Re: DSL "aggregation".... NO

[Anton Kapela <tkapela () gmail com>]

On Apr 15, 2010, at 5:39 PM, Jack Carrozzo wrote:

> You can balance over DSL by putting different L2TPv3 tunnels over each
> physical device and agg it at someplace with real connections and
> such. It's possible to do it with GRE or OpenVPN too, but much less
> classy.

As Jack points out, "aggregating" xDSL links via l2tpv3 is possible. I'd like to suggest you consider this for a few 
other reasons, and mention that you needn't use v3 -- in fact, l2tp as implemented by Cisco IOS VPDN guts will 
transport layer-2 PPP in IP (or UDP+IP) without fuss. Here's a few reasons why you should consider l2 tunnel 
abstractions over your existing IP access:

a) l2tp + vpdn permits the use of MLPPP over IP -- which means, you get *packet sequencing* and packet ordering, for 
free, because this is what ML does when added to PPP.

b) with ML, you also get packet fragmentation support (i.e. split a single user 1500 byte packet into halves, each 
transported over l2tp tunnel sessions to the upstream/off-network box) -- this removes the need for l2tp endpoints to 
process fragments, at least when you have both DSLs (and 2 link members) up.

c) even if you were not using fragmentation + mlppp, the "inside" IP packet's DF field is not copied into the PPP 
header (it can't be), and so outer lt2p packet does not get its DF set either. Fragmentation would be confounded by an 
inner IP packet of a full MTU size + DF set, and thus, it is not supported.

Failing this, you can slum it with ECMP 0/0 route over both DSL links + NAT, or OER-type junk. This example doesn't 
suck and is easily adapted to "dialer" or other interfaces: 
http://www.blindhog.net/cisco-dual-internet-connections-without-bgp/

Same thing, restated in a more cisco-y way: 
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_configuration_example09186a00808d2b72.shtml

Finally, a great full-on OER (i.e. connection aware multi-egress point polling + FIB adjustment) example, which maybe 
more in line with what you want or expect your "dual dsl edge router" to provide: 
http://www.netcraftsmen.net/resources/archived-articles/468.html

-Tk