Understanding Basics of Multicast RPF (Reverse Path Forwarding)

Document

Sat, 02/11/2017 - 02:17
Mar 5th, 2013
User Badges:
  • Gold, 750 points or more

Introduction:

In normal routing i.e. in Unicast routing packet forwarding decisions are typically based on the destination address of the packet arriving at a router. The unicast routing table is organized by destination subnet and mainly set up to forward the packet toward the destination.

In IP multicast routing, the router forwards the packet away from the source to make progress along the distribution tree and prevent routing loops. The router's multicast forwarding state runs more logically by organizing tables based on the reverse path, from the receiver back to the root of the distribution tree. This process is known as reverse-path forwarding (RPF).

In short, Incoming multicast packet will not be accepted/forwarded unless it is received on an interface that is the outgoing interface for unicast route to the source of the packet.

 

Configuration Example:

In below example multicast server S1 sends a multicast packet, with R1 flooding it to R2 and R3.R2 received its copy, and floods it as well. As a result R3 receives the same packet from two routers:

a) On its interface fa0/0 from R1.

b) On its interface s0/0 from R2.

Topology Diagram:

multicast_rpf1.jpg

 

Without the RPF check, R3 would forward the packet it got from R1 to R2, and vice versa, and begin the process of looping packets also with the same logic, R1 and R2 also keep repeating the process. This duplication creates multicast routing loops and generates multicast storms that waste bandwidth and router resources.

Before I dive into multicast configuration, let me share with you the initial configuration of our network. All relevant configurations are below.

 

R1

R2

R3

hostname R1

 

ip cef

!

ip multicast-routing

!

interface FastEthernet1/0

ip address 1.1.1.1 255.255.255.0

ip pim dense-mode

!

interface FastEthernet0/0

ip address 10.1.1.1 255.255.255.252

ip pim dense-mode

speed 100

full-duplex

!

interface FastEthernet0/1

ip address 10.1.1.5 255.255.255.252

ip pim dense-mode

speed 100

full-duplex

!

router eigrp 1

network 1.1.1.1 0.0.0.0

network 10.1.1.0 0.0.0.255

no auto-summary

 

hostname R2

!

ip multicast-routing

!

interface FastEthernet0/0

ip address 10.1.1.2 255.255.255.252

ip pim dense-mode

speed 100

full-duplex

!

interface Serial0/0

ip address 10.1.1.9 255.255.255.252

ip pim dense-mode

clock rate 2000000

!

router eigrp 1

network 10.1.1.0 0.0.0.255

no auto-summary

!

 

 

 

 

 

 

 

hostname R3

!

ip cef

!

ip multicast-routing

!

interface FastEthernet0/0

ip address 10.1.1.6 255.255.255.252

ip pim dense-mode

no ip route-cache

no ip mroute-cache

speed 100

full-duplex

!

interface FastEthernet0/1

ip address 3.3.3.3 255.255.255.0

ip pim dense-mode

ip igmp join-group 239.1.1.1

!

interface Serial0/0

ip address 10.1.1.10 255.255.255.252

ip pim dense-mode

no ip route-cache

no ip mroute-cache

clock rate 2000000

!

router eigrp 1

network 3.3.3.3 0.0.0.0

network 10.1.1.0 0.0.0.255

no auto-summary

!

 

When R3 performs the RPF check the following things will happen:

1) R3 examines the Source address of each incoming packet, which is 1.1.1.1.

2) R3 determines the reverse path interface based on its route used to forward packets to 1.1.1.1

In our case R3's route to 1.1.1.1/24 is matched, and it is lists an outgoing interface fa0/0, making fa0/0 R3's RPF interface for IP address 1.1.1.1

R3#sh ip route | beg Gate

Gateway of last resort is not set

 

     1.0.0.0/24 is subnetted, 1 subnets

D       1.1.1.0 [90/156160] via 10.1.1.5, 02:01:51, FastEthernet0/0

     3.0.0.0/24 is subnetted, 1 subnets

C       3.3.3.0 is directly connected, Loopback0

     10.0.0.0/30 is subnetted, 3 subnets

C       10.1.1.8 is directly connected, Serial0/0

D       10.1.1.0 [90/30720] via 10.1.1.5, 04:24:40, FastEthernet0/0

C       10.1.1.4 is directly connected, FastEthernet0/0

 

R3#sh ip rpf 1.1.1.1

RPF information for ? (1.1.1.1)

RPF interface: FastEthernet0/0

RPF neighbor: ? (10.1.1.5)

RPF route/mask: 1.1.1.0/24

RPF type: unicast (eigrp 1)

RPF recursion count: 0

Doing distance-preferred lookups across tables

 

R3#sh ip mroute | beg Interfac

Interface state: Interface, Next-Hop or VCD, State/Mode

 

(*, 239.1.1.1), 00:38:46/stopped, RP 0.0.0.0, flags: DCL

Incoming interface: Null, RPF nbr 0.0.0.0

Outgoing interface list:

