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BECNs and FECNs aren’t just important to know for your Cisco CCNA and CCNP certification exams – they’re an important part of detecting congestion on a Frame Relay network and allowing the network to dynamically adjust its transmission rate when congestion is encountered.

The Forward Explicit Congestion Notification (FECN, pronounced “feckon”) bit is set to zero by default, and will be set to 1 if congestion was experienced by the frame in the direction in which the frame was traveling. A DCE (frame relay switch) will set this bit, and a DTE (router) will receive it, and see that congestion was encountered along the frame’s path.

If network congestion exists in the opposite direction in which the frame was traveling, the Backward Explicit Congestion Notification (BECN, pronounced “beckon”) will be set to 1 by a DCE.

If this is your first time working with BECNs and FECNs, you might wonder why the BECN even exists – after all, why send a “backwards” notification? The BECN is actually the most important part of this entire process, since it’s the BECN bit that indicates to the sender that it needs to slow down!

For example, frames sent from Kansas City to Green Bay encounter congestion in the FR cloud. A Frame Switch sets the FECN bit to 1. In order to alert KC that it’s sending data too fast, GB will send return frames with the BECN bit set. When KC sees the BECN bit is set to 1, the KC router knows that the congestion occurred when frames were sent from KC to GB.

Frame Relay BECN Adaptive Shaping allows a router to dynamically throttle back on its transmission rate if it receives frames from the remote host with the BECN bit set. In this case, KC sees that the traffic it’s sending to GB is encountering congestion, because the traffic coming back from GB has the BECN bit set. If BECN Adaptive Shaping is running on KC, that router will adjust to this congestion by slowing its transmission rate. When the BECNs stop coming in from GB, KC will begin to send at a faster rate.

BECN Adaptive Shaping is configured as follows:

KC(config)#int s0

KC(config-if)#frame-relay adaptive-shaping becn

To see how many frames are coming in and going out with the BECN and FECN bits set, run show frame pvc.
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Whether you’re just starting to think about passing the CCNA or CCNP exams, or you’ve been on the certification track for a while, you’ve got to have a plan for success. If you wanted to drive your car from Florida to California, you’d create a plan to get there. You’d get a map and decide how far you wanted to drive per day, and maybe even make some hotel reservations in advance. You certainly wouldn’t get in your car, just drive it randomly down the nearest highway, and hope you ended up in California, would you?

Certainly not. Earning your CCNA certification is the same way. It’s not enough to just study a few minutes “when you feel like it”, or tell yourself that you’ll start studying for the exams “when I get such-and-such done”. The perfect time to start on the road to Cisco certification is not tomorrow, and it’s not next week. It’s today.

You’re much better off with one hour of solid study than three hours of interrupted, unfocused study. Here are a few ways to go about getting the kind of quality study time that will get you to the CCNA or CCNP (or any Cisco certification, for that matter!).

Schedule your study time, and regard this study time as you would an appointment with a client. If you were to meet a customer at 10:00 to discuss a network install, would you just decide not to show up and watch television instead? Not if you wanted the job. The same goes for your study time. That’s an appointment with the most important customer of all – YOU. Read the rest of this entry »

A prime topic of your CCNA and CCNP CIT exams will be connecting Cisco routers directly via their Serial interfaces, and while the configuration is straightforward, there are some vital details and show commands you must know in order to pass the exams and configure this successfully in production and home lab networks. Let’s take a look at a sample configuration.

Connecting Cisco routers directly via their Serial interfaces works really well once you get it running – and getting such a connection up and running is easy enough. You can use show controller serial x to find out which endpoint is acting as the DCE, and it’s the DCE that must be configured with the clockrate command.

R3#show controller serial 1

HD unit 1, idb = 0x11B4DC, driver structure at 0×121868

buffer size 1524 HD unit 1, V.35 DCE cable

R3(config)#int serial1

R3(config-if)#ip address 172.12.13.3 255.255.255.0

R3(config-if)#clockrate 56000

R3(config-if)#no shut

Failure to configure the clockrate has some interesting effects regarding the physical and logical state of the interfaces. Let’s remove the clockrate from R3 and see what happens.

R3(config)#int s1

R3(config-if)#no clockrate 56000

R3(config-if)#

18:02:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down

The line protocol doesn’t drop immediately, but it does drop. Let’s run show interface serial1 to compare the physical and logical interface states. Read the rest of this entry »

As a CCNA or CCNP, one thing you’ve got to get used to is that change is constant. Cisco regularly issues new IOS versions, not to mention the many different kinds of hardware they produce! While it’s always nice to have “the latest and the greatest” when it comes to routers, switches, firewalls, etc., we have to be prepared for the fact that not all our clients are going to have that latest and greatest!

For instance, there are still quite a few Catalyst 5000 switches out there humming away, and if you’re used to working on IOS-driven switches like the 2950, the same command can have dramatically different results.

Let’s say you’re going to examine the spanning tree protocol (STP) setup of a new client. You’re used to working with newer 2950 switches, and you’ve always run show span on those switches to display spanning-tree information. Then, you run show span on a Catalyst 5000 – and something like this shows:

switch (enable) show span

Destination : Port 6/1

Admin Source : Port 6/2

Oper Source : Port 6/2

Direction : transmit/receive

Incoming Packets: disabled

Learning : enabled

Multicast : enabled

Filter : -

Status : active

Total local span sessions: 1

What’s going on here?

The command show span on a 5000 will not show spanning tree stats – instead, what you’re going to see are statistics relating to Switched Port ANalyzer (SPAN). Surprise!

Consider an example where you’re used to running show span on 5000 switches to see SPAN information. When you run that on a 2950, you know now what you’re going to get – spanning tree information! On a 2950, you’ll need to run show monitor session, followed by the SPAN session number.

SW1#show monitor session 1
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