Cisco CNS Network Registrar User's Guide, 6.0
Understanding Network Registrar Concepts
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Understanding Network Registrar Concepts

Table Of Contents

Understanding Network Registrar Concepts

Domain Name System

How DNS Works

Domains

Learning ExampleCo's Address

Establishing a Domain

Difference Between Domains and Zones

Name Servers

Reverse Name Servers

Dynamic Host Configuration Protocol

How DHCP Works

Sample DHCP User

Typical DHCP Administration

Leases

Scopes

Policies

Network Registrar's DHCP Implementations

Virtual Private Networks and Namespaces

Subnet Allocation and DHCP Address-Blocks

Dynamic DNS Update

Effect on DNS of Obtaining a Lease

Effect on DNS of Releasing a Lease

Effect on DNS of Re-Acquiring a Lease

DHCP Failover

How Failover Works

Failover States and Transitions

Allocating Addresses Through Failover

Client-Class Quality of Service

DHCP Processing Without Client-Class

DHCP Processing With a Client-Class

Defining Scopes for a Client-Class

Choosing Networks and Scopes

Trivial File Transfer Protocol


Understanding Network Registrar Concepts


Cisco CNS Network Registrar provides the tools to configure and control the servers necessary to manage your IP address space. This chapter provides an overview of the related network concepts and protocols. Table 2-1 lists the topics in this chapter and their associated sections.

Table 2-1 Network Registrar Network Concepts and Protocols 

If you want to learn about...
See...

Domain Name System (DNS)

"Domain Name System" section

Dynamic Host Configuration Protocol (DHCP)

"Dynamic Host Configuration Protocol" section

Trivial File Transfer Protocol (TFTP)

"Trivial File Transfer Protocol" section


Domain Name System

The Domain Name System (DNS) was designed to handle the growing number of Internet users. DNS translates names, such as www.cisco.com, into Internet Protocol (IP) addresses, such as 192.168.40.0, so that computers can communicate with each other. DNS makes using Internet applications, such as the World Wide Web, easy. The process is as if, when phoning your friends and relatives, you could autodial them based on their names instead of having to remember their phone numbers.

How DNS Works

To understand how DNS works, imagine a typical user, John, logging on to his computer. He launches his Web browser so that he can view the website at a company, ExampleCo. He enters the name of their website—http://www.example.com. Then (see Figure 2-1):

1. John's workstation sends a request to the DNS server about the IP address of www.example.com.

2. The DNS server checks its database to find that www.example.com corresponds to 192.168.1.4, and returns this address to John's browser.

3. The browser uses the address to locate the website and displays it on John's monitor.

Figure 2-1 Domain Names and Addresses

Domains

John can access ExampleCo's website because his DNS server knows the www.example.com IP address. The server learned the address by searching through the domain namespace. DNS was designed as a tree structure, where each named domain is a node in the tree. The top-most node of the tree is the DNS root domain (.), under which there are subdomains (Figure 2-2), such as .com (commercial), .edu (education), .gov (government), and .mil (military).

Figure 2-2 The Domain Name System Hierarchy

The fully qualified domain name (FQDN) is a dot-separated string of all the network domains leading back to the root. This name is unique for each host on the Internet. The FQDN for the sample domain is example.com., with its domain example, parent domain .com, and root domain "." (dot).

Learning ExampleCo's Address

When John's workstation requests the IP address of the website www.example.com (Figure 2-3):

1. The DNS server looks for the www.example.com domain in its database, but cannot find it, indicating that the server is not authoritative for this domain.

2. The server asks the root name server that is authoritative for the top-level (root) domain "." (dot), which directs the query to a name server for the .com domain that knows about its subdomains.

3. The name server for the .com domain responds that example.com is one of its subdomains and responds with its server's address.

4. The DNS server asks example.com's name server for www.example.com's location, which replies that its address is 192.168.1.4.

5. The server sends this address to John's Web browser.

Figure 2-3 DNS Hierarchical Name Search

Establishing a Domain

ExampleCo has a website that John could reach because it registered its domain with an accredited domain registry. ExampleCo also entered its domain name in the .com server's database, and requested a network number, which defines a range of IP addresses. In this case, the network number is 192.168.1.0, which includes all addresses in the range 192.168.1.1 through 192.168.1.255. You can only have the numbers 0 through 256 (28) in each of the address fields, known as octets. However, the numbers 0 and 256 are reserved for network and broadcast addresses, and are not used for hosts.

