Many CCNA and CCNP candidates hear about reverse telnet, but arent quite sure what it is. Learn what it is, how it works, and how to configure it in this free Cisco tutorial from Chris Bryant, CCIE #12933.

Cisco CCNA / CCNP Home Lab Setup: How To Configure Reverse Telnet

Cisco CCNA / CCNP Home Lab Setup:   How To Configure Reverse TelnetOccasionally, during your CCNA and CCNP studies, you'll run into a term that just doesn't quite make sense to you. (Okay, more than occasionally!) One such term is "reverse telnet". As a Cisco certification candidate, you know that telnet is simply a protocol that allows you to remotely connect to a networking device such as a router or switch. But what is "reverse telnet", and why is it so important to a Cisco CCNA / CCNP home lab setup?Where a telnet session is started by a remote user who wants to remotely control a router or switch, a reverse telnet session is started when the host device itself imitates the telnet session.In a CCNA / CCNP home lab, reverse telnet is configured and used on the access server. The access server isn't a white box server like most of us are used to; an access server is a Cisco router that allows you to connect to multiple routers and switches with one session without having to move a rollover cable from device to device.Your access server will use an octal cable to connect to the other routers and switches in your home lab. The octal cable has one large serial connector that will connect to the access server, and eight rj-45 connectors that will connect to your other home lab devices. Your access server then needs an IP Host table in order to perform reverse telnet.An IP Host table is easy to put together (and you better know how to write one to pass the CCNA!). The IP Host table is used for local name resolution, taking the place of a DNS server. A typical access server IP Host table looks like this:ip host FRS 2007 host R3 2003 host R1 2001 host R2 2002 host R4 2004 host R5 2005 host SW1 2006 Loopback0 ip address no ip directed-broadcastThis configuration will allow you to use your access server to connect to five routers, a frame relay switch, and a switch without ever moving a cable. When you type "R1" at the console line, for example, you'll be connected to R1 via reverse telnet. If you have a smaller lab, an access server is still a real timesaver and an excellent investment. And by getting a static IP address to put on your access server, you can even connect to your home lab from remote locations!

Cisco CCNP Certification: The BGP Weight Attribute

Cisco CCNP Certification:  The BGP Weight Attribute

When you're studying for the CCNP certification, especially the BSCI exam, you must gain a solid understanding of BGP. BGP isn't just one of the biggest topics on the BSCI exam, it's one of the largest. BGP has a great many details that must be mastered for BSCI success, and those of you with one eye on the CCIE must learn the fundamentals of BGP now in order to build on those fundamentals at a later time.Path attributes are a unique feature of BGP. With interior gateway protocols such as OSPF and EIGRP, administrative distance is used as a tiebreaker when two routes to the same destination had different next-hop IP addresses but the same prefix length. BGP uses path attributes to make this choice.The first attribute considered by BGP is weight. Weight is a Cisco-proprietary BGP attribute, so if you're working in a multivendor environment you should work with another attribute to influence path selection.The weight attribute is significant only to the router on which it is changed. If you set a higher weight for a particular route in order to give it preference (a higher weight is preferred over a lower one), that weight is not advertised to other routers.BGP uses categories such as "transitive", "non-transitive", "mandatory", and "optional" to classify attributes. Since weight is a locally significant Cisco-proprietary attribute, it does not all into any of these categories.The weight can be changed on a single route via a route-map, or it can be set for a different weight for all routes received from a given neighbor. To change the weight for all incoming routes, use the "weight" option with the neighbor command after forming the BGP peer relationships.R2(config)#router bgp 100R2(config-router)#neighbor remote-as 10R2(config-router)#neighbor weight 200Learning all of the BGP attributes, as well as when to use them, can seem an overwhelming task when you first start studying for your BSCI and CCNP exams. Break this task down into small parts, learn one attribute at a time, and soon you'll have the BGP attributes mastered.

