Cisco Discovery Protocol – CDP

The Cisco Discovery Protocol is of use in most networked environments however it is probably most useful in a Cisco switched environment. This is because one the difficulties in troubleshooting an environment with mostly switches is that they usually provide a lot less information than routers. It can also pose further difficulties because a switched environment is sometimes more confusing because these networks are not always clearly segmented and can be confusing. CDP provides a useful tool for identifying and detecting Cisco switches and routers and to build up a picture of the complete network topology of all the Cisco hardware.

The Cisco Discovery Protocol is a data link multicast protocol, which uses a standard multicast MAC address. These broadcasts are formed as SNAP type 200 packets and as such they have no layer 3 component. CDP must be enabled on a Cisco router or switch before it can be used for detecting other Cisco devices which can be seen on all it’s interfaces. It should be noted that CDP is not like extended network management protocols like SNMP it can only detect directly attached devices. Even if two routers were plugged into a single Cisco switch, CDP will allow each router to see the switch but it would not allow them to see the other router. The routers will appear to the switches as CDP neighbours, the primary reason for this is that the protocol is not designed to forward frames between devices.

Although this sounds of limited use, the information that CDP provides about directly attached devices is extensive and extremely useful for troubleshooting things like network problems or anonymous torrenting taking place on the network. You can for example use the protocol for completely defining whole wiring closers, and detailing the exact models and versions of routers and switches. The information is extensive and you’ll soon know even the versions of Catalyst switches that are connected and which port/interface. Other protocols will normally go little further than identifying whether the hardware is a router or switch.

There are a host of commands which can be used to provide extensive information on all Cisco hardware. The most used ones are the SHow and Clear commands which can be used to bring up general information on the devices plus details of the software versions, modules installed, port configuration and any error messages. The command can also be used to display the full routing and switching tables plus much more information.

The show config command is used to display the configuration information which is stored in NVRAM of the device. For example in Catalyst 5000 series of routers, the entire configuration is stored automatically in this memory, the command write terminal can also be used to display the same information. To clear the configuration information the clear config command will erase the information on a specified module. The displayed information can be categorized into the following broad categories:

  • switch management parameters
  • IP configuration
  • Virtual LANS
  • Bridging Parameters

There is further information which can be accessed including extensive information on module specific information. You can also use the show cam command which refers to the special memory on the switches which has a low access time. THe information stored here is normally bridging and switching tables, it needs a fast response time because it can be updated very quickly in fast dynamic environments.

Further Reading

No Comments Networks, Protocols

Troubleshooting the Border Gateway Protocol (BGP)

It’s often the more complex IP routing protocols which are the most difficult to diagnose and troubleshoot and BGP (Border Gaetway Protocol) is no exception.  Like many such protocol, BGP has a fairly specialised application in that it is used specifically for routing between different routing domains and autonomous systems.    You’ll normally find BGP being used in advanced or specialised network environments like Internet Service Providers (ISP) or global corporate networks with advanced routing requirements.

Another situation where you may encounter BGP is when companies have merged, it is ideally suited to bring disparate computer networks together without starting from scratch.    During the end of the last century there was a huge amount of these sort of corporate mergers and huge networks needed to be joined together – BGP provided the optimum solution for many of these situations and indeed is still commonly used today. Many a network administrator will have spent hours analysing at the end of residential VPN trying to determine the complexities behind a long established BGP routing tables.

When troubleshooting issues that may be related to BGP it’s important to understand the fundamental characteristics of the protocol.  Without knowing these core concepts it can be very difficult to analyse a complex and specialised protocol like BGP:

Neighbour Formation : Like many routing protocols, BGP creates neighbour adjacency between routers before it starts exchanging information.  These neighbours though are almost always defined statically rather than dynamically by the protocol.  Their formation is normally determined by the setting up of a simple TCP connection, the command for determining a list and status of BGP neighbours is as follows:

show ip bgp neighbor

Most of the important data is found in the first few lines of the output of this show command. The most useful parameter for troubleshooting is the BGP state which will switch from Idle-Active-Open-Established as the formation of the neighbour state takes place. Remember this process can take a little time to complete, especially compared to some modern day routing protocols, so give it time, however if the state ends up as anything other than Established then the formation has not completed successfully.

Other relevant information that is important are the BGP version. There are quite a few different versions of BGP being used in the wild and they will always establish on the lowest common version when establishing a connection. If you see these version constantly changing and switching it is usually indicative of some fundamental network configuration problem.