Difference Between Domains and Zones

The domain namespace is divided into areas called zones that are points of delegation in the DNS tree. A zone contains all domains from a certain point downward except those for which other zones are authoritative.

A zone usually has an authoritative name server, often more than one. In an organization, you can have many name servers, but Internet clients can query only those that the root name servers know. The other name servers only answer internal queries.

The ExampleCo company registered its domain, example.com. It established three zones—example.com, marketing.example.com, and finance.example.com. ExampleCo delegated authority for marketing.example.com and finance.example.com to the DNS servers in the Marketing and Finance groups in the company. If someone queries example.com about hosts in marketing.example.com, example.com directs the query to the server for marketing.example.com.

In Figure 2-4, the domain example.com includes three zones, with the example.com zone being authoritative only for itself.

Figure 2-4 Example.com With Delegated Subdomains

ExampleCo could choose not to delegate authority to its subdomains. In that situation, the example.com domain consists of the example.com zone, which is authoritative for the subdomains marketing and finance (Figure 2-5). The example.com server answers all outside queries about marketing and finance.

Figure 2-5 Example.com Without Delegation

As you begin to configure zones using Network Registrar, as described in "Configuring DNS Servers," you will find that you must configure a name server for each zone. Each zone has one primary server, which loads the zone's contents from a local configuration database. Each zone can also have any number of secondary servers, which load the zone contents by fetching the data from the primary server. Figure 2-6 shows a configuration with one secondary server.

Figure 2-6 Primary and Secondary Servers for Zones

Name Servers

DNS is based on a client/server model. In this model, name servers store data about a portion of the DNS database and provide it to clients that query the name server across the network. Name servers are programs that run on a physical host and store zone data. As administrator for a domain, you set up a name server with the database of all the resource records describing the hosts in your zone or zones (Figure 2-7). For details about resource records, see "Resource Records."

Figure 2-7 Client/Server Name Resolution

The DNS servers provide name-to-address translation, or name resolution. They interpret the information in a fully qualified domain name (FQDN) to find its address. If a local name server does not have the data requested in a query, it asks other name servers until it finds it. For commonly requested names, this process can go quickly, because name servers continuously cache the information they learn from queries about the domain namespace.

Each zone must have one primary name server that loads the zone contents from a local database, and a number of secondary servers, which load a copy of the data from the primary server (Figure 2-8). This process of updating the secondary server from the primary server is called a zone transfer.

Figure 2-8 DNS Zone Transfer

Even though a secondary name server acts as a kind of backup to a primary server, both types of servers can be authoritative for the zone. They both learn about host names in the zone from the zone's authoritative database, not from information learned while answering queries. Clients can query both servers for name resolution.

As you configure Network Registrar's DNS name server, as described in "Configuring DNS Servers," you specify what role you want the server to perform for a zone—as primary, secondary, or caching-only. The type of server is meaningful only in context to its role. A server can be a primary for some zones and a secondary for others. It can be a primary or secondary only, or it can serve no zones and just answer queries by means of its cache.

Although all servers are caching servers, because they save the information until it expires, a caching-only server is one that is not authoritative for any zone. This server answers internal queries and asks other authoritative servers for the information. Sites create caching-only servers to unburden the authoritative servers so that they do not need to have every query directed to the authoritative servers.

To configure the:

Primary name server, see the "Configuring a Primary DNS Server" section.

Secondary server, see the "Configuring Secondary Servers for a Zone" section.

Caching-only server, see the "Configuring a Caching-Only Server" section.

Reverse Name Servers

The DNS servers described so far perform name-to-address resolution. They can do this easily by searching through their database for the correct address, because they index all the data by name. However, there are times when you need address-to-name resolution so that you can interpret certain output, such as computer log files.

Finding a domain name when you only know the address, however, would require searching the entire namespace. DNS solves this problem by supporting a domain namespace that uses addresses as names, known as the in-addr.arpa domain. This reverse zone contains subdomains for each network based on the network number. For consistency and natural grouping, the four octets of a host number are reversed.

When you read the IP address as a domain name, it appears backwards, because the name is in leaf-to-root order. For example, ExampleCo's example domain's network number is 192.168.1.0. Its reverse zone is 1.168.192.in-addr.arpa. If you only know the DNS server address (192.168.1.1), the query to the reverse domain would find the host entry 1.1.168.192.in-addr.arpa that maps back to example.com (Figure 2-9).