Cisco CCNP / BSCI Certification: BGP Route Reflector Tutorial

Cisco CCNP / BSCI Certification:  BGP Route Reflector Tutorial

When you're studying for your BSCI exam and CCNP certification, you quickly realize that BGP is a whole new world from anything you've previously studies. One topic that sometimes confuses CCNP candidates is when a BGP route reflector needs to be configured.In the following example, the routers R1, R2, and R3 are all in BGP AS 100. This is not a full mesh, however. There are peer relationships between R1-R2 and R1-R3, but not between R2 and R3. R3 is advertising network via BGP, and the route is seen on R1. R1's iBGP neighbor, R2 does not see the route.A basic rule of BGP is that a BGP speaker cannot advertise a route to an iBGP neighbor if that route was learned from another iBGP neighbor. Configuring R1 as a route reflector will allow us to circumvent this rule. The entire route reflector process is transparent to the clients, and no configuration is necessary on those clients. We'll configure R1 as a route reflector for both R2 and R3.R1(config)#router bgp 100R1(config-router)#neighbor route-reflector-client3d18h: %BGP-5-ADJCHANGE: neighbor Down RR client config changeR1(config-router)#neighbor route-reflector-client3d18h: %BGP-5-ADJCHANGE: neighbor Down RR client config changeThe BGP adjacencies do come down when this configuration is added, so this isn't something you want to do during a peak traffic time.Once the adjacencies come back up, R2 will have the route to are other possible solutions to this iBGP limitation, such as configuring BGP confederations. Those solutions are generally used on larger BGP deployments and with other concerns in mind, though, and configuring route reflectors serves this purpose just as well.

Cisco CCNA Certification: Cisco Switching Modes Tutorial

Cisco CCNA Certification:  Cisco Switching Modes Tutorial

To pass the CCNA exam and earn that coveted certification, you've got to know Cisco switches inside and out. Among the many important details you've got to know are the three methods that Cisco switches use to forward frames, and the differences between the three.The first switching method is Store-and-Forward. The name is the recipe, because that's just what the switch does - it stores the entire frame before beginning to forward it. This method allows for the greatest amount of error checking, since the Frame Check Sequence (FCS) can be run before the frame is forwarded. As always, there is a tradeoff, since this error checking process makes this the slowest of the three frame forwarding methods.The quickest method is Cut-Through, where only the destination MAC address of the frame is examined before the forwarding process begins. This means that the part of the frame is actually being forwarded as it is still being received! The tradeoff here is that the FCS does not run, so there is absolutely no error checking with Cut-Through switching.The middle ground between these two extremes is Fragment-Free, so named since fragmented frames will not be forwarded. The switch examines only the first 64 bytes of the frame for errors, since that is the part of the frame that will be damaged in case of a collision. There is error checking, but it is not as thorough as Store-and-Forward.Keeping these three switching schemes straight is vital to your CCNA exam efforts, and it will help you in working with Cisco switches in the real world as well. Keep studying!

Cisco CCNP Certification / BCMSN Exam Tutorial: The HSRP MAC Address

To pass the BCMSN exam and earn your CCNP, you've got to know HSRP inside and out! Part of that is knowing how the MAC address of the virtual router is derived, and another part is knowing how to change this address. We'll look at both features in this tutorial.We've got two routers on a segment running HSRP, so first we need to find out what the MAC address of the HSRP virtual router is. The show command for HSRP is show standby, and it's the first command you should run while configuring and troubleshooting HSRP. Let's run it on both routers and compare results.R2#show standbyEthernet0 - Group 5 Local state is Standby, priority 100 Hellotime 3 sec, holdtime 10 sec Next hello sent in 0.776 Virtual IP address is configured Active router is, priority 100 expires in 9.568 Standby router is local 1 state changes, last state change 00:00:22R3#show standbyEthernet0 - Group 5 Local state is Active, priority 100 Hellotime 3 sec, holdtime 10 sec Next hello sent in 2.592 Virtual IP address is configured Active router is local Standby router is expires in 8.020 Virtual mac address is 0000.0c07.ac05 2 state changes, last state change 00:02:08R3 is in Active state, while R2 is in Standby. The hosts are using the address as their gateway, but R3 is actually handling the workload. R2 will take over if R3 becomes unavailable.An IP address was statically assigned to the virtual router, but not a MAC address. However, there is a MAC address under the show standby output on R3, the active router. How did the HSRP process arrive at a MAC of 00-00-0c-07-ac-05?Well, most of the work is already done before the configuration is even begun. The MAC address 00-00-0c-07-ac-xx is reserved for HSRP, and xx is the group number in hexadecimal. That's a good skill to have for the exam, so make sure you're comfortable with hex conversions. The group number is 5, which is expressed as 05 with a two-bit hex character. If the group number had been 17, we'd see 11 at the end of the MAC address - one unit of 16, one unit of 1.On rare occasions, you may have to change the MAC address assigned to the virtual router. This is done with the standby mac-address command. Just make sure you're not duplicating a MAC address that's already on your network!R2(config-if)#standby 5 mac-address 0000.1111.22221d12h: %STANDBY-6-STATECHANGE: Ethernet0 Group 5 state Active -> LearnR2#show standbyEthernet0 - Group 5 Local state is Active, priority 150, may preempt Hellotime 4 sec, holdtime 12 sec Next hello sent in 3.476 Virtual IP address is configured Active router is local Standby router is expires in 10.204 Virtual mac address is 0000.1111.2222 configured 4 state changes, last state change 00:00:001d12h: %STANDBY-6-STATECHANGE: Ethernet0 Group 5 state Listen -> ActiveThe MAC address will take a few seconds to change, and the HSRP routers will go into Learn state for that time period. A real-world HSRP troubleshooting note: If you see constant state changes with your HSRP configuration, do what you should always do when troubleshooting - check the physical layer first. Best of luck on your BCMSN exam!

