on outbound, notice source IP address is Customer Router 1, destination IP address is Customer Router 2.
And on egress or outbound out of the router, now have a GRE header and a encapsulated IP header. So the original inbound Protocol Data Unit or inbound information, this IP information is encapsulated inside an external header.
So going right to the top, Ethernet 2, this is an Ethernet link between Customer Router 1 and ISP 1. External IP header is this. Encapsulated in that we have GRE and we have the encapsulated original IP packet. Click capture forward, on this router something very similar happens. Notice inbound PDU contains the outer headers of Customer Router 1 talking to Customer Router 2 with the encapsulated original IP packet.
Outbound something very similar is going to be used but what you’ll notice is the MAC addresses will change. So from one interface to another, on the router, the source MAC address will change because we’re sending the traffic across a separate Ethernet link.
When it gets to this router, something very similar, all that happens here is the MAC addresses are being changed, other information changes but we won’t worry too much about that. The big thing to note is the routers are routing based on the external IP headers.
It gets to Router 3, something very similar, inbound PDU is still using these IP addresses and so is the outbound PDU or Protocol Data Unit. When it gets to this router however, things change. So on this router, inbound PDU has the eight network IP addresses and the encapsulated IP addresses. But the outbound PDU has the GRE information stripped.
So what you’ll notice is inbound, we have these IP addresses. We have the GRE header and we have the internal IP packet information. But outbound, there is no GRE and we don’t see Network 8 in the list of IP addresses. The outer headers have been removed from the packet and that’s what the client is going to see.
The client PC only sees the RFC 1918 addresses. It doesn’t see Network 8 and these routers on the Internet use network 8. So, if I go back to see this router here and look at to the packet, notice inbound PDU and outbound PDU both are using public IP addresses. But if I go back again to this router, this is where the encapsulation is taking place. Inbound PDU contains RFC 1918 addresses, outbound PDU has public IP addresses, a GRE header and the original encapsulated packet.
When we go forward to here, packet is being encapsulated. So inbound we have IP header , GRE IP header but outbound we don’t see the public IP addresses, we don’t see the GRE headers. That’s been stripped. So headers are added here, headers are removed here.
So how did you get on? Were you able to complete the lab? Did you get your GRE tunnel up and running?