   Loopback0, Forward/Dense, 00:38:46/00:00:00

   FastEthernet0/0, Forward/Dense, 00:38:46/00:00:00

   Serial0/0, Forward/Dense, 00:38:46/00:00:00

(1.1.1.1, 239.1.1.1), 00:00:26/00:02:37, flags: LT

Incoming interface: FastEthernet0/0, RPF nbr 10.1.1.5

Outgoing interface list:

   Loopback0, Forward/Dense, 00:00:26/00:00:00

   Serial0/0, Prune/Dense, 00:00:26/00:02:34, A

 

3) R3 compares the reverse path interface fa0/0 on which multicast packet arrives .If they match, it accepts the packets and forward it; otherwise ,it drops the packet .In this case ,R3 floods the packets received on fa0/0 from R1 but it ignore the packets received on s0/0 from R2.

Verification:

1) To verify we will be sending ICMP echo to group 239.1.1.1 from R1 with source 1.1.1.1.It's always safe to collect debugging logs in buffer rather than on console hence we will be debugging multicast packet and collect it in logging buffer as shown below:

R3#conf t

Enter configuration commands, one per line. End with CNTL/Z.

R3(config)#logging console informational

R3(config)#logging buffer 7

R3(config)#logging buffer 64000

R3(config)#no ip cef

R3(config)#end

*Mar 1 04:44:41.670: %SYS-5-CONFIG_I: Configured from console by console

R3#debug ip mpacket

IP multicast packets debugging is on

 

R1#ping 239.1.1.1 source 1.1.1.1

 

Type escape sequence to abort.

Sending 1, 100-byte ICMP Echos to 239.1.1.1, timeout is 2 seconds:

Packet sent with a source address of 1.1.1.1

 

Reply to request 0 from 10.1.1.6, 24 ms

Reply to request 0 from 10.1.1.6, 128 ms

 

R3#sh logging | beg Log

Log Buffer (64000 bytes):

 

IP(0): s=10.1.1.5 (FastEthernet0/0) d=239.1.1.1 (Serial0/0) id=19, ttl=254, prot=1, len=100(100), mforward

IP(0): s=10.1.1.1 (Serial0/0) d=239.1.1.1 id=19, ttl=253, prot=1, len=104(100), not RPF interface

IP(0): s=10.1.1.5 (FastEthernet0/0) d=239.1.1.1 (Serial0/0) id=20, ttl=254, prot=1, len=100(100), mforward

IP(0): s=10.1.1.1 (Serial0/0) d=239.1.1.1 id=20, ttl=253, prot=1, len=104(100), not RPF interface

IP(0): s=1.1.1.1 (FastEthernet0/0) d=239.1.1.1 (Serial0/0) id=20, ttl=253, prot=1, len=100(100), mforward

IP(0): s=1.1.1.1 (Serial0/0) d=239.1.1.1 id=20, ttl=252, prot=1, len=104(100), not RPF interface

 

From the above logs we can see that R3 forwarded the packets received on fa0/0 from R1 but it ignore the packets received on s0/0 from R2.

2) Let’s look it same with mtrace from R1 and capturing packet with the help wireshark on  R3’s interface Fa0/0 and S0/0.

R1#mtrace 1.1.1.1 3.3.3.3 239.1.1.1

Type escape sequence to abort.

Mtrace from 1.1.1.1 to 3.3.3.3 via group 239.1.1.1

From source (?) to destination (?)

Querying full reverse path...

0 3.3.3.3

-1 10.1.1.6 PIM  [1.1.1.0/24]

-2 10.1.1.5 PIM [1.1.1.0/24]

-3 1.1.1.1

 

On R3’s interface fa0/0 we capture trace route query and request as mark in black box below diagram:

1.jpg

Let's open the traceroute request packet to get more detail inside view.

2.jpg

As show in above figure “FORWORDING CODE: NO_ERROR” field shows that after the router receives a multicast packet it performed an RPF check as the RPF check succeeds, the packet is forwarded.

Now let’s view capture taken on interface S0/0:

3.jpg

 

It is only showing trace route query not request as packets are drop due to RPF check failure.

Hence conclusion is the RPF check is a strategy by which router accept packets that arrives over the shortest path and discard those that arrive over longer routes and thereby avoid routing loops and duplication.

IP Multicast

IP Multicast Technology Overview

Configuring IP Multicast Routing

Loading.
ashirkar Wed, 03/06/2013 - 22:16
User Badges:
  • Gold, 750 points or more

Hello Tim,


Thanks for your positive feedback.Keep on sharing your views and knowledge on community.


Regards,

Ashish Shirkar

Technical community manager (Network Infrastructure)

ashirkar Mon, 02/24/2014 - 04:48
User Badges:
  • Gold, 750 points or more

Welcome Amjad,Hope it helps you


Regards,

Ashish Shirkar

Balaji Murthy Sat, 08/01/2015 - 05:42
User Badges:

Hi Ashish,

 

Very simple and clear explanation,

able to understand easily and quickly in one reading.

Thanks for sharing the knowledge :) 

 

Regards,

Balaji.M

Actions

This Document

Related Content