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Silver

How Host A talks to Host B

Hi Experts ,

I would like to know how Host A talks to Host B (on what factors & commands comes into picture) in the following scenarios.

1.Host A - switch(L2) - Host B

2.Host A -switch(L2)-switch(L2) - Host B

3.Host A-switch(L3)-switch(L3)-Host B.

when trunk is configured between L3's.

when access link is configured between L3's.

Thanks,

satish

5 REPLIES
Silver

Re: How Host A talks to Host B

1.Host A - switch(L2) - Host B

host A compares its IP @ with the one of host B. if they belong to the same subnet, host A broadcasts an ARP request to know the mac @ of host B. host B responds with ARP reply. now host A can send the frame and the switch switches it according to its switching table.

2.Host A -switch(L2)-switch(L2) - Host B

same thing but in this case sw1 will send the frame to sw2. this later sends the frame to B.

3.Host A-switch(L3)-switch(L3)-Host B.

in this case host A and B are in different subnets. sw1 is the gateway of A and sw2 is the gateway of B. when A detects it is in a different subnet, it will send the frame to sw1 (mac destination = sw1 mac ; IP dest= B). so sw1 IP must be configured as gateway for A.

sw1 routes the paquet to sw2. sw2 broadcasts ARP request to search for B mac....

when trunk is configured between L3's.

no sens. trunk is configured between L2's.

when access link is configured between L3's.

usually

Cisco Employee

Re: How Host A talks to Host B

Hi Satish,

Lot depends if the host B and Host A are on same vlan/subnet or different? Also it really does not matter if you use L2 switches between host A & B or L3 switch between them if you have layer 2 link between the switches let it be l3 switch or l2 switch behavior remains same.

So please confirm if your hosts are in same vlan or different. If they are in different scenario 1 and 2 will not work and scenario 3 will work if you have configured routing (inter vlan routing).

Also assuming they are on same vlan , host A and B source mac addresses will be listed in switch mac address table so when host A want to communicate to host B , it will try to learn host B mac address sending an ARP request and once host B response to host A arp request , host A will send the frame out with host B mac address as destination mac address and once that frame reaches the switch it will check the mac address table and find out that mac address is listed in mac address table for host B and then will also come to know the interface number where host B is connected and frame will be sent to that interface only.

It will work the same way when host A and host B are on 2 different switches but now the mac address of host B will be learned on switch A uplink port so any frame when hits switch A it will fidn out that mac address in destination feild in frame is learned on uplink port and this frame will leave switch A and hit switch B and then switch B will be having host B mac address already listed on port to which it is connected and will receive the frame from on that interface.

HTH

Ankur

*Pls rate all helpfull post

Silver

Re: How Host A talks to Host B

Thanks Ankur ..

I have few doubts and get back to you soon.

HostA-Switch(L3)(accesslink)switch(L3)-host b

Assume that access link is configured between two L3 switches and uplink port is in vlan 15.

in first switch we have vlan 15 vlan 2 and vlan 3 , 10.10.10.1 / 24 , 10.10.20.1/24 & 10.10.30.1 / 24 respectively.

In Second switch we have vlan 15 , vlan 5 & Vlan 6 , 10.10.10.2 /24 , 10.10.50.1/24 , 10.10.60.1 /24.

How come 10.10.50.5 of 2'nd switch knows about 10.10.20.2 of 1'st switch.It is necessary to do the intervlan routing on both switches ?

I know that access link carries only one vlan info.

Thanks,

satish

Cheers :)

satish

Cisco Employee

Re: How Host A talks to Host B

Hi Satish,

Good design is to have inter vlan routing on one switch and configure that switch with all vlans as in your case vlan 15, vlan 2, vlan 3 and vlan 6 layer 3 interfaces should exist on same vlan for inter vlan routing to work.

Or else you can play with routing protocols configured on both the switches and they will advertise vlan 2 and vlan 5 information across the switches.

Or else you can configure static routes also on sw 2 and sw 1 to reach respective interface.

HTH

Ankur

*Pls rate all helpfull post

Bri
New Member

Re: How Host A talks to Host B

Hi Ank,

Good Day dude.

I've got a question for you.

How would I restrict and route multiple voice VLANs to Data VLANs?

Im using do1q encapsulation for voice switch (3560) and data switch(2950). Both these switches are connected to L3 Core Switch (3750). Lastly L3 switch is connected to 2851 which will be used to route VLANs to Head Office.

How would I configure 2851 router to route different VLAN if hosts (IP Phone and PC) to the head office?

Also, does Native VLAN carries traffic of different VLANS?

Your help will be very much appreciated.

Cheers.

Regards,

Brian

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