CCNP Switch Question

hurricane1091

Hello all,

Working on Chapter 2 in the OCG. It references things like a TCAM which is difficult to understand even after going on Google. Also, does anyone actually mess with SDM templates? In fact, on thede 3650s, I'm being told it used the advanced template and doing "sdm prefer ?" yields only that option, which I have not read anywhere.

My boss, a Cisco guru, could not really recall either one. Which makes me question the relevancy of both, but maybe someone else could shed some light.

networker050184

You definitely need to know about TCAM. Most importantly it's size and limitations on your device and it's role in the network.

Once you understand this part the SDM templates will make much more sense.

FYI if you boss doesn't know anything about TCAM he's FAR from a guru!

hurricane1091

The guy worked on the network for that satellite that went to pluto, taught CCIE's at Cisco, and designed this entire network that is completely redundant with detailed QoS, strict redistribution, route influencing, etc and has an iWan lab set up and designed better than the idea presented by Cisco themselves (who was here today) so I don't really question his resume at all.

But, I do find it odd about the lack of information out there on this subject and specifically the lack of detailed information. So, I'm left confused. I believe you when you say it is important, but I question how important it can be or how he could possibly know about it when he seemingly knows everything. As I mentioned, it appears that there's not even a choice except the "advanced" template for SDM at our branch switches. Maybe for whatever reason it just doesn't become relevant, either out of luck or for how everything is designed. I don't know, help me understand please.


We have 7ks at the core, and it appears that "show sdm prefer" is not a command here. So, what is going on? Perhaps that is different in some sort or what? It distinctly mentions in the book that you the TCAM is self sufficient, so I mean I don't know. Just confused over here.

Update: I'm looking at our IDF switches, and they just use the default. So, I'll just go ahead and assume that this works for most people, most of the time. I would totally love to hear scenarios where it did not work and why, and would help me understand better.

networker050184

The SDM templates are a device specific way to configure TCAM allocations. That part isn't really important. Understanding TCAM and the role it plays in the device is the important part.

7ks certainly have TCAM as well. TCAM will also determine things like MAC entries a device can hold, routes, etc. This is extremely important information when designing a network. Take Cisco 6500's for example. It is very common to configure your TCAM to fit your network needs. A SUP720 for instance defaults to holding 512k IPv4 routes. This isn't enough to hold the modern internet table. You need to steal some from IPv6 for example.

Getting even deeper into TCAM you can understand the way a device runs queries against it. Longest match through subsequent trees vs specific look up of a host route etc.

hurricane1091

Hello,

I spoke with my boss and we he looked it up to refresh. Basically said we never had to adjust the SDM templates from default and doing a show SDM prefer and actually looking at it, I can see why that would be the case. Unrelated but my boss talked about VDC templates which was interesting. Anyways, I guess the actual process of how a tcam is used is what confuses me. From a high level it makes sense but I guess knowing how it all computes out confuses me lol.

theodoxa

Yes, people do change SDM templates. A good example would be if you wanted to implement IPv6. You would need to change to a Dual Stack SDM template. SDM templates define how the TCAM is partitioned. A VLAN template for example would allow for more MAC Addresses to be learned at the expense of having little or no Layer 3 (route learning) support. A Routing template would set aside space to store routes, but at the expense of having less space for learned MAC Addresses.