Mac address daily limit

setch · 06-02-2010, 22:19

Hi, Does anybody know what the maximum number of MAC addresses that can connect in a single day?

I have an issue whereby my router will not obtain an ip from the NTL Modem, but if I connect a laptop directly it gets an ip ok.

I several laptops and have connected each one directly to try and resolve my connection problem. So it is possible that I may have busted this limit and will need to power everything down overnight.

Thanks
Setch

Sephiroth · 06-02-2010, 22:28

You have to set up the router dcorrctly for Dynamic IP with DHCP server on. Is that what you've done?

Then the router will share the VM issued IP address among the laptops.

The cable modem is concerned then only with the router's MAC address not that of the PCs.

Ignitionnet · 06-02-2010, 22:36

Not really anything to do with his question Seph, which was that he's connected various devices directly to his modem and has now used all the MAC addresses up that he's allowed to have associated with his modem.

Could you perhaps clone the MAC of one of the laptops, preferably one you connect via wireless

to the router perhaps?

The limit used to be either 3 or 4 so you've almost certainly broken it. Powering stuff down doesn't make them go away and won't make any difference, time will.

Try the MAC cloning. I can't remember offhand the timeout.

Sephiroth · 06-02-2010, 23:52

Quote:

Originally Posted by Broadbandings

Not really anything to do with his question Seph, which was that he's connected various devices directly to his modem and has now used all the MAC addresses up that he's allowed to have associated with his modem.

Could you perhaps clone the MAC of one of the laptops, preferably one you connect via wireless

to the router perhaps?

The limit used to be either 3 or 4 so you've almost certainly broken it. Powering stuff down doesn't make them go away and won't make any difference, time will.

Try the MAC cloning. I can't remember offhand the timeout.

Didn't know there was such a limit. Well, well.

Ignitionnet · 06-02-2010, 23:58

Prevents a single modem from noshing down all the available DHCP leases - remember each new MAC address connected to a modem = an IP address from the DHCP scope (subnet of IP addresses available to a particular cable interface or bundle of cable interfaces) occupied for the duration of the DHCP lease. Have to draw the line somewhere.

KingDaveRa · 07-02-2010, 00:48

The modems are layer 2 devices then? They have the local management IP on them, which I can see, but I'm guessing Virgin can't. Unless there's another IP on the network-facing side.

Ignitionnet · 07-02-2010, 09:35

Quote:

Originally Posted by KingDaveRa

The modems are layer 2 devices then? They have the local management IP on them, which I can see, but I'm guessing Virgin can't. Unless there's another IP on the network-facing side.

Yep that's why they're modems not routers

The modems do have an IP address reachable on the network facing side, they grab one for their own use for downloading their configuration file, etc, on some kit you see the modem's gateway when you ping your first hop. Your traffic doesn't go through that hop it's just where some kit responds from. The modems' network facing connectivity is purely a layer 2 interface.

Cable downstream, remember, is a broadcast medium. All modems in a segment see all traffic. The Virgin kit switches traffic at layer 2 to customers' PCs but this is actually a broadcast, similar to an old school hub, on the physical cable. If a modem doesn't have that MAC address behind it it simply drops the traffic, if it does it switches the traffic to your PC / router. Likewise if the traffic is destined to the 10.x address the modem took at boot up it'll do the necessary.

Peter_ · 07-02-2010, 10:07

The host table on the uBR carries a maximum of 4 MAC's and if you exceed this you can wait 24 hours or call in and 2nd line will clear the host table.

KingDaveRa · 07-02-2010, 10:49

Quote:

Originally Posted by Broadbandings

Yep that's why they're modems not routers

Oh yeah.... duh

Quote:

Originally Posted by Broadbandings

The modems do have an IP address reachable on the network facing side, they grab one for their own use for downloading their configuration file, etc, on some kit you see the modem's gateway when you ping your first hop. Your traffic doesn't go through that hop it's just where some kit responds from. The modems' network facing connectivity is purely a layer 2 interface.

Cable downstream, remember, is a broadcast medium. All modems in a segment see all traffic. The Virgin kit switches traffic at layer 2 to customers' PCs but this is actually a broadcast, similar to an old school hub, on the physical cable. If a modem doesn't have that MAC address behind it it simply drops the traffic, if it does it switches the traffic to your PC / router. Likewise if the traffic is destined to the 10.x address the modem took at boot up it'll do the necessary.

