Re: switch speed question
Not every bit in results in just one bit out. Broadcast, multicast, flooding for unknown MACs (or switching failures), ... They were talking about a simple scenario where a bit that enters a port will leave a port. With 24 gigabit ports, for all intents and purposes, you will only ever have 24 gigabits at the most traversing the backplane.
Re: switch speed question
Doesn#39;t that assume that the communicarion is unidirectional? If two hosts are exchanging 1Gbps flows, the traffic across the bus will be 2Gbps, right? And of course, this doesn#39;t include any bus-intensive operations like multicast or things which require cpu processing - those can consume a lot more resources than the input rate of the port. -David Barak Tom Storey wrote: Not every bit in results in just one bit out. Broadcast, multicast, flooding for unknown MACs (or switching failures), ... They were talking about a simple scenario where a bit that enters a port will leave a port. With 24 gigabit ports, for all intents and purposes, you will only ever have 24 gigabits at the most traversing the backplane.
Re: switch speed question
Were not considering anything other than basic switching in this scenario, as is my understanding. 2 hosts will create 2gbps of traffic as each host is inputting 1gbps into the switch (just multiply it by 12 to give you 24 ports). 3 hosts will create 3gbps of traffic as each inputs 1gbps into the switch (e.g. each host could be sending 500mbps to each of the other hosts). And thus and so forth. :-) You can only input a maximum of 24gbps into the switch, which means that only 24gbps will cross the backplane. Yes there is 48gbps if you combine tx and rx of each port, but traffic only has to cross the backplane once, from rx on one port to tx on another. Sorry if I have hijacked this thread from the OP. :-) Tom On 26/02/2009, at 12:18 AM, David Barak wrote: Doesn#39;t that assume that the communicarion is unidirectional? If two hosts are exchanging 1Gbps flows, the traffic across the bus will be 2Gbps, right? And of course, this doesn#39;t include any bus-intensive operations like multicast or things which require cpu processing - those can consume a lot more resources than the input rate of the port. -David Barak Tom Storey wrote: Not every bit in results in just one bit out. Broadcast, multicast, flooding for unknown MACs (or switching failures), ... They were talking about a simple scenario where a bit that enters a port will leave a port. With 24 gigabit ports, for all intents and purposes, you will only ever have 24 gigabits at the most traversing the backplane.
Re: switch speed question
On 26/02/2009, at 2:48 AM, David Barak wrote: Doesn#39;t that assume that the communicarion is unidirectional? ... No. If two hosts are exchanging 1Gbps flows, the traffic across the bus will be 2Gbps, right? Yes. 1Gbps backplane impact per host. You have two hosts, right? One host per port? That's 1Gbps per port. So, 24 ports = 24Gbps, right? Let's try look at it another way: - A 24 port gig switch can receive at most 24Gbps. - That same switch can transmit at most 24Gbps. You don't get to add transmit and receive together to get 48Gbps. Packets don't go across the backplane once to receive, and then once more to transmit. They go across once, from the receiving port to the transmitting port. (sure, sometimes perhaps packets do go across twice, but not normally) And of course, this doesn#39;t include any bus-intensive operations like multicast or things which require cpu processing - those can consume a lot more resources than the input rate of the port. Of course multicast/broadcast consumes more resources than the input rate. That's the point. If you receive multicast or broadcast at 1Gbps, and the multicast needs to go out all the ports, you need to transmit at 24Gbps. That's 24 x the transmit resources (and probably backplane resources, depending on architecture etc. etc.) than a single 1Gbps unicast stream. Of course, with unicast it is only getting to one host. Let's assume we have data at 1Gbps that we need to get to 24 hosts. - If we unicast, we need 24 input ports, and 24 output ports, assuming we only have gig ports (or say 3x10GE, or whatever). - If we multicast, we need 1 input port, and 24 output ports. When you compare the end result, multicast uses significantly less resources, right? In fact, perhaps some bus architectures know about how multicast works, and it consumes *less* resources than doing the same thing with many unicast streams. If the bus does not know about multicast, then the bus would treat it as 24 unicast streams, surely. -- Nathan Ward
Re: switch speed question
Nathan Ward wrote: On 26/02/2009, at 2:48 AM, David Barak wrote: If two hosts are exchanging 1Gbps flows, the traffic across the bus will be 2Gbps, right? You don't get to add transmit and receive together to get 48Gbps. Packets don't go across the backplane once to receive, and then once more to transmit. They go across once, from the receiving port to the transmitting port. (sure, sometimes perhaps packets do go across twice, but not normally) Assuming a crossbar switch, sure. If your ports individually look up the outgoing port for an incoming packet, request backplane to that port, and transmit, then you only need 24Gbps. If your ports need to connect to an intelligent entity on the backplane to do your routing/switching/IGMP snooping/QoS enforcement/etc, then you are indeed going to cross the backplane twice, and need both transmit and receive bandwidth. Since many of us are routing goons with store-and-forward roots, we tend to think along those lines. And it is still wise, even in this day and age, to make sure that backplane bandwidth doesn't include a central switching point, or, if it doesn't, the marketing folks haven't doubled the backplane numbers because they took it out. -Dave
RE: switch speed question
Hi, It depends on how heavily loaded your switch is expected to be, for instance two machines using the switch will be able to get a full 1Gbps, however depending on the backplane (switching fabric), it limits how many ports will receive full 1Gbps when the switch is congested, e.g. a 2 gig backplane against a 24 gig. Regards, Bruce -Original Message- From: Deric Kwok [mailto:deric.kwok2...@gmail.com] Sent: Monday, February 23, 2009 5:08 PM To: nanog@nanog.org Subject: switch speed question Hi Can you share your experience what is fastest Gig switch? I see there is CEF feature in cisco. ls it big different when i enable it in switch vs other switch? ls there any problem? Thank you
