The OSPF Default Mysteries
Default routing should be a simple concept, but becomes surprisingly complex in routing protocols that have multiple layers of default routes. In this IP Corner article, Ivan Pepelnjak describes how the Open Shortest Path First (OSPF) protocol uses default routes and how various OSPF-generated default routes interact in typical network scenarios. 1.7.2008
Servers in Small Site Multi-Homing
If you want to deploy high-availability public servers within your network, you should implement proper multi-homing solution including BGP routing with the Service Providers. But even if you use alternative solutions, like the ones presented in the previous IP corner article Small Site Multi-Homing, there are ways to deploy public servers within your site. In this IP Corner article, Ivan Pepelnjak describes the challenges you’ll face and give you several design and deployment guidelines. 1.6.2008
Scalable Policy Routing
Network designers and implementers try to avoid policy routing, as its common implementation in Cisco IOS requires a complex mix of access-lists and route-maps that have to be deployed on a hop-by-hop basis. In most cases, distance vector routing protocols can be used to implement policy routing requirements in large networks. In this IP Corner article, Ivan Pepelnjak describes how you can use BGP to implement an architecture where a set of applications should prefer a different subset of links than other applications. 1.5.2008
It's Good to be on Time
The importance of having accurate time on distributed servers and even personal workstations has been recognized long time ago by the IT managers, but it hasn’t been applied consistently to the networking devices. In this IP Corner article, Ivan Pepelnjak, describes the importance of time synchronization for networking devices, the basics of Network Time Protocol (NTP) that is commonly used to synchronize IP hosts and routers, how to use it on Cisco routers and IOS-based switches and how to implement it in a highly scalable way. 1.4.2008
Designing Fast Converging BGP Networks
Border Gateway Protocol (BGP) was always considered a mastodonic routing protocol: huge, complex, hard to understand and configure, and very slow to converge. When Cisco decided to use it to implement layer-3 Virtual Private Networks (VPNs) due to its enormous route carrying capabilities, the slow convergence of BGP became a liability. The Cisco engineers decided to fix the BGP code, resulting in a routing protocol with decent convergence times. In this IP Corner article, Ivan Pepelnjak illustrates how you can optimize BGP convergence in your network without overloading the routers running it. 1.3.2008
When OSPF Becomes a Distance Vector Protocol
Contrary to common wisdom, OSPF is not a pure link-state protocol. It uses link state algorithms within an area, but behaves almost like a distance vector protocol between the areas. This distinction introduces temporary routing instabilities into multi-area OSPF network that does not use inter-area summarization. In today's IP Corner article, Ivan Pepelnjak illustrates how this unexpected behavior can affect the convergence of your network and how you can use proprietary extensions of Cisco IOS to alleviate the undesired side effects of OSPF. 1.2.2008
The Never-Ending Story of IP Fragmentation
After years of struggles, the IP fragmentation remains one of the challenges in IP network deployment, particularly if you have to implement extra layers in the protocol stack (like PPP over Ethernet) or if you use any IP-over-IP encapsulation or IP encryption techniques. In this IP Corner article, Ivan Pepelnjak describes the reasons behind IP fragmentation, how the Path MTU Discovery works and how the various mechanisms can be used on Cisco routers to alleviate the IP fragmentation-related problems. 1.1.2008
Bring your Network Closer to Five Nines with Graceful Shutdown
The five nines (99.999% availability of a service) is the holy grail of many Chief Information Officers (CIO). To reach this goal, the average monthly downtime should be less than 25 seconds, which is extremely hard to achieve even in a fully redundant architecture. The scheduled router outages (upgrades, hardware maintenance), while being necessary, can also impact the safety margin you have. In this IP Corner article, Ivan Pepelnjak describes how you can reduce the network downtime caused by scheduled router outages if your network uses OSPF (Open Shortest Path First) routing protocol. 1.12.2007
Load Balancing in BGP Networks
A few years ago, the traditional wisdom was that you could not do load balancing in networks using Border Gateway Protocol (BGP) as their core routing protocol. The technology, actually its implementation in Cisco IOS, has evolved since then, resulting in a number of load balancing options for BGP-based networks. However, even though it is possible to load-balance in BGP networks, it is still not as easy as Interior Gateway Protocol (IGP; for example OSPF or EIGRP)-based load balancing, which happens automatically. In this IP Corner article, Ivan Pepelnjak describes load-balancing options available with BGP. 1.11.2007
Changing the Routing Protocol in Your Network
Selecting the right IP routing protocol is one of the most important decisions in the network design phase. But even after careful consideration of all facts known to you at that time, you might get it wrong and have to change the routing protocol after the network has already been in production for some time. In this IP Corner article, Ivan Pepelnjak will give you some suggestions on how to migrate from one routing protocol to another in a moderately complex network. 1.10.2007
Increase the Stability of your Network
