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Aspiring Contributor
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BT Broadband network architecture

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Hi all,

I'm new to DSL, having had cable since forever, and I'm struggling a little with working out the concepts of the network design from both a physical and an IP perspective. I have Fibre 250. As far as I can work out, it looks (at a simplistic level)  like this:

            SH2 --> ONT --> DSLAM --> BRAS --> (ISP network) --> Internet router

The SH2 is clearly a router. The ONT looks like an L2 bridge (i know it's a lot more than that, the user guide is 400 pages...). the DSLAM is an L2 concentrator of some kind, and the BRAS seems to route traffic into the ISP network. I'm probably assuming that the PPPoE connection is to the BRAS.

Some questions:

1) My SH2's default gateway is 172.12.something. I think it is the same address that my Sophos UTM got when it was connected to the ONT. as both devices had the same address, it might seem that it is assigned to me as a subscriber or something else unique to me or my premises - is that the case?

2) my SH2 (and my UTM) both know what my internet address is (the SH2 says it's a 5.x.x.x address, UTM shows as an 86.y.y.y). How do they know that? advertised route from the BRAS? something inherent in PPPoE (no experience of it at all)

3) Why do the SH2 and UTM get in totally different ranges? I did consider that it might be related to the BT public wifi service, just a guess.

4) which device and interface in my simplified  description above hosts my 5.x.x.x internet address? the BRAS' ISP-side interface?

just want to understand a little more about what's happening, and hopefully draw a diagram of how it fits together.

Thanks

DM

 

 

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Sage
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Message 2 of 5

Re: BT Broadband network architecture

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Distinguished Sage
Distinguished Sage
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Message 3 of 5

Re: BT Broadband network architecture

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@darthmuppet wrote:

Hi all,

I'm new to DSL, having had cable since forever, and I'm struggling a little with working out the concepts of the network design from both a physical and an IP perspective. I have Fibre 250. As far as I can work out, it looks (at a simplistic level)  like this:

            SH2 --> ONT --> DSLAM --> BRAS --> (ISP network) --> Internet router

There is no DSLAM involved with FTTP, the DSLAM is just a means of injecting the DSL signal onto the copper voice channel and is located at the exchange for ADSL and at the cabinet for VDSL, its where copper becomes fibre. The ONT will connect directly to a fibre aggregation node which may or may not be your local exchange, via various splitter/combiners.

The SH2 is clearly a router. The ONT looks like an L2 bridge (i know it's a lot more than that, the user guide is 400 pages...). the DSLAM is an L2 concentrator of some kind, and the BRAS seems to route traffic into the ISP network. I'm probably assuming that the PPPoE connection is to the BRAS.

The ONT is just a fibre modem. Yes, PPPoe connection is to the BRAS

Some questions:

1) My SH2's default gateway is 172.12.something. I think it is the same address that my Sophos UTM got when it was connected to the ONT. as both devices had the same address, it might seem that it is assigned to me as a subscriber or something else unique to me or my premises - is that the case?

No, it is just the IP address of the BRAS and as such in the private 172.xx range.

2) my SH2 (and my UTM) both know what my internet address is (the SH2 says it's a 5.x.x.x address, UTM shows as an 86.y.y.y). How do they know that? advertised route from the BRAS? something inherent in PPPoE (no experience of it at all)

Your public IP address is dynamic and thus will change if you disconnect and re-connect to line.

3) Why do the SH2 and UTM get in totally different ranges? I did consider that it might be related to the BT public wifi service, just a guess.

See above, you will only get 1 address but BT have many different IP ranges

4) which device and interface in my simplified  description above hosts my 5.x.x.x internet address? the BRAS' ISP-side interface?

The WAN port of your router

just want to understand a little more about what's happening, and hopefully draw a diagram of how it fits together.

Thanks

DM

 

In addition to @pippincp  link, have a look at the various links within this page https://kitz.co.uk/adsl/


 

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Aspiring Contributor
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Message 4 of 5

Re: BT Broadband network architecture

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That's excellent, many thanks. Now I look a little closer, the internet address is presented on my end of the ppp virtual interface - I'd not seen that before, probably because I wasn't looking for it!

 

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Aspiring Contributor
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Message 5 of 5

Re: BT Broadband network architecture

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Not totally, but that's exactly the kind of information I am interested in. i see I have a lot of reading to do 🙂

Many thanks.

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