Reverse domains are handled through Pointer (PTR) resource records, as indicated by the command in the Figure 2-9. PTR records are described in the "Adding a Primary Reverse Zone for the Server" section.

Figure 2-9 Reverse Domains

Dynamic Host Configuration Protocol

All hosts seeking Internet access must have an IP address. As Internet administrator, you must do the following for every new user and for every user whose computer was moved to another subnet:

1. Choose a legal IP address.

2. Assign the address to the individual workstation.

3. Define workstation configuration parameters.

4. Update the DNS database, mapping the workstation name to the IP address.

These activities are time-consuming and error-prone, hence the Dynamic Host Configuration Protocol (DHCP). DHCP frees you from the burden of individually assigning IP addresses. It was designed by the Internet Engineering Task Force (IETF) to reduce the amount of configuration required when using TCP/IP. DHCP allocates IP addresses to hosts. It also provides all the parameters that hosts require to operate and exchange information on the Internet network to which they are attached.

DHCP localizes TCP/IP configuration information. It also manages allocating TCP/IP configuration data by automatically assigning IP addresses to systems configured to use DHCP. Thus, you can ensure that hosts have Internet access without having to configure each host individually.

How DHCP Works

DHCP makes dynamic address allocation possible by shifting workstation configuration to global address pools at the server level. DHCP is based on a client/server model. The client software runs on the workstation and the server software runs on the DHCP server.

Sample DHCP User

After Beth's workstation (bethpc) is configured to use DHCP, these actions occur when she first starts her workstation (Figure 2-10):

1. Her workstation automatically requests an IP address from a DHCP server on the network.

2. The DHCP server offers her a lease that is an IP address with the configuration data necessary to use the Internet. Nobody else uses the leased address, and it is valid only for her workstation.

3. Before the address's lease expires, bethpc renews it, thereby extending the expiration time. It continues to use the lease right up to its expiration or if it cannot reach the server.

4. If Beth relocates to another department and her workstation moves to a different subnet, her current address expires and becomes available for others. When Beth starts her workstation at its new location, it leases an address from an appropriate DHCP server on the subnet.

As long as the DHCP server has the correct configuration data, none of the workstations or servers using DHCP will ever be configured incorrectly. Therefore, there is less chance of incurring network problems from incorrectly configured workstations and servers that are difficult to trace.

Figure 2-10 Hosts Request an IP Address

The example illustrates the DHCP protocol with a set of DHCP servers that provide addresses on different subnets. To further simplify the administration of address pools, network routers are often configured as DHCP relay agents to forward client messages to a central DHCP server. This server is configured with address pools for a group of subnets.

Typical DHCP Administration

To use DHCP, you must have at least one DHCP server on the network. After you install the server:

Define a scope of IP addresses that the DHCP server can offer to DHCP clients. You no longer need to keep track of which addresses are in use and which are available. For details about defining scopes, see the "Defining and Configuring Scopes" section.

Configure a secondary server to share the distribution or handle leases if the first DHCP server goes down. For details, see "Configuring DHCP Failover."

Leases

One of the most significant benefits of DHCP is that it can dynamically configure workstations with IP addresses and associate leases with the assigned addresses. DHCP uses a lease mechanism that offers an automated, reliable, and safe method for distributing and reusing addresses in networks, with little need for administrative intervention. As system administrator, you can tailor the lease policy to meet the specific needs of your network.

Leases are grouped together in an address pool, called a scope, which defines the set of IP addresses available for requesting hosts. A lease can be reserved (the host always receives the same IP address) or dynamic (the host receives the next available, unassigned lease in the scope). The ExampleCo DHCP server is configured to lease addresses 192.168.1.100 through 192.168.1.199 (Figure 2-11).

Figure 2-11 DHCP Hosts Requesting Leases from a DHCP Server

If you plan not to have more network devices than configured addresses for the scope, you can define long lease times, such as one to two weeks, to reduce network traffic and DHCP server load. For details about leases, see the "Configuring Leases in the Scope" section.

Scopes

A scope contains a set of addresses for a subnet, along with the necessary configuration parameters. You must define at least one scope for each subnet for which you want dynamic addressing. For details about scopes, see the "Defining and Configuring Scopes" section.

