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TOPIC: Cisco Catalyst 3550 VLAN Routing

Re: Cisco Catalyst 3550 VLAN Routing 14 years 3 months ago #9492

Thank you very much :!: I believe I am back on track now. I did not setup the static route entry on the linksys router before. I did not realize that it needed to provide a separate protocol to 'get back' to the 3550.

I am able to ping the linksys from both vlans now. :D
Internet still does not work on VLAN2, but that is most likely because the linksys is not directly connected to the internet. The router upstream has not been setup to route that information back.

I would like to know the commands for DHCP setup on the 3550. I do not have the field manual for catalyst routers. It would definitely be more simplistic if the DHCP server was moved to the 3550 I think. Chris, if you have some extra time ever, will you pass that information along to me?

After I get areas separated into VLANs, I will be working to incorporate the VLANs with our domain controllers. Then my goal is to setup two 3550s with redundant connections to the 2950s using rapid STP.
What would you classify as the difficulty of setting up redundant 3550s compared to the setup and routing of the vlans?

Thank you again for your help. I will probalby be back with some other questions/comments to post on this forum in the future.

Re: Cisco Catalyst 3550 VLAN Routing 14 years 3 months ago #9493

  • tiamat
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What he means is that your router (not the 3550) probably does not know where to send packets destined for hosts on VLAN2. The static route needs to be added to your router, directing any traffic destined for the VLAN2 network towards the VLAN1 ip address of the 3550. In essence, there is no route back to the hosts on VLAN2.

edit: doh! I didn't realize I hadn't reached the end of this thread (page2) and responded to a problem that was already solved... my bad...

edit2: here's a link to configuring DHCP servers on cisco hardware:

Re: Cisco Catalyst 3550 VLAN Routing 14 years 3 months ago #9494

  • Chris
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To setup DHCP on your 3550 simply follow these commands:

ip dhcp pool vlan1
dns-server (assuming your router supports DNS requests)
lease 7 (The number indicates how many days the lease is valid)

ip dhcp pool vlan2
lease 7

Now, because you would naturally want to exlude a few IP Addresses from your DHCP server, enter the following commands:

ip dhcp excluded-address
ip dhcp excluded-address

This will exclude the range to & to

Save your configuration and your done!

p.s Vlan topics and Catalyst 3550's are my favourite topic - can't you tell ?

If you have any problems or questions, please do not hesitate to ask, were here to help!

Oh, and when the time comes, we will help you setup the STP for your redundant links - its simple and quite nice.

Chris Partsenidis.
Founder & Editor-in-Chief

Re: Cisco Catalyst 3550 VLAN Routing 14 years 3 months ago #9495

  • jwj
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I was just wondering how exactly does a router in this case treat the ethernet frames? Are they stripped at a certain point, and later encapsulated with brand new ones that would be untagged?

I get it this far: frame comes from PC untagged, ingresses on access port that is configured for the vlan, as it egresses, it is tagged as belonging to that vlan. When it ingresses to the vlan interface that routes it, what exactly happens as far as the ethernet header?

Re: Cisco Catalyst 3550 VLAN Routing 14 years 3 months ago #9496

I have realized that you must enjoy this topic and are very knowledgeable about it. I haven't received this much support on any other forum about any topic that I can recall. :D

I may try the DHCP option on the 3550 sooner than later. I have not yet figured out how to get my sonicwall to assign addresses other than on its own subnet.

In addition to the commands you wrote. What is the procedure to disable dynamic dhcp requesting and create a static dhcp table only for individual MAC addresses?


Re: Cisco Catalyst 3550 VLAN Routing 14 years 3 months ago #9497

  • Chris
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The whole process is quite simple if you look at it from a different point of view.

An Ethernet frame is 'tagged' only when it triverses a Trunk Port.
When the frame leaves a PC and ingresses to an Access Link, it does not contain any VLAN information. Similarily, while inside the switch, no VLAN ID (from what I know) is applied, as long as it egresses out a Access Link.

In case of the 3550 now, because this is a layer 3 switch and can perform routing, the Ethernet packet simply gets routed by the switche's processor and exits the access link port to find its destination.

This doesn't mean the solution lotus-eater was asking for couldn't have been done with the use of Trunk ports - it's possible, but just a bit more complicated.

In this case, as you so correctly noted, he would have to use a trunk link to the router and the 802.1q encapsulation method to maintain the VLAN information in the frames. The router would then perform the routing and send back the packets to the appropriate network - This method is called 'Router on a stick' if I recall correctly.

I hope I haven't confused you as its 2 am at the moment here in Greece :)

Cheers mate,
Chris Partsenidis.
Founder & Editor-in-Chief
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