[mini] PIM Dense State Refresh

Been brushing up on multicast recently.  It was one of the first topics I ever deep-dived and some of the material is rusty now... two and a half years later.

Came across PIM-DM State Refresh.  This is an interesting attempt to make dense mode PIM more scalable. If you ask a CCNP student what the common detriment with using dense mode is, he'd probably tell you "It floods all its groups to every PIM device every three minutes".  That is true, but that's an attempt to solve a problem, not the problem itself.

The real problem is that dense mode has no way of letting potential receivers know what groups are available, or where to find them. It's easy to lose site of this when labbing: you control every device and know exactly where all the transmitters and receivers are, and know what groups are on the network.  The 3-minute flooding is only present because that's how dense mode tells the network what groups are available and where to find them.

State Refresh is not a new technology - it was proposed in the late 90's - but I'd never heard of it before today.  With it enabled, you still have the initial densing of the actual multicast stream, but after the initial prune, instead of just firing the stream off every 3 minutes, it instead sends a state refresh every X number of seconds, where X is defined by the command that enables it:

interface FastEthernet1/0
 ip pim dense-mode
 ip pim state-refresh origination-interval 10

In this sample, we would send state refresh messages every 10 seconds.

In this fashion, all PIM routers in the network are still aware of the stream, but they don't get the annoying densing out of the traffic followed by having to prune it constantly.

Also note, this process only works if the transmitter is still sending traffic. It does not do this "keepalive" signaling for multicast streams that are no longer in use.

Where you place the state-refresh command is important.  It should always go on the PIM interface closest to the transmitting host. If you put it anywhere else, it does not work.  You do not need to enable it on other routers on the host.  In my lab, I had three interfaces, one pointing at a host endlessly pinging 239.0.0.100, and the other two pointing towards PIM routers. I only have it enabled on the interface pointing towards the host.

All PIM Dense routers/interfaces will automatically relay state-refresh messages.  This command only needs to be enabled on interface facing transmitting hosts.

If you don't want a PIM router to relay these messages, use this global command:

ip pim state-refresh disable

Cheers,

Jeff

The Woz!

Totally off topic this time - but tonight, I met Steve Wozniak, and it was amazing.

He went to a small networking event that I attended.  When I signed up for it, I was on the fence about attending - he hadn't been signed at that time - but I decided to go anyway (drug along was more like it, but that's another story).  Then the message came out that he was the guest speaker, and tickets sold out in the blink of an eye.

He talked for about 30 minutes, took Q&A for another 30.  What a super guy.  Incredibly personable.

Brief recap of the discussion:
- He spoke several times about Steve Jobs.  Some insightful things, including the mentor Apple had in the early days, and how Steve J learned most of his technique from that mentor (one of their early investors).  Interestingly, it wasn't all positive - he mentioned on several occasions how he wished Steve J had been more generous both in his personality and financially.
- He's such a nerd! (in a good way).  This was a technology business networking function, so inevitably the topics tended to lean towards business. He'd start off on a business topic, and really just end up saying that if you have a cool engineering idea, it'll probably do well.  Then he'd go off on a story about a neat engineering idea.
- He has some interesting ideas about wearable technology.  He doesn't think we've got it nailed yet, and that the smartwatch in particular doesn't have a big enough screen to be usable.
- He's disappointed that computers in schools didn't result in smarter students.  He talks a lot about how people had interest in them as they displayed something new, but the interest didn't stick unless the "new" kept coming.
- He talked a lot about the education system in general.  Interesting ideas about Singularity - http://en.wikipedia.org/wiki/Technological_singularity - and how that might help in learning someday.  Also he hypothesized that Moore's Law (http://en.wikipedia.org/wiki/Moore's_law) is going to fail soon, and that Singularity may be a long way off because of that.
- He made a few good jokes, including a reference that we now ask all questions of the Internet, which was clearly never designed to be an "ask me a question" resource.  He says we now ask all questions of something that starts with "Go" and it's not "God".
-  He had some really positive comments about Google, even a vague reference that there perhaps should have been an Apple/Google merger ... ?  He said Google definitely had Apple licked on human phrase interpretation, indicating Siri did a relatively poor job of taking a human idea and producing an answer, but Google took human phrasing very well and produced ... a page of links (Google doesn't answer questions, for the most part).

He was signed up for a photo op with the sponsors, but let everyone know in advance that he'd be coming back down to mingle afterwards.

He came back down and gave everyone a chance to take photos, and chatted with everyone, as best he could, for a person that was being mobbed by 60+ fans.

This was my best photo, which was (thank God) taken unbeknownst to me by an old friend, who I happened to bump into there.  Because all my other ones came out terrible.  You can't see it, but I did get to shake his hand.

This is a very hastily cleaned up version of one of my other photos --

All my other ones had severe lightning problems, unfortunately.

What a great evening!

Jeff