SVI ip needed?

[Scott_O'Brien]

Hey all,

I have 2 layer 3 switches connected to each other over an l3 WAN link, switch A has a DHCP server connected to it with Svi's set up and dhcp helper addresses, my qustion is on the second switch to get to that dhcp server will it need ip addresses on its SVI or will a DHCP helper and a static route be fine?

[Vaibhava Varma]

Hi Scott

From the above description the setup seems to be like this as below :

DHCP_Server----------L3_Switch1-----------L3_Link--------------L3_Switch2---SVI_LAN---------LAN_Users

SInce the Link between the L3 Switches is L3 I would presume that the LAN Users are connecting to L3_Switch2 and they have SVI for them whose IP Address will be defined as the GW for the DHCP Pool for that particular SVI_LAN. So, Yes we would need IP on the SVI on L3_Switch2. Also the same SVI_LAN on L3_Switch2 has to be configured with IP Helper Address command to relay the DHCP request to the DHCP server.

Apart from this routing will be required (e.g Static) on L3_Switch2 to reach the DHCP Server and on L3_Switch1 to reach the SVI_LAN Pool  on L3_Switch2 for which we want to enable DHCP.

Hope this helps to answer your query.

Regards

Varma

[Scott_O'Brien]

Thanks for that, Switch 1 is set up with the gw as its SVI ip so im guessing i have to just assign an IP to the SVI of switch 2 that is in the same scope. 

[Vaibhava Varma]

Hi Scott

If Switch1 is the Gateway for LAN users then it that case Switch2 should be only doing L2 Transport for the Frames to reach Switch1. Do we have an L2 Link between Switch1 and Switch2 here. There is no IP needed on Switch2 SVI and actually per my understanding we do not need SVI at all on Switch2 when Switch1 is the GW for that LAN Pool.

Hope this answers your question.

Regards

Varma

[Scott_O'Brien]

its an Layer 3 link so this is way i am a little confused as to what is needed.

[Vaibhava Varma]

Hi Scott

If the Link between the Switches is an L3 Link as shown below then the Gateway for the LAN users can not be on L3_Switch1 but only on L3_Switch2 and for that case my first response is valid.

DHCP_Server----------L3_Switch1-----------L3_Link--------------L3_Switch2---SVI_LAN---------LAN_Users

As explained earlier the LAN_Users GW does the function of DHCP_Relay using Helper-Address.

Hope this clarifies you on the setup requirement. Do let me know for any more questions.

Regards

Varma

[Ganesh Hariharan]

its an Layer 3 link so this is way i am a little confused as to what is needed. 

It will be helpful if you can post your switch configuration for better understandings

Hope to Help !!

Ganeshh Iyer

[Scott_O'Brien]

thanks guys, above is the diagram of the network, the 3750 has the dhcp server and the GW Svi's on it. the 4507 is the new switch that is being implemented and os on the other side of the l3 link. the 4507 needs to have the same vlan's ( will need to create at VTP wount work over an l3 link).

[Reza Sharifi]

Since the link between the 3750 and the 4507 is L3, then you do not need to have the same vlan (SVI) on the 4507. The vlan terminates on the 3750, from there it is routed via your L3 link to the 4507. If you have a users connected to the 4507 on the remote location, and need to access the 3750 site, all you need is to define DHCP scope for them.  As long as your routing is working and you can reach the DHCP server, then the users in your remotes site can grab an IP address

HTH

[Scott_O'Brien]

thanks for that, on the 4507 will i still need the helper-addresses under the interfaces? ( im only putting the interfaces on the 4507 for cosmetic value).

[Vaibhava Varma]

Hi Scott

On the New 4507, on the New SVI for the LAN Users the helper-address command needs to be enabled and DHCP server has to be routed on 4507 and same way New SVI Subnet has to be routed on the 3750 via the L3 Link.

Also on the DHCP Server DHCP scope has to be defined for the new SVI subent and GW for that would be the new SVI IP on the new 4507.

Hope this brings clarity on the requirement. Do let me know for any questions.

Regards

Varma

[Scott_O'Brien]

the 4507 is using the same vlans that are all ready on the 3750. no new vlans will be going on the 4507. 

[Vaibhava Varma]

Hi Scott

When the Link between the 4507 and 3750 is a L3 Link then we can not pass L2 Traffic between 3750 and 4507. Even though there is no restriction on using the same VLAN IDs on either them when connected via L3 as VLANs are locally significant within an L3 domain.

Now as I understand your requirement the 4507 needs to connect to 3750 in L2 Mode only and L3 link is not needed. We just need to pass the vlans which are configured on 4507 on the trunk link between them. The GW for the respective SVI stays on 3750 and the helper-address command also stays on 3750 SVI's. 4507 will just need one VLAN SVI for MGMT purpose.

Hope this helps to clarify on the requirement.

Regards

Varma

[Scott_O'Brien]

thanks, the only reason i am keen to make it an l3 link is to stop spanning tree loops ( if one ever happens)

[Vaibhava Varma]

Hi Scott

I understand the spanning tree concern but unfortunately we can not have an L3 Link when we have an L2 Traffic Flow.

As explained earlier for L3 Link we need to have a different and new SVI on 4507 and the IP conifugred on this SVI will be the GW IP for the LAN Users being part of this new SVI and will also be declared as the GW in DHCP Scope on the DHCP Server for this new SVI Subnet. Also helper-address command will need to be configured on the new SVI on the 4507.

Then routng needs to be enabled between the 4507 for DHCP Server Reachability and the new SVI reachability.

So if the Link between 3750 and 4507 is L3 we can not have same vlans on them as L2 Traffic Flow.

Also looking at the Spanning-Tree effect if we have just 2 Switches and no redundant Link as shown in above diagram there is no worries on SPT front.

Hope this helps to answer your query.

Regards

Varma