all serial interface with default config,1T on 3620 :sh ver Serial0 is administratively down, line protocol is down Hardware is HD64570 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation...

I have a 2948G and 2 firewall connect to it,the 2 firewall need multicast communication to backup eath other.but it seems 2948G cann't support,even a hub can do it. the 2948G have default configuration.