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How many EIGRP neighbors on 3750 over layer 2 WAN?


My  customer has bought a layer 2 WAN service  from a service provider and  already purchased 3750 switches to act as CE  devices. They now want me  to specify a design of how to implement what  they have bought.  They  have about 50 sites but due to QoS limitations  of the service provider  they need to use 1 physical interface to route  voice traffic towards  the WAN at each site and a different interface for  data traffic. They  wish to have a dynamic routing environment so in my  opinion they have  two basic options for routing protocols that could  send the voice and  data out of different interfaces:-

1) Run EIGRP and use distribute lists to advertise voice subnets out of 1 link and data subnets out of the other.

2)   Run 2 different routing protocols, e.g. OSPF for data and EIGRP for   voice. I don't think there should be any need to redistribute between   them.

I  think that either of these solutions is likely  to give me scalability  issues due to the number of neighbors required  (ie approximately 100  per switch) but I haven't managed to find any  numbers for what should  be achievable. Can anyone give me any indication  of the likely number  of eigrp neighbors I could reasonably support? I  have seen some people  mention best practice is 20 but I have also seen  references to a  Networkers presentation where they discussed a live  environment with  800 neighbors on a 7200.

I have also been considering ways to improve scalability. I have identified the following:-

1) Most sites can run as stubs because there is only a single switch per site.

2)  I am not sure yet but I am hoping that the LAN addresses  within the  sites can be summarised towards the WAN so instability in the  LAN will  not cause instability in the WAN.

3) I think I could  possibly  reduce the number of neighbors required on some devices by  configuring  static neighbors and configuring switches at key sites as  hubs for the  smaller sites. I think this would probably require me to  turn off split  horizon. I think I could also use the 'no ip  next-hop-self eigrp'  command to stop traffic having to route via the hub  site. I am nervous  of turning off split horizon and using this command  in an environment  with multiple hubs. Please could you give me any  guidance?

4) I  could implement multiple VLANs within the WAN and implement a  hierarchical design over the top of the layer 2 WAN. I am sure I could   get a solution like this to work and it is the one I will most likely   fall back on if I don't get an answer to this query, but the exact   solution would also depend on how many neighbors I could configure on   the 3750s. The main problem with this solution, however, is that  traffic  will route via the hub sites rather than going directly to the   destination site. This will cause an overhead on the WAN connections  at  the key sites, which is exactly where I don't want it.

Failing   all of the above, I could push back on the customer's requirement to   have dynamic routing and suggest static routing over half or all of the   network. I obviously don't want to do this if these is a feasible   solution.

All suggestions appreciated.



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