crossover cable or serial cable on back to back routers in a live network?

So, I'm having a little trouble with my WAN knowledge and had a couple ?'s. What I'm asking here is on a live network, do you usually see routers connected together via crossover or serial/T1 cable? and when would you use one over the other? I know it probably depends on the network but from what I understand you generally connect two routers together via serial cable for a connection to an isp(this is point to point I think). I think you also use a serial link for long distance connection to another site/remote network because you wouldn't use an ethernet crossover cable long distance. If the routers are in the same physical location obviously you would use a crossover cable. Sometimes I get confused with this concept because I like to look at other people's lab/gns3 topologies and I see some topologies where there are like 10+ routers and all are connected via serial cables and I see the same topology, only this time it's crossover cables, and that kinda throws me off. Plus, I hear that it's quite different in a lab environment than on a live network.

One more somewhat related question...when connecting to an isp, you don't have acces to the isp router right? You have a router with a default route that points to the isp router and that's how you get internet. If someone can correct me if I'm wrong that would be great, thanks.

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Comments

theodoxa

I saw alot of T1 at sites I worked at. I have never seen Serial (DB60 or SS) cable used outside of a Lab environment. It used to be used to connect a router to an external CSU/DSU, but now you can buy a WIC/HWIC (WIC-1DSU-T1-V2, etc...) with integrated CSU/DSU (think of it as a T1 modem), so there is no need for an external CSU/DSU anymore. Some companies will also use Ethernet, but typically over Fiber. Cisco has some "LRE" switches that can do Ethernet over Phone lines at distances of nearly a mile (Long Reach Ethernet - Wikipedia, the free encyclopedia). Many small businesses will use Cable or DSL and just connect the Cable/DSL modem to their router [or Firewall - e.g. Cisco ASA 5500 Series] with a regular old Ethernet Patch cable.

TehToG

Serial is quite an old technology but it's still out there. I'll explain how real world compares to a lab setup.

In a lab scenario you see a serial connection on one router going to the other (with one end being DTE and the other being DCE). This is because one end provides the clocking signal (the DCE).

In the real world the serial connection on the router (DTE) is connected to an ISP router/modem. At the other end it's identical (DTE again). As far as you are concerned there's a magic cable between the two ISP modems. The ISP handle the connection between the modems. In this scenario the clocking signal is provided by the ISP modem.

In the real world you'll see ethernet used mostly but the concept is the same. The ethernet of the router will connect to the Modem provided by the ISP. Sometimes you'll see ISR routers which use ASDL connections or ISDN connections.

The other issue you had was understanding crossover and it's very simple. There are two standards for wiring an ethernet cable. TIA/EIA 568A and TIA/EIA 568B. If you connect like devices together (Router-Router or Switch-Switch) you'll find that both devices have their ethernet ports wired the same way. This means that the input of one device goes to the input of the other so signals are never received. The crossover cable is one where one end is 568A and the other is 568B. This means that the input from device one goes to the output of device 2 and vice versa. Connecting differing devices with a normal cable these inputs and outputs are already connected.

davenull

since we are on this topic, if my routers have WIC-1DSU-T1 cards, am I correct in thinking that a plain crossover is not going to work, and I'd need a special T1 crossover cable?

cisco wic 1dsu t1
wic 1dsu t1 crossover

I've been using GNS3 to do my routing labs, but I'll need to get one these eventually.

Cisco Inferno

davenull wrote: »

since we are on this topic, if my routers have WIC-1DSU-T1 cards, am I correct in thinking that a plain crossover is not going to work, and I'd need a special T1 crossover cable?

cisco wic 1dsu t1
wic 1dsu t1 crossover

I've been using GNS3 to do my routing labs, but I'll need to get one these eventually.

correct. Plain crossovers would not work. You would need a T1 Crossover.
They are both technically crossovers. But you would need the T1 Pinout for your T1 WIC module which is slightly different.

This is how I connect my routers instead of serial. So much neater. Also, cheaper!

WIC1DSU-T1 V2 can be found on Ebay for $5.49 Shipped. LINK
T1 Crossover can be found on Ebay for $3.79 Shipped. LINK

You will need to put in some special commands as if you were to set clock rate and what not on serial.
One will need to set clock and one will need to receive. clock source internal/line will be the equivalent.

T1s don't use the "clock rate" command, but instead use two commands "clock source {line | internal}" to indicate whether one end is a DCE or DTE and "service-module t1 timeslots 1-24" to indicate the desire speed for the link (each timeslot is 64 Kbps, so using all 24 = a Full T1 or 1.544 Mbps). One WIC-1T to WIC-1T link [or even Packet Tracer] is more than sufficient to play around with the clock rate command. All you really need to know is that it has to be set on the DCE side.

Here is an example of how to set up.

R1
=====
interface Serial0/0
description BACK-TO-BACK T1 CONNECTION - CSU/DSU LINK 1544KBPS
ip address 192.168.1.1 255.255.255.0
service-module t1 timeslots 1-24
service-module t1 linecode b8zs
service-module t1 framing esf
service-module t1 clock source internal

R2
=====
interface Serial0/0
description BACK-TO-BACK T1 CONNECTION - CSU/DSU LINK 1544KBPS
ip address 192.168.1.2 255.255.255.0
service-module t1 timeslots 1-24
service-module t1 linecode b8zs
service-module t1 framing esf
service-module t1 clock source line

davenull

Cisco Inferno, thank you very much for the exhaustive reply! I'm gonna save your configs for later.

theodoxa

davenull wrote: »

since we are on this topic, if my routers have WIC-1DSU-T1 cards, am I correct in thinking that a plain crossover is not going to work, and I'd need a special T1 crossover cable?

cisco wic 1dsu t1
wic 1dsu t1 crossover

I've been using GNS3 to do my routing labs, but I'll need to get one these eventually.

Flip the Orange (1, 2) and Blue (4, 5) Pairs for T1 instead of Orange (1, 2) and Green (3, 6) for Ethernet.