I'd forgotten about the fact they broadcasted traffic. How big would a segment be though? I know the old thing used to be people would say it could be slow if everybody in your street were downloading, but there must be a slightly more specific level for deciding how big a segment is than just an entire street. There's about 40-odd houses in my street, yet there's others round here into the hundreds. Also, there's two types of cabs round here; there's the smaller pitched-roof ones in each street, then larger flat-roofed ones which seem to sit on the main roads. I've seen inside the smaller ones and it's just the taps, and some Krone strips for the telephones. Afaik they then all link up to the bigger ones which do the fibre coax conversion. My guess would be each segment is multiplexed into the fibre at that point, then punted up to the CMTS location where it's split back out, then fed into it. All the CMTS I've seen online have been coax-based, but I'm sure there must be some that can take the fibre coax link as is.

That's my guess anyway

Ignitionnet · 07-02-2010, 10:55

The segment size would vary. From 250 up to 4000 homes passed maybe?

The only real limitation is the quality of the amplification and optics. So long as a signal can be amplified and kept within acceptable noise levels one can cover as many homes as they wish but the above is a practical limit.

Just to really cook your noodle each broadcast domain doesn't necessarily correspond to fibre, in the case of the DOCSIS 3 network its' downstream signal is split between multiple optical transmitters at the hubsite and goes to multiple fibre optic nodes.

The Cisco 10k's DOCSIS 3 output on the VM network is optical, however it feeds into an external QAM modulator which converts the digital signal into digital over RF and outputs it coaxially.

Simplified it's CMTS <Coax> Optical Transmitter <Fiber> Fibre Optic Node <Coax> Home.

The placement of the fibre reflects its' original intention when all coaxial networks were overbuilt, it is there to reduce the amount of coaxial amplifiers needed in the network, thereby reducing the amount of amplifier induced distortion on the network. Instead of a string of amplifiers from hubsite to customer a single laser replaces a load of them leaving just a few in series to get from node to customer premise.

KingDaveRa · 07-02-2010, 11:04

Well there's got to be a finite amount of bandwidth on a wire with a given standard, and depending on the users you could cover more or less.

I'm not overly surprised it does get split out into multiple places. It makes sense to catch those oddball places out on a limb with a handful of connections, or use spare capacity in a segment to cover a new housing estate on the other side of town for example. It's probably a form of VLANning.

Ignitionnet · 07-02-2010, 11:07

No, it's not VLANing really, the closest to that you find on the cable network is frequency division multiplexing. For example My DOCSIS 3 cable modem runs on 4 downstreams, 299, 307, 315, 323MHz. A DOCSIS 1.1 cable modem can go onto any one of those and not share downstream capacity with other DOCSIS 1.1 modems on other channels.

The cable network really isn't that smart, think of an old BNC thicknet and you're about there.

The combining of DOCSIS 3 network over multiple nodes was as much as anything else VM being cheap and wanting to save cash on line cards. You should be mindful of the distinction between physical segmentation, how many homes are connected to a node and therefore on the same physical broadcast domain, and logical segmentation, such as taking the DOCSIS 3 output and splitting it to feed multiple physical nodes.

KingDaveRa · 07-02-2010, 11:20

Oh now I see.

Well I'd imagine those line cards are pretty pricey. I know most Cisco blades for their big switches are usually five figures, so they probably saved a lot of money. Plus all the extra chassis and infrastructure they wouldn't have had to buy. From a purely commercial standpoint, I can see why they did it.

Ignitionnet · 07-02-2010, 11:51

Quote:

Originally Posted by KingDaveRa

Oh now I see.

Well I'd imagine those line cards are pretty pricey. I know most Cisco blades for their big switches are usually five figures, so they probably saved a lot of money. Plus all the extra chassis and infrastructure they wouldn't have had to buy. From a purely commercial standpoint, I can see why they did it.

Me too, shame as I alluded to back in 2008 it's caused issues with upstream congestion now. Someone forgot the stop off in between DOCSIS 1 and DOCSIS 3, the upstream increasing DOCSIS 2, and just bunged DOCSIS 3 kit either end of a load of networks that were only really DOCSIS 1 ready.

Fine until too many modems got onto the DOCSIS 3 platform and killed the single DOCSIS 1.1 upstream channel available to each node, then BT were rude enough to start rolling out 10Mbps upstreams which necessitates DOCSIS 2 upstream to compete with so thanks to that commercial decision it's now catch up time both in terms of present capacity requirements and future ones.