On Jul 6, 2004, at 4:52 PM, Joshua Graessley wrote:
I think there is some confusion between VLANs and subnets. The multiple
subnets Jens is referring to are IP subnets. An IP address is
associated with a subnet. The subnet effectively defines which other IP
addresses are local at the IP layer (don't have to go through a router
to reach).
I think it is better to define a VLAN as a layer 2 broadcast domain. A
subnet is usually associated with a specific ip address block/subnet
mask. Multiple subnets can be overlayed in the same VLAN. Link local
multicast will be independent of the specific subnet the a host is
using for its unicast traffic. I will be able to iChat with people from
ten different subnets if they are all overlayed on the same VLAN.
Typically, though, most networks only use one subnet per VLAN. Our
network actually uses two per VLAN, a subnet for registered users and
one for unregistered users, but I digress...
Rendezvous uses IP link-local multicasts. These multicast packets are
sent to every node on the "link". A link, for ethernet, is defined by
how far an ethernet multicast packet will travel.
I would define it by how far an ethernet broadcast will travel. IGMP
snooping and CGMP can constrain multicast traffic to only ports that
are interested in receiving the traffic by sniffing the IGMP join
messages or getting multicast group information directly from the links
multicast router.
VLANs let you create multiple networks at the link layer. This
effectively takes one link and adds a special header to make it look
like there are actually multiple links. Multicast DNS will not work
beyond a single link. Since a VLAN effectively limits the scope of a
link, the VLAN limits the scope of dot-local names and services.
I think you are thinking more of 802.1q and ISL which allow you to
trunk multiple VLANs across the same point to point link. In this case
you have a situation where multiple VLANs are existing on the same
physical link, different from the multiple subnets on the same logical
network.
Peter John Hill
Senior Network Engineer
Carnegie Mellon University
-josh
On Jul 6, 2004, at 1:24 PM, Matt Harrington wrote:
On Tuesday, June 29, 2004, at 01:49 PM, Jens Alfke wrote:
mDNS uses link-local multicast and (to avoid spoofing) discards any
incoming packets with a TTL < 255. This means you won't be able to
use
mDNS through a router. It's possible to set up multiple subnets on
the
same switch and mDNS would work with those; but you probably don't
want
to do that for an entire building with 20 subnets.
So it's impossible to use the current implementation of Rendezvous
across a router, even when VLANs are implemented? When I say VLAN, I
mean a collection of nodes in the same broadcast domain even if
they're on different network segments separated by a router. I spent
some time investigating exactly what link-local multicast is, but I
didn't see how it ties in with VLANs.
---Matt
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