difference "real router" and NAT

ncage

Golden Member
Jan 14, 2001
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Ok guys just out of curiosity what is the difference between a real router like a cisco pix router and one of the cheap cable routers you buy? NAT translates ip addresses between two disjoint networks. What does a high end router do? High end routers don't use NAT at all correct?


thanks,
Ncage
 

RebateMonger

Elite Member
Dec 24, 2005
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"Real" routers can usually do NAT. It's just that they aren't limited to NAT, like most home-grade "routers".

The choice of NAT vs. Routing depends on what you are trying to accomplish. If you want full two-way traffic, allowing conversations to be initiated from either direction, then routing is appropriate. If you want to limit conversations so that they can only be initiated from one direction, then NAT is often appropriate. NAT is also good if, for some reason, you need to share a network address among many devices.

Wikipedia.org: Router
 

spidey07

No Lifer
Aug 4, 2000
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"real" routers route. Meaning they maintain a routing table that they activey exchange with other routers via some kind of routing protocol. They maintain route/path state and actively control these paths. Most of the time they do NOT perform NAT as nat can cause problems with various protocols. Also a PIX is a firewall and NOT a router. firewalls and routers are very different types of devices.

Another big difference is performance. "real" routers can forward packets in the 100s of millions of packets per second (PPS) regardless of how many layer4 flows are going on.

That's a real vague answer but I'd need a white board to go into it deeper. Also routers have a whole slew of features other than basic routing. All of these features revolve around manipulating packets and where they are sent and how/if they are modified.
 

n0cmonkey

Elite Member
Jun 10, 2001
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Originally posted by: spidey07
"real" routers route. Meaning they maintain a routing table that they activey exchange with other routers via some kind of routing protocol. They maintain route/path state and actively control these paths. Most of the time they do NOT perform NAT as nat can cause problems with various protocols. Also a PIX is a firewall and NOT a router. firewalls and routers are very different types of devices.

Another big difference is performance. "real" routers can forward packets in the 100s of millions of packets per second (PPS) regardless of how many layer4 flows are going on.

That's a real vague answer but I'd need a white board to go into it deeper. Also routers have a whole slew of features other than basic routing. All of these features revolve around manipulating packets and where they are sent and how/if they are modified.

"Real" routers need to be running something like RIP, OSPF, BGP, etc. to be in that "real router" category?

Even low end "real" routers can forward that many pps?

Or by real did you mean "relatively high end" in these instances?
 

spidey07

No Lifer
Aug 4, 2000
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Sorry noc, don't know much about the lower models. All I deal with is the big iron.

Although I will say it's mostly a design issue. Real routers place a priority on control plane and data plane separation. This means their top priority is getting packets in and getting them out as quickly as possible. I guess I'd place the top half of juniper and cisco's model lines "real" routers.

Also to add, real routers can take any kind of interface/media there is. They aren't limited to ethernet. That would probably be a nice, easy differentiation.
 

n0cmonkey

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Jun 10, 2001
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Originally posted by: spidey07
Sorry noc, don't know much about the lower models. All I deal with is the big iron.

Although I will say it's mostly a design issue. Real routers place a priority on control plane and data plane separation. This means their top priority is getting packets in and getting them out as quickly as possible. I guess I'd place the top half of juniper and cisco's model lines "real" routers.

Also to add, real routers can take any kind of interface/media there is. They aren't limited to ethernet. That would probably be a nice, easy differentiation.

Cool, had to ask. I haven't had a chance to get into the cisco stuff yet...
 

Genx87

Lifer
Apr 8, 2002
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Originally posted by: spidey07
Sorry noc, don't know much about the lower models. All I deal with is the big iron.

Although I will say it's mostly a design issue. Real routers place a priority on control plane and data plane separation. This means their top priority is getting packets in and getting them out as quickly as possible. I guess I'd place the top half of juniper and cisco's model lines "real" routers.

Also to add, real routers can take any kind of interface/media there is. They aren't limited to ethernet. That would probably be a nice, easy differentiation.

Oh yeah, serial connection baby!
 

spidey07

No Lifer
Aug 4, 2000
65,469
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POS (OC12, OC192, OC768)
HSSI
ATM
IMA
Token Ring
FDDI
10 gig ethernet
T1
T3

basically any kind of network there is.
 