The introduction of real-time mission-critical applications (like voice-over-IP) into data networks has prompted many network designers to tune their routing protocols for faster convergence. The resulting network usually becomes highly susceptible to repetitive failures (e.g. a flapping interface), which can cause recurring instabilities in large parts of the network and significant data loss. In this IP Corner article, Ivan Pepelnjak describes how the IP Event Dampening, introduced in Cisco IOS release 12.3, can be used to increase the stability of your network, as well as how you can cope with scenarios that are beyond the scope of this feature. 1.9.2007
Redundant Small Site Multi-Homing
The February IP Corner article Small Site Multi-Homing described how to implement the small site multi-homing with existing Cisco technologies in the existing ISP environment. That article has generated lots of responses, most of them being questions about redundant implementation of the same principles. Therefore Ivan Pepelnjak decided to describe how to extend the small site multi-homing design with a set of redundant routers. The final design still retains the administrative simplicity of the original solution – with no need to own public IP address space, autonomous system number or to run Border Gateway Protocol (BGP). 1.7.2007
Cisco Router: the Swiss Army Knife of Network Services
The cost optimization in the IT industry is affecting all segments of network design and implementation. For example, some IP services like DHCP and DNS, which were previously distributed throughout the network, are now concentrated on central servers. As these services are vital for the proper operation of IP networks, your remote sites might lose even intra-site connectivity if their link to the core site fails. Fortunately Cisco routers can provide most network services locally, including DNS and DHCP. In this IP Corner article, Ivan Pepelnjak describes how you can use a Cisco router as a local DNS server. 1.6.2007
Enhance the IOS User Interface
Have you ever wanted to fine-tune the IOS show commands to provide you with the exact information you need instead of having to dig through long screens full of data you are not interested in to find what you need? In this IP Corner article, Ivan Pepelnjak describes how to use the simple filters provided by Cisco IOS to pick only the information you need from the printouts, as well as how to generate tailored printouts (even combining outputs from multiple show commands) with Tcl shell introduced in IOS release 12.3(2)T. 1.5.2007
Scaling EIGRP Networks with Stub Routers
Enhanced Interior Gateway Routing Protocol (EIGRP), Cisco’s proprietary yet hugely successful and widely deployed routing protocol is known to behave disappointingly in inadequately designed networks. Cisco has improved EIGRP’s behavior dramatically with the introduction of stub routers in Cisco IOS release 12.0(7)T. However, this feature has remained a well-hidden mystery. In this IP Corner article, Ivan Pepelnjak explores the typical problems that the EIGRP stub routers help to solve, describes how the introduction of stub routers improves network stability and implements a fully redundant remote location (stub site); yet another very common design requirement that is not documented anywhere. 1.4.2007
Replacing Configuration on a Working Router
Have you ever faced a situation where you have badly misconfigured your router and had to roll back the configuration to a previous known state? Assuming that the working configuration was still saved in the NVRAM, you only had two options - to manually work out the configuration commands to bring the router back to the previous state or to reload the router. In both cases the time was running too fast - both for the users who were facing the network downtime and especially for you. In this month`s IP Corner Ivan Pepelnjak describes how to use the Cisco IOS Configuration Replacement and Configuration Rollback feature to replace the current running configuration with any saved Cisco IOS configuration file. 1.3.2007
Small Site Multi-Homing
High-availability seems a de-facto requirement of enterprise networks, even more so today as the network managers have to migrate from traditional highly robust technologies to MPLS/VPN- or Internet-based transport networks. Usually these migrations result in multi-homed central sites, while the small remote sites end up having a single best-effort upstream connection. In this month's IP Corner article Ivan Pepelnjak describes how to implement the small site multi-homing with existing Cisco technologies in the existing ISP environment. 1.2.2007
Keep Track of Router Configurations with Configuration Archive
Can you answer these questions when faced with a network-down situation:
Do we have a backup of a working configuration?
What was the router configuration before the last mistake was committed to the startup configuration?
Do we have a copy of the configuration that was used a week (or a month) ago? If not, maybe it's time you deploy Configuration Archive feature described in this article. 1.1.2007
Router Configuration Management ... Too Good to be True?
In Cisco IOS release 12.4, Cisco finally gave us fundamental router configuration management tools that we've been sorely missing in the last 20 years. In this month's IP corner Ivan Pepelnjak describes how he tested them and uncovered a few unpleasant surprises along the way. 1.12.2006
Perfect load-balancing: How close can you get?
Multi-protocol Label Switching (MPLS) is usually regarded as a Service Provider technology, but in this month's article, Ivan Pepelnjak shows you how you can deploy MPLS Traffic Engineering (MPLS-TE) in your enterprise network to achieve optimal load-balancing in a highly redundant setup. 1.11.2006
Using a Web Server to Manage Your Router Configurations
Starting with IOS release 12.3(2)T, you can download and upload software and configuration of your Cisco router to a web server, greatly simplifying router management and enabling the network managers to use the same infrastructure as the rest of their IT department. In this article, you’ll find the description of the required configuration steps for both the Cisco routers and the web server on which you want to store the router configurations. 1.10.2006
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