Native VLAN and trunk port.

[subharojdahal]

I read in one of the forum that it may create a problem if I allow native vlan through the trunk port connecting two switch.

But I am still not getting exact consquences of doing so?

[Jon Marshall]

Switch A will forward traffic to vlan 25 on it's root port if C/D are STP root for vlan 25. Root ports lead to the root switch. Designated ports lead away from the root switch.

If native vlan was 25 then the packet would be sent across the trunk link untagged. Not a good idea to have servers in the native vlan though.

Jon

[subharojdahal]

Jon

As I told you before there is active EIGRP protocol among those core switches. I can see different route from one switch to other ( and from one vlan to other)

I didn't see any sense of configuring EIGRP in MSFC of those switch. I am sorry but kinda confused in L2 trunk and L3 routing.

I hope you will help me to get the hell out from this confusion.

[Jon Marshall]

Yes it is a bit confusing.

When you run the command "sh trunk" on switch A what do you see ?

You should see a link to B.

And if you are correct you should see a link to C and a link to D.

But i'm thinking you may be using L3 routed links from A&B -> C&D and using EIGRP to exchange routes between your switches. If that is the case then ignore what i said before as to how the packet get from A1 to S1.

Key thing to work out is what are the links from A&B to C&D.

Jon

[subharojdahal]

Let me give you clear picure of what I have and what I am trying to do.

Currently, I have link from A to B, B to C, C to D. I have EIGRP running and trunk links among all connection. C and D are core switches. All are 6513 catalyst.

What I wanted to do

--------------------

As I told you earlier, I wanted to make core network fully redundant by connecting A<-->C, A<-->D, B<--->C, and B<-->D.

On the basis of above scnario

-------------------------------

I just wanted to know from you that what are the isseues that i have to know other than just configuring each new link as trunk and assignging same native vlan on either side of new trunk (redundant) trunk link.

I would appreciate your help jon.

[Jon Marshall · Accepted solution]

Subharoj

You need to decide whether you want to route or switch between the 2 pairs of switches.

What i would do with the information you have given so far is to connect A -> C and A -> D, B -> C, B -> D with L2 trunk links.

A & B would not have a trunk link between them but C & D would be interconnected via a L2 trunk.

Then migrate the L3 interfaces + HSRP configuration off A&B and move it on C&D. So A&B are not routing for any vlans any more they are simply access switches connecting back via L2 trunks to C&D.

This would give you a more standard setup.

Alaternatively you can leave the link between A&B and leave it routing for vlan 125 etc. and make the links back to C&D L3 routed links. And then run EIGRP between all the switches. Note that with CatOS you cannot actually have a L3 routed port ie. "no switchport". What you do is use a /30 subnet and create an SVI on each switch eg.

Switch A

int vlan 200

description L3 connection to C

ip address 192.168.5.1 255.255.255.252

Switch C

int vlan 200

description L3 connection to A

ip address 192.168.5.2 255.255.255.252

and then assign the port on A & C into vlan 200. Then repeat with a different subnet for

A -> D

B -> C

B -> D

Each has their advantages and disadvantages. If your switches are all running CatOS i would use the first option of L2 trunk uplinks and migration of L3 interfaces off A&B to C&D.

Jon

[subharojdahal]

Thanks lot jon

You gave me a very good solutions. but i got a different work place where I work as consultant. Its hard for them to tolarate downtime other that larger than time taken by STP to converge.

My concerned is what happen if I create trunk link for my new connection and let the EIGRP do its work. Considering the fact that I know root switch for each vlan and wondering if EIGRP could forward packet through designated trunk port among core switches.

They have more than 20 local vlan under switch A and B, so they wanna move to core C and D.

jon, Is that possible keeping current configuration intact and creating new trunk line and let the EiGRP does its work??

[subharojdahal]

jon

I know you must be busy. I would appreciate if you look at my issue if you few moment.

[Jon Marshall]

I appreciate that you are trying to avoid downtime so we need to be careful. What you can do is take option 2 from my previous e-mail. That would be the least disruption to your network.

I would suggest using an unused vlan for the uplinks. Do not make them trunk links because they don't need to be.

This is assuming that vlans on C&D don't need to be on A&B and vice-versa. If you have vlans that need to be on all 4 switches at the same time then you will have to make the links trunk links.

Jon

[subharojdahal]

thanks lot.