Cooky

Golden Member
Apr 2, 2002
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VoIP / voice gateway is becoming a hot trend nowadays so don't forget about it...like spidey has pointed out, a router can accomodate many different interfaces.
I can't imagine if anyone here still deals w/ token ring or FDDI though...pretty old stuff.
I've never heard of IMA; what is it?
 

cmetz

Platinum Member
Nov 13, 2001
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There is an important distinction to be made between "routing" and "forwarding." Briefly put, "forwarding" is the data-plane activity of taking a L3 packet in on an interface and (usually) sending it out some L3 interface. However, "routing" is the control-plane activity of deciding how to get to a certain L3 destination. This distinction is so important because people often use the term "routing" or "router" to describe boxes that basically don't and aren't.

In my personal opinion, static routing alone does not count. That is to say, a device that runs no routing protocols or has any other smart metrics to make routing decisions is never actually making any routing decisions; they have all been made for the device by the device's administrator. If you consider static routing to be routing, then every desktop system you have is also a router, because it uses a static routing table to decide where to output packets and what the destination L2 address needs to be. Since I don't consider your desktop PC to be a router, static routes don't count.

Now, that said, you can see that most SOHO devices don't qualfy as routers, because, again, they don't make any routing decisions at all. They have been explicitly configured with static routing information and that's it. They typically have an unusual forwarding path that is geared towards port address translation (PAT) rather than pure IP forwarding, though some allow you to turn the "firewall" PAT off and get a normal IP forwarding path.

As a matter of pure routing protocol bigotry, RIP does not a "real" router make. Yes, technically a device with a forwarding path and RIP would be considered a router, because it can make routing decisions. But because it's using RIP to do it, I would never be confident in it making the *right* routing decisions. So I consider that case a toy router. You will see some SOHO devices and even entry-level business devices that support RIP, primarily because they think that makes them a real router. But they're wrong.

So in my personal opinion, a "real router" is a device that has an L3 forwarding path (e.g., IP forwarding) and also makes its routing decisions using one or more useful routing protocols (e.g., OSPF, IS-IS, BGP).

L3 switches are a special case to consider. Most L3 switches do not have full hardware L3 forwarding paths, only hardware cache-flow engines. They do, however, do implement a full L3 forwarding path in software, which populates the hardware cache. By my definition, L3 switches are in fact real routers (if they implement real routing protocols, too), but they have architectural limitations that make them only appropriate in certain situations. The same goes for Cisco's NSEs.

Now that's slightly different than when you see hard-core networking guys who talk in chest-thumping terms about how they use the biggest/fastest. When you're having that kind of argument, where "real router" means biggest and the best and evolves over time, that's not at all a technical term and can't be defined. (And anyone who says that the AGS+ isn't a real router, well, them's fightin' words)

Cooky, IMA is inverse-multiplexed ATM, usually where they use multiple T1s as a fat ATM channel. Because ATM is Evil Incarnate(tm) you need not worry about it. This is not a technology you want to use.

FDDI is dead because the chips haven't been available for a few years now. FDDI did rock hard back in the day, though. I know some folks who are trying very hard not to give up their FDDI but will be forced to convert as their spare gear stockpiles run out.
 

Cooky

Golden Member
Apr 2, 2002
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cmetz, good point on static routing and desktop PC.
I know ATM never got popular in LAN, but many providers and military use it so I'm not sure if you can just dismiss it, even though it's "evil incarnate".
 

phatrabt

Senior member
Jan 28, 2004
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Originally posted by: Cooky
VoIP / voice gateway is becoming a hot trend nowadays so don't forget about it...like spidey has pointed out, a router can accomodate many different interfaces.
I can't imagine if anyone here still deals w/ token ring or FDDI though...pretty old stuff.
I've never heard of IMA; what is it?

IMA is Inverse Multiplexing over ATM. It allows a bridge in bandwidth between a T1 and a T3. Example is that you have an office that needs 3Mb\s bandwidth but a T1 is too small (it's only 1.54Mb\s). If you're using ATM you can use it to multiplex 2 T1's together virtually into an IMA group so that there's a 3Mb\s pipe. On the Cisco routers I've worked on you have a limit of 8 T1's in the IMA group, giving you a bandwidth of approx 12Mb\s. It's cheaper than a T3 and also allows some redundancy in that if one of the ATM T1's goes down the other(s) will remain up (depending on the config).