I have a router (actually a pair of them in HSRP but that's irrelavent) that connects two networks in non-contiguous IP address ranges through a 100MB F/E TX port on the inside to an ISP network on the outside, also via a 100MB F/E TX port, at a colo facility. I have a half of a hundred MB pipe to the outside world. The two networks behind the router are independent of each other, seperated by PIX's behind the routers but, on occasion, they do communicate with each other. I currently have a primary and secondary IP address set on the inside F/E interface, one for each network. I've never seen any mention if sub-interfaces being used in Ethernet or Fast Ethernet interfaces in any Cisco literature. They primarily seem to be mentioned in regards to serial interfaces. Is there an advantage to using sub-if's here over a primary and secondary IP address? Any packet filtering is handled by the PIX's so I don't ever foresee the use of access-lists on the router. This router simply routes packets. I don't foresee the use of more than two networks inside but I suppose that's a slight possibility down the road if I would need more IP addresses and couldn't get contiguous addresses. I'm not sure if you can use more than a single secondary address on an interface or if you can pnly use a single one. I guess I'm not sure if "recommended practice" would be to always use sub-if's when connecting more than one network to any interface, use sub's only with serial i/f's and use primary/secondary addresses with F/E interfaces or if it's time to consider adding more F/E modules with 2 or more networks. I've used this primary/secondary config for a couple of years and it's worked fine but, as my colo facility is Sprint and they've decided to get out of the hosting biz, it would be a good time for me to reconfigure things during the move if there is an advantage in doing so. Thanks.
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