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SG300 & 150M Docsis 3.0 Cable Modem Multiple External IP

I have recently purchased a 150M internet pipe from our ISP, along with 2 IP Addresses.  Cable Modem > Cisco 8 Port 1G Switch > Router 1 ASUS RT66U Guest Wireless Network Only with unique IP Address 1 and Router 2, Cisco ISA550 Network Appliance Serving a Secure Business Network with unique IP Address 2.  Two discrete networks, one that is a guest network and one that is secure.  I do not want the traffic for the Guest Network to impact performance of the ISA550 secure business network.  Completely discreet networks.

I am getting 150M to and from the Motorola 6141 Cable Modem, but not after my basic Cisco 8 port 1G Switch.  I can connect a PC directly to the modem and get 150M Download Speeds.  I have an SG300-10 available.  I also have an older HP and 3Com 1G Switch and no matter what I do, I cannot get 150M download after the switch.

I was reading somewhere that it may be necessary to use a Layer 3 switch to get the 150M speed from the Modem to the two networks.  Therefore, I want to configure the SG300-10 in Layer 3 and configure respectively so each of the two routers go to the WAN, get their IP and do their thing.  However, I am not a guru with this type of configuration.

Regardless of what I do, or what 1G switch I put after the Cable Modem, I can only get 50M out of the switch even if I connect a PC directly to the switch.  I guess basically, there is no way a Layer 2 switch can get the fast bandwidth across it's ports.  However, read an article elsewhere that stated to get the high bandwidth a layer 3 router or layer 3 switch is required.  Good thing I have an extra SG300-10.

I should add that I only get 50M Download after the switch, no matter which layer 2 switch is used.

Thanks in advance for any opinions.


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