So I have a question on this. it is stated that dummy multicast are flooded out all ports that contain the switch with the broken links Mac address table. in addition this mechanism will cause the 20 second timer enacted by the topology table to stop and the other switches can go to receiving the new mac address information immediately. quote from web lesson:
Take a look again at the MAC address table for SW2. The MAC address (000c.29e2.03ba) that I highlighted belongs to H2. When SW2 receives an Ethernet Frame for H2 it will be forwarded to SW1 and it will be dropped! (Well at least for 15 seconds until the topology change mechanism kicks in…).
I tend to really learn when I have to explain this back to myself or another person as if I was teaching. When I did so the I saw an inquisitive explorative question that might arise if we just did not take something as fact because it was said.
why does the dummy multicast cause the switches to stop holding those MAC addresses contained within the dummy multicast??? why does it not drop the dummy multicast like it does in the quote above from the web lesson??
Now if asked this I would simply say because the dummy multicast are a special address that the switches recognize so they simply drop those from its aging table and then they are able to receive.
however, I am just making the most logical guess that I can make as I did not see that posted anywhere. the true answer would be I don’t know for certain.
Anyway diving deep into questions like this helps me to remember the content better as its all embedded behind the reasoning and must be known to get to this point so I figured why not ask =)
I actually am being naughty, I have not read my book or tried to watch the videos to see if they state this detail. so I am being lazy. What I will do is this; I will go back read my book and try to go through and find the answer they give in my video and post here. I think its worth having the question on the forums as others can benefit as well. If someone provides an answer here before I can get back and post that’s great to as I a:) may not be able to find my answer or b:) will confirm what I find when I come back to post my findings!
Ok first information non related to my question but I will post as its useful:
Configuring Uplink Fast Uplink Fast is enabled on Access layer switches and keeps track of possible paths to the Root Bridge. Once the Uplink Fast feature is enabled globally, it is enabled for the entire switch and all VLANs. By default, when Uplink Fast is enabled, Cisco IOS software performs the following actions on the local switch:
- The Bridge Priority of the switch is raised to 49,152
- The Port Cost of all VLANs is increased by 3,000
These two actions ensure that the switch will never be elected Root Bridge, and it makes the path through this switch as undesirable as possible for any downstream switches. For this reason, Uplink Fast should never be enabled on the Root Bridge because it will lose its Root status or lose switches that have other downstream switches connected to them.
Tafa, Farai. Cisco CCNP SWITCH Simplified (Kindle Locations 2588-2596). Reality Press Ltd. Kindle Edition.
Next is the answer to my question. was not in the book basically the same as the web lesson it did have a extra piece of information or two:
By transitioning the port to a Forwarding state almost immediately, the Uplink Fast feature presents the potential problem of incorrect entries in the CAM tables of the other switches because they have not had an opportunity to re-learn the new path for the MAC addresses of the devices connected to the Access switch.
To prevent this, the Access layer switch on which the Uplink Fast feature is enabled ﬂoods dummy frames with the different MAC addresses that it has in its CAM as a source. The frames are sent to the Multicast address 01-00.0C-CD-CD-CD and appear to originate from the hosts connected to the switch so all the upstream switches can learn of these addresses through the new port.
By default, the switch sends out these Multicast frames at a rate of 150 packets per second (pps). However, this value can be adjusted by using the spanning-tree uplinkfast max-update-rate [rate] global conﬁguration command.
Tafa, Farai. Cisco CCNP SWITCH Simplified (Kindle Locations 2618-2620). Reality Press Ltd. Kindle Edition.
so next I will try to scan through my INE CCNP video and see if they mention the answer to my question!
They did not give an answer on INE as well they just said it sent dummy multicast and that fixed it. so I am only left to guess that it fixes it and is not dropped because the address is special and the software code says hey when you see this address drop the MAC addresses and add these Mac addresses from this location or maybe it does not drop but just changes but the effect is the same.
if someone is able to find this answer feel free to post at least I now did my due diligence in trying to find the answer!
note: I did find the following it does not spell out the answer of how either, it does once again logically suggest that the “how” is because its a special address and strengthens it with terms such as “ensures” ect…
In order to solve this problem, switch A begins to flood dummy packets with the different MAC addresses that it has in its CAM table as a source. In this case, a packet with C as a source address is generated by A. Its destination is a Cisco proprietary multicast MAC address that ensures that the packet is flooded on the whole network and updates the necessary CAM tables on the other switches.