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Exchange 5.5 Intersite Connectivity: The Basics 

Added: 01/22/2002, Hits: 3,585, Rating: 0, Comments: 0, Votes: 0
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By Brian Talbert

Exchange Server sites can be linked together via "connectors". Normally servers within a site communicate directly through the MTA in order to exchange messages or from on DS to another to exchange directory information. However, this isn't directly possible for communication between sites. Therefore, a "connector" must be used to establish a logical tranport link between the sites.

There are connectors for both messaging and for directory replication. Since directory replication occurs through the transfer of "messages", a messaging connector must first be in place before direcotry replication can be configured. In the following sections, we'll take a look at the various types of connectors.

Messaging Connections
Message Connectors are really at the heart of intersite communication. All intersite communication will take place via a messaging connector. Even Directory Replication between sites depends upon the underlying existence of a messaging connector. There are four standard types of message connectors:

  • Site Connector

  • X.400 Site Connector

  • Dynamic RAS Connector

  • Internet Mail Service (IMS)

Note that some of the connectors, notably the X.400 and IMS, can be used to connect Exchange Server to NON-Exchange systems. It is for this reason that Directory Replication must be explicity configured, via a seperate connector, over these pathways.

The SITE Connector
The SITE connector is by far the easiest connector to configure between sites. It is ONLY used to connect EXCHANGE systems and therefore doesn't require the Directory Replication connector be configured. Communication is done via RPCs, just as with intRAsite communication, and is able to take place via the raw Exchange messaging format. Other connectors, such as X.400 require tranlation into an appropriate format. When connecting an Exchange server to another Exchange server via x.400, the translation must occur twice, once on the sending end from Exchange to x.400 and again on the receiving end from x.400 back to Exchange. For this reason, SITE is the most efficient connector and about 20% faster than X.400. When configuring SITE connectors, the configuration can be done from one side. Also, by default ALL servers in the remote site are potential TARGET servers and ALL servers in the local site are potential Bridgehead (sending) servers.

It is not without drawbacks though. A SITE connector requires a high-speed connection. The actual speed is somewhat arbitrary but consider it must be above 128k ISDN (e.g. LAN connection, or full T1). It also can not SCHEDULE connections. It assumes it is on a high-speed, high-availability network and therefore wants to make connections on its own schedule. It also can not restrict message size or restrict user access to the connector.

SITE connectors can use any available transport that supports RCP (TCP/IP, NetBEUI, IPX/SPX) communication. It uses TCP/IP as first preference if available.

The X.400 Connector
X.400 is a ITU (International Telecommunications Union - formerly CCITT) messaging standard. It is also now referenced as standard F.400, but Microsoft and the Exchange exam currently use the more commonly known X.400.

Using this connector, Exchange can send messages to another system compatible with the X.400 Message Handling System standard. This other system can be a private or public X.400 network, or could be another Exchange system, as is the case for intersite connectivity. When configuring this type of connector, it is done on both sides of the connection, and the name of the remote MTA is specified. It essentially allows for MTA to MTA communication, just as you have intrasite, to occur between sites.

The X.400 connector has some distinct advantages over the SITE connector. First is it's tolerance of slower speed networks, including dial-up connections. This connector also allows for scheduling, message size restriction and restricting the users that can send messages through the connector.

On the down side, the X.400 connector is not quite a swift as the SITE connector. It required additional configuration. It requires configuration on BOTH sides of the connection. Because of the requirement to specify a remote MTA for the connection, you have a single server to single server communication path. These servers are known as BridgeHead servers, and all X.400 traffic must go through these designated bridgehead servers. This can be a performance bottleneck, though it is possible to install MULITPLE connectors between sites, each connector can only have a single pair of bridgehead servers.

The X.400 connector requires the use of TCP/IP, TP0 (x.25), or TP4 (CLNP) transport stacks.

The Dynamic RAS Connector
Dynamic RAS offers the ability for Exchange servers to exchange messages automatically over dial-up lines. It takes advantage of a RAS server to do this. This connector is obviously intended to be used when no type of permanent connectivity between sites exists. Specifically, this connector is meant for non-permanent ISDN, Analog Modem, or X.25 connectivity. The RAS server can be the same physical machine as the Exchange server but does not have to be.

This connector is similar to the X.400 connector in that it does provide the ability to schedule connections, limit user connectivity, and limit message size. The main advantage is the ability to take advantage of non-permanent connectivity lines. It is also useful for ISDN back-up lines. The disadvantages are obvious, it's slow, expensive, and requires a lot of configuration.

Dynamic RAS can take advantage of any RAS support transport, such as TCP/IP, IPX/SPX, and NetBEUI.

The IMS Connector
The Internet Mail Service essential provids the ability to handle the Simple Mail Transport Protocol (SMTP). SMTP is the protocol used on the internet to transfer messages. Therefore, with IMS you can connect two Exchange sites over an existing internet connection or SMTP network, simply by using the IMS.

The advantage of the IMS service is that it allows the use of an existing internet connection instead of requiring that a dedicated line be used purely for Exchange traffic. It also allows the restriction of users, message size, and can be scheduled IF a dial-up connection is used.

The downside is that, like X.400, you have message bridgeheads at each site, presenting a potential bottleneck, you are at the mercy of the stability of the internet, and you have a potential privacy issue with messages traveling via an Internet backbone.

IMS requires the TCP/IP protocol.

Directory Replication Connections
Directory Replication Connectors allow sites to share directory information. Within a site, directory replication is able to take place via direct DS to DS communication. This is not possible, however, between sites. Instead it takes place over the Directory Replication connector.

Directory Replication between sites occurs via messages over an existing messaging connector. Now remember that some connectors don't necessary have to connect two exchange sites together. X.400, for instance, can be used to connect Exchange to a publix X.400 network, and the IMS can simply be used to provide Internet email capabilities. Therefore, if you are using X.400, IMS, or even Dynamic RAS, you must explicity configure these messaging connectors (through the Connected Sites tab) to allow Directory Replication. The only connector that doesn't require this is the SITE connector, which is only ever used between Exchange sites.

Once the messaging connectors are in place and properly configured to facilitied directory replication, the Directory Replication Connector can be installed. The Directory Replication connector creates a logical directory connection between two sites. One specific server in each site must be the designated as the Directory Replication Bridgehead Server.

Quick Connector Comparison

FeatureConnector Type
Format of Transferred MessagesMSEDB (Native Exchange)X.400X.400SMTP
Required BandwidthHIGH (LAN)Low-Med.Dial-UpLow-Med.
Supported ProtocolsRPC Supporting such as IPX, TCP/IP, and NetBEUITCP/IP, TP0, TP4Ras Supporting such as IPX, TCP/IP, and NetBEUITCP/IP
Requires Specified BridgeheadsNoYesYes, Because of the Point to Point nature or RASYes
Can Schedule ConnectionsNoYesYesNo, Unless using a Dial-Up Connection
User Access ControlNoYesYesYes
Message Size RestrictionsNoYesYesYes
Level of ConfigurationLowHighHighHigh

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