First attempt at L3 InterVlan routing

The picture of the lab is hopefully attached.

Please critique it to see if my understanding is sound. I'm using RIP as the routing protocol, because I have yet to learn any of the others.

My biggest question and the reason for this post is about DHCP and ip helper-addresses. The only way I was able get a DHCP address to the Mac and PC was to set an ip helper-address on each of the VLAN interfaces. Now the question. Is that the only way or is there an easier way of routing DHCP, like only putting the ip helper-address on one router interface, instead of every VLAN interface?

I only have three VLANs here, but it would seem if I had a lot, say 60 VLANs, that it would be kind of troublesome to add an ip helper-address to every VLAN interface.

Any thoughts are appreciated.

L3 lab.jpg

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Comments

networker050184

You have to think of the way DHCP works. What is the destination address going to be? What interface is it going to be received on? Once you look at it logically you will understand where the helper address will be needed.

chmorin

networker050184 wrote: »

You have to think of the way DHCP works. What is the destination address going to be? What interface is it going to be received on? Once you look at it logically you will understand where the helper address will be needed.

Bang-Zoom-Wow.

Also, if you are in a situation where you need to add a single command to many interfaces, use the interface-range command. It is handy.

IRONMONKUS

What I know:

-VLANs seperate broadcast domains.
-The gateway for each computer on a VLAN is on the L3 switch as a VLAN interface.
-DHCP uses udp broadcasts.

So, since VLANs are separate broadcast domains and any computer on a given VLAN broadcasts a udp DHCP request, which then gets sent to their gateway that is a VLAN interface setup on the L3 switch, I have to put the ip helper-address on the VLAN interface of that give VLAN, so that any computer on that VLAN can get a DHCP address?

I ran wire shark on the computer I was trying to get a DHCP request on and the VLAN interface would try and forward the request to 224.0.0.9. I have no idea what that is. When I set the ip helper-address on the VLAN interface, I received a DHCP address on the client computer.

It makes sense now after reading your responses, thank you!

Think logically and take baby steps!

chmorin

IRONMONKUS wrote: »

I ran wire shark on the computer I was trying to get a DHCP request on and the VLAN interface would try and forward the request to 224.0.0.9. I have no idea what that is. When I set the ip helper-address on the VLAN interface, I received a DHCP address on the client computer.

The 224.0.0.9 is the helper address sending the frame out at as a multi-cast to its new destination. It does not actually forward the broadcast.
^^^^^^^^
See that? I'm a moron. Don't believe me. See below.

IRONMONKUS

chmorin wrote: »

The 224.0.0.9 is the helper address sending the frame out at as a multi-cast to its new destination. It does not actually forward the broadcast.

Excellent! Thank you for explaining that. I still have a lot to learn and am very thankful to have this community to go to.

networker050184

chmorin wrote: »

The 224.0.0.9 is the helper address sending the frame out at as a multi-cast to its new destination. It does not actually forward the broadcast.

Actually 224.0.0.9 is RIPv2, but we'll let you slide this time.

chmorin

networker050184 wrote: »

Actually 224.0.0.9 is RIPv2, but we'll let you slide this time.

-slaps face- this is why I failed BSCI. XD

I could of sworn helper-address multi-casted the broadcasts out the next interface, but I cant find it anywhere. Someone help my stupid!

IRONMONKUS

networker050184 wrote: »

Actually 224.0.0.9 is RIPv2, but we'll let you slide this time.

Thank you for correcting that.

chmorin

Alright, I'll correct myself. helper address does forward broadcasts, but only on certain ports and you have the ability to limit them.

-sigh- I really need to pick up my SWITCH book again. I read it two books ago and never took the damn test.

IRONMONKUS

chmorin wrote: »

Alright, I'll correct myself. helper address does forward broadcasts, but only on certain ports and you have the ability to limit them.

Okay, thank you! I'm going to try and limit them next. I'm going to move the server over to 3R, make that my server farm VLAN switch, and introduce some pruning. I'll probably move the PC to 2R and make that the user switch and leave S1 as a management switch.

I don't really know what I'm doing, but it's all about trying, sniffing the network, and having fun.

Thank you for everyones input!

chmorin

IRONMONKUS wrote: »

Okay, thank you! I'm going to try and limit them next. I'm going to move the server over to 3R, make that my server farm VLAN switch, and introduce some pruning. I'll probably move the PC to 2R and make that the user switch and leave S1 as a management switch.

I don't really know what I'm doing, but it's all about trying, sniffing the network, and having fun.

Thank you for everyones input!

I can't remember, but you may be reaching beyond the CCNA level of switching here (with helper-addresses at least) unless they started to add those things in the curriculum after I got it.

What I mean to say is, your attitude is amazing and it seems like you have a passion for the subject and what you are doing. Keep it up and you will get far!:D

IRONMONKUS

chmorin wrote: »

I can't remember, but you may be reaching beyond the CCNA level of switching here (with helper-addresses at least) unless they started to add those things in the curriculum after I got it.

What I mean to say is, your attitude is amazing and it seems like you have a passion for the subject and what you are doing. Keep it up and you will get far!:D

Thank you for your kind words. You are right, this is CCNP level stuff, but I feel I need to push my learning a little bit farther than what's just on the CCNA, even though I have yet to take it. Hopefully that won't get me into trouble on the CCNA test.

I want to be able to jump into a network with a mind set linked with the network, instead of jumping in only knowing router-on-a-stick (CCNA level) and trying to configure L3 switching in a live environment (CCNP level).

My passion for networking has been driven by working in Desktop Support for the last six years and finally coming to the conclusion of what I want to do as a career. I am very thankful for my time as a Desktop tech and I've learned so much that has helped me understand the way things work, but I feel that it is my time to move on.

I am also thankful for this community and the respect people show towards others in obtaining similar goals. I have learned a lot from reading others posts and it helps in keeping myself focused. I also enjoy reading the CCNP/CCIE threads, because in my eyes at this level in my training, you guys and gals are the super-heroes of networking and when you run into issues or it's taking you years to prepare for the CCIE, it makes me feel better knowing that you are human like me.