I have a Cisco 2960, IOS 15.0(2)SE2, between the fiber switch of the ISP and the Sonicwall firewall of the ISP. This works fine. I have connected a PC with Windows XP to the switch, and a Juniper SRX-240 cluster (for redundancy), to replace the Sonicwall. All devices have an external address in the subnet, exept the 2960, and are in the same vlan on the 2960. From the PC and the new SRX-240 cluster, I can't reach any of the other devices: arp requests are send, but the replies are blocked by the 2960. When I run Wireshark on the PC, I can see traffic from the SRX-240s and the 2960, but not from the fiber switch or the Sonicwall (I suspect they do send broadcasts). The ISP says that he sees on the Sonicwall broadcasts from the SRX-240s or the PC and the that replies are send. But they are blocked too on the 2960.
What can be the cause of this?