Policies

A policy includes lease times and other configuration parameters that a DHCP server communicates to clients. Use policies to configure DHCP options that the DHCP server supplies to a client upon request. They ensure that the DHCP server supplies all the correct options for scopes without having to do so separately for each scope. For details about policies, see the "Creating a Policy" section.

Policies are especially useful if you have multiple scopes on a server. You can create policies that apply to all or selected scopes. In practice, you usually specify a router for each policy, which means that you need a policy for each scope.You can also write extensions to handle policy assignments. For details, see "Using Extension Points."

The difference between scopes and policies (Figure 2-12) is that scopes contain server information about addresses, such as which address is leasable and whether to ping clients before offering a lease. Policies contain client configuration data, such as the lease duration and address of the local DNS server.

Figure 2-12 Scopes and Policies

Network Registrar's DHCP Implementations

The Network Registrar DHCP server provides a reliable method for automatically assigning IP addresses to hosts on your network. You can define DHCP client configurations, and use the Network Registrar database to manage assigning client IP addresses and other optional TCP/IP and system configuration parameters. The TCP/IP assignable parameters include:

IP addresses for each network adapter card in a host.

Subnet masks for the part of an IP address that is the physical (subnet) network identifier.

Default gateway (router) that connects the subnet to other network segments.

Additional configuration parameters you can assign to DHCP clients, such as a domain name.

Network Registrar automatically creates the databases when you install the DHCP server software. You add data through the Web UI, CLI, or GUI as you define DHCP scopes and policies.

The Network Registrar DHCP server also supports allocating addresses in virtual private networks (VPNs) and subnets to pool manager devices for on-demand address pools. These features are described in the following sections.

Virtual Private Networks and Namespaces

Virtual private networks (VPNs) allow the possibility that two pools in separate networks can have the same address space, with these two pools having overlapping private network addresses. This can save address resources without having to use valuable public addresses. These VPN addresses, however, require a special designator to distinguish them from other overlapping IP addresses. Network Registrar DHCP servers that are not on the same VPN as their clients can now allocate leases and addresses to these clients, and can distinguish the addresses from one VPN to another.

Through changes made to the Network Registrar DHCP server and Cisco IOS DHCP Relay Agent, the DHCP server can service clients on multiple VPNs. The configuration concept in Network Registrar is known as a namespace. A namespace distinguishes a set of DHCP server objects, making them independent of otherwise identical objects in other address spaces. You can define multiple namespaces containing the same addresses. You create a namespace based on the VPN identifier configured in the Cisco IOS Relay Agent. Configuring namespaces is described in "Configuring the DHCP Server for Virtual Private Networks and Subnet Allocation."

Figure 2-13 shows a typical VPN-aware DHCP environment. The DHCP Relay Agent services two distinct VPNs, blue and red, with overlapping address spaces. The Relay Agent has the interface address 192.168.1.1 on VPN blue and is known to DHCP Server 1 as 172.27.180.232. The server, which services address requests from DHCP Client 1 in VPN blue, can be on a different network or network segment than the client, and can be in a failover configuration with DHCP Server 2 (see the "DHCP Failover" section). The Relay Agent can identify the special, distinguished route of the client's address request to the DHCP server, as coordinated between the Relay Agent and Network Registrar administrators. The DHCP servers can now issue leases based on overlapping IP addresses to the clients on both VPNs.

Figure 2-13 Virtual Private Network DHCP Configuration

Subnet Allocation and DHCP Address-Blocks

Network Registrar supports creating on-demand address pools as a network infrastructure for address provisioning and VPNs. Traditionally, the DHCP server is limited to interact with individual host devices. Through subnet allocation, the server can interact with VPN routers and other provisioning devices to provision entire IP subnets. This Network Registrar feature enhances the on-demand address pool capability currently supported by the Cisco IOS Relay Agent.

Network Registrar supports explicitly provisioned subnets. You must explicitly configure the DHCP server's address space and subnet allocation policies before the server can allocate pools or leases. You can thereby configure a server as a pool manager to manage subnets and delegate them to client devices.

You manage DHCP subnet allocation using DHCP server address-block objects in Network Registrar. A DHCP address-block is a range of contiguous IP addresses delegated to the DHCP server for assignment. The server expects to subdivide these addresses into pools so that it or other servers or devices can allocate them. DHCP address-blocks are parents to subnets. These DHCP address-blocks are distinct from the address blocks you can create using the Network Registrar Web UI, which are static. DHCP address-blocks cannot include static address ranges or lease reservations. Configuring DHCP address-blocks and managing subnets is described in "Configuring the DHCP Server for Virtual Private Networks and Subnet Allocation."

