Just for clarity, VLAN's are a layer 2 concept and IP is of course a layer 3 (please
do not start with the "but what layer is arp again" :)
Despite subnets and VLAN's generally happening on a 1:1 basis in a lot of theoretical
and practical discussions, the two concepts are totally unrelated and altogether
unaware of each others presence. An IP host will not detect a node is on another VLAN
and hence send to the gateway, it will detect a node is on another subnet. It doesn'
t really care if the node is in the same broadcast domain or halfway around the world,
if its not on the network, its sent via the gateway. This is very strict behavior.
Nodes on different IP subnets do not communicate directly in any case without the use
of an intermediary, layer 3 device.
VLANs as a concept are of trivial complexity. VLAN membership, particularly dynamic
membership along with protocols like 802.1q, ISL, PVST etc that leverage and support
VLANs do offer some element of challenge and opportunity for best practise designs.
I just felt that the line between VLANs (broadcast domains) and IP subnets was getting
somewhat blurry when it really shouldn't be.
*********** REPLY SEPARATOR ***********
On 1/16/2001 at 10:19 AM Curtis Call wrote:
>Keep in mind that seperate VLANs will be seperate subnets. Which means
>that by default a host will encapsulate any IP packet destined for a
>different VLAN within an ethernet packet with a destination MAC address of
>the default gateway. So a layer 2 switch will never get the chance to try
>and "switch" between VLANs since everytime a host needs to get to a
>different VLAN (subnet) it will just send a packet to the router which is
>on the same VLAN in order for it to be routed.
>
>
>
>
>>-----Original Message-----
>>From: Bob Vance [mailto:[EMAIL PROTECTED]]
>>Sent: Tuesday, January 16, 2001 8:35 AM
>>To: CISCO_GroupStudy List (E-mail)
>>Subject: why is routing needed with VLANs
>>
>>
>>OK.
>>I must be brain dead, today.
>> (and, yes, Chuck, I *have* had my morning dose of Diet Coke :)
>> and, yes, I know, "What's so special about 'today' "?
>> )
>>As far I can understand it so far, about the only benefit that I see
>>from VLANs is reducing the size of broadcast domains.
>>
>>Suppose that I have a switch in the closet with one big flat address
>>space (well, it couldn't be that big with only one switch, now, could
>>it ?>). Then someone says,
>> "You know, we're getting a lot of blah-blah broadcast traffic.
>> Let's VLAN.
>> "
>>OK, fine. We VLAN and put whatever services in each VLAN that are
>>required to handle the broadcasts (e.g., DHCP service). So, now the
>>switch doesn't send broadcasts outside a particular VLAN.
>>
>>But, what's so magic about a VLAN that the switch also decides not to
>>send unicasts outside a VLAN. Before the VLANs, the switch maintained
>>a MAC table and knew which port to go out to get to any unicast address
>>in the entire space. So, why can't it continue to do that after we
>>arbitrarily implement some constraint on broadcast addresses?
>>It seems to me that the same, exact MAC table, with an additional VLAN
>>field would not require that restriction. If it's a broadcast, send =
>>the
>>packet only out ports with a VLAN-id that matches the source port's
>>VLAN-id. If it's a unicast, handle it just like we used to.
>>
>>
>>Similarly, even if we have 5 switches, I just don't see the requirement
>>that we (as switch-code designers) must block unicasts and resort to a
>>routing requirement.
>>
>>Even with 500 switches ... well, let's not get ridiculous :)
>>
>>
>>I feel that there is a simple point that I've overlooked, so I will
>>continue to RTFM while I await your responses.>)
>>
>>
>>-------------------------------------------------
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>>BV=A0=A0=A0 =A0=A0=A0=A0 | <mailto:[EMAIL PROTECTED]>
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>>=3D
>>
>>
>>
>>
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Peter A. van Oene
Juniper Networks Inc.
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