exchanging vlan information between two layer 3 switches

[zaidiatifs]

hi people ,

i have a query about exchanging vlan information between two layer three switches , so in packet tracer i have connected two layer 3 switches, they have their own vlans configured .

each switch's vlans are able to ping each other using inter vlan routing ,  the ports which connect both switches together are configured with trunk ports, still the Vlan information is not getting transferred from one switch to another .

am i doing something wrong here .

i am also attaching the running config of both the switches .

please advise and thanks in advance .

[Bilal Nawaz]

Hello Atif, you may need to have some sort of routing protocol between your layer 3 switches so they can exchange routing information. This means, instead of your trunks you have routed interfaces.

If you have a trunk, the trunk will only be carrying layer 2 information, it is not aware of layer 3 at this stage, so how will it route to other vlans? Since you have inter-vlan routing you are able to ping from the vlan on one switch to another vlan on the same switch. But you do not have routing between switches!

To make this work, I would implement a routing protocol like OSPF as an example. I would configure it like this:

on SW0:

conf t

!

interface fa0/7

no switchport

ip address 10.10.10.1 255.255.255.252

no shut

!

router ospf 1

router-id 10.10.10.1

network 10.10.10.1 255.255.255.255 area 0

redistribute connected subnets

on SW1:

conf t

!

interface fa0/5

no switchport

ip address 10.10.10.2 255.255.255.252

no shut

!

router ospf 1

router-id 10.10.10.2

network 10.10.10.2 255.255.255.255 area 0

redistribute connected subnets

• First we made the trunk interfaces instead to be routed interfaces.
• Then we enabled OSPF. This should create an adjacency with both layer 3 switches so that routing information can be exchanged.
• Then we specified the 'redistribute connected subnets' command.

Now all the connected subnets i.e. all vlans will be advertised to the other switch, and vice versa, so you should have end to end connectivity.

I hope I understood your question correctly.

Hope this helps

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[zaidiatifs]

thanks very much Bilal ...i can now see the end to end connectivity

just to let you know i am training for CCNA .....one more question that i have for you is what is the significance of using the command  "redistribute connected subnets" under OSPF process  .

i am sure we are trying to advertise external routes beyond the switches , but just wanted to be clear on this

firstly i did not enable this command , on the OSPF process >> and i could not see the complete routes

but once i enabled it i can see the complete routes  ( thanks to you again ).

[Bilal Nawaz · Accepted solution]

Hello Atif,

With the 'redistribute connected subnets' command, we are telling the OSPF process to advertise all the connected routes that we have, in to OSPF.

So that means any routes that look like this:

C        1.1.1.0/24 is directly connected, Vlan 100

because this is a connected network direct to a vlan, it will advertise this out also in OSPF. Otherwise if you did not do this, then how will OSPF know how to advertise it to its neighbour? And as you saw, it did nothing, until you told it to redistribute : )

Hope you're CCNA studies go well, and best of luck - keep it up!

Please see here for more details of redistribution:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009487e.shtml

Hope this helps.

Please rate useful posts and remember to mark any solved questions as answered. Thank you.

[zaidiatifs]

thanks very much and   all the best for   CCIE !!!!

[Bilal Nawaz]

Thank you Atif

Please rate useful posts and remember to mark any solved questions as answered. Thank you.