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PIM and NAT conflict ?

Hi all !

I use a CISCO 1720/IP plus router for ISDN dialup connection to Internet. I implemented NAT for the LAN and everything works fine when we navigate to Internet...

I want to use a multicast (PIM) application to a PC directly connected to Ethernet. I setup the multicast routing, but the application didn't work... I checked that problem with the ISP technicians, who claimed that there should be a conflict between PIM and NAT. Is that true? Is there any mistake in the following configuration?


Cisco1720#sh conf

Using 1540 out of 29688 bytes


version 12.2

service timestamps debug uptime

service timestamps log uptime

service password-encryption


hostname Cisco1720


memory-size iomem 25

ip subnet-zero


ip multicast-routing


isdn switch-type basic-net3


interface BRI0

description connected to Internet

ip address negotiated

ip nat outside

ip pim nbma-mode

ip pim sparse-mode

encapsulation ppp

no ip route-cache

ip igmp join-group

no ip mroute-cache

no keepalive

dialer idle-timeout 300 either

dialer string 89625xxxxx

dialer hold-queue 10

dialer-group 1

isdn switch-type basic-net3

no peer default ip address

no fair-queue

ppp authentication chap callin

ppp chap hostname

ppp chap password


interface FastEthernet0

description *** connected to EthernetLAN ***

ip address

ip nat inside

ip pim dense-mode

speed auto


ip nat inside source list 1 interface BRI0 overload

ip classless

ip route BRI0 permanent

no ip http server


access-list 1 permit


access-list 101 permit ip any any

dialer-list 1 protocol ip list 101


snmp-server community public RO

snmp-server enable traps tty


password xxxxx


line aux 0

line vty 0 4

password xxxxx



no scheduler allocate



Thanks in advance,


VIP Purple

Re: PIM and NAT conflict ?

Hello George,

I think you have to configure the same PIM mode on both interfaces, first of all (I would use pim sparse-dense mode on both interfaces). Second, I think you need to have an RP configured. NAT and PIM shouldn´t be an issue (the attached link might provide some more info on NAT and PIM).