Cisco CCNA / CCNP Certification Exam Tutorial: Configuring PPP Callback

You may run into situations where a router in a remote location needs to dial in to a central router, but the toll charges are much higher if the remote router makes the call. This scenario is perfect for PPP Callback, where the callback client places a call to a callback server, authentication takes place, and the server then hangs up on the client! This ensures that the client isn't charged for the call. The server then calls the client back.In the following example, R2 has been configured as the client and R1 is the callback server. Let's look at both configurations and the unique commands PPP Callback requires.Client:username R1 password CCIEinterface BRI0 ip address encapsulation ppp dialer map ip name R1 broadcast 5557777 dialer-group 1 isdn switch-type basic-ni ppp callback request ppp authentication chapMost of that configuration will look familiar to you, but the ppp callback request command might not. This command enables the BRI interface to request the callback.Simple enough, right? The PPP Callback Server config requires more configuration and an additional map-class as well.Server:username R2 password CCIEinterface BRI0 ip address encapsulation ppp dialer callback-secure dialer map ip name R2 class CALL_R2_BACK broadcast 5558888 dialer-group 1 isdn switch-type basic-ni ppp callback accept ppp authentication chapmap-class dialer CALL_R2_BACK dialer callback-server usernameExamining the PPP Callback Server command from the top down...dialer callback-secure enables security on the callback. If the remote router cannot be authenticated for callback, the incoming call will be disconnected.The dialer map statement now calls the class CALL_R2_BACK, shown at the bottom of the config excerpt. ppp callback accept enables PPP callback on this router.dialer callback-server username tells the callback server that the device referenced in the dialer map statement is a callback client.The only way to find out if the config works is to test it, so let's send a ping from R2 to R1 and see if the callback takes place.R2#ping escape sequence to abort.Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:02:45:42: BR0 DDR: Dialing cause ip (s=, d= BR0 DDR: Attempting to dial 555777702:45:42: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up02:45:42: BR0:1 DDR: Callback negotiated - Disconnecting now02:45:42: BR0:1 DDR: disconnecting call02:45:42: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 5557777 R102:45:42: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down02:45:42: DDR: Callback client for R1 5557777 created02:45:42: BR0:1 DDR: disconnecting call.....Success rate is 0 percent (0/5)R2#02:45:57: %LINK-3-UPDOWN: Interface BRI0:1, changed state to upR2#02:45:57: BR0:1 DDR: Callback received from R1 555777702:45:57: DDR: Freeing callback to R1 555777702:45:57: BR0:1 DDR: dialer protocol up02:45:58: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to upThe callback was successfully negotiated, and the call then disconnected. R1 then called R2 back, and show dialer on R1 confirms the purpose of the call.R1#show dialerBRI0 - dialer type = ISDNDial String Successes Failures Last DNIS Last status5558888 2 4 00:00:20 successful0 incoming call(s) have been screened.0 incoming call(s) rejected for callback.BRI0:1 - dialer type = ISDNIdle timer (120 secs), Fast idle timer (20 secs)Wait for carrier (30 secs), Re-enable (15 secs)Dialer state is data link layer upDial reason: Callback return callTime until disconnect 99 secsConnected to 5558888 (R2)Pretty cool! PPP Callback isnt just important for passing your CCNA and CCNP exams in circumstances such as shown in this example, it can save your organization quite a bit of money!


Many CCNA and CCNP candidates hear about reverse telnet, but arent quite sure what it is. Learn what it is, how it works, and how to configure it in this free Cisco tutorial from Chris Bryant, CCIE #12933.