External BGP : This is usually run between two different but autonomous systems which are defined on networks which must be directly connected. The neighbors are established by specifying the address of the link, for example you could configure by naming the address of a serial links between two routers on the two networks. You may have to use the ebgp-multihop parameter in these situations as often interfaces are not directly connected as specified by the loopback address. In order to ensure that there is a loop-free topology, BGP will ignore any BGP routes which has originated in any autonomous systems (AS).

Further Reading:

Designing Multimedia Networks

Moving voice and video over any data network can be a challenge, if you’ve ever sat through a stuttering video conference you’ll appreciate that you have to do it well.     Fortunately it’s becoming more of a reality nowadays with efficient compressions techniques, high bandwidth networks and of course QoS.   Compression is probably the most important factor as it radically reduces the volume of traffic that needs to be transmitted over network links.

Genuine multimedia networks are rarer than you would think, and indeed some of the best which have integrated ATM (Asynchronous Transfer Mode) can be extremely fast.   One of the most important factors apart from the increased speed ATM can bring to both WAN and LAN networks is it’s support for QoS.   This guarantees a certain bandwidth and performance levels for the multimedia connections, although this has to be reserved to be effective.  Not only can administrators reserve their multimedia requirements but they can also set up virtual circuits to separate their video conference, multimedia or voice calls.   Although it should be noted that this will require either ATM compatible applications, adapters fitted to the workstations or software that emulates ATM on standard network interface cards.

Whatever technology is incorporated the main issue with adding multimedia applications to a network is simply the traffic load.  It’s pointless letting users have access to real time multimedia applications without a very fast data network and some sort of QoS guarantee.   The network also needs the capacity to provide these guarantees without affecting the rest of the normal data traffic.    Capacity planning is crucial and until this is carried out you will have little idea how even a modest set of multimedia applications will effect your network speeds.

For any long term use there are a variety of techniques which can radically boost network performance for multimedia.  Core switched networks which connect to existing departmental hubs is a start and these can be upgraded to provide switched services to different departments as required.   Any videoconferencing equipment should be connected directly to high performance switches, on no account should the traffic be allowed to broadcast out through out the network through a simple hub or repeater.  Most high performance networks now try to standardise on Gigabit ethernet although often this can be slowed by legacy network hardware. Iso-Ethernet is an emerging technology which can incorporate  voice and standard 10 mbit ethernet on the same cable.

There are a variety of methods and technologies which will provide quality of service over existing networks if you don’t have access to ATM.  In fact often it is easier to use one of these bespoke methods as ATM does require modification and support in all applications, transports and software.  A technology called RSVP (Resource Reservation Protocol) has been developed by the IETF (Internet Engineering Task Force) which allows any IP host to request directly a specified amount of bandwidth on a network.

Further Reading

 

HTTP (Hypertext Transfer Protocol)

For many of us a network is either our little home setup consisting of perhaps a modem and wireless access point and a few connected devices, or perhaps that huge global wide network – the internet.  Whatever the size all networks need to allow communication between the various devices connected to them.  Just like human beings need languages to communicate so do networks only in this context we call them ‘protocols’.

The internet is built primarily using TCP/IP protocols to communicate, this is used to transport information between ‘web clients’ and ‘web servers’.   It’s not enough though to enable the media rich content delivered to our web browsers and a host of secondary protocols site above the main transport protocol – the most important one which enables the world wide web is called HTTP.

This provides a method for web browsers to access content stored on web servers, which is created using HTML (Hypertext Markup Language).  HTML documents contain text, graphics and video but also hyperlinks to other locations on the world wide web.   HTTP is responsible for processing these links and enabling the client/server communication which results.

Without HTTP the world wide web simply wouldn’t exist and if you want to see it’s origins search for RFC 1945 where you’ll find HTTP defined as an application level protocol designed with the lightness and speed necessary for distributed, collaborative, hypermedia information systems.   It is a stateless, generic and object orientated protocol which can be used for a huge variety of tasks – crucially it can be used on a variety of platforms which means it doesn’t matter whether you’re platform your computer is on (linux, Windows or Mac for instance) – you can still access the web content via HTTP.

So what happens? When someone types a web name or address into the address field of their web browser, the browser attempts to locate the address on the network it is connected to.  This can either be a local address or more commonly it will look out on to the internet looking for the designated web server.   HTTP is the command and control protocol which enables communication between the client and the web server allowing commands to be passed between the two of them.   HTML is the formatting language of the web pages which are transferred when you access a web site.

The HTTP connection between the client and server can be secured in two specific ways – using secure HTTP (SHTTP) or Secure Sockets Layer (SSL) which both allow the information transmitted to be encrypted and thus protected.  It should be noted though that the vast majority of communication is standard HTTP and is transmitted in clear text insecurely which is why so many people use proxies and VPNs like this to protect their connections.