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When you begin your CCNA studies, you get hit with a lot of different networking terms right away that you might not be familiar with. What makes it a little more confusing is that a lot of these terms sound a lot alike. Here, we're going to discuss the differences between broadcasts, multicasts, and unicasts at both the Data Link (Layer 2) and Network (Layer 3) layers of the OSI model.A broadcast is simply a unit of information that every other device on the segment will receive. A broadcast is indicated by having every bit of the address set to its highest possible value. Since a hexadecimal bit's highest value is "f", a hexadecimal broadcast is ff-ff-ff-ff-ff-ff (or FF-FF-FF-FF-FF-FF, as the upper case does not affect hex value). The CCNA exam will demand you be very familiar with hex conversions, so if you're not comfortable with these conversions, get comfortable with them before taking the exam!At layer 3, a broadcast is indicated by setting every bit in the 32-bit binary string to "1", making the dotted decimal value 255.255.255.255. Every host on a segment will receive such a broadcast. (Keep in mind that switches will forward a broadcast, but routers do not.) In contrast to a broadcast, a unicast is a packet or frame with only one destination. There is a middle ground between broadcasts and unicasts, and that is a multicast. Where a broadcast will be received by all, and a unicast is received by only one host, a multicast will be received by multiple hosts, all belonging to a "multicast group". As you climb the Cisco certification pyramid, you'll be introduced to creating multicast groups and controlling multicast traffic, but for your CCNA studies you need only keep certain multicast groups in mind.Class D addresses are reserved for multicasting this range is 220.127.116.11 - 18.104.22.168. The addresses 22.214.171.124 - 126.96.36.199 are reserved for use by network protocols on a local network segment, and like broadcasts, routers will not forward these multicast packets. (Packets with these addresses are sent with a Time To Live of 1.)As a CCNA candidate, you should know that OSPF routers use the address 188.8.131.52 to send hellos, EIGRP routers use 184.108.40.206 to send updates, and RIP version 2 uses 220.127.116.11 to send routing updates. RIP version 1 and IGRP both broadcast their updates.Multicasting gets a bit more complicated as you go from your CCNA to the CCNP and CCIE, but by simply understanding what multicasting is, you go a long way toward securing the CCNA.
In the first part of this free CCNP / BSCI tutorial, we looked at how leaving one simple word out of our route redistribution configuration - "subnets" - resulted in an incomplete routing table when redistributing routes from RIP to OSPF. (If you missed that part of the tutorial, visit my website's "Free Tutorials" section.) Today, we'll look at redistributing OSPF routes into RIP and identify another common redistribution error.We are using a three-router network. R5 is running RIP, R1 is serving as a hub between R5 and R3 and is running RIP and OSPF, and R3 is running OSPF. To begin this lab, we'll add three loopbacks to R3 and advertise them to R1 via OSPF.R3(config)#int loopback33R3(config-if)#ip address 18.104.22.168 255.255.255.255R3(config-if)#int loopback34R3(config-if)#ip address 22.214.171.124 255.255.255.255R3(config-if)#int loopback35R3(config-if)#ip address 126.96.36.199 255.255.255.255R3(config-if)#router ospf 1R3(config-router)#network 188.8.131.52 0.0.0.0 area 1R3(config-router)#network 184.108.40.206 0.0.0.0 area 1R3(config-router)#network 220.127.116.11 0.0.0.0 area 1R1 sees all three of these routes in its routing table.R1#show ip route ospf 18.104.22.168/32 is subnetted, 1 subnetsO IA 22.214.171.124 [110/65] via 126.96.36.199, 00:00:55, Serial0 188.8.131.52/32 is subnetted, 1 subnetsO IA 184.108.40.206 [110/65] via 220.127.116.11, 00:00:45, Serial0 18.104.22.168/32 is subnetted, 1 subnetsO IA 22.214.171.124 [110/65] via 126.96.36.199, 00:00:55, Serial0We'll now redistribute these routes into RIP on R1. Remember the "subnets" option we talked about in the first part of this tutorial? There is no such option when redistributing OSPF routes into RIP, as IOS Help shows us.R1(config)#router ripR1(config-router)#redistribute ospf 1 ? match Redistribution of OSPF routes metric Metric for redistributed routes route-map Route map reference vrf VPN Routing/Forwarding Instance
If a Layer Two switch doesn't have the capabilities to run IGMP Snooping, it will be able to run CGMP - Cisco Group Membership Protocol. CGMP allows the multicast router to work with the Layer Two switch to eliminate unnecessary multicast forwarding. CGMP will be enabled on both the multicast router and the switch, but the router's going to do all the work. The router will be sending Join and Leave messages to the switch as needed. PIM must be running on the router interface facing the switch before enabling CGMP, as you can see:R1(config)#int e0R1(config-if)#ip cgmp WARNING: CGMP requires PIM enabled on interfaceR1(config-if)#ip pim sparseR1(config-if)#ip cgmpWhen CGMP is first enabled on both the multicast router and switch, the router will send a CGMP Join message, informing the switch that a multicast router is now connected to it. This particular CGMP Join will contain a Group Destination Address (GDA) of 0000.0000.0000 and the MAC address of the sending interface. The GDA is used to identify the multicast group, so when this is set to all zeroes, the switch knows this is an introductory CGMP Join, letting the switch know that the multicast router is online.The switch makes an entry in its MAC table that this router can be found off the port that the CGMP Join came in on. The router will send a CGMP Join to the switch every minute to serve as a keepalive.A workstation connected to the switch on port 0/5 now wishes to join multicast group 188.8.131.52. The Join message is sent to the multicast router, but first it will pass through the switch. The switch will do what you'd expect it to do - read the source MAC address and make an entry for it in the MAC address table as being off port fast 0/5 if there's not an entry already there. (Don't forget that the MAC address table is also referred to as the CAM table or the bridging table.)The router will then receive the Join request, and send a CGMP Join back to the switch. This CGMP Join will contain both the multicast group's MAC address and the requesting host's MAC address. Now the switch knows about the multicast group 184.108.40.206 and that a member of that group is found off port fast 0/5. In the future, when the switch receives frames destined for that multicast group, the switch will not flood the frame as it would an unknown multicast. Instead, the switch will forward a copy of the frame to each port that it knows leads to a member of the multicast group.Two major benefits of CGMP are the explicit Join and Leave Group messages. In the next part of this BCMSN exam tutorial, well take a look at the Leave Group messages.
As a CCNA and/or CCNP candidate, you've got to be able to spot situations where Cisco router features can save your client money and time. For example, if a spoke router is calling a hub router and the toll charges at the spoke site are higher than that of the hub router, having the hub router hang up initially and then call the spoke router back can save the client money (and make you look good!) A popular method of doing this is using PPP callback, but as we all know, it's a good idea to know more than one way to do things in Cisco World! A lesser-known but still effective method of callback is Caller ID Screening & Callback. Before we look at the callback feature, though, we need to know what Caller ID Screening is in the first place!This feature is often referred to simply as "Caller ID", which can be a little misleading if you've never seen this service in operation before. To most of us, Caller ID is a phone service that displays the source phone number of an incoming call. Caller ID Screening has a different meaning, though. Caller ID Screening on a Cisco router is really another kind of password - it defines the phone numbers that are allowed to call the router. The list of acceptable source phone numbers is created with the isdn caller command. Luckily for us, this command allows the use of x to specify a wildcard number. The command isdn caller 555xxxx results in calls being accepted from any 7-digit phone number beginning with 555, and rejected in all other cases. We'll configure R2 to do just that and then send a ping from R1 to R2. To see the results of the Caller ID Screening, debug dialer will be run on R1 before sending the ping. Ive edited this output, since the output you see here will be repeated fire times once for each ping packet.R2(config-if)#isdn caller 555xxxxR1#debug dialerDial on demand events debugging is onR1#ping 220.127.116.11Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 18.104.22.168, timeout is 2 seconds:03:30:25: BR0 DDR: Dialing cause ip (s=22.214.171.124, d=126.96.36.199)03:30:25: BR0 DDR: Attempting to dial 8358662.Success rate is 0 percent (0/5)R1 doesn't give us any hints as to what the problem is, but we can see that the pings definitely aren't going through. On R2, show dialer displays the number of screened calls.R2#show dialerBRI0 - dialer type = ISDNDial String Successes Failures Last DNIS Last status8358661 1 0 00:03:16 successful7 incoming call(s) have been screened.0 incoming call(s) rejected for callback.The callback option mentioned in the last line shown above enables the router to reject a phone call, and then call that router back seconds later.R2 will now be configured to initially hang up on R1, and then call R1 back. R2(config-if)#isdn caller 8358661 callbackR1 will now ping R2. The pings aren't returned, but seconds later R2 calls R1 back.R1#ping 188.8.131.52Success rate is 0 percent (0/5)R1#03:48:12: BRI0: wait for isdn carrier timeout, call id=0x8023R1#03:48:18: %LINK-3-UPDOWN: Interface BRI0:1, changed state to upR1#03:48:18: BR0:1 DDR: dialer protocol upR1#03:48:19: %LINEPROTO-5-UPDOWN: Line protocol on Interface BRI0:1, changed state to upR1#03:48:24: %ISDN-6-CONNECT: Interface BRI0:1 is now connected to 8358662 R2show dialer on R2 shows the reason for the call to R1 is a callback return call.R2#show dialerBRI0 - dialer type = ISDNDial String Successes Failures Last DNIS Last status8358661 3 0 00:00:48 successful7 incoming call(s) have been screened.10 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 71 secsConnected to 8358661 (R1)The drawback to Caller ID Callback is that not all telco switches support it, so if you have the choice between this and PPP Callback, you're probably better off with PPP Callback. However, it's always a good idea to know more than one way to get things done with Cisco!