Figure 2-14 shows a sample environment where a DHCP server allocates entire subnets to access concentrators or other provisioning devices, in addition to servicing individual clients. The traditional client/server relationship is shown on the left of the diagram, while the subnet allocation to access concentrators is shown on the right of the diagram. Dialup customers, for example, connect to the service provider's network at two ISP gateways (routers), which connect to the management network segment where the DHCP server resides. The gateways provision addresses to their connected clients based on the subnet requested from the DHCP server.

Figure 2-14 Sample DHCP Subnet Allocation Configuration

Dynamic DNS Update

Although DHCP frees you from the burden of distributing IP addresses, it still requires updating the DNS server with DHCP client names and addresses. Dynamic DNS update automates the task of keeping the names and addresses current. With Network Registrar's dynamic DNS update feature, the DHCP server can tell the corresponding DNS server when a name-to-address association occurs or changes. When a client gets a lease, Network Registrar tells the DNS server to add the host data. When the lease expires or when the host gives it up, Network Registrar tells the DNS server to remove the association.

In normal operation, you do not have to manually reconfigure DNS, no matter how frequently clients' addresses change through DHCP. Network Registrar uses the host name that the client workstation provides. You also can have Network Registrar synthesize names for clients who do not provide them. For details, see "Configuring Dynamic DNS Update."

Effect on DNS of Obtaining a Lease

For ExampleCo, the administrator creates a scope on the DHCP server and allocates 100 leases (192.168.1.100 through 192.168.1.199). Each workstation gets its owner's name. The administrator also configures the DHCP server to use dynamic DNS update and associates it with the correspondingly configured DNS server. The administrator does not need to enter the names in the DNS server database.

Monday morning, Beth (user of bethpc) tries to log on to a website without having an address. When her host starts up, it broadcasts an address request (Figure 2-15).

Figure 2-15 Dynamic DNS Update at ExampleCo Company

The DHCP server then:

1. Gives bethpc the next available (unassigned) IP address (192.168.1.125).

2. Updates her DNS server with the host name and address (bethpc 192.168.1.125).

Beth can now access the website. In addition, programs that need to translate the name of Beth's machine to her IP address, or the other way around, can query the DNS server.

Effect on DNS of Releasing a Lease

Later that day, Beth learns that she needs to travel out of town. She turns off her host, which still has a leased address that is supposed to expire after three days. When the lease expires, the DHCP server:

1. Acknowledges that the IP address is now available for other users (Figure 2-16).

2. Updates the DNS server by removing the host name and address. The DNS server no longer stores data about bethpc or its address.

Figure 2-16 Relinquishing a Lease

Effect on DNS of Re-Acquiring a Lease

When Beth returns from her trip to start up her host again:

1. Her workstation broadcasts for an IP address.

2. The DHCP server checks if the host is on the correct network. If so, the server issues an address. If not, the server on the correct network issues the address.

3. The DHCP server updates the DNS server again with the host and address data.

DHCP Failover

Because DHCP, as described in RFC 2131, provides for multiple servers, you can configure these servers so that if one cannot provide leases to requesting clients, another one can take over. Network Registrar provides this capability in its DHCP failover feature, where two servers operate as redundant partners. Existing DHCP clients can continue to keep and renew their leases without needing to know or care which server is responding to their requests.

How Failover Works

Failover is based on a partner server relationship. The partners must have identical scopes, leases, policies, and client-classes. After the servers start up, each contacts the other. The main server provides its partner with a private pool of addresses and updates its partner with every client operation. If the main server fails, then the partner takes over offering and renewing leases, using its private pool. When the main server becomes operational again, it re-integrates with its partner without administrative intervention.

The failover protocol keeps DHCP operational if:

The main server fails—The partner takes over services during the time the main server is down. The servers cannot generate duplicate addresses, even if the main server fails before updating its partner.

Communication fails—A partner can operate correctly even though it cannot tell whether it was the other server or the communication with it that failed. The servers cannot issue duplicate addresses, even if they are both running and each can communicate with only a subset of clients.

