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 224.0.0.0 - 239.255.255.255.
The addresses 224.0.0.0 - 224.255.255.255 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 224.0.0.5 to send hellos, EIGRP routers use 224.0.0.10 to send updates, and RIP version 2 uses 224.0.0.9 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.
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Monday, December 05, 2005
Cisco Router Security
Network security is a hot topic today, and will only increase in importance in the months and years ahead.While most of the attention is paid to exterior threats, there are some steps you can take to prevent unwanted Cisco router access from within your organization.Whether you want to limit what certain users can do and run on your routers, or prevent unauthorized users in your company from getting to config mode in the first place, here are four important yet simple steps you can take to do so.Encrypt the passwords in your running configuration.This is a basic
Cisco router security command that is often overlooked. It doesn’t do you any good to set passwords for your ISDN connection or Telnet connections if anyone who can see your router’s running configuration can see the passwords. By default, these passwords are displayed in your running config in clear text.One simple command takes care of that. In global configuration mode, run service password-encryption. This command will encrypt all clear text passwords in your running configuration.Set a console password.If I walked into your network room right now, could I sit down and start configuring your Cisco routers? If so, you need to set a console password. This password is a basic yet important step in limiting router access in your network.
Go into line configuration mode with the command “line con 0”, and set a password with the password command.Limit user capabilities with privilege level commands.Not everyone who has access to your routers should be able to do anything they want. With careful use of privilege levels, you can limit the commands given users can run on your routers.Privilege levels can be a little clumsy at first, but with practice you’ll be tying your routers down as tight as you like. Visit www.cisco.com/univercd for documentation on configuring privilege levels.Configure an “enable secret” password.It’s not uncommon for me to see a router that has an enable mode password set, but it’s in clear text. By using “enable secret”, the enable mode password will automatically be encrypted. Remember, if you have an enable password and enable secret password set on the same router, the enable secret password takes precedence.These four basic steps will help prevent unwanted router access from inside your network. If only preventing problems from outside your network was as simple!
Author: Chris Bryant
Cisco router security command that is often overlooked. It doesn’t do you any good to set passwords for your ISDN connection or Telnet connections if anyone who can see your router’s running configuration can see the passwords. By default, these passwords are displayed in your running config in clear text.One simple command takes care of that. In global configuration mode, run service password-encryption. This command will encrypt all clear text passwords in your running configuration.Set a console password.If I walked into your network room right now, could I sit down and start configuring your Cisco routers? If so, you need to set a console password. This password is a basic yet important step in limiting router access in your network.
Go into line configuration mode with the command “line con 0”, and set a password with the password command.Limit user capabilities with privilege level commands.Not everyone who has access to your routers should be able to do anything they want. With careful use of privilege levels, you can limit the commands given users can run on your routers.Privilege levels can be a little clumsy at first, but with practice you’ll be tying your routers down as tight as you like. Visit www.cisco.com/univercd for documentation on configuring privilege levels.Configure an “enable secret” password.It’s not uncommon for me to see a router that has an enable mode password set, but it’s in clear text. By using “enable secret”, the enable mode password will automatically be encrypted. Remember, if you have an enable password and enable secret password set on the same router, the enable secret password takes precedence.These four basic steps will help prevent unwanted router access from inside your network. If only preventing problems from outside your network was as simple!
Author: Chris Bryant
Tips On Buying A Cisco CCNA / CCNP Home Lab Kit
Buying a CCNA / CCNP home lab is the best way to be totally prepared for your Cisco exams.
Most home labs are put together one router or switch at a time, but many CCNA / CCNP candidates prefer to buy kits where you get multiple routers and switches, along with all the cables and other connection devices you'll need.
While this is a good idea, keep a few things in mind when purchasing Cisco home lab kits.
Don't buy anything you don't need. The problem is that when you're first starting out with your Cisco home lab, you don't know everything that you need. (I sure didn't!) Keep in mind that you only need one transceiver per AUI port on a Cisco router, so if you're getting routers with two AUI ports in all, you don't need five transceivers in the kit. It doesn't hurt to have one spare, but three is a little too much.
More importantly, don't buy kits with old CCNA or CCNP study guides included. I've seen kits with books that were three years old and were of no use to the candidate. If you see a kit that looks good but includes books or manuals you just don't want, ask the vendor for a price that doesn't include the books. It never hurts to ask.
Watch the IOS version. Unless you've got access to IOS upgrades, you'll be working with the
IOS version that's on the routers and switches when you buy the kit for a while. You don't necessarily need the latest and greatest IOS version for CCNA study, but don't buy routers with IOS versions beginning with "10" unless you have an IOS to upgrade them with. (And make sure the routers have enough memory to handle the IOS you plan on putting on them.)
Purchasing a Cisco CCNA / CCNP Home Lab is one of the best investments in your career that you will ever make. Exercise just a bit of caution when purchasing your kit, and you'll be on your way to true Cisco success, in the exam room and on your network!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!
Most home labs are put together one router or switch at a time, but many CCNA / CCNP candidates prefer to buy kits where you get multiple routers and switches, along with all the cables and other connection devices you'll need.
While this is a good idea, keep a few things in mind when purchasing Cisco home lab kits.
Don't buy anything you don't need. The problem is that when you're first starting out with your Cisco home lab, you don't know everything that you need. (I sure didn't!) Keep in mind that you only need one transceiver per AUI port on a Cisco router, so if you're getting routers with two AUI ports in all, you don't need five transceivers in the kit. It doesn't hurt to have one spare, but three is a little too much.
More importantly, don't buy kits with old CCNA or CCNP study guides included. I've seen kits with books that were three years old and were of no use to the candidate. If you see a kit that looks good but includes books or manuals you just don't want, ask the vendor for a price that doesn't include the books. It never hurts to ask.
Watch the IOS version. Unless you've got access to IOS upgrades, you'll be working with the
IOS version that's on the routers and switches when you buy the kit for a while. You don't necessarily need the latest and greatest IOS version for CCNA study, but don't buy routers with IOS versions beginning with "10" unless you have an IOS to upgrade them with. (And make sure the routers have enough memory to handle the IOS you plan on putting on them.)
Purchasing a Cisco CCNA / CCNP Home Lab is one of the best investments in your career that you will ever make. Exercise just a bit of caution when purchasing your kit, and you'll be on your way to true Cisco success, in the exam room and on your network!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!