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Posts Tagged ‘ccie’

When you’re studying to pass the BSCI exam and earn your CCNP certification, you’re going to be introduced to ISIS. ISIS and OSPF are both link-state protocols, but ISIS works quite differently from OSPF. You must master these details in order to earn your CCNP.

One of the major differences between OSPF and ISIS will be evident to you when you first begin your BSCI exam studies, and that is the terminology. ISIS uses terms that no other protocol you’ve studied to date uses, and learning these new terms is the first step to BSCI and CCNP exam success.

First off, what does “IS” stand for in “ISIS”? It stands for “Intermediate System”, which sounds like a group of routers. As opposed to Autonomous Systems, which are logical groups of routers, an Intermediate System is simply a single router. That’s it.

You’ll also become familiar with End Systems, referred to in ISIS as an “ES”. The End System is simply an end host.
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While routers accept and generate broadcasts, they do not forward them. This can be quite a problem when a broadcast needs to get to a device such as a DHCP or TFTP server that’s on one side of a router with other subnets on the other side.

If a PC attempts to locate a DNS server with a broadcast, the broadcast will be stopped by the router and will never get to the DNS server. By configuring the ip helper-address command on the router, UDP broadcasts such as this will be translated into a unicast by the router, making the communication possible. The command should be configured on the interface that will be receiving the broadcasts.

R1(config)#int e0

R1(config-if)#ip helper-address ?

A.B.C.D IP destination address

R1(config-if)#ip helper-address 100.1.1.2

Now, you may be wondering if this command covers all UDP services. Sorry, you’re not getting off that easy! The command does forward eight common UDP service broadcasts, though.

TIME, port 37

TACACS, port 49

DNS, port 53

BOOTP/DHCP Server, port 67

BOOTP/DHCP Client, port 68

TFTP, port 69

NetBIOS name service, port 137

NetBIOS datagram service, port 138

That’s going to cover most scenarios where the ip helper-address command will be useful, but what about those situations where the broadcast you need forwarded is not on this list? You can use the ip forward-protocol command to add any UDP port number to the list.

Additionally, to remove protocols from the default list, use the no ip forward-protocol command. In the following example, we’ll add the Network Time Protocol port to the forwarding list while removing the NetBIOS ports. Remember, you can use IOS Help to get a list of commonly filtered ports!

R1(config)#ip forward-protocol udp ?

<0-65535> Port number

biff Biff (mail notification, comsat, 512)

bootpc Bootstrap Protocol (BOOTP) client (68)

bootps Bootstrap Protocol (BOOTP) server (67)
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In this section, you’re going to be reintroduced to a networking model you first saw in your CCNA studies. No, it’s not the OSI model or the TCP/IP model – it’s the Cisco Three-Layer Hierarchical Model. Let’s face it, just about all you had to do for the CCNA was memorize the three layers and the order they were found in that model, but the stakes are raised here in your CCNP studies. You need to know what each layer does, and what each layer should not be doing. This is vital information for your real-world network career as well, so let’s get started with a review of the Cisco three-layer model, and then we’ll take a look at each layer’s tasks. Most of the considerations at each layer are common sense, but we’ll go over them anyway!

Today we’ll take a look at the core layer of the Cisco model.

The term core switches refers to any switches found here. Switches at the core layer allow switches at the distribution layer to communicate, and this is more than a full-time job. It’s vital to keep any extra workload off the core switches, and allow them to do what they need to do – switch! The core layer is the backbone of your entire network, so we’re interested in high-speed data transfer and very low latency – that’s it!
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QoS is a big topic on your BCMSN and CCNP exams, and for good reason. As more and more traffic flows through today’s networks, accurately applying QoS to both your routers and switches becomes more important.

Note the phrase “accurately applying”. You must have a plan in place before you start configuring QoS on your switches, and to create such a plan you should use Cisco’s Three-layer Hierarchical Model.

This model breaks switches down into three main groups – Access, Distribution, and Core. You’re familiar with these groups from your CCNA studies, and now you’ve got to apply this knowledge.

The QoS workload should be borne by the Access and Distribution layers, because the Core layer switches need to be left alone as much as possible to their primary purpose – switching!

Traffic should generally be classified and marked at the Access layer. This allows traffic to be assigned the desired QoS values and carry that value throughout the network.

If you choose to change CoS-DSCP mappings, this will generally be done at the Distribution layer. Since distribution layer switches will be receiving frames and packets with QoS values from the access layer switches, the appropriate “trust” and “no trust” statements should be configured on the appropriate distribution layer switches.
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