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ip dhcp snooping - dialin routers, WAN/ISPs, old/new cisco gear

1) At one site, all switches but one are 2960 switches that support ip dhcp snooping. However, one switch is an old 3500XL switch that doesn't support ip dhcp snooping. Will this switch be able to pass through dhcp requests to the DHCP server if it is interconnected to a fiber switch that is configured for ip dhcp snooping.

2) I enabled ip dhcp snooping at our main office that has several 2960 switches and a fiber switch. However, I had to roll back ip dhcp snooping because two sites that are part of our corporate WAN could no longer get dhcp info. These sites are very small, about 4 computers. They are connected to our WAN via an ISP router that is configured with an ip dhcp helper address for the dhcp server at our main office. The switches at the remote sites, behind the ISP router, are not Cisco switches. What do I have to ask our ISP to add to their router to allow these sites to get dhcp info when ip dhcp snooping is enabled at the main office?

3) Similarily, I have a 2621 router at our main office that is used as a dial in router. It passes through ms-chap and ms-chap-v2 dialin requests to an internal RAS server that authenticates the dialin attempts and hands out dhcp info. However, when I invoked ip dhcp snooping, the dialin's failed. The 2621 also does not support ip dhcp snooping. What must be added to the 2621 config to make it work?