Re: ip igmp static unlike IGMP ?join? and CPU issue

zillah2004 · ‎09-28-2006

Can anyone shed some light on the quote below ?

http://www.internetworkexpert.com/downloads/iewb-rs.sample.lab.pdf#search=%22unlike%20igmp%20static%20interface%20command%20when%20a%20router%20issues%20an%20IGMP%20join%22

" Unlike the ip igmp static interface command, when a router issues an IGMP ?join?, the CPU must actually process the multicast group. Therefore, a router configured with an IGMP join statement will respond to ICMP echo requests sent to the group that is has joined. In order to prevent looping of multicast feeds the router performs a reverse-path ".

Regards

vladrac-ccna · ‎09-28-2006

Ive seen the same in other ccie wb.

Sometimes they will ask you to make the router respond to the selected multicast group, So you'd use ip igmp join-group.

In other times, they will ask you to join the group but dont respond to multicast traffic sent to it, then you'd use ip igmp static command.

joing group:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a00800ca76d.html#wp1018571

static group:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_command_reference_chapter09186a00800ca76d.html#wp1065158

so, it really depend on the task:

In order to help track down the source of this problem configure R5?s

Ethernet interface attached to VLAN 58 to join the multicast group

226.26.26.26.

Ensure that R5 responds to ICMP echo-requests sourced from VLAN 2

which are sent to 226.26.26.26.

IN this case you can only use the join command (cant use the static as the router would not respond, and you'd only met the first part of the task).

HTH,

Vlad

zillah2004 · ‎09-29-2006

>>IN this case you can only use the join command (can't use the static as the router would not respond, and you'd only met the first part of the task.<<

Thanks Vlad for your explaination. I was trying to find the resource for what you have mentioned above, and I have done googling for "ip igmp join-group vs ip igmp static-group",,,,,

http://www.cme.com/files/MarketDataPlatform_DeveloperGuide.pdf#search=%22%20ip%20igmp%20static-group%20vs%20ip%20igmp%20join-group%20%22

[cut]--

What is the difference between the ip igmp join-group and the ip igmp static-group?

The join-group command forces the router to respond as a host on its own interface. This is processswitched. The static-group command causes the router to put the interface permanently in the OIF (output interface) list. This is not process-switched.

[/cut]

http://blog.sazza.de/?cat=21

[cut]

ip igmp join-group the router will accept multicast packets and also forward them

ip igmp static-group the router will not accept and only forward a multicast stream[/cut]

http://ftp.uth.gr/Mbone/guides/IP_Multicast_Com_Ref.pdf

[cut]

ip igmp join-group

Informs the router to join group on the interface. IP packets that are addressed to this group address will be passed up to the IP client process in the router as well forwarded out the interface. If you do not want packets forwarded out the interface, join the group on a loopback interface. Packets are not sent on the loopback interface.

ip igmp static-group

Configures a static connected member for the group on the interface. Packets to the group will get fastswitched, unlike the ?ip igmp join-group? command. If ?ip igmp join-group? command is used for the same group address, it will take precedence and the group will behave like a locally joined group.

[/cut]

http://www.hpl.hp.com/research/mmon/specifications.html

[cut]

When the Cisco configuration command "ip igmp static-group" is used to force a router interface to join a multicast group, that self-subscription of the router is not reflected properly in the IGMP MIB.

A workaround is to use the command "ip igmp join-group", which properly shows self-subscription in the IGMP MIB

[/cut]

http://www.oreilly.com/catalog/cisiosnut/chapter/refI.html

[cut]

This command (ip igmp static-group) enrolls the router in the multicast group specified by the group-address. Unlike the ip igmp join-group command, this command allows packets to the group to be fast-switched out the interface

[/cut].

1- Is the processswitch same as fastswitched ?

2- Is quote #2 authenticated ?

3- I could not find where it has been mentioned that :"the static as the router would not respond"

Regards

zillah2004 · ‎09-29-2006

>>1- Is the processswitch same as fastswitched ?<<

Yes, I found it is same.

>>2- Is quote #2 authenticated ?<<

Yes. The link below :

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800ca794.html

[cut]

.Use the ip igmp join-group interface configuration command. With this method, the router accepts the multicast packets in addition to forwarding them. Accepting the multicast packets prevents the router from fast switching.

?Use the ip igmp static-group interface configuration command. With this method, the router does not accept the packets itself, but only forwards them. Hence, this method allows fast switching. The outgoing interface appears in the IGMP cache, but the router itself is not a member, as evidenced by lack of an "L" (local) flag in the multicast route entry.

[/cut]

>>3- I could not find where it has been mentioned that :"the static as the router would not respond"<<

Still I am looking for it ??