Re: switch speed question
On Tue, Feb 24, 2009 at 2:33 AM, Bruce Grobler br...@yoafrica.com wrote: Hi, It depends on how heavily loaded your switch is expected to be, for instance two machines using the switch will be able to get a full 1Gbps, however depending on the backplane (switching fabric), it limits how many ports will receive full 1Gbps when the switch is congested, e.g. a 2 gig backplane against a 24 gig. Regards, Bruce Note that the traffic to a switch is bi-directional (full duplex) - so a 24 port gigabit switch can max out its 32 Gig backplane, if all 24 ports have a gig coming in and going out (24 X 2 is 48, more than the 32 gig backplane). This isn't immediately apparent - the other day someone at my work asked the exact question Why's the 32 gig backplane the 24 ports on the switch? -- Eric http://nixwizard.net
Re: switch speed question
That isn't always true. Some switches are already speced as full. It's best to read the product docs or speak with a rep to be sure. tv - Original Message - From: Eric Gearhart e...@nixwizard.net To: NANOG list nanog@nanog.org Sent: Tuesday, February 24, 2009 9:51 AM Subject: Re: switch speed question On Tue, Feb 24, 2009 at 2:33 AM, Bruce Grobler br...@yoafrica.com wrote: Hi, It depends on how heavily loaded your switch is expected to be, for instance two machines using the switch will be able to get a full 1Gbps, however depending on the backplane (switching fabric), it limits how many ports will receive full 1Gbps when the switch is congested, e.g. a 2 gig backplane against a 24 gig. Regards, Bruce Note that the traffic to a switch is bi-directional (full duplex) - so a 24 port gigabit switch can max out its 32 Gig backplane, if all 24 ports have a gig coming in and going out (24 X 2 is 48, more than the 32 gig backplane). This isn't immediately apparent - the other day someone at my work asked the exact question Why's the 32 gig backplane the 24 ports on the switch? -- Eric http://nixwizard.net
RE: switch speed question
Arista claims to have the fastest 1/10 Gig 24 and 48 port 1RU switch, with a backplane capacity guaranteeing 10 Gig full duplex line rate per port. Cisco's CEF is local only and functions to download the arp cache and routing table into ASICs for hardware switching; but look at Cisco's NSF/SSO for cases where adjacent devices are all defined in the same packet forwarding state machine. -Original Message- From: Deric Kwok [mailto:deric.kwok2...@gmail.com] Sent: Monday, February 23, 2009 5:08 PM To: nanog@nanog.org Subject: switch speed question Hi Can you share your experience what is fastest Gig switch? I see there is CEF feature in cisco. ls it big different when i enable it in switch vs other switch? ls there any problem? Thank you
Re: switch speed question
Eric Gearhart wrote: Note that the traffic to a switch is bi-directional (full duplex) - so a 24 port gigabit switch can max out its 32 Gig backplane, if all 24 ports have a gig coming in and going out (24 X 2 is 48, more than the 32 gig backplane). I think your math is faulty. While there may be 24G going in and 24G going out, there is only 24G crossing the backplane. You can't count a bit twice (once on in and once on out). Its the same bit.
Re: switch speed question
Once upon a time, Roy r.engehau...@gmail.com said: I think your math is faulty. While there may be 24G going in and 24G going out, there is only 24G crossing the backplane. You can't count a bit twice (once on in and once on out). Its the same bit. Not every bit in results in just one bit out. Broadcast, multicast, flooding for unknown MACs (or switching failures), ... -- Chris Adams cmad...@hiwaay.net Systems and Network Administrator - HiWAAY Internet Services I don't speak for anybody but myself - that's enough trouble.
Re: switch speed question
Switches like this and the force10 2410 and the like are cut through so do sub micro second versus a 'regular' store and forward switch --Original Message-- From: Holmes,David A To: Deric Kwok To: nanog@nanog.org Subject: RE: switch speed question Sent: Feb 24, 2009 1:03 PM Arista claims to have the fastest 1/10 Gig 24 and 48 port 1RU switch, with a backplane capacity guaranteeing 10 Gig full duplex line rate per port. Cisco's CEF is local only and functions to download the arp cache and routing table into ASICs for hardware switching; but look at Cisco's NSF/SSO for cases where adjacent devices are all defined in the same packet forwarding state machine. -Original Message- From: Deric Kwok [mailto:deric.kwok2...@gmail.com] Sent: Monday, February 23, 2009 5:08 PM To: nanog@nanog.org Subject: switch speed question Hi Can you share your experience what is fastest Gig switch? I see there is CEF feature in cisco. ls it big different when i enable it in switch vs other switch? ls there any problem? Thank you Sent from my BlackBerry device on the Rogers Wireless Network
switch speed question
Hi Can you share your experience what is fastest Gig switch? I see there is CEF feature in cisco. ls it big different when i enable it in switch vs other switch? ls there any problem? Thank you
Re: switch speed question
Can you elaborate a bit on your question? The fastest Gig switches can do 1GB full speed on the port. There are many that can do that. Do you have a particular density you need to do full speed with? Any particular features? Are you looking at any particular models now, in others words have you even begun to explore this before posting here? Are you looking at Layer 3 switching, I assume you are since you are asking about CEF. Every manufacturer has a way of switching the packets, Cisco uses CEF, and yes if you enable CEF its a big difference vs. a netgear gig switch from best buy, but I think you are wanting more of an answer than that and you just need to give us some more info. Brian On Feb 23, 2009, at 10:08 AM, Deric Kwok wrote: Hi Can you share your experience what is fastest Gig switch? I see there is CEF feature in cisco. ls it big different when i enable it in switch vs other switch? ls there any problem? Thank you
