From: Gerry Hilton (gerry.hilton@rogers.com)
Date: Tue Jul 20 2004 - 22:14:16 GMT-3
Thanks, Kenneth. Those were very helpful tips on how to determine which
mode to configure. My question had to be quite deliberately vague but
it produced a lot of good information. Thanks for the help.
Gerry
Kenneth Wygand wrote:
>Gerry,
>
>In light of the lab, here are my comments and thoughts.
>
>First, the PIM mode is configured on an interface. That mode restricts the forwarding of group traffic in a certain fashion. Running pim sparse-dense mode on an interface allows each group to receive streams in either fashion: sparse or dense mode. Running pure sparse or pure dense mode PIM on an interface only allows multicast traffic to flow from source to group in that fashion.
>
>What you are saying is correct. If you run PIM sparse-dense mode on an interface, groups -WITH- an RP will run in Sparse mode, which groups -WITHOUT- an RP will run in Dense Mode. Thus the auto-rp groups, which obviously don't have an automatic RP themselves, will run in dense mode and all other groups can run in either fashion.
>
>If all groups have an RP, all groups will be running in sparse mode. However, you must read what the requirements of the question are. Does it say "the interface should run Pim Sparse-mode"? If so, even if all the groups are running in sparse mode, Pim is running in sparse-dense mode if you configure it that way.
>
>Look for other subtle clues, such as "ensure shared trees are never used". Since sparse-mode initially starts using a shared tree (even if it switches to a SPT after that), running any kind of sparse mode would violate these requirements. Maybe you are to "ensure traffic switched over the frame-relay network does not impact critical routing processes". In this case, you must run "ip pim nbma-mode" so multicast traffic is not "accidentally" put into the hardware strict priority queue, and in order to run "ip pim nbma-mode", you must be in sparse-only mode.
>
>Yes, there are a lot of times where multiple solutions can satisfy the requirements for a particular set of exam questions. Maybe you can configure subinterfaces on an interface or run a protocol on a physical interface and both solutions satisfy a given set of requirements. However, with a topic as large as multicast, I'd be hard-pressed to think the requirements they set forth on you will be loose enough that you can choose either version of PIM and still satisfy all of the requirements. Now, whether or not you actually -realize- the implied requirement to know which way to configure it, that's a whole different ball game. ;-)
>
>I know on my attempts thus far, I thought I could satisfy a set of requirements in multiple ways, so I just picked one way of doing it. However, since that time I've learned a lot more, and usually (sometimes several weeks apart), I'll learn something new and say "Ahh, -THAT'S- why I couldn't configure it the way I did and still satisfy the requirements". That's a hard lesson to learn when you give up 2, 3 or 4 points per concept. God Bless the CCIE exam and all those who have gone before me! :)
>
>Hope this helps a bit,
>Ken
>
>________________________________
>
>From: Gerry Hilton [mailto:gerry.hilton@rogers.com]
>Sent: Tue 7/20/2004 7:45 PM
>To: Kenneth Wygand
>Cc: ccielab@groupstudy.com
>Subject: Re: Multicast - Sparse Mode
>
>
>
>Hi. I agree - if it is clearly stated to configure sparse mode, and
>there are no other requirements, then I too would configure sparse mode,
>and probably use the ip pim auto-rp listener command.
>
>In my email though, I was referring to the less clear case where there
>were additional requirements so that configuring sparse mode only would
>probably not work. I keep reading that interfaces are treated as sparse
>if they know of an RP, so in that case I was thinking that configuring
>them as sparse-dense and making sure that an RP was known, would meet
>the requirements of sparse mode as well as some additional requirements.
>
>Thanks,
> Gerry
>
>Kenneth Wygand wrote:
>
>
>
>>Gerry,
>>
>>My opinion... If "one is asked to configure sparse mode", hey, you'd better be configuring SPARSE mode... not Dense mode, not Sparse-Dense mode, but SPARSE mode... This is just my opinion.
>>
>>However, take a look at the following command in the Doc CD:
>>
>>"ip pim auto-rp listener"
>>
>>It may answer some of your question for things that previously "required" dense-mode...
>>
>>HTH,
>>Ken
>>
>>________________________________
>>
>>From: nobody@groupstudy.com on behalf of Gerry Hilton
>>Sent: Tue 7/20/2004 7:12 PM
>>To: ccielab@groupstudy.com
>>Subject: Multicast - Sparse Mode
>>
>>
>>
>>Hi there. A few questions about multicast and sparse mode:
>>
>>If one is asked to configure sparse mode, then is it valid to configure
>>ip pim sparse-dense mode and have a known RP, as then the interfaces
>>would be treated as sparse mode (this would be if there was a case where
>>just configuring sparse mode would not meet all requirements)?
>>
>>If the interfaces are configured as sparse-dense and the RP is known
>>via auto-RP, then are they still considered to be acting in sparse
>>mode? If not, then is the only way to know the address of the RP and
>>still be considered sparse mode, to manually configure the RP address on
>>each router?
>>
>>This is only in the case where one requires the interfaces to be
>>configured in sparse-dense mode but treated as sparse.
>>
>>Thanks,
>> Gerry
>>
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