Unanswered Question
Jun 21st, 2008

Dear all

i have 3750 which does interVLAN routing and a default route for internet with DHCP enable

now what should be good for redundancy purpose


one more thing in VTP does the VTP update the IP address in the VLAN to the client

i have some more question that will come after this

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mahmoodmkl Sat, 06/21/2008 - 04:43


Do u have another 3750 or a router so that u can implement redundancy.

Only HSRP is used for default gateway redundancy.

VTP is used for centralize management of Vlan information.

STP is used for loop avoidance.

'one more thing in VTP does the VTP update the IP address in the VLAN to the client

i have some more question that will come after this'

Can u please elaborate more on this.



Richard Burts Sat, 06/21/2008 - 07:44


No VTP does not update any IP addresses. VTP works only at layer 2 and does not do anything with IP addresses.

Your question about VTP and IP addresses sounds more like it is related to DHCP. Your original post says something about DHCP. But I am not clear whether the 3750 is using DHCP to learn an IP address on its outside interface or whether the 3750 is set up to run DHCP for clients. Perhaps you can clarify?

I agree with Mahmood that a more clear explanation of your objectives would be helpful. What kind of redundancy are you looking for? Is it redundancy at layer 2, or at layer 3, or what? And his point about whether there is a single device or multiple devices is an excellent point. If there is a single device how can there be redundancy?



bh@rg@vdes@i Mon, 06/23/2008 - 02:42

Thanks for your replies,

Here is the senario -------

i have two 3750 switches.

i have configure 5 Vlan with different ip pools.

one 3750 switch is configured as L3 for interVLAN routing and a default route to the internet.

i want redundancy for that switch.

now my questions are as below

1.) if i run VTP on the switch, now in the VTP client does the VLAN have the same VLAN ip as in the server, as the VLAN database is update?

2.)Now If i configured my switches in HSRP for redundancy. IF any of my Active or Standby path goes down, it will give me SNMP alert or not ?

3.)can we do ether channel on two different physical switches?


mahmoodmkl Mon, 06/23/2008 - 03:39


See u can provide redundancy to the clients in u r vlans ie. using HSRP u can provide default gateway redundancy.

1.VTP is used for centralize management of vlans.If one of u w switch is acting as VTP server n the other one as a vtp client and a trunk link is placed between them then the vlans created on the vtp server will be propogated to the client.

No the vlan IP's will not be propogated to the other switch u need to define the vlan IP as on the other switch as well.

U can configure SNMP to send HSRP traps.

If u r saying that using two ports on a switch and connecting them to the other two yes u can create ether channel.



Richard Burts Mon, 06/23/2008 - 04:17


If you have 2 3750 switches then you could configure both of them to operate in layer 3 mode and configure HSRP on the various VLAN interfaces of both switches. This would provide gateway redundancy to the end stations. If one switch were to fail then end stations connected to the other switch would still be able to communicate with each other and to route between VLANs of that switch.

It is not clear in your description whether both switches have a connection to outside or whether only 1 switch has the connection to outside. Be aware that if it is only 1 switch with the outside connection then your redundancy is limited. If that switch goes down then every end station loses its ability to talk to the outside world.

As Mahmood points out, VTP does not have anything to do with IP addresses. And if you want to do Etherchannel between the switches that is supported on the 3750 switches.



bh@rg@vdes@i Mon, 06/23/2008 - 05:00

Thx for early replies,

i got all the points and that will do for me. Thanks again all the genius.

Now about Ether Channels, My question is about three Switch Ether channel means, a port from Switch A and Switch B can make ether channel to Switch C as Switch and Switch B is connected as for the HA.


cyphur353 Mon, 06/23/2008 - 06:03

Etherchannel technology allows you to bundle multiple ethernet links together. This creats a single logical link, which load balances traffic across the multiple physical links.

You can mesh the switches, where A<->B, B<->C, A<->C, so that all switches have connectivity to each other, and then let STP sort the topology out to be loop-free. Or you can statically set the root via priority if you prefer.

vsaavedra Mon, 06/23/2008 - 20:04

Are your 3750 stacked or setup via a trunk. If they are stacked the redundancy you are looking for is pretty much built in. In a stacked solution the configuration is kept the same across both switches. If the Master switch in the stack were to fail the secondary switch takes over the Master role. You would just need to dual home your router to both switches and run a dynamic routing protocol between the switches and router advertising the default route to the web. If they aren't stack you can stick dual home your router and run the dynamic routing protocol between the router and switches and then VTP between your two switches.

bh@rg@vdes@i Mon, 06/23/2008 - 21:12

i dont have swiches stacked.

can give me idea of DUAL HOME and how that can be done.

How we can do dual home if i do not have switches stacked.



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