A host connected to switch (2950) cannot communicate to A host which is connected to another switch (2950)

[chiragom2341]

Hi All,

I'm preparing the CCNA exam and using cisco packet tracer to implement the network. I know the question what I'm going to ask is very common but I couldn't find out exactly what is going on, I need your help !

The connection in packet tracer looks like this :- host A connected to Switch A -> Switch A connected to Switch B -> Switch B connected to host B

I can see the active links between two switches but cannot see the active link from the switch to hosts. Why is that ? I have also set up the ip address into both host. Is there problem of spanning-tree or any kind of loop ? Generally, when we connect the host to switch, just after few seconds we can see the active connection (green light).

Also, in VLAN we use trunk line to carry the all vlans traffic then why we need to use ip address for vlans to  set up inter vlans communication.

Example : there are two switches connected each other through cross over cable and each one has two vlans. now if we set up trunk mode on cross over cable by using the command -> switchport mode trunk then why we need here to set up ip address for vlan 1 and 2 ? as we know here the trunk mode will carry the all vlans traffic.

Please clarify me !

Many Thanks,

Chirag

[Reza Sharifi · Accepted solution]

Hi,

why we need here to set up ip address for vlan 1 and 2 ? as we know here the trunk mode will carry the all vlans traffic

That is because since you have 2 vlans, each vlan needs to have an SVI interface on one of the switches, so the hosts can talk to their gateways (svi interfaces) and than SVIs can communicate with each other. The SVI interface is the IP address you assign to the host's default gateway. If you just has one vlan, than there would not be any need for an SVI with IP address.

As for your first question why the host physical interface are not coming up, check the ports that the hosts are connected to and make sure they are not shut down.  Also, make sure there is no speed and negotiation mismatch between the hosts ans the switches.

HTH

[chiragom2341]

Many thanks Reza for replying !

So, I can say Vlan creates the separate network and as we know to communicate between separate network we need "Router or layer 3 switch".

What is purpose of trunk then ?  Will it use just to carry the vlans traffic ? if yes then same things can be achieve through router or layer 3 switch, right ?

[Reza Sharifi · Accepted solution]

Hi,

That is correct. If you want the vlans to communicate you need a layer-3 interface or an SVI.  SVI applies to a switch with layer-3 capability or a router with a physical port and and an IP address. 

The purpose of a trunk is to carry multiple vlans between the switches.  If you only have one vlan that needs to be carried between the switches, you don't have to have a trunk.  You can simply have a link between the switches and configure it as an access port.  If you need to carry more than one vlan you need a trunk port.

Hope it all makes sense.

[chiragom2341]

Thank you ! All clear now :-)

[Reza Sharifi]

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Thanks,