IPv6 Multicast Trouble Shooting Guide

SUZUKI, Shinsuke at KAME Project
$Id: index.html,v 1.2 2005/07/18 04:55:37 suz Exp $


This troubleshooting guide classifies the tips for IPv6 multicast troubleshooting into three categories.

  1. Issues in Sending Hosts
  2. Issues in Receiving Hosts
  3. Issues in Routing Configuration
From my experiance, I often encounters the first two issues. So please be careful to confirm that they don't exist in your environment, before going into the analysis of the complex behavior of the PIM protocol.

1. Possible Issues in Sending Hosts

(a) Hoplimit is large enough?

KAME transmits IPv6 multicast packets with hoplimit = 1, unless application explicitly specifies some hoplimit value. This fact means that IPv6 multicast packets don't go over a router by default due to a lack of hoplimit.

So if you cannot receive packets from a multicast packet sender, please check the hoplimit first!

(b) Packet size is less than 1280?

Some router does not generate ICMPv6 Packet-Too-Big Error for multicast packets, to prevent ICMPv6 Error packet generation for multicast packets (Precisely speaking, it's against ICMPv6 Spec(draft-ietf-ipngwg-icmp-v6-04.txt section 2.4 (e)). Such systems, however, assumes that multicast senders transmit multicast packets less than 1280 Byte).

If your network has such a router, you should take a great care to see if the size of the transmitted packets shouldn't exceed more than 1280 bytes.

2. Possible Issues in Receiving Hosts

(a) Have you joined a multicast group?

Sometimes people forget to start IPv6 multicast receiver application:-)

(b) MLD version is correct?

Especially when there's a MLD snooping switch between a host and its accomodating router, you should take care of the MLD version.

3. Possible Issues in Routing Configuration

(a) RP-related issues

In case of pim6sd, you can check the mapping using "pim6stat" command.

 # pim6stat
 Current BSR address: 2001:200:0:8000::2000:1 Prio: 10 Timeout: 160
 RP-address(Upstream)/Group prefix             Prio Hold Age
      ffxx:x:xxxx::/96             0    150  150
c.f.) the equivalent command on other implementations. (Other vendor info is welcome!)
  • AlaxalA AX-series: show ipv6 pim rendezvous-point mapping
  • Allied Telesys SB-series: show ipv6 pim rendezvous-point mapping
  • Cisco IOS-series: show ipv6 pim group-map
  • Cisco IOS XR-series: show pim ipv6 group-map
  • Foundry: show ipv6 pim group(?)
  • Hitachi GR/GS-series: show ipv6 pim rendezvous-point mapping
  • Juniper JunOS: show pim rps inet6
  • NEC IP8800-series: show ipv6 pim rp-mapping

If you cannot find out the right Group-RP mapping, you should check the following things:

(b) RPF-related issues
To be written
(c) pim6sd-specific issues
pim6sd does not support static-RP at RP router itself. So at RP, you have to configure cand_bsr and cand_rp option as well.