>"Vicky Rode" shaped photons and electrons to say:
>
>
>see comments in-line:
>
>
>-----Original Message-----
>From: Stephen Hoover [mailto:[EMAIL PROTECTED]]
>Sent: Monday, February 17, 2003 11:20 AM
>To: Vicky Rode
>Cc: [EMAIL PROTECTED]
>Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
>
>
>> > Say for instance I have 2 hosts on the same layer 3 switch, but the >
>two
>> hosts are on 2 different IP subnets (No VLANs are defined).
>>
>> That's not possible! if you are talking about 2 IP subnet, than:
>> -------------------------
>> actually it is by doing secondaries, but i would highly recommend doing
>> vlans if possible. keep it clean and simple.
>>
>
>
>Vicki,
>
> You mention the use of secondary IP's. On a L3 switch (a switch with
the
>router engine in it) is it not possible to define Ethernet sub interfaces
>instead of using secondary IPs - without VLANs defined?
Yes and no. Secondaries and VLANs serve different purposes.
Basic IP assumption: 1 physical medium[1] = 1 subnet
Secondary assumption: 1 physical medium[2] = multiple subnets
Basic VLAN assumption: multiple phyical media [3] = 1 subnet
VLAN with secondaries: multiple physical media = multiple subnets on all
Notes
-----
[1] Based on the "local versus remote" IP assumption: if a host is on
your subnet, you have layer 2 connectivity to it. if a host is on
a different subnet, you need to reach it through a router.
This works nicely for broadcast and point-to-point media. NBMA
and demand circuits break the local-vs-remote assumption.
If you do assume a broadcast* medium, then the physical medium =
1 broadcast domain = 1 subnet
(* broadcast is used loosely -- multicast is often closer. Some
stupid NICs don't recognize multicasts and treat all multicasts
as a broadcast. Broadcasts, indeed, are special cases of
multicasts.)
[2] The medium simultaneously must support a broadcast domain for each
subnet, unless it is a non-broadcast medium.
[3] The media in different locations are assumed to be linked by L2**
trunking, typically IEEE 802.1q. While the trunks do contain
traffic from multiple subnets, they are effectively tunneled. The
only multicasts on the trunk medium are for layer management functions,
such as 802.1d, 802.1q, VTP, etc.
(** there are exotic variants where you could carry trunking over
a conventionally routed tunnel, but let's not go there.)
>----------------------------
>yes you can but when you create sub-interfaces it ask for encapsulation type
>and this is where vlans come into play.
Encapsulation type is one reason to use VLANs, because it does create
different broadcast domains for each encapsulation. This is
preferred, but Cisco certainly has supported secondaries for
different encapsulations -- more an IPX than an IP support technique.
>whereas with secondaries it will
>route between the subnets.
>
>
>
> I'm sorry to be so thick, I'm just not getting it. If a L3 switch (with
>a routing module/engine in it) is essentially a wire speed router, then the
>VLAN just seems like an additional identifier on top of the L3 address - and
>doesn't really serve any purpose.
Not exactly. It lets you have the _same_ broadcast domain in several
L2 switches. That's what gives you the portability of hosts from
VLAN (same subnet) to same VLAN in different buildings. There need be
only one router on the subnet, but there can be multiple VLAN
segments connected by trunking.
>In my previous example, 2 hosts on the
>same L3 switch, but on 2 different IP subnets - wouldn't a defined Ethernet
>subinterface be each clients respective gateway, and thus normal L3 routing
>would occur, just at switch speeds????
>---------------------
>....well let me you ask this, why not just supernet and put all stations on
>the same subnet (don't do this i'm being facetious).
>
>that's because you do not want to create this huge broadcast domain. that's
>the whole purpose of having vlans.
>
>if this still doesn't make sense, feel free to ask...would love to help.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=63259&t=63147
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