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CCNP Enterprise ENSLD (300-420): Designing OSPF Routing

2. OSPF Neighbor Adjacencies and LSA's

Now, OSPF network design can include full mesh, partial mesh, or hub and spoke. The scalability of OSPF is really influenced by the number of LSAs and the neighbours in an OSPF area. Route summarisation will help and without someof these mechanisms, the amount of bandwidthconsumed these routers with this OSPF processing. Now remember we talked about "network convergence," which is the time that it takes for a network to actually respond to any changes or failures in the network. I always want to improve our network convergence time. The route workload also depends on how much information there is within the area and the routing domain. The factors that influence ospalibility include the number of routers in the area, the average number of links and the network type, the type of area, the amount of summarization done, and the number of external routers in the routing domain. The tech tools used to reduce information are area design, area type selection, routetype selection summarization, and interrefiltering. The number of routers and links to adjacent routers in an area determine how much information is in the LSA database or how much routing information is in the area. The type of area and the amount of summarization are factors that influence the amount of routing information. As well, the number of areas and types of areas that are supported by each router also influence how mutating information is handled, as in a Dom domain. There are techniques and tools to reduce this information. Stub and totally stubby areas import less information about destinations outside of the routing domain or the area than L areas do. Therefore, using stubs and totally stubby areas further reduces the workload. Each area advertises OSPF router interregional routes and costs into an area. Totally stubby areas keep not only external routes but also this interrelated information from flooding into and within an area. Another technique to reduce the number of prefixes that are exchanged between areas is interarea filtering using prefix lists. This method can be used instead of speed-stubby areas if specific routing information is needed for some prefixes but not for others. One way to think of ASPRS in OSPF is that each provides a distance vectorlike, lit destination, and associated costs. The more external prefixes and the more SBRs there are, the higher the vote for type. Five or seven LSAs keep all of this information from flooding an area. The conclusion is that area size and layout, design types, redistribution, and summarization all affect the size of the LSA database in an area. The general advice on the OSPF design is to keep it simple and utilise stubby areas. Now there are some timers that have to do with these SPF processes. These are the timings of successive SPF calculations that can be throttled just like we throttle LSAs in OAF. So when a router receives a topology change, it's going to generate a NESPF process to update the routes. Now, if I have a flapping interface in a network These flapping interfaces could cause consecutive SPF recalculations, and this could actually cause issues with the convergence of the network. But by default, what happens as Ciscorouters will schedule the shortest pathfirst runfive secnds after receiving an updated LSA. If the updated LSA arrives after the run, the subsequent delay then grows, which they call dampening the topology. So we want to consider adjusting these timers very clearly. The calculation is set to a particular time for very good reasons. Do not schedule the shortest Pathf first run until the previous calculation is complete, and you must ensure that the entire timer is running as well as the time it takes for the run to finish. And then you want to estimate the runtimes before you do anything, and then also account for future growth. The timers involved in the absence of SPF SPFThrottling works similarly to LSA throttle timers. There are three tunable timers. SPF star. This is the initial delay to schedule an SPF calculation after a change. SPF Hold This is the minimum hold time between two consecutive SPF calculations, similar to, say, a hold timer. This timer is used as an incremental value in an exponential buckoff algorithm. SPF max Weight this is the maximum weighttime between two consecutive S PF calculations. You.

3. OSPF Scalability Issues

OSPF scaling is determined by the utilisation of three router resources: memory, CPU, and AZ bandwidth. The workload that OSPF imposes on a router depends on many factors. Number of prefixes: The number of prefixes that OSPF carries is arguably the most impressive. Stability and therefore scalability factors Connection stability will see unstable connections as flapping links. These flapping links will introduce recalculating the routing process and, therefore, instabilities in the number of adjacent neighbours for any one route. OSPF floods all link state changes to all routers in an area. Routers with many NAB have the most work to do when link state changes occur. The number of adjacent routers inan area uses a Cpuintensive algorithm. The number of calculations that must be performed given a set of state packets is proportional to n log n. As a result, the larger and more unstable the air, the greater the likelihood for performance problems that are associated with the periodic recalculation of the number of areas that are supported by any one router. A router must run the link state algorithm for each link state change that occurs in the area where the router resides. Every ABR is in at least two areas: the backbone and one adjacent area. The first and most important decision when designing an LAN is to determine which routers and links will be included in the backbone area and which routers and links will be included in each adjacent area. OSPF scalability area routers' number of adjacent neighbours has far more impact than the total number of routers in the area. The most important consideration is the amount of information that has to be distributed within the area. One network might have, for example, 200 Wan routers with one fast Ethernet's subnet in one area. Another network has fewer routers and more subnets. The number of routers in an area affects its scalability. The amount of information in the LSA increases with the size of the LSA. It is a good idea if the OSPF router LS stays under the IPMTU size. When the maximum transmission unit size is exceeded, the result is IP fragmentation. IP fragmentation is at best a less efficient way to transmit information and requires extra router processing. Router LSAs also imply that there are many interfaces and perhaps neighbors, which is an indirect indication that they may have become too large. Stability and redundancy are the most important criteria for the back. Stability is increased by keeping the size of the backbone reasonable. If link quality is high and the number of routes is small, the number of routers can be increased. Redundancy is important in the bone to prevent partition when a link fails. Good backbones are designed so that no single link failure can cause a partition. Due to several complexity factors, it is difficult to specify a minimum of routers per area. A well-developed area zero with the latest Cisco hardware could never have more than about 300 routers. This number is intended as an approximate indication that OSPF design is getting into trouble and should be reconsidered by focusing on a smaller area zero. OSPF's scalