LAN<-->WAN<-->LAN ; L3 switches or Routers??

We have a case, where there is a LAN in the local place and a separate LAN in the remote place. This also means, there is a WAN in between these 2 LANs and they communicate via this WAN. I would like to know - What is most preferred among 'Routers' and 'L3 switches', which are to be used for routing the messages from the LAN to the other LAN, via the WAN. Could anybody give a rationale behind that choice?

Also, I would like to know whether this choice depends on - whether the WAN is our own or 3rd party network? - whether there is only one VLAN, for which some of the machines are kept locally and the other ones remotely

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It is quite common to use layer 3 switches connected directly to a carrier provided WAN transport as many carrier deliver the WAN via Ethernet..

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True, we traditionally use Frame-Relay WAN between our main office and remote LAN's ... requiring a HSSI at either end ... easilly slotted into a 'router' but we just recently comissioned our first BDSL service which the Telco delivers via an RJ45 Ethernet jack ... so a two Ethernet port router is simple but so is using a 'layer-3 switch'.

So something to consider is what hardware you need to interface to the WAN.


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This is a good point as while Cisco high-end switches like the 6500 have FLEXWAN card to support more than just Ethernet connectivity this is not the case on their low end switches.

So you might need routers and switches depending on the WAN transport options available.

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this is a good point to mention to the OP that the difference between routers and L3 switches these days is more about marketing and the "bias" in the box design than engineering.

originally - routers were basically software, and switches were hardware. now boxes with reasonable performance are usually a "blend" somewhere between those 2 extremes.

the rule of thumb is that if all the interfaces are "lan like" - then it probably gets called a switch, and if you are using older style WAN interfaces such as T1 / E1, Frame Relay or ATM it probably is called a router - but as other have said there are exceptions to both of these.

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This is not entirely true. A "L3 switch" does not do QoS. Yes a "L3 Switch" can do QoS marking, but it cannot do policing, queuing (at layer 3) WRED, etc. A L3 switch does CoS (class of service), which is limited by hardware (number or receive queues, output queues and priority queues) on each port. FLEXWAN and OSM cards for the 6500 series are boards that let you do the same QoS stuff that you can do on router interface. Its also why they are big bucks.

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as with most computer things "it depends" - it doesnt have to, but any high end network equipment these days that doesnt "do" QoS doesnt get bought - QoS features are mandatory when a company generates a tick list for a big procurement, and the manufacturers want their kit to get bought.

Yes a "L3 Switch"

FWIW i just did a lot of lab testing on a Cat6, Sup 720-3B, 6724-SFPs.

policing, Q managment (2 Qs inbound, 4 out), and WRR (which is sort of modified WRED) are all there.

it is the same rich set of features as you get on a software router, but it did everything we wanted, apart from shaping to fractional Gig E while also doing QoS queue management.


in our case it was cheaper to use more bandwidth and do GigE "wire speed" across a WAN than to have the richer features needed to handle lower speed links.

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Comparing a 6500 to a just a router or just a switch is not a fair comparison. A 6500 contains both a router and a switch (plurality in some cases). There is no apples to apples comparison in a chassis that has MLS capabilities. If you want to compare a basic router and a basic switch then compare a 3825 without any EtherSwitch modules and a

3750-EMI. That's a fair comparison between a router and a L3-switch.

This makes good sense. I'm also opposed to blending network layers in large chassis. You could run an entire ISP out of a single 7600 but that would involve blending core, distribution, access, and border router functions (I always separate border routers into a 4th network layer) into a single chassis. That breaks most of the design principals of modern day LAN/WAN design. Just because the device you're working with can do routing, it doesn't make it a good router. Likewise for interfaces; just because you can put 16 FastEthernet interfaces in a router doesn't mean it makes a good switch. This also applies to circuits and their purposes. Just because you can get an Ethernet hand-off for your Internet circuit doesn't mean you should forego a real router and plug it into a L3 switch.

One place that a WAN Ethernet hand-off does allow you to fudge on is firewalls. I don't have any problem at all with eliminating the border router for a small non-multi-homed office and replacing it with a purely-Ethernet firewall. There are design applications that may require a connection outside of the firewall but most of our customers don't have these requirements. If the hand-off had been a couple T1s they would have been stuck with a border router.

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you are tlkaing about hybrid mode where you use CatOS for the L2 aspects of the switch and IOS on the MSFC to provide an embedded router. but you dont have to run a Cat6k like that - and in some configs you cant.

when you use the DFC blade you run the Cat6k as an IOS only switch - it basically follows the same design as a 3750, just extra scale and some more flexibility.

FWIW a 7600 is Cat6k, AFAIR just different chassis, vertical mounted blades and restricted to IOS only mode.

That breaks most of the design

Actually we are delivering Internet access to customers at work - and often that is exactly what they do.

For 10 and 100 M, it doesnt matter that much whether you have a router or a switch, since a mid range router like a 3845 is fast enough.

A "real" router with the horsepower to terminate a GigE WAN link is an expensive toy - A stackable Cat has good enough forwarding to substitute (although there isnt as much software processing available, memory for routing tables etc) at a small fraction of the cost.

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As I am aware, the following are the possible WAN transport options: - Point to point private links - Gigabit Ethernet - ATM - Frame Relay - SDH/SONET

What do you suggest for each of the above-mentioned WAN transport options? Why? Also, should the replacement cost also be considered while taking this decision?

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i was trying to point out that just because this is possible on a 6500 doesnt mean it is a good idea.

Personally i would try to use "real" routers for low end stuff, and LAN optimised kit like a 6509 for Ethernet WAN connections.

this could be almost anything - usually PDH or SDH links - E1, T1, E3, STM-1. easiest used to be flexWAN with 7200 style port adaptors (basically a VIP if you know the 75xx routers). SPA can do the same for some interfaces.

for "low speed" say up to 2 Mbps / 34 Mbps it may be cheaper and more flexible to use a separate router.

just plug it into the Cat. Main issue is if you buy a rate limited port (ie.

20 Mbps of bandwidth presented on a 1 Gbps link). Normal LAN type ports on the Cat cannot do traffic rate limiting in conjunction with QoS, but the high end modules such OSM support this if you need it.

i understand cisco now recommend the SPA for this.

Same reasoning goes for the different speed ports 10G, 100M and 10M as long as they are presented as Ethernet.

flexWAN adaptor or SPA.

pretty much obsolete for new links, but lots around and you might want it if you already have

same as a serial (up to 34 Mbps, although not many high speed F/R links.


If you have lots of SDH, then you probably dont want a 6509.

However that much derided animal the SDH mux can give you very cheap wire speed GigE thru SDH...... and if you have 10G SDH (STM-64 or SONET 192), then a modern lambda transport will normally also support 10G Ethernet LAN PHY - so use the Ethernet presentation instead.

not sure what you mean here - there is an "oppotunity cost" - ie putting low speed ports into a high speed chassis like a 6509 eats up slots but doesnt give much thruput.

But - the cost of a slot in a 6509 is much less than in a more conventional router - at work the "real" high end routers used to be7513s, and GSR12008. Now new units are 10008s, GSR1241x, CRS-1.... (this is where the phrase "carrier class" is used to justify 5 times the price for 1/3 or less ports, although to be fair the boxes support much richer QoS, aggregated interfaces and so on).

For some of the big iron routers a single card with a couple of ports costs more than a 6509 full of LAN blades, so if the relatively reduced feature set of the 6509 is "good enough", the cost per Gbps of the 6509 is much better

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