Failover configurations are usually in a basic, back office, or symmetrical fashion. See "Configuring Three Types of Failover" section. Once configured:

1. The partners connect.

2. The main server supplies data about all existing leases to its partner.

3. The backup server requests a pool of backup addresses from the main server.

4. The main server replies with a percentage of available addresses from each scope to its partner.

5. The backup server ignores all DHCPDISCOVER requests, unless it senses that the main server is down. In normal operations, it handles only DHCPRENEW and DHCPREBINDING requests. A DHCPDISCOVER request is a broadcast to locate available servers.

6. The main server updates its partner with the results of all client operations.

Failover States and Transitions

During normal operation, the failover partners transition between states. They stay in their current state until all the actions for the state transition are completed and, if communication fails, until the conditions for the next state are fulfilled. The states and their transitions are described in Table 2-2.

Table 2-2 Failover States and Transitions 

State
Server Action

STARTUP

Tries to contact its partner to learn its state, then transitions to another state after a short time, typically seconds.

NORMAL

Can communicate with its partner. The main and backup servers act differently in this state:

The main server responds to all client requests using its pool. If its partner requests a backup pool, the main server provides it.

The backup server only responds to renewal and rebinding requests. It requests a backup pool from the main server.

COMMUNICATIONS-
INTERRUPTED

Cannot communicate with its partner, whether it or the communication with it is down. The servers cycle between this state and NORMAL state as the connection fails and recovers, or as they cycle between operational and nonoperational. During this time, the servers cannot give duplicate addresses.

During this state, you usually do not need to intervene and move a server into the PARTNER-DOWN state. However, this is not practical in some cases. A server running in this state is not using the available pool efficiently. This can restrict the time a server can effectively service clients.

A server is restricted in COMMUNICATIONS-INTERRUPTED state:

It cannot re-allocate an expired address to another client.

It cannot offer a lease or renewal beyond the maximum client lead time (MCLT) longer than the current lease time. The MCLT is a small additional time added that controls how much ahead of the backup server's lease expiration the client's is. See the "Allocating Addresses Through Failover" section.

A backup server can run out of addresses to give new clients, because it normally has only a small pool, while the main server has most of them.

 

The server is limited only by the number of addresses allocated to it and the arrival rate of DHCPDISCOVER or INIT-REBOOT packets for new clients. With a high new client arrival or turnover rate, you may need to move the server into PARTNER-DOWN state more quickly.

PARTNER-DOWN

Acts as if it were the only operating server, based on one of these facts:

The partner notified it during shutdown.

The administrator put the server into PARTNER-DOWN state.

The safe period expired and the partner automatically went into this state. For the risks involved, see the "Moving a Server into PARTNER-DOWN State" section.

 

The server ignores that the other server might still operate and could service a different set of clients. It can control all its addresses, offer leases and extensions, and re-allocate addresses. The same restrictions to servers in COMMUNICATIONS-INTERRUPTED state do not apply.

Either server can be in this state, but only one should be in it at a time so that the servers do not issue duplicate addresses and can properly resynchronize later on. Until then, an address is in a pending-available state.

POTENTIAL-
CONFLICT

Might be in a situation that does not guarantee automatic re-integration, and is trying to re-integrate with its partner. The server might determine that two clients (who might not be operating) were offered and accepted the same address, and tries to resolve this conflict.

RECOVER

Has no data in its stable storage, or is trying to re-integrate after recovering from PARTNER-DOWN state, from which it tries to refresh its stable storage. A main server in this state does not immediately start serving leases again. Because of this, do not reload a server in this state.

RECOVER-DONE

Can transition from RECOVER or PARTNER-DOWN state, or from COMMUNICATIONS-INTERRUPTED into NORMAL state.

PAUSED

Can inform its partner that it will be out of service for a short time. The partner then transitions to COMMUNICATIONS-INTERRUPTED state and begins servicing clients.

SHUTDOWN

Can inform its partner that it will be out of service for a long time. The partner then transitions to PARTNER-DOWN state to take over completely.


Allocating Addresses Through Failover

To keep your partners operating in spite of a network partition, in which both can communicate with clients, but not with each other, you must allocate more addresses than are needed to run a single server. Configure the main server to allocate a percentage of the currently available (unassigned) addresses in each scope's address pool to its partner. These addresses become unavailable to the main server. The partner uses them when it cannot talk to the main server and does not know if it is down.

How many additional addresses are needed? There is no single percentage for all environments. It depends on the arrival rate of new DHCP clients and the reaction time of your network administration staff. The backup server needs enough addresses from each scope to satisfy the requests of all new DHCP clients that arrive during the period in which the backup does not know if the main server is down.

Even during PARTNER-DOWN state, the backup server waits for the lease expiration and the maximum client lead time (MCLT), a small additional time buffer, before re-allocating any leases. When these times expire, the backup server offers:

Leases from its private pool of addresses.

Leases from the main server's pool of addresses.

Expired leases to new clients.

During the day, if the administrative staff can respond within two hours to a COMMUNICATIONS INTERRUPTED state to determine if the main server is working, the backup server needs enough addresses to support a reasonable upper bound on the number of new DHCP clients that might arrive during those two hours.

During off hours, if the administrative staff can respond within 12 hours to the same situation, and considering that the arrival rate of previously unheard from DHCP clients is also less. The backup server then needs enough addresses to support a reasonable upper bound on the number of DHCP clients that might arrive during those 12 hours.

Consequently, the number of addresses over which the backup server requires sole control would be the greater of these numbers of addresses, expressed as a percentage of the currently available (unassigned) addresses in each scope.

Client-Class Quality of Service

Assigning classes to clients is an important adjunct to DHCP addressing. You can use the Network Registrar client or client-class facility to provide differentiated services to users that are connected to a common network. You can group your user community based on administrative criteria, and then ensure that each user receives the appropriate class of service.

Although you can use Network Registrar's client-class facility to control any configuration parameter, the most common uses are for:

Address leases—How long a set of clients should keep its addresses.

IP address ranges—From which lease pool to assign clients addresses.

DNS server addresses—Where clients should direct their DNS queries.

DNS host names—What name to assign clients.

Denial of service—Whether unauthorized clients should be offered leases.

One way to use the client-class facility is to allow visitors access to some, but not all, of your network. For example, when Joe, a visitor to ExampleCo, tries to attach his laptop to the example.com network, Network Registrar recognizes it as being foreign. ExampleCo creates one class of clients known to Network Registrar as having access to the entire network, and creates another visitor class with access to a subnet only. If Joe needs more than the standard visitor's access, he can register his laptop with the Network Registrar system administrator, who adds him to a different class with the appropriate service.

The following sections describe how DHCP normally processes an address assignment, and then how it would handle it with the client-class facility in effect.

DHCP Processing Without Client-Class

To understand how you can apply client-class processing, it is helpful to know how the DHCP server handles client requests. The server can perform three tasks:

Assign an IP address.

Assign the appropriate DHCP options (configuration parameters).

Optionally assign a fully qualified domain name (FQDN) and update the DNS server with that name.

Here is what the DHCP server does:

1. Assigns an address to the client from a defined scope—To choose an address for the client, the DHCP server determines the client's subnet, based on the request packet contents, and finds an appropriate scope for that subnet. See the "Scopes" section.

If you have multiple scopes on one subnet or several network segments, known as multinetting, the DHCP server may choose among these scopes in a round-robin fashion. After the server chooses a scope, it chooses an available (unassigned) address from that scope.

2. Assigns DHCP option values from a defined policy—Network Registrar uses policies to group options. See the "Policies" section. There are two types of policies—scope-specific and system default. For each DHCP option the client requests, the DHCP server searches for its value in a defined sequence:

a. If the scope-specific policy contains the option, the server returns its value to the client and stops searching.

b. If not found, the server looks in the system default policy, returns its value, and stops searching.

c. If neither policy contains the option, the server returns no value to the client and logs an error.

d. The server repeats this process for each requested option.

3. With dynamic DNS update in effect, assigns an FQDN to the client—If you enabled dynamic DNS update, Network Registrar enters the client's name and address in the DNS host table. See the "Dynamic DNS Update" section. The client's name can be:

Its name as specified in the client's lease request (the default).

Its MAC address (hardware address; for example, 00:d0:ba:d3:bd:3b).

A unique name using the default prefix dhcp or a specified prefix.

DHCP Processing With a Client-Class

When you enable the client-class facility for your DHCP server, the request processing performs the same three tasks of assigning IP addresses, options, and domain names as described in the "DHCP Processing Without Client-Class" section, but with added capability. The DHCP server:

1. Considers the client properties and client-class inclusion before assigning an address—As in regular DHCP processing, the DHCP server determines the client's subnet. The server then checks if there is a MAC address for this client in its database. If there is:

a. No MAC address, it uses the default client specification. For example, if the client is assigned guest access, its client specification is Guest.

b. No MAC address and no default client, the server handles the client through regular DHCP processing.

c. A MAC address, the server checks if the client is a member of a client-class, determines its subnet based on the request packet, and applies the appropriate access properties based on its scope assignment.

The scopes must have addresses on client-accessible subnets. For example, a scope would either have a scope-selection tag of Employee or Guest, but not both. In other words, there are two scopes for each subnet—one with the scope-selection tag Employee, the other with Guest. Each scope has a different associated policy that provides the appropriate access rights for the user group.

2. Checks for client-class DHCP options—In regular DHCP processing, the server checks the scope-specific and system default DHCP options. With client-class, it also first checks the client-specific and client-class-specific options.

3. Provides additional FQDN assignment options—Beyond the usual name assignment process of using the host name the client requests, the server can:

Provide an explicit host name that overrides it.

Drop the client-requested host name and not replace it.

Synthesize a host name from the client's MAC address.

Defining Scopes for a Client-Class

To fully use the client-class capability on a subnet, you must define more than one scope for that subnet. Essentially, you must have more than one address pool to offer new clients. Each pool should have different characteristics. The usual distinction, and motivating factor for using client-classes, is to offer an address from one or another scope to a client. Secondary distinctions might be to provide clients with different option values or lease times.

To get more than one scope on a subnet, they must come from the same network segment. Networks are not configured directly in Network Registrar, but are inferred from scope configurations. Scopes become related (end up in the same network):

Implicitly—Two scopes have the same network number and subnet mask. These scopes naturally end up on the same network without explicit configuration.

Explicitly—One scope is marked as a secondary to another. This is required when the scope marked as a secondary has a network and subnet mask unrelated to the primary. An example is putting a set of 10.0.0.0 network addresses on a normal, routable network segment.

When the Network Registrar DHCP server reads the scope configuration from its database, it places every scope on a network, and logs this information. Scopes with the same network number and subnet mask end up on the same network, while a secondary scope ends up on the primary scope's network.

Choosing Networks and Scopes

When a DHCP packet arrives, the server determines the address from which it came by:

Gateway address (giaddr), if there was one, for packets sent through a BOOTP relay.

Interface address of the interface on which the broadcast packet arrived, if the DHCP client is on a network segment to which the DHCP server is also directly connected.

In all cases, the DHCP server determines a network from the gateway or interface address. Then, if the network has multiple scopes, the server determines from which scope to allocate an address to the DHCP client. It always looks for a scope that can allocate addresses to this type of client. For example, a DHCP client needs a scope that supports DHCP, and a BOOTP client needs one that supports BOOTP. If the client is a DHCP client and there are multiple scopes that support DHCP, each with available (unassigned) addresses, the DHCP server allocates an IP address from any of those scopes, in a round-robin manner.

The Network Registrar scope-selection tag and the client-class features enable you to configure the DHCP server to allocate IP addresses from:

One or more scopes on a network to one class of clients.

A different set of scopes to a different class of clients.

In the latter case, the gateway or interface address determines the network. The client-class capability, through the mechanism of the scope-selection tags, determines the scope on the network to use.

Trivial File Transfer Protocol

The Trivial File Transfer Protocol (TFTP) is a way of transferring files across the network using the User Datagram Protocol (UDP), a connectionless TCP/IP transport layer protocol. Network Registrar maintains a TFTP server so that systems can provide device provisioning files to cable modems that comply with the Data Over Cable Service Interface Specification (DOCSIS) standard. The TFTP server buffers the DOCSIS file in its local memory as it sends the file to the modem. When the TFTP transfer is completed, the server flushes the file from local memory. TFTP also supports other, non-DOCSIS configuration files.

Here are some of the features of the Network Registrar TFTP server:

Complies with RFCs 1350 and 1123.

Includes a high performance multithreaded architecture.

Caches data for performance enhancements.

Is configurable and controllable using the CLI. See the tftp command in the Network Registrar CLI Reference Guide for details.

Includes flexible path and file access controls.

Includes audit logging of TFTP connections and file transfers.

The default root directory for Network Registrar TFTP files is install-path/data/tftp.

See also the "Troubleshooting and Optimizing the TFTP Server" section.