nanog mailing list archives
Re: PPPoE over L2TP over GigE questions
From: Francois Menard <francois () menards ca>
Date: Tue, 17 Jun 2008 07:20:14 -0400
Actually, with AGAS, there are no tunnel switches anymoremultiple tunnels are set-up directly netween Juniper ERXes aggregating DSLAMs and acting as LAC's and the ISPs LNS's receiving the L2TP tunnels.
This is one giant step towards TR-101, but Bell won't accept to do this f. On 11-Jun-08, at 3:37 PM, Robert E. Seastrom wrote:
Jean-François Mezei <jfmezei () vaxination ca> writes:Pardon my ignorance on the subject, but I would need to know how packetsbetween a BAS/LAC and an ISP's router are transported (this is within Bell Canada ADSL territory).Bell uses L2TP to link each BAS/LAC to the ISP. Some of the ISPs get aGigabit Ethernet link to the Bell cloud.Actually, they don't set up connections directly from the BASes and SMSes anymore. I'm quite sure they've got some old Redback kit still out there too, as well as perhaps some other ancient stuff. You're going to be talking to a tunnel switch (TSW2-TORONTO63 for instance). These are all Juniper ERXes to the best of my knowledge. N number of BAS/SMS devices talk to a TSW, which talks to your LNS. This cuts down drastically on the number of tunnels that you have to manage (Bell has a couple of hundred BASes out there last I checked). Brings the number of tunnels (and VLANs) down to a couple of hundred. The tunnel switch is smart enough to look inside the authentication packets at session start time and switch you properly based on the realm the customer is logging into.Would the L2TP payload be an ethernet packet which contains a PPPoE packet, or would the L2TP payload be the PPPoE packet only ?My recollection is that it includes the src/dst MAC addresses and the rest of the ethernet header in the L2TP frame.Also, while I am at it: Architecturally, is a BAS considered a router, or a bridge/switch ? (since the PPPoE packet has no routing information (source, destination), it is the BAS which maintains the table ofsource/destination for each PPPoE session ID. Yet, the BAS machines aresupposedly Juniper ERX routers in Bell territory...I'd call them VPN endpoints for a layer 2 VPN; thus the functionality they're providing is more like a bridge than a router, notwithstanding their peeking into layer 5.And while I am at it:From the end user point of view, the ADSL modem sends all ATM frames toa predetermined ATM destination (VPI/VCI). I assume that VPI/VCI pointsto the BAS.Yes, and at that point it's PPPoEoATM. Which gets turned into PPPoEoATMoL2TP on the upstream side of the BAS.How does the BAS address ATM packets back to an individual subscriber ? Do each subscribers get their own VPI/VCI that points to the right porton the right DSLAM ?Nothing that's visible on the upstream side of the BAS - it's all src/dst mac address at that point.And in cases where the telcos are extending the ethernet to the DSLAM, with the fragmentation into multiple ATM frames limited to the ADSL link itself, how does the BAS address invididual customers ? Does each ADSLport on the DSLAM get its own ethernet address ?the ADSL router has its own ethernet address.(since some services do not use PPPoE, I have to assume that the DSLAMdoesn't base its packet switching on PPPoE session IDs.)These other services are VLAN-per-customer and don't use PPPoE or L2TP at all. I think we looked at these and decided not to use 'em. You may be thinking too deeply about this though. Contact me offline if you want a working redacted config for Cisco kit talking to Bell Canada. :-) -r
-- François D. Ménard francois () menards ca
Current thread:
- PPPoE over L2TP over GigE questions Jean-François Mezei (Jun 11)
- Re: PPPoE over L2TP over GigE questions Ingo Flaschberger (Jun 11)
- Re: PPPoE over L2TP over GigE questions Robert E. Seastrom (Jun 11)
- Re: PPPoE over L2TP over GigE questions Francois Menard (Jun 17)
- Re: PPPoE over L2TP over GigE questions Robert E. Seastrom (Jun 17)
- Re: PPPoE over L2TP over GigE questions Charles N Wyble (Jun 17)
- Re: PPPoE over L2TP over GigE questions Francois Menard (Jun 17)
- Re: PPPoE over L2TP over GigE questions Francois Menard (Jun 17)