TCAM Utilization on SG300 28P

[brettmason]

Hi There,

I was wondering if you could help me aviod a situation where the limit of 100IP's is reach on my client new site using 3 x SG300 28P switches.

I have 1 x SG300 28P in Layer 3 mode which is the default gateway for all the IP phones that will be installed. The PC's ont he network will use the existing default gateway which is another router. I will have another 2 x SG300 28P devices in layer 2 mode which are connected to the Layer 3 SG300 28P. 

My question  - Are the IP's that registered against the TCAM limit only the devies which physically plug into the SG300 28P switches ? I assume other computers on the network which are plugged into another switch and don't use the default gateway of the SG300 (its only for voice) they then wouldn't be registered in the TCAM ?

The site has around 65 computers currently and obviously plugging in 65 IP phones we're going to hit a limit of over 100 IP's. My thoughts were to potentially keep the computers and Phones seperate on a couple of the switches to keep the IP's in the TCAM to a minimum..  Is this possible?

Any advice would be welcomed!

Brett                  

[Tom Watts]

Hi Brett, the Ternary Content Addressable Memory is a part of the RAM and has an allocation block. The switch should be able to hold around 16,000 MAC address entries. However, the 100 IP addresses should not affect the layer 2 switch as it simply forwards the packet. The TCAM will support 510 entries. The TCAM has information about the QoS and ACL's.

In the layer 3 mode, the switch can hardware switch 100 IP addresses, however, it can software switch up to 510 IP addresses which would result in the switch slowing down.

I think you'll be okay. The TCAM will store the information locally however with a 130 client network and even more, this won't flood the switch.

-Tom

[brettmason]

Hi Thomas,

Thanks for the quick reply.

Just to confirm though, I want to be sure that the Layer 3 SG300 28P will have have all the IP phones from the other Layer switches using it as the default gateway for the voice VLAN - Obviously this will then register 60 + IP addresses. If I have the computers plugged into the back of the Phones (which is then into the SG300 switches) this will then register another 60 IP's correct? If I don't patch these computers into the phones and have them in a seperate switch then the TCAM address list doesn't care about these computer IP's? I do believe we'll have traffic routing from the computer to the phones even if they are on a different switch so would that then add these addresses to the TCAM?

The reason I ask this to be clear is that I read someone else going over the 100 limit and causing the network to slow down which with voice traffic I want to avoid...

Brett

[Tom Watts]

Hi Brett,

The network traffic may not nescessarily suffer. However, if you exceed the 100 IP addresses, you may see error logging "SFFT overflow". Although this error would indicate network performance is suffering, it doesn't necessarily mean you are having issues.

I had one case about 8 months back a gentlement had 300 IP addresses' through the switch. This resulted in such an error but his network performance was okay. Of course this is a case by case basis, which the answer to you is "may be."

General wisdom may dictate, may be is not good enough. I'd recommend you err on the side of caution and go with a bit larger switch as, since it is over 100 IP addresses, I couldn't garauntee your performance one way or the other.

-Tom