VLANs on L3 Switches

Unanswered Question
Oct 24th, 2007

Just by enabling IP Routing on a L3 Cisco Switch Inter Vlan Communication takes place,then what is the use of creating Vlan's on a L3 Switch.Then why to create Vlan's at all,why not to use a single LAN Subnet then,except from Access-Lists is there anyother way to stop Inter Vlan communication.

I have this problem too.
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bvsnarayana03 Wed, 10/24/2007 - 07:10

Significance of L3 VLANs is to segregate a network in to various subnets. Simply by turning on "ip routing", u cant have VLANs communicating with each other.U 'll have to run a routing protocol & advertise those subnets that need intercommunication.

& if u want to restrict communication between specific hosts, u do that by ACLs.

susheel.p Tue, 10/30/2007 - 06:57

i have checked it today on Cisco 3560 Switch.Just by enabling IP Routing and not using any Routing Protocol Inter VLAN Communication took place.I was able to Ping between 2 PC's connected in two different VLAN's.

Jon Marshall Tue, 10/30/2007 - 08:25

Hi

This is what you would expect. If all the inter-vlan routing is happening on the same switch you do not need a routing protocol because all the L3 vlan interfaces are directly connected.

Where a routing protocol comes in use is if you have multiple L3 switches/routers that all need to exchange routes.

HTH

Jon

MUHAMMAD SHAHEEN Wed, 10/24/2007 - 10:53

Hi,

You can also do this by not assigning the Ip address to that particular Vlans SVI. Say for example you have L2 Vlan 45 created on the Switch but do not create Interface VLAN45 and do not assign IP address. And if you have created Interface VLAN45 the shut it down.

Alternatively to this will be to use Access-Lists to stop/filter the IP addresses as you wish.

HTH

Shaheen

Joseph W. Doherty Wed, 10/31/2007 - 17:51

One advantage of L3 subnets on the same L3 switch is containment of broadcasts and multicast.

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