Global Knowledge Instructor Kevin Schweers reviews the fundamentals of spanning tree, including the three primary steps.
Archive for August, 2009
Address Resolution Protocol (ARP)
Published August 28, 2009 David Stahl , Routing & Switching 1 CommentTags: addressing
When sending data end-to-end through a network, routers are used in internetworking to create a virtual network from one device to another, either locally or globally. Routers are configured to operate with most common network protocols. That means they know the format of the addresses, how many bytes are in the basic package of data sent out over the network, and how to make sure all the packages reach their destination and get reassembled. For the routers that are part of the Internet’s main “backbone,” this means looking at and forwarding millions of information packages every second.
As I discussed in my last blog, for end-to-end packet delivery addressing is done at two different layers of the OSI model. These two different layers use different types of addresses for different purposes. Layer 2 addresses, such as IEEE 802 MAC (Media Access Control) addresses, are used for local transmissions between hardware devices that can communicate directly. They are used to implement basic LAN, WLAN, and WAN technologies. In contrast, Layer 3 addresses, which are most commonly 32-bit Internet Protocol (IP) addresses, are used in internetworking to create a virtual network at the network layer.
The odds are very good that you will never see the MAC addresses for any of your equipment, because the software that helps your devices communicate with a network takes care of matching a Layer 2 MAC address to a Layer 3 logical address. In computer and device networking, the Address Resolution Protocol (ARP) is the method for finding a host’s link layer (hardware) address when only its Internet Layer L-3 Internet Protocol (IP) address is known. ARP is defined in RFC 826.
ARP has been implemented in many types of networks since it is not an IP-only or Ethernet-only protocol. It can be used to resolve many different network layer protocol addresses to interface hardware addresses, although, due to the overwhelming prevalence of Ipv4 and Ethernet, ARP is primarily used to translate IP addresses to Ethernet MAC addresses. ARP is also used for IP over other LAN technologies, such as Token Ring, FDDI, and IP over ATM.
ARP is a Data Link Layer protocol because it only operates on the local area network or point-to-point link that a host is connected to. The purpose of ARP is to resolve addresses by finding the MAC address that corresponds to an IP address. Even though a sending system may know the IP address it wants to ultimately send data to, it may not know the MAC address. On an Ethernet LAN, systems communicate directly using CSMA/CD and must know the MAC address of the system that data is being be sent to next.
Remember that MAC addresses are fixed. Ethernet network adapters, or Network Interface Cards (NIC), are produced with a physical address embedded in the hardware. Manufacturers take care to ensure that these 6-byte (48-bit) addresses are unique, and Ethernet relies on these unique identifiers for message delivery.
IP addresses, on the other hand, are not fixed on any interface. Systems can be manually configured with an IP address, or they can obtain one using the Dynamic Host Configuration Protocol (DHCP). As such, it doesn’t make much sense to have a static mapping between the two, since IP addresses may change. Instead, when a system needs to obtain the MAC address associated with an IP address, it sends out a Layer 2 broadcast message asking that the system with the specified IP address reply with its MAC address. Once it receives a reply, the answer is cached for a limited period of time (typically between 2 and 20 minutes) in the system’s ARP table.
Since an ARP request is a Layer 2 broadcast, it will be seen by every system in the same broadcast domain. When any system receives an ARP request, it will check to see if it is the intended recipient. If it is, the system will process the Ethernet frame. If not, the Ethernet frame is ignored. Caching helps to ensure that ARP broadcasts don’t continually flood a network.
Author: David Stahl
IOS Tips and Tricks, Part 2
Published August 26, 2009 Al Friebe , Routing & Switching Leave a CommentTags: IOS
Continuing with our examples of IOS Command Line Interface (CLI) tips, here are the new and old versions of several config-related commands, along with some common shortcuts. Note that these shortcuts are not necessarily the most concise possible. Use the question mark to find shortcuts that you like.
To see the startup config (stored in NVRAM):
- New#show startup-config (“s start”)
- Old#show config (“s conf”)
Note that “show config” does not display the running config, it displays the startup config. To erase the startup config:
- New#erase startup-config (“erase start”)
- Old#write erase (“wr er”)
To display the running config (also referred to as the “active” or “current” config):
- New#show running-config (“s run”)
- Old#write terminal (“wr t”)
To save the running config to NVRAM:
- New#copy running-config startup-config (“copy run start”)
- Old#write memory (“wr”, you don’t need the “mem”)
Although Cisco no longer officially endorses the old commands (due to confusion over what “show config” does), you might see “old-timers” using them, especially “wr”. Why type fourteen or so keystrokes (“copy run start”) when two (“wr”) will do?
Global Knowledge Instructor Kevin Schweers reviews the fundamentals of layer 2 devices.
Check out some of the photos we got from our Beat the Geek champions.
Jacob Hess shows off some moves
Trying on Daddy’s (Michael Abrahim) belt
Did you get your picture taken at our World of Solutions booth? Just enter the 100-xxxx number written on the back of your card to pull up your photo. If you put it someplace public (facebook, LinkedIn, Twitter, etc) let us
know. We’d love to see how you use it.
Cisco UC Integration for Microsoft Office Communicator
Published August 24, 2009 Dennis Hartmann , Unified Communications Leave a CommentTags: cucimoc, integration
Cisco UC Integration for Microsoft Office Communicator (CUCIMOC – pronounced “cookie-mock”) is a Cisco client that integrates with the Microsoft Office Communicator (MOC) client at the desktop computer. CUCIMOC is an add-on to MOC 2007 that allows phone calls to be made through Cisco Unified Communications Manager (CUCM).
CUCIMOC allows enterprises to leverage their investment in a Microsoft OCS 2007 server for instant messaging and presence, while utilizing the investment in Cisco UC for voice calls. CUCM and MOC integration has been supported for many years, but required a Cisco Unified Presence Server (CUPS) before the release of CUCIMOC.
CUCIMOC provides an integrated soft phone (SIP-based) to make and receive phone calls, but the CUCIMOC client can also provide desk-phone control allowing incoming calls to a Cisco hardware phone to be picked up with the MOC client. The MOC client can also initiate phone calls from the MOC client that take place on the Cisco IP phone (via the CTI feature).
The mid-call control features allow instant message sessions to be promoted to audio calls or multiple instant messaging sessions promoted to conference calls. The full list of features can be found in the CUCIMOC data sheet link in the links section of this blog.
As of the time of this writing, the only version of CUCIMOC is 7.1. The 7.1 version number was used to align to the version number used in the rest of Cisco’s UC platform. CUCIMOC requires CUCM version 6.1(3) or higher, and three device license units (DLU) in CUCM 6.1(3) regardless of the deployment model. CUCM 6.1(4) and later (as well as CUCM 7.x and later) only require one DLU with adjunct licensing and three DLUs when deployed as a standalone phone. Adjunct licensing requires the end user to be associated to both CUCIMOC and a standard Cisco IP phone.
More Information
Cisco Unified Communications Integration for Microsoft Office Communication
CUCIMOC 7.1 Release Notes
CUCIMOC Data Sheet
Cisco and Microsoft Collaboration in Unified Communications
Author: Dennis Hartmann
Network Layer Utilities: End-to-End Data Delivery
Published August 22, 2009 David Stahl , Routing & Switching Leave a CommentTags: layer 2, layer 3, OSI Model
From our studies, we know that the International Organization for Standards (ISO) created the Open Systems Interconnection (OSI) networking model to standardize data networking protocols, to enable communication between all computers and devices across any network anywhere in the world. The OSI model is now mainly used as a point of reference for discussing the specifications of protocols used in network design and operation. The upper layers of the OSI reference model (application, presentation, and session = Layers 7, 6, and 5) define functions focused on the application. The lower four layers (transport, network, data link, and physical = Layers 4, 3, 2, and 1) define functions focused on end-to-end delivery of the data.
When we consider the seven layers of the OSI Reference Model, there are two that deal with addressing the data link layer and the network layer. The physical layer is not strictly concerned with addressing at all, only sending at the bit level. The layers above the network layer all work with network layer addresses.
When we discuss end-to-end delivery of data, we must necessarily talk about how datagrams are addressed. We find out that addressing is done at two different layers of the OSI model and two different layers are used, which are very different types of addresses that are used for different purposes. Layer 2 addresses, such as IEEE 802 MAC addresses, are used for local transmissions between hardware devices that can communicate directly. They are used to implement basic LAN, WLAN, and WAN technologies. In contrast, layer 3 addresses, which are most commonly 32-bit Internet Protocol addresses, are used in internetworking to create a virtual network at the network layer.
The most important difference between these types of addresses is the distinction between layers 2 and 3 themselves. Layer 2 MAC addresses enable communication between directly-connected devices residing on the same physical network. Layer 3 IP addresses allow communications between both directly and indirectly-connected devices.
For example, say you want to connect to the Web server at http://www.cisco.com. This is a Cisco Web site that resides on a server that has an Ethernet card used for connecting to its Internet service provider site. However, even if you know its Layer 2 MAC address, you cannot use it to talk directly to this server using the Ethernet card in your home PC. This is because these two devices are on different networks. In fact, they may even be on different continents!
Instead, these devices communicate at layer 3, using the Internet Protocol and higher layer protocols such as TCP and HTTP. Your request is routed from your home machine through a sequence of routers to the Cisco server. The response is then routed back to you. The communication is, logically, at layers 3 and above. You send the request, not to the MAC address of the server’s network card, but rather to the server’s IP address.
While we can virtually connect devices at Layer 3 through routers, these connections are really conceptual only. When you send a datagram that has been created using the OSI 7-Layer-Model, it is sent one hop at a time, from one router to another, from one physical network to the next. At each of these hops, an actual transmission occurs at the physical and data link layers.
When your request is sent to your local router at layer 3, which is usually referred to as your default gateway, the actual request is encapsulated in an Ethernet frame using whatever method you use to physically connect to the router. It is addressed and sent to the default gateway router using the router’s data link layer MAC address. The same happens for each subsequent step until, finally, the router nearest the Cisco Web server, sends the datagram to the destination using the data link (MAC) address of the NIC card of the Cisco Web server.
In my next blog, I will discuss the Address Resolution Protocol (ARP) that is a method used for finding a device’s link layer MAC hardware address when only its Internet Layer IP address is known.
Author: David Stahl
Which Exams to Take: CCNA Composite vs ICND1 & ICND2
Published August 21, 2009 Routing & Switching 1 CommentGlobal Knowledge Instructor Kevin Schweers reviews the basic differences between the two paths to achieving CCNA certification.
IOS Tips and Tricks – Part 1
Published August 20, 2009 Al Friebe , Routing & Switching 1 CommentTags: IOS
Let’s look at some things we can do with Cisco’s IOS to make our lives easier. Here are some useful commands, followed (in parentheses) by their most concise shortcuts at the time of this writing. Although I’m showing them on a router, these commands also work on the IOS-based switches.
As you may know, when interacting with IOS from the command line interface (CLI), there are two main EXEC modes, “user” and “privileged” (the latter is also referred to as “enabled” mode). In “user” mode, you can do limited examination of the device (via “show” commands), and the prompt appears as the device’s hostname (“Router” in this example) followed by the “greater-than” symbol:
- Router>
Call Admission Control
Published August 20, 2009 Paul Stryer , Unified Communications Leave a CommentTags: CAC, call control
Call Admission Control (CAC) is often times included as part of the same topic as Quality of Service (QoS), when in actuality CAC is a separate and complete topic itself.
QoS is defined as traffic engineering on a packet switched network. This definition means moving the IP Packets on to the wire and across the network in the fastest time possible, with the least amount of dropped packets. QoS manages this process by prioritizing different data flows. Packets with high sensitivity to the amount of time it takes to traverse the network receive a higher priority; such as voice and video packets.
CAC prevents over-subscription of VoIP networks. It is used in the call set-up phase and applies to Real-Time Transport Protocol traffic also known as the media portion of a call. CAC compliments QoS, however, it’s mechanisms of operation are very different from those of QoS operations. CAC protects voice traffic from the negative effects of excess voice traffic on the VoIP network, by ensuring there is enough bandwidth for all authorized flows.
In most cases CAC is done on Wide Area Networks (WAN) where there is typically only enough bandwidth to support a small volume of calls. For example, if a WAN can only support five G.729 calls and six or more calls come in on that WAN there would be degraded call quality for all calls on the WAN, not just for calls six and above. The reason call quality suffers for all calls is because of shared bandwidth. Generally, without any device to provide CAC, the system would continue to allow calls on the WAN circuit and exceed the bandwidth specifications of the WAN. With the insertion of a CAC device, the number of calls on the VoIP networks is counted with a limit set to how many calls can be placed on each WAN network. The CAC device would start rejecting call set-up messages when the limit is reached. It would be up to the initiating system to reroute the call onto another network, such as the Public Switched Telephony Network (PSTN).
Within Cisco’s IP Telephony systems there are two main types of topology unaware CAC. Topology unaware CAC is defined as any mechanism that is based on a static configuration within a call processing agent or IP-Based PBX, aimed at limiting the number of simultaneous calls to or from a remote site connected via the IP WAN. Due to the reliance on static configurations, topology unaware CAC mechanisms must be designed in a simple hub-and-spoke topology.
Location Based Call Admission Control
Cisco Unified Communications Manager (CUCM) provides a simple mechanism known as static locations for implementing CAC in the centralized call processing deployment. When you configure a device in CUCM, the device can be assigned to a location. A certain amount of bandwidth will be allocated for calls to or from each location. CUCM can define a voice and video bandwidth pool for each location. If the location’s audio and video bandwidths are configured as “Unlimited”, there will be unlimited bandwidth available for that location and every audio or video call to or from that location will be permitted by CUCM. On the other hand, if the bandwidth values are set to a finite number of kilobits per second (kbps), CUCM will allow calls in and out of that location as long as the aggregate bandwidth used by all active calls is less than or equal to the configured values.
When an inter-site call is denied by CAC, CUCM can automatically reroute the call to the destination via the PSTN connection by means of the Automated Alternate Routing (AAR) feature. AAR is invoked only when the locations-based CAC denies the call due to a lack of network bandwidth. AAR is not invoked when the IP WAN is unavailable or other connectivity issues cause the called device to become unregistered with CUCM. In such cases, the calls are redirected to the target specified in the Call Forward No Answer field of the called device.
Gatekeeper Based Call Admission Control
A Cisco IOS gatekeeper can provide call routing and CAC between devices such as CUCM, Cisco Unified Communications Manager Express (CME), or H.323 gateways connected to a legacy PBX. The gatekeeper uses the H.323 Registration Admission Status (RAS) protocol to communicate with these devices and route calls across the network.
Gatekeeper CAC is a policy-based scheme requiring static configuration of available resources. The gatekeeper is not aware of the network topology, so it is limited to simple hub-and-spoke topologies.
The CAC capabilities of a Cisco IOS gatekeeper are based on the concept of gatekeeper zones. A zone is a collection of H.323 devices, such as endpoints, gateways, or Multipoint Control Units (MCUs) that register with a gatekeeper.
The bandwidth command is used to manage the number of calls that the gatekeeper will allow, thus providing call admission control functionality. This command has several options, but the most relevant are the following:
- The interzone option controls the amount of bandwidth for all calls into or out of a given local zone.
- The total option controls the amount of bandwidth for all calls into, out of, or within a given local zone.
- The session option controls the amount of bandwidth per call for a given local zone.
- The remote option controls the total amount of bandwidth to or from all remote zones.
The bandwidth value deducted by the gatekeeper for every active call is double the bit-rate of the call, excluding Layer 2, IP, and RTP overhead.
Resource Reservation Protocol – Topology Aware CAC
CUCM Release 5.0 introduces a topology aware CAC mechanism based on the Resource Reservation Protocol (RSVP). Topology aware CAC is applicable to any network topology and eases the restriction of a traditional hub-and-spoke topology. The Cisco RSVP Agent is a Cisco IOS feature that enables CUCM to perform the RSVP-based CAC. The Cisco RSVP Agent feature has been introduced into Cisco IOS Release 12.4(6)T and is available on the Cisco 2800 Series and 3800 Series Integrated Services Routers platforms.
The Cisco RSVP Agent registers with Unified CM as either a media termination point (MTP) or a transcoder device with RSVP support. When an endpoint device makes a call in need of a bandwidth reservation, CUCM invokes a Cisco RSVP Agent to act as a proxy for the endpoint to make the bandwidth reservation.
Calculating Bandwidth for CAC
In both cases of topology unaware CAC you will have to inform the CAC mechanism how much bandwidth is available on each WAN link. You can do this by reading two of my previous posts:
Fudge Math of CAC
Calculating VoIP Bandwidth
Sources
SRND for CUCM 7.x
Author: Paul Stryer
