Chapter 22 Advanced DHCP Server Properties

Defining Advanced Server Parameters

In the CLI, use dhcp show and dhcp get to show the current server parameters, then use dhcp set, dhcp unset, dhcp enable, and dhcp disable to change them (see Table 22-1).

Step 2 In the Web UI, click the name of the server.

Step 3 In the Web UI, add or modify attributes on the Edit DHCP Server page.

Step 4 In the Web UI, click Modify Server to make the changes.

Deferring Lease Extensions

The defer-lease-extensionsattribute allows the DHCP server to optimize its response to a sudden flood of DHCP traffic. The parameter is enabled by default. An example of a network event that could result in such a traffic spike is a power failure at a cable internet service provider (ISP) data center that results in all of its cable modem termination systems (CMTS) rebooting at once. If this were to happen, the devices attached to the CMTSs produce a flood of DHCP traffic as they quickly come back online.

When you enable the defer-lease-extensionsattribute, a DHCP client that renews a lease that is less than halfway to its expiration does not have its lease extended to an additional full lease time. Instead, the client receives a lease corresponding to the remaining time on its existing lease. Because the absolute lease expiration time does not change, the server can avoid database updates that result in a significantly higher server throughput.

If a client is more than halfway to its expiration, this setting has no effect, and the lease is extended to the full configured lease interval as usual, including the database writes.

Note Deferring lease extensions significantly increases the server’s performance while remaining in compliance with the DHCP RFC, which stipulates that client binding information is committed to persistent storage when the lease changes.

When deferring lease extensions, it is advisable to leave the policy attribute allow-lease-time-overrideto its default of disabled, or to change it to disabled if it is enabled.

These three specific situations are described from the server’s point of view:

•Client retries—When the server gets behind, it is possible for a client to retransmit requests. The DHCP server does not maintain enough information to recognize these as retransmissions, and processes each to completion, granting a full lease duration again and updating the database. When the server is already behind, doing extra work worsens the situation. To prevent this, the DHCP server does not extend leases that are less than 30 seconds old, regardless of the state of the defer-lease-extensionsattribute.

•Client reboots—The effective renew time for a client’s lease is really the minimum of the configured renew time and the time between client reboots. In many installations this may mean that clients get fresh leases one (in a typical enterprise) or two (in a typical cable network) times per day, even if the renew time is set for many days. Setting the defer-lease-extensionsattribute can prevent these early renews from causing database traffic.

•Artificially short renewal times—Because there is no way for a DHCP server to proactively contact a DHCP client with regard to a lease, you might configure short lease times on the DHCP server to provide a means of doing network renumbering, address reallocation, or network reconfiguration (for example, a change in DNS server address) in a timely fashion. The goal is to allow you to do this without incurring unacceptable database update overhead.

Cisco CNS Network Registrar User’s Guide

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OL-6240-02

 

 

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Cisco Systems OL-6240-02 manual Deferring Lease Extensions, 22-6
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OL-6240-02 specifications

Cisco Systems OL-6240-02 is a highly regarded certification exam that delves into the realm of enterprise networking, particularly focusing on the Cisco Certified Network Professional (CCNP) Routing and Switching track. This particular exam serves as a critical test for network professionals aiming to validate their skills and knowledge in advanced routing technologies and network design.

One of the main features of OL-6240-02 is its emphasis on in-depth understanding of IP routing principles. Candidates are expected to demonstrate proficiency in configuring, maintaining, and troubleshooting complex routing protocols like OSPF (Open Shortest Path First), EIGRP (Enhanced Interior Gateway Routing Protocol), and BGP (Border Gateway Protocol). This includes comprehending routing algorithms, path selection, and the implementation of route redistribution, which are essential for optimizing network performance.

The exam also covers the characteristics of network security and infrastructure, where candidates must exhibit knowledge of implementing security protocols and best practices to safeguard network communications. This includes configuring virtual private networks (VPNs), utilizing access control lists (ACLs), and applying encryption techniques to secure data in transit.

Another crucial technology tested in OL-6240-02 is Quality of Service (QoS). Professionals must be adept at configuring QoS mechanisms to prioritize traffic, ensuring that critical applications receive the necessary bandwidth and latency requirements. This is increasingly important as businesses rely on a multitude of applications that have varying performance needs.

Cisco's emphasis on network automation and programmability is also a focal point of this certification. Candidates are expected to be familiar with using Cisco's software-defined networking (SDN) solutions and automation tools, which are essential for modern network management. This involves understanding how to leverage APIs and modeling tools to streamline network operations and enhance efficiency.

To succeed in OL-6240-02, candidates should possess a solid foundation in network fundamentals and have hands-on experience with Cisco devices. The certification not only validates expertise but also prepares professionals for real-world challenges in designing, implementing, and troubleshooting enterprise networks. Overall, OL-6240-02 is a stepping stone for network professionals seeking higher-level certifications and career advancements in the ever-evolving field of networking.
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