Questions about: Jumbo Frame and Link-Aggregation vs STP

[johnk3r]

Hi !

1º Already set up and read the documentation on the feature Jumbo-Frame. My question is:  When do I use this feature ? It's worth setting in host ports ? And uplink ports ? I have a problem enabling this feature?

2º In environments that have redundant paths, it is best to use the STP to have a backup link or use the link-aggregation between switches? I know that in the STP topology the link-aggregation is seen as a single port. Imagining that I have a IRF topology, which is the best scenario?

3º Is there a specific feature that can be used in switches that operate in industrial environments (PLC machines and the like)?

[Apachez- · Accepted solution]

1) You use it to more efficiently push large amount of data between two hosts.

The background is that there is a limit of how many interrupts a cpu can handle (just above 250k per core or something like that for x86 even if that have increased slightly over the years). There are of course various ways to overcome this limit but one of them is to push more data per interupt (1 arriving packet/frame = 1 interrupt if you dont use polling etc).

So jumbo frames got "invented" where the MTU is changed from 1500 to 9000 (which is the common one today, well actually 9216 or something like that) or even 12000 have existed over the years. This gives that instead of sending 6 frames @1500 bytes with the equal number of interrupts along with other processing per frame (checksum calculations etc) you just send 1 frame @9000 bytes. Note that just because you fully enabled jumbo frame support doesnt neccessary mean that all traffic will fully utilize maxsize at all time.

In order for jumbo frames to work the infrastructure between the hosts must support it along with the hosts themselfs.

When it comes to jumbo frames for infrastructure (routers/switches) there are two things to keep an eye on. Jumbo frames within a VLAN (L2) and jumbo frames across VLANs (L3). Sometimes jumbo frames are supported within a VLAN but not by the VLAN-interface itself which means that this particular device cannot route jumbos, only switch them.

You should be fine enabling jumbos but at the same time if the traffic is for example towards internet (which doesnt supports jumbos anyway) then there is no need to enable it.

2) Personally I would avoid relying on STP at all costs. That is better to use static or dynamic link-aggregation along with a loop-free design like with IRF and where you use L3 at all links. STP can still be enable as a safeguard but you will then not rely on STP and all links will be used at all time (compared to with STP where all links except one will be "down").

3) You mean configuration wise? Na, the same applies as for regular switches. SEC (Secure Enduser Connections) should be a good read on recommendations https://secureenduserconnection.se/

[16again]

Jumbo frames:
Is there really any CPU advantage? Decent hardware NICs already do TCP offloading.
Remains the advantage of slightly less overhead (1 instead of 6 headers) , resulting in somewhat higher throughput.
I'd only use on links with heavy traffic  like iSCSI SAN traffic or towards backup device

[johnk3r]

@apachez, thanks for the help !!!

About Jumbo Frame: to study some more to get into a technical discussion. I know most of switches comes with this feature enabled. Is there any moment or situation that need to disable?

About STP: I agree with you !!!

About industrial machines : My question would be for industrial machines, you must configure specific parameters of broadcast-suppression, edge-port stp and similar features ...

[johnk3r]

@16again ,  thanks for the answer !!!

How do you usually do the configuration of this feature? Most switches already comes with it enabled ! You disables and leaves only

the necessary ports?

display interface GigabitEthernet 1/0/2
 GigabitEthernet1/0/2 current state: UP
 IP Packet Frame Type: PKTFMT_ETHNT_2, Hardware Address: b8af-67ef-0051
 Description: GigabitEthernet1/0/2 Interface
 Loopback is not set
 Media type is twisted pair
 Port hardware type is  1000_BASE_T
 1000Mbps-speed mode, full-duplex mode
 Link speed type is autonegotiation, link duplex type is autonegotiation
 Flow-control is not enabled
 The Maximum Frame Length is 9216
 Broadcast MAX-ratio: 100%
 Unicast MAX-ratio: 100%
 Multicast MAX-ratio: 100%
 Allow jumbo frame to pass 

[16again]

Johnk3r 

I just keep the switch default setting (1500 or 9k).  
Only when enabling jumbo frames, I'd look into config.
As a rule of thumb, all endpoints default to 1500, which will work right away.