First of all, if I send this twice, excuse me...I am trying out outlook
express and I am not sure it is sending anything...but I have a couple of
questions and comments.
Questions:
So the only reason vlans are implemented then is for a "type of subnet" that
controls broadcasts from a layer 2 standpoint and for no other reason other
than that. And the benefits would be increased bandwidth for the network
since it is a form of broadcast control?? Is that correct?
Routers (layer 3 switches) are only necessary when communication between vlans
is necessary?
And if that is so, routers are unnecessary in this type of network, unless
they are there only for the purpose of connecting different vlans in the same
network.....
I am reading the last sentence you wrote and it is confusing to me. "VLAN's
that determined membership based on IP address would be a challenging thing to
accomplish." In school, I did this..at least I think I did..if I understand
you correctly. When you say based on Ip address, do you mean they are
implemented at router level based on the ip addresses and that is how
membership is determined?? By way of router access lists? And then routed to
the correct ports of the switch?
Jennifer
-----Original Message-----
From: Peter Van Oene [SMTP:[EMAIL PROTECTED]]
Sent: Thursday, January 18, 2001 8:08 AM
To: Ruben Arias; [EMAIL PROTECTED]
Subject:
To me, there is no concept of a layer three VLAN. If you chose to route IP,
you need a router, whether you have dynamic or statically configured broadcast
scopes is fully irrelevant. If you are talking about dynamic VLAN membership
based on IP address (or protocol for that matter), then I will agree that some
level of layer 3 and potentially above awareness is required to identify the
address or protocol. However, any such application that I have seen (mostly
Xylan) performed this at the switch level.
Given most networks are running DHCP, or moving in that direction, VLAN's that
determined membership based on IP address would be a challenging thing to
accomplish.
*********** REPLY SEPARATOR ***********
On 1/18/2001 at 9:21 AM Ruben Arias wrote:
>VLANs can be defined by MAC address or IP address.
>When MAC address is used, you have a layer 2 VLAN, when IP address is used
you have a layer 3 VLAN and a router is needed.
>Layer 2 VLANs mostly used for filtering (never done, I supose is a hard work
to mantain)
>
>
>Peter Van Oene wrote:
>
>> 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.>)
>> >>
>> >>
>> >>-------------------------------------------------
>> >>Tks=A0=A0=A0 =A0=A0=A0 | <mailto:[EMAIL PROTECTED]>
>> >>BV=A0=A0=A0 =A0=A0=A0=A0 | <mailto:[EMAIL PROTECTED]>
>> >>Sr. Technical=A0Consultant,=A0 SBM, A Gates/Arrow Co.
>> >>Vox 770-623-3430=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A011455 Lakefield Dr.
>> >>Fax 770-623-3429=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 Duluth, GA 30097-1511
>> >>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
>> >>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
>> >>=3D
>> >>
>> >>
>> >>
>> >>
>> >>_________________________________
>> >>FAQ, list archives, and subscription info:
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>> >>_________________________________
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>> >_________________________________
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>>
>> ---
>> Peter A. van Oene
>> Juniper Networks Inc.
>>
>> _________________________________
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>
>_________________________________
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Peter A. van Oene
Juniper Networks Inc.
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Have a Good Day!!
Jennifer Cribbs
[EMAIL PROTECTED]
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