Serial connection on LAN ?

hendychow

Hello All,

I have questions, please help...

For lab practice, I use a book CCNP Building Scalable Internetworks (BSCI 642-901) Lab Portfolio (Cisco Networking Academy) by David Kotfila.

Please see the topology in the attachment.

My questions are:
1. Are those routers and switches located on the same location / LAN ?

2. If yes, why is a serial connection used for LAN ? -- If No, why ?

3. Can we use a serial connection for LAN OR serial connection is always used for WAN ?

4. Is Ethernet interface always used for LAN ?

5. If no, why ?
Because I've seen sometimes where Ethernet interface is used for routing
and it's assigned an ip address and the there is an ospf ip routing configured for that ip
address. -- If Yes, why ?

Please suggest.. I really appreciate any help

Thank You,

Hendy

kryolla

that diagram can be a little confusing. Yes ethernet are only lan and serial are only wan. Why because of the encapsulation type for their respective links you cant use ethernet encapsulation on a serial interface and vice versa on a ethernet link. You can have routing in house between devices thus ethernet and routing between building, cities, etc thus wan links. To be honest with you this is CCNA type stuff

tech-airman

hendychow,

hendychow wrote: »

Hello All,

I have questions, please help...

For lab practice, I use a book CCNP Building Scalable Internetworks (BSCI 642-901) Lab Portfolio (Cisco Networking Academy) by David Kotfila.

Please see the topology in the attachment.

My questions are:
1. Are those routers and switches located on the same location / LAN ?

Physically, for the purposes of the lab, yes all the routers and switch are located in the same site. Keep in mind that whenever a serial link is used for labs, it's supposed to REPRESENT a WAN link by using a back to back serial cable. A back to back serial cable is a LOT cheaper to work with instead of ordering two T-1 lines to be installed into your site, the following costs associated with the two T-1 lines, the costs associated with the other end of either the physical (leased line) or logical (frame relay) link, such as another router. Here's an attempt of graphically what I'm talking about...

back to back serial cable
[R1]----z-----[R2]

point to point leased line T-1 service
[R1]---z---{ISP network}----z---[R2]

Two leased line links to a remote network site
[R1]---z---{ISP network}---z---[Remote Router]---z----{ISP network}---z---[R2]

Two leased line links to two remote network sites that are interlinked
[R1]--z--{ISP network}--z--[Remote Site 1]---z--{Internal network}--z--[Remote Site 2]--z--{ISP network}--z--[R2]

As you can see, things can get a LOT more complicated. So, just keep in mind that for the purposes of the labs, the serial links REPRESENT WAN links.

hendychow wrote: »

2. If yes, why is a serial connection used for LAN ? -- If No, why ?

One possible purpose of a crossconnection using a serial cable is if the WAN links are T-1. The purpose of the crossconnection is either for backup purposes or load balancing. In that case, it may be undesirable to crossconnect using even a 10Mbps Ethernet link because that would result in a backlog of the WAN interface buffers of the router working harder because it has to support two routers worth of traffic. So by having a serial crossconnection, the backup/load balancing link is operating at the same bandwidth as the WAN interface.

A reason why not to use a serial crossconnect is if the WAN link is at a bandwidth that's different from a serial link, for example DSL, Cable, dialup, etc.

hendychow wrote: »

3. Can we use a serial connection for LAN OR serial connection is always used for WAN ?

Yes, you can use a serial connection for LAN but you must have a good reason to do so. As mentioned above, you could possibly use a serial crossconnect if the WAN link(s) are the same bandwidth as the serial crossconnect. However, if you have a gigabit network segment then decide to use a serial connection from that network segment to the WAN link, you're probably going to end up with a lot of upset gigabit network users about network performance.

You have to keep in mind that within the WAN module of the Enterprise Edge functional area of the Cisco Enterprise Architecture, usually refers to direct connections to remote sites using leased line, frame relay, or some other kind of long haul connection. Unfortunately, "WAN" has become meaning any kind of long distance network connection, such as DSL, Cable, and similar long distance network access.

hendychow wrote: »

4. Is Ethernet interface always used for LAN ?

Not always. What if you use the Ethernet interface to connect to a DSL modem or Cable modem? That's clearly an extension of the "WAN" side of the network, even though physically it's located at the LAN site.

hendychow wrote: »

5. If no, why ?

See above.

hendychow wrote: »

Because I've seen sometimes where Ethernet interface is used for routing
and it's assigned an ip address and the there is an ospf ip routing configured for that ip
address. -- If Yes, why ?

It's possible to have OSPF routing for the Building Distribution, Core, or even in the Building Access. So if that segment of the network uses Ethernet, then you're going to have to include that into the ospf routing configuration.

hendychow wrote: »

Please suggest.. I really appreciate any help

Thank You,

Hendy

Did my reply help?

hendychow

Hello All,

Thank your for the answers. I really appreciate that.

Mr. tech-airman,
so are you saying that the reason serial connection was used for LAN in this lab is because just for "lab" purpose by using "a back to back serial cable" ?

So, the Cisco Lab book does not really represent a "real world" scenario.

About using Ethernet for WAN,
In the company I work, most of the WAN connections are using a leased line using "Ten Gigabit Ethernet" (XENPAK card on 6500),
which connecting up to 20 miles for the network between buildings in different locations.
I don't know whether this connection is considered as WAN or LAN, since It's running Layer 3 Routing Protocol (OSPF).

Mr. kryolla,
Yes, It's CCNA level, but It seems that in real world scenario, companies does not always follow book or Cisco recommendation.
You might only think that serial = WAN, ethernet = LAN, well that doesn't not apply to the company where I work.
If It would be that easy, I might have just found it on my CCNA book.

Thank You,

Hendy

tim100

hendychow wrote: »

Hello All,

About using Ethernet for WAN,
In the company I work, most of the WAN connections are using a leased line using "Ten Gigabit Ethernet" (XENPAK card on 6500),
which connecting up to 20 miles for the network between buildings in different locations.
I don't know whether this connection is considered as WAN or LAN, since It's running Layer 3 Routing Protocol (OSPF).

Hendy

Serial and Ethernet interfaces are not exactly bound to the terms WAN and LAN. Remember WAN is Wide Area and LAN is Local area. Serial interfaces are used in WAN environments whereas Ethernet interfaces are used in LAN environments. 10 GigE is Ethernet but it is also used as a WAN transport because it can cover a wide area between endpoints. It is also referred to as Long Range Ethernet or Extended Range Ethernet. As far as running a Layer 3 Routing Protocol it has no significance on whether it is a WAN or LAN connection since it must run over Layers 1 and 2 in either environment.

APA

tim100 wrote: »

Serial and Ethernet interfaces are not exactly bound to the terms WAN and LAN. Remember WAN is Wide Area and LAN is Local area. Serial interfaces are used in WAN environments whereas Ethernet interfaces are used in LAN environments. 10 GigE is Ethernet but it is also used as a WAN transport because it can cover a wide area between endpoints. It is also referred to as Long Range Ethernet or Extended Range Ethernet. As far as running a Layer 3 Routing Protocol it has no significance on whether it is a WAN or LAN connection since it must run over Layers 1 and 2 in either environment.

^ what he said...

There is nothing stopping a company from buying two routers hooking them up via serial and using it to route traffic within their LAN... No idea why you would want to do this but you can......

Metro Ethernet is a WAN solution delivered via fibre\copper.... see where i'm going with this??

You're looking at things the wrong way..... If two devices are confined to the same area it can be determined to be a LAN essentially. Two endpoints geographically diverse could either be a MAN(if in the same metro area) or a WAN(if across cities,states,countries)

hendychow

I just wanna say thank you for the help and the enlightenment
I really appreciate that.


Hendy Chow

aryandart2001

Maybe I am blind or am too drunk.. but i do not see a serial connection for LAN anywhere in the topology attached. I see a Se0/0/0 on 10.1.100 /30 network. Are you misreading the topology.? or am I?

If you are preparing for CCNP BSCI exam, you will not encounter such topologies where you will use serial links for LAN. It is unheard of in huge Enterprise networks.

Sepiraph

aryandart2001 wrote: »

Maybe I am blind or am too drunk.. but i do not see a serial connection for LAN anywhere in the topology attached. I see a Se0/0/0 on 10.1.100 /30 network. Are you misreading the topology.? or am I?

If you are preparing for CCNP BSCI exam, you will not encounter such topologies where you will use serial links for LAN. It is unheard of in huge Enterprise networks.

No, you are neither drunk nor blind. In that diagram the serial link is between the routers (WAN).

Serial link is really slow and quite old technology, if all the circuits were fibers then there would be no reason to use any serial link. Unfortunately we are often still using T1 circuit so we are stuck with serial but Ethernet WAN interface will definitely become more widely adopted.

I think once upon a time, LAN connection was also serial (before Ethernet was widely adopted). If someone old enough to remember can confirm this.

w4nn4b1337

hendychow wrote: »

Hello All,

I have questions, please help...

For lab practice, I use a book CCNP Building Scalable Internetworks (BSCI 642-901) Lab Portfolio (Cisco Networking Academy) by David Kotfila.

Please see the topology in the attachment.

My questions are:
1. Are those routers and switches located on the same location / LAN ?

2. If yes, why is a serial connection used for LAN ? -- If No, why ?

3. Can we use a serial connection for LAN OR serial connection is always used for WAN ?

4. Is Ethernet interface always used for LAN ?

5. If no, why ?
Because I've seen sometimes where Ethernet interface is used for routing
and it's assigned an ip address and the there is an ospf ip routing configured for that ip
address. -- If Yes, why ?

Please suggest.. I really appreciate any help

Thank You,

Hendy

It looks like your questions have already been answered but I felt compelled to say that the topology represented shows the LAN being on the routers R1, R2 and R3 fast ethernet interfaces on the 10.1.200.0/24 network. The WAN is represented by the loopback interfaces on the routers. Therfore, the serial connection would be for creating a partial mesh for LANs on 10.1.1.1/24 on R1 and 10.1.2.1/24 on R2 to the 10.1.200.0/24 LAN.

I see that the serial connection is a redundant path between 3 LAN's. However, it still doesn't make sense as to why a slower serial connection is used when in all practicality it appears the routers would be close enough to each other to use 100baseT. Considering the switch is connected to both via 100baseT I could assume the routers would be close enough to use 100baseT as well. I guess it is possible routers R1 and R2 could be on seperate ends of the campus with a distance greater then 300m leaving the use of the serial connection but, that would be an assumption.

I work in a rural area with not very many choices with Internet access. We have recently subscribed to a Metro-e circuit that comes in via ATM over fiber to the demarc but is then converted to ethernet at the CPE. Esentially what comes in off the wall is a 100baseT ethernet connection. I understand the Metro-e circuit can be used as a point to point circuit essentially creating an ethernet WAN interface. (These circuits are new to me so I am just learning about them.) Like it was prevously stated associating the terms LAN and WAN with a layer 1 technology is not a good idea.

In these modern days where routers do switching and switches doing routing the line is getting blurred as to what is LAN and what is WAN. Especially since T circuits will be probably start to go away as more affordable ethernet options become available.

In my mind the key is to say the LAN ends at the routers interface that provide a gateway to another network via an ISP to another router. In contrast, I see that multiple VLAN's on an MLS is still the "LAN" despite the distances. If you can create a VTP trunk between sites/buildings on the campus then it is still the LAN. When you have to install a router and subscribe to a circuit via an ISP then we're talking WAN. I'm sure someone can find an arguement against it but, that helps me keep it straight in my head.