03.30.08

Cisco CCNP / BSCI Certification: The BGP Attribute “MED”

Posted in House Of Technology at 9:42 am by admin

When you’re preparing to pass the BSCI exam and earn your CCNP certification, one of the biggest challenges is learning BGP. BGP is totally different from any protocol you learned to earn your CCNA certification, and one of the differences is that BGP uses path attributes to favor one path over another when multiple paths to or from a destination exist.

Notice I said “to or from”. In earlier free BGP tutorials, I discussed the BGP attributes “weight” and “local preference”. These attributes are used to favor one path to a destination over another; for example, if BGP AS 100 has two paths to a destination in AS 200, these two attributes can be set in AS 100 to favor one path over another. But what if AS 100 wants to inform the routers in AS 200 as to which path it should use to reach a given destination in AS 100?

That’s where the BGP attribute “Multi-Exit Discriminator”, or MED, comes in. The MED value can be set in AS 100 to tell AS 200 which path it should use to reach a given network in AS 100.

As with many BGP attributes, the MED can be set with a route-map. What you need to watch is that there is no “set med” value in route maps. To change the MED of a path, you need to change the metric of that path. Let’s say that there are two entry paths for AS 200 to use to reach destinations in AS 100. You want AS 200 to use the 100.1.1.0/24 path over the 100.2.2.0/24 path. First, identify the two paths with two separate ACLs.

R1(config)#access-list 22 permit 100.1.1.0 0.0.0.255

R1(config)#access-list 23 permit 100.2.2.0 0.0.0.255

Next, write a route-map that assigns a lower metric to the more-desirable path.

R1(config)#route-map PREFER_PATH permit 10

R1(config-route-map)#match ip address 22

R1(config-route-map)#set metric 100

R1(config-route-map)#route-map PREFER_PATH permit 20

R1(config-route-map)#match ip address 23

R1(config-route-map)#set metric 250

Finally, apply the route-map to the neighbor or neighbors.

R1(config-route-map)#router bgp 100

R1(config-router)#neighbor 22.2.2.2 route-map PREFER_PATH out

The key points to keep in mind is that while many BGP attributes prefer a higher value, the MED is basically an external metric - and a lower metric is preferred, just as with the protocols you’ve already studied to earn your CCNA certification.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of free CCNA and CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages. Video courses and training, binary and subnetting help, and corporate training are also available. Pass the CCNA exam and BSCI exam with Chris Bryant, CCIE #12933!

For a copy of his FREE ebooks, “How To Pass The CCNA” and “How To Pass The CCNP”, visit the website and download your copies! Get your CCNP certification with The Bryant Advantage!

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03.26.08

Cisco Certification: A Survival Guide To The Cisco Cable Jungle

Posted in House Of Technology at 1:43 pm by admin

One of the most confusing parts of beginning your Cisco studies is keeping all the cable types separate in your mind, and then remembering what they’re used for. This often occurs when a CCNA or CCNP candidate starts putting together their own home practice lab, and they suddenly realize that they have the equipment to run labs, but not the cables.

With this in mind, here are some common Cisco cable types and their primary use.

First, there’s the regular old “straight-through cable”, so named because the eight wires inside the cable go straight through the wire. While the wires may be twisted inside to reduce electromagnetic interference (EMI), the wire that’s connected to Pin 1 on one end is connected to Pin 1 on the other end, and so on. In a home lab, a straight-through cable is often used to connect a switch port to an Ethernet port on a router, with a transceiver attached to the Ethernet port. Straight-through cables are also good for connecting a BRI interface to an ISDN simulator.

The “crossover cable” is so named because the wires do cross over between pins. This allows the devices to both send and receive at the same time, and crossover cables are a must for directly connecting ports on Cisco switches to create a trunk.

The “rollover cable” allows you to connect directly to a Cisco console port with your laptop or PC. This is the blue cable that comes with new Cisco devices, and it’s the one that engineers tend to hold on to with their lives. Without a rollover cable (also commonly called a “console cable”), you can’t connect your laptop directly to a Cisco device.

Finally, there’s the DTE/DCE cable. To create a frame relay cloud in your home lab (using one of your Cisco routers as a DCE), or to directly connect two Cisco routers via their serial interfaces, you will need a DTE/DCE cable. Remember that the DCE interface will need to supply clockrate to the DTE interface.

The different cable types can be confusing when you first read about them, but after tearing down or building your home lab a few times, you’ll definitely have them straight come test day!

Best of luck in your lab and your exams.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com), home of free CCNA and CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages. Video courses and training, binary and subnetting help, and corporate training are also available. Pass the CCNA exam with Chris Bryant, CCIE #12933!

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