Frame Relay Multicast (RPF check failure)

Frame Relay Multicast 

When doing multicast in frame relay network, the very important thing we need to take a close look at is RPF check. Since in some types of Frame Relay network, it is not a full mesh topology. 

What is RPF? 

RPF is used to prevent multicast loop in the network. RPF uses routing table to check whether it prunes the interface or not. When it receives multicast traffic, it checks where the source of multicast traffic is and compare to its routing able and make sure that the received interface and the forwarding interface to the source is the same interface. 

For example, if router received the multicast packet on f0/0. It performs RPF check by looking at its routing table to see the forwarding interface back to the multicast source is the f0/0. If it is not, RPF is failed. If it is, RPF check is passed.

In the frame relay Hub and spoke network, RPF check will be failed since the multicast traffic is received from spoke by hub and forward out by hub to another spoke. Therefore we need a tunnel interface to help accomplished this.

Example

Configuration

R1

interface Serial1/0
 no ip address
 encapsulation frame-relay
 serial restart-delay 0
 frame-relay lmi-type ansi
!
interface Serial1/0.1 multipoint
 ip address 192.168.1.1 255.255.255.0
 ip pim dense-mode
 ip igmp join-group 239.1.1.1
 ip ospf network point-to-multipoint
 snmp trap link-status
 frame-relay map ip 192.168.1.2 101 broadcast
 frame-relay map ip 192.168.1.3 102 broadcast
 no frame-relay inverse-arp

R2

interface Serial1/0
 ip address 192.168.1.2 255.255.255.0
 ip pim dense-mode
 encapsulation frame-relay
 ip ospf network point-to-multipoint
 serial restart-delay 0
 frame-relay map ip 192.168.1.3 202 broadcast
 frame-relay map ip 192.168.1.1 202 broadcast
 no frame-relay inverse-arp
!

R3

interface Serial1/0
 ip address 192.168.1.3 255.255.255.0
 ip pim dense-mode
 encapsulation frame-relay
 ip igmp join-group 239.1.1.1
 ip ospf network point-to-multipoint
 serial restart-delay 0
 frame-relay map ip 192.168.1.2 303 broadcast
 frame-relay map ip 192.168.1.1 303 broadcast
 no frame-relay inverse-arp
!
R4
A source of 239.1.1.1 multicast group

This configuration will not work in this case since it is a hub and spoke  network RPF check is failed. Therefore, we need to configure the tunnel interface  between R1 and  R3.

Building Tunnel interface

R1

interface Tunnel0
 ip address 1.1.1.1 255.255.255.0
 ip pim dense-mode
 tunnel source Serial1/0.1
 tunnel destination 192.168.1.3

R3

interface Tunnel0
 ip address 1.1.1.2 255.255.255.0
 ip pim dense-mode
 tunnel source Serial1/0
 tunnel destination 192.168.1.1

Now, multicast traffic is going through tunnel between R1 and R3. To pass the RPF check in R3 routing table should forward packet to source via Tunnel as well. However, R3 uses s1/0 at this time. To make it work, we require to add static multicast route to R3 as follow.

ip mroute 10.10.10.1 255.255.255.255 tunnel0



Then, we pass RPF check and  accomplish this task.