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Multicast in a layer 2 to layer 3 migration

Posted on 2016-10-18
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I have 10 buildings and they are connected via fiber in a ring topology. It is currently layer 2 with the core at one building doing the routing. I am in the process of implementing EIGRP between the building to keep STP locally to the building. I have a couple of questions:
- Will it be a downtime when I start configuring EIGRP?
- Can I perform the work remotely?
- My network currently has multicast routing running with auto-RP. Will multicast be affected by the migration to layer 3?

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Question by:leblanc
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Predrag Jovic earned 500 total points
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I have 10 buildings and they are connected via fiber in a ring topology. It is currently layer 2 with the core at one building doing the routing.
Extremely bad STP is created for 7 "hops" diameter max.-
Will it be a downtime when I start configuring EIGRP?
Yes. Since, most liklely, you can't just implement EIGRP you also will need to change IP address ranges on local VLANs in buildings. In the case that you don't have to rearrange IP addresses you still need to change location of default gateway so default gateway will have different MAC address and you can expect at least 15 to 45 seconds of downtime for hosts, in some cases even longer (arp need to be cleared on hosts and switches - manually or automatically). However, with a careful planning it could be smooth implementation but sure there will be downtime... Do not forget that if you want move links between buildings to EIGRP most likely you will need do reconfigure VLANs, since you can't have the same IP address ranges on 2 locations.  
Can I perform the work remotely?
Yes, with extremely good planning of the steps (one wrong step order and you cut yourself from the network or a part of the network permanently). The first issue reload command and start to work. :)
My network currently has multicast routing running with auto-RP. Will multicast be affected by the migration to layer 3?
Sure it will. You are moving from L2 to L3 topology between buildings, that fact alone is enough to disrupt traffic flow and there may be need to reconfigure multicast configurations.

Another architectural part:
It would be highly recommendable for more than 3 buildings to implement some other topology (with redundant core switches), not just to move links between buildings to L3. Ring topology is simply not flexible enough.
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