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I'm having a hard time understanding some "basic" things...

I know a bit about configuring routing using CISCO packet tracer exercises (static routers, rip, ripv2, ospf, eigrp), but I have a doubt that I really need help figuring it out.

During the CISCO routing lessons I had, I always configured LAN's with routers on the edge of the LAN, connected using Serial ports between them and using only L3 (routing only) between every router (static routes or some routing protocol configured to reach other networks). I always thought that was what my ISP router did to reach other networks, routing my traffic only. But from what I see, my ISP delivers the Internet traffic to the WAN port on my ISP router using a VLAN, so they tag the Internet packets. Shouldn't be pure routing? I never used a VLAN when configuring routing protocols, I only used it inside LANs (so behind the edge router), to route vlans inside a LAN (routing on a stick), but never to route them outside.

The other question is:

I have a FTTH connection and my Public IP is (I will change it because of security purposes): 82.125.100.200 and from what I can see on the router configuration the subnet mask is /24, and the gateway is 82.125.100.1.

The thing is: why when I do a tracert on my machine, to for example, google.com (or any other website), the 2 hop (the 1º is my router 192.168.1.254) is a private ip: 10.252.128.1? The 2 hop shouldn't be the gateway of the router: 82.125.100.1?

I'm sorry my questions doesn't make much sense? But right now I'm very confused :(

Thanks to every one!!

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  • "my ISP delivers the Internet traffic to the WAN port on my ISP router using a VLAN, so they tag the Internet packets." As Ron pointed out, you are confusing the network layers. Layer-2 frames may have tags, but layer-3 packets do not. The layer-3 packets can be carried by any layer-2 protocol, included VLAN-tagged ethernet, and the packet have no idea, nor do they even care, in which layer-2 protocol they are carried. A router strips off the frame, and it routes the packets, so the routing process never sees the frame with the VLAN tags.
    – Ron Maupin ♦
    Apr 5, 2018 at 20:18
  • To answer your last question, we would need to know your router model and configuration. Simply edit your question to include that.
    – Ron Maupin ♦
    Apr 5, 2018 at 20:19
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin ♦
    Jan 7, 2021 at 0:55

1 Answer 1

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But from what I see, my ISP delivers the Internet traffic to the WAN port on my ISP router using a VLAN, so they tag the Internet packets. Shouldn't be pure routing?

You're confusing layer 2 and layer 3 (A common mistake). The layer 2 technology -- serial port, ethernet, ethernet trunk, DSL, satellite, or piece of string -- does not affect layer 3. You can have IP use any of those layer 2 media. It's still IP routing.

In the past, it was easy to define LAN and WAN by their technologies: serial ports were always "WAN." Now that Ethernet is so ubiquitous, the distinctions are harder to identify which is which, because Ethernet is everywhere.

The 2 hop shouldn't be the gateway of the router: 82.125.100.1?

Can't say for sure without seeing the router configuration. If you don't manage the router, the question is off-topic here.

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  • The thing is: I always thought VLAN's weren't routed after leaving my router, via the WAN port, to the ISP network, because on CISCO exercises we only configure routes to forward packets between routers, interfaces have IP's, not VLAN's. We only do VLAN routing inside the LAN, never past the router to the WAN. I'm still having trouble understanding this... Feels bad. About the router, I "manage" the router configuration but with limitations, because the router is the one provided by the ISP. Although I can see the configuration and his public IP add + his gateway to the ISP network.
    – John
    Apr 5, 2018 at 20:00
  • I really don't know how to explain myself better, but in a simple way, I always thought ISP's routed the traffic without using VLAN's. From what I see, that's not the case, they tag VLAN's and then they route them... Like a intervlan routing in big scale?
    – John
    Apr 5, 2018 at 20:12
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    You're confusing your terms. VLANS are not routed. They are layer 2 constructs. The VLAN only exists between your router and the ISP router. VLANs can carry IP, which is routed.
    – Ron Trunk
    Apr 5, 2018 at 21:28
  • @John Here's an example that might clarify things - in one room at work (not an ISP, we are testing network equipment) we have a core router with about 800 VLANs on it. Then all of those VLANs are distributed using a bunch of switches so that one VLAN goes to each port. At layer 2 we have a bunch of switches, but at layer 3 it just looks like one router with 800 ports. The IP protocol only sees what's on the other end of each VLAN. Like a tunnel.
    – user253751
    Sep 11, 2018 at 10:34

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