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How does VTP work between broadcast domains?

OREOspeedwagon (MIS) (OP)
13 May 03 18:44
I was just wondering about this. Say you have two subnets separated by a router. On both sides of the router lay many switches. If I wanted to implement a VTP domain in order to replicate VTP information, how would those multicast VTP messages pass through the router in order to sync up the other side?

I'm assuming they can't.

So, how do you do this? Can you enable VTP on a router so it can pass the VTP advertisements?
IPKONFIG (Instructor)
13 May 03 22:04
VTP operates through VTP messages (multicast messages) sent to a particular MAC address. Note that VTP advertisements only travel through trunk ports. Therefore, VTP information only flows through the Inter-Switch Link (ISL), 802.1q port, or LAN emulation (LANE), when the trunk is up, after Dynamic Inter-Switch Link (DISL) or DTP convergence.

VTP messages are only carried through VLAN 1.

"I can picture a world without war.  A world without hate.  A world without fear.  And I can picture us attacking that world, because they'd never expect it."
- Jack Handey, Deep Thoughts

OREOspeedwagon (MIS) (OP)
14 May 03 12:01
Thanks for the reply!

But...

All the things you've mentioned are things that I already understand regarding VTP. But it still doesn't answer my question. Maybe I should rephrase my question...

How does VTP information get advertised through routers?

I realize that VTP is layer 2 data and is only carried via trunk ports. But, what happens in a routed environment? You may want to have one VTP domain, but if you have several routers, they would obviously block the layer 2 VTP multicasts.

So, I'm wondering... Can a router be added to a VTP domain so then it too will pass VTP advertisements, therefore giving contiguous VTP connectivity throughout the network?
IPKONFIG (Instructor)
14 May 03 12:17
Theoretically…yes.  You can pass VTP information through routers.  Although I don’t understand why you would want a single huge VTP domain.   This would be like removing your routers from your network and have one flat layer 2 network.  

To answer your question more specifically, you’d have to bridge the traffic with bridge groups.  But this would bridge all your traffic and not just the VTP portion that you’re interested in.  

I hope this helps.

"I can picture a world without war.  A world without hate.  A world without fear.  And I can picture us attacking that world, because they'd never expect it."
- Jack Handey, Deep Thoughts

IPKONFIG (Instructor)
14 May 03 12:26
Oh...I forgot to tell you that you can bridge on any interface, including any serial interface, regardless of encapsulation.

Bridging can be configured between interfaces on different cards, although the performance is lower compared with interfaces on the same card. Also note that serial interfaces must be running with HDLC, X.25, or Frame Relay encapsulation.

"I can picture a world without war.  A world without hate.  A world without fear.  And I can picture us attacking that world, because they'd never expect it."
- Jack Handey, Deep Thoughts

Twinmoon (TechnicalUser)
14 May 03 23:32
I'm curious on how one would do that.  I didn't think you could bridge traffic, could you fill in the commands on how to do that?  My first thoughts went to ip-helper address but that's for broadcasts.

--I guess I'm just nosey.
irasman (Instructor)
14 May 03 23:37
(Shoop) sound made by putting boots on.
Ok so VTP as you know is a layer 2 trunking protocol that is advertised on all trunk ports via 802.1q. Propagation is controlled by VTP domains. Messages have to be the same version. VTP members must be locally adjacent.
So how do get from one VLAN to another?
Well you have to have a router with at least one fast ethernet interface. then configure the interface.
int f0/whatever.1
encap isl
ip add <give it. a .valid. ip address>
Configure additional interfaces as necessary

Router <protocol>
network
no auto-summary
wala we are now trunked.
IPKONFIG (Instructor)
15 May 03 8:26
Here is how you bridge traffic on your routers.  Cisco explains it really well and gives examples for various types of bridging.  

http://www.cisco.com/en/US/products/sw/iosswrel/ps1824/products_command_reference_chapter09186a0080080681.html

"I can picture a world without war.  A world without hate.  A world without fear.  And I can picture us attacking that world, because they'd never expect it."
- Jack Handey, Deep Thoughts

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