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Cisco inter-VLAN routing and OSPF

Posted on 2008-06-24
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Last Modified: 2012-06-22
Hi Experts,

I'm setting up a new branch office very similar to the one shown in the attached diagram, and wanted to run it by you guys before buying any equipment.

My two questions are:

1.  What's best way to link the layer 3 switch and the router? Should I use a /252 subnet mask so the link has it's own subnet, or use a trunk and sub-interfaces?

2.   Would it be best to use OSPF on the layer 3 switch, or just a default route to the router; then just use OSPF on the router?


Thanks Experts!
howto-3550-intervlanrouting.gif
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Question by:steven_maher
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donmanrobb earned 125 total points
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Probably will not be too much benefit with running OSPF on the network shown since there doesn't look like theres any redundant layer 3 paths to take advantage with. Trunking should be fine.
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by:mabutterfield
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I prefer to create a small routing vlan that's a .248 (/29) or .252 (/30) to connect my routers and switches on an internal VLAN.  I think it's simpler to setup and support than trunking a VLAN off to the router.  Also, you get the added benefit of having a seperate address space for the routers, which you can use in ACLs.

For a network of this size, i see no reason to use OSPF.  Your router could route 10.1.x.x/16 to the 3550, and give your 3550 a default route of the router and you're done.  

Additionally, if I remember correctly, you need the enhanced image on the 3550, which can be a costly option.  (I may be mixing this up with another device/vendor, but i'm pretty sure.)
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by:kdearing
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1. It depends on which device you want to do the routing. I would trunk the VLANs to the router and let it do it's job. The other method you are asking about is perfectly acceptable, but be aware you are adding an extra hop.

2. I am assuming the cooporate network you are attaching to uses OSPF. If you are going to have the router NAT the traffic, then nothing is required. If you are just routing the traffic, then configure the router for whatever the corporate network uses.

Note that if corporate network is under someone else's control, you would be advised to get with them before implementing any routing protocol.
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by:Jan Springer
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I like to plan for growth.

I create subnets of /28 for router use.  

I use vlans on the switch for all subnets (to include the router).

And I run OSPF on the backbone devices to distribute connected and static routes.
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by:steven_maher
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Hi guys,

This office is unlikely to grow very much (it's only to be open for a year) so I've decided to use trunking from the layer 3 switch to router with sub-interfaces. Only the router with participate in OSPF.

Thanks.
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