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Home > Archive > CCNA > February 2002 > Virtual lans Questions
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Virtual lans Questions
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| Tekwannabe 2002-02-07, 5:48 pm |
| Hi all I'm studying virtual lans right now and i'm using CCNA study guide by todd lamle
my question is:
I have a 1900 series switch
port 1-5: vlan A
port 6-10:vlan B
IF hostA (connected to port1)wants to ping hostD(connected to port6)but hostA doesn't know the MAC address(encapsulation) of the hostD how will hostA will ping hostD, I hostA is on VlanA which will only broadcast from port1-5.
I'm a little bit confused with Vlan but i hope i will really understand it. | |
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|  Tekwannabe
Here goes; you ready?
Think of VLANs A & B as separate ethernet LANS, to which you have connected 4 devices each. Each LAN will have its own IP address, and each device will have an address within that subnet.
Now, any device on either LAN can 'talk' to any device on the same LAN by sending an ARP request. Say for example, device #1 wants to talk to device #2 on LANA, but doesn't know #2 MAC address, the ARP request will resolve the MAC address of #2 from the IP address in the ARP request sent to #2.
Stay with this, now.
Becuase the LANs are separated, they will have different IP addresses, so to get an ARP request from device #1 on LANA to device #8 on LANB, takes another piece of equipment.
Remember that these LANS have been created 'virtually' within a Catalyst switch, but never the less, behave as if they were separate LANs. To get an IP address from one LAN to another requiires a routing function. In traditional etherent LANs this is achieved by connecting each LAN to an ethernet port on a router and providing routing between the LANs. In a Catalyst switch, the same can be achieved by installing an RSM blade - Router Switch Module - in the switch. This is often called a 'router on a stick'. On the RSM blade, you then create Virtual LAN interfaces, which are the virtual equivalent of physical ethernet interfaces. The simply provide routing between the interfaces and LANA can now talk to LANB.
Did this answer your question?
Cheers
Hippo
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| Tekwannabe 2002-02-07, 8:42 pm |
| What if they are on the same network, example hostA(vlanA) 192.168.1.1 255.255.255.0 and hostD(vlanB) has 192.168.1.2/24,Do they still need to go to router? | |
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| That is a very good question !
First of all, the PC in a different VLAN can be in the same subnet. But this is not a good design and should be avoid.
VLAN is layer 2 stuff. IP is layer 3 stuff.
In you question, when host A want to talk with host B. Host A will find that host B is on the same network (layer 3), then it will think that it can reach host B without routing.
Host A will broadcast an ARP request to solve host B IP address to MAC address. As these two host are on different VLAN, broadcast will not get to host B. Of course, the communication will fail.
But if these two VLAN are bridged together, the broadcast then can get across VLAN. This communication will success.
See, these two host are on the same network (layer 3) but different VLAN (Layer 2), you can utilize bridging to make it work.
Again, this is not a good design ! | |
| Hippo 2002-02-08, 12:38 am |
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The idea of VLANs is to separate LAN segments, so it would be counter-productive to have two VLANs in the same subnet. The switch could be configured for bridging as Hoon states, but I have researched that yet. Switching is my next CCNP topic - when I finally get through the Routing guide.
Hippo | |
| Tekwannabe 2002-02-08, 5:24 pm |
| quote:
Originally posted by Hoon
That is a very good question !
But if these two VLAN are bridged together, the broadcast then can get across VLAN. This communication will success.
See, these two host are on the same network (layer 3) but different VLAN (Layer 2), you can utilize bridging to make it work.
Again, this is not a good design !
How do you do bridging?
On real life scenerio how many host on the local network before i do VLAN? | |
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| One of the easy way to bridge these two vlan.
Connect two-arm router, each arm to each vlan.
!
interface e0
bridge-group 1
!
interface e1
bridge-group 1
!
!
bridge 1 protocol ieee
!
That's all.
Basically, if you find a lot of broadcast flooding in the LAN segment. That's the time to consider VLAN implementation. Some implement VLAN for policy-base networking. | |
| Tekwannabe 2002-02-11, 4:46 pm |
| Thanks... |
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