Cisco Routers and Serial Interfaces/Connections

Inter-networker

I see network diagrams all the time that show routers connected using serial interfaces and I have some questions.

When you see a network diagram like the following:




Does the diagram mean that the routers are connected using a RS232 cable or a V.35 cable or something?

Wouldn't that be terribly slow?

Does a "serial interface" mean it has physical RS232 or V.35 cabling?

Is it common for routers to be connected using serial interfaces?

I see it in nearly every diagram that has more than one router. Routers in the diagrams are never connected using ethernet or fiber optics. I don't have access to a lot of physical cisco equipment, but I have seen plenty of networking equipment, even racks, and I have never seen routers connected with RS232 or V.35 cables. I thought that type of physical cabling was something that you use as console management on older equipment, not to connect routers.

I hope I've made sense. Thanks for the help.

networker050184

Yes it would be slow in comparison. It is not very popular anymore to connect using serial.

james43026

Yeah, most diagrams and practice exercises use those serial links as a way to simulate a legacy WAN connection, like frame relay or a T1 line. Most companies aren't going to connect their campuses together with a slow link like this, unless they are being used in a network for voice calls, but not really for data. You do also have things like HSSI (high speed serial interface), which could be used for a T3 line, which would be decently fast for dedicated internet access / private line depending on what you buy. There are still networks that might be using frame relay, which would present a typical serial interface (please correct me if I'm wrong), there are still plenty of companies that are running T1 lines for voice as well.

NetworkNewb

My 1841 routers I use have some WIC DSU-T1 cards that show up as serial interfaces in the CLI. And just use them like any other of the FastEthernet interfaces on there... Use regular CAT cables.

james43026

Yeah, physically the difference between a ethernet port using RJ-45, and a T1 serial port using RJ-48, would be wiring and shielding, they both us 8p8c modular connectors, they both use cat cable, but the one big difference between the two, is that the RJ-48 standard always calls for STP, and RJ-45 is fine with UTP or STP. As long as you are using a straighthrough cable, then an RJ-45 would work just the same, unless you are spanning anything more than a short distance with UTP, you could have some signal degradation as well. The important wiring part for RJ-48 is that pins 1&2 are paired and pins 4&5 are paired, the crossover if necessary will be done by the carrier at the jack that they provide for you, but a crossover may be necessary if connecting directly to a csu-dsu.

theodoxa

Nobody connects sites with RS232 or V.35 cables. Those were only meant to connect the router to an external CSU/DSU back in the days before the CSU/DSU was built right into the WIC/HWIC card. From the CSU/DSU, most folks just run a regular old Cat. 5e/6 cable to a Smart Jack on the wall (despite the standard calling for a special shielded cable). From the Smart Jack, various twisted pairs of cable go back to the Telco. The Telco creates a dedicated circuit or virtual circuit through their network to the other site. There is not one long cable, though functionally it acts like one. Using Serial cables to directly connect routers is a lab trick.

nice343

The last time I saw a serial connection in any data center was about 5 years ago but to answer your question Yes, those links are low speed wan links.
v.35 cables (DTE side) connects the router to a CSU/DSU (DCE side)

Internet connections have become so cheap to the point where dedicated WAN links are going the way of the dodo bird because of VPN technologies.

james43026

nice343 wrote: »

The last time I saw a serial connection in any data center was about 5 years ago but to answer your question Yes, those links are low speed wan links.
v.35 cables (DTE side) connects the router to a CSU/DSU (DCE side)

Internet connections have become so cheap to the point where dedicated WAN links are going the way of the dodo bird because of VPN technologies.

I think this depends greatly on the use case. Most dedicated WAN links are still in use for specific reasons, like connecting sites together for VOIP, or connecting your datacenter to multiple sites, to ensure you have the bandwidth when you need it. Although most ISP's will offer both uptime and a CIR with their SLA's for business class internet service, and a VPN connection is great for connecting sites that you don't have a dire need to have a guaranteed bandwidth. Places like a University, or a Hospital system, will definitely use dedicated WAN links for connecting their major sites to one another, usually dark fiber and or metro ethernet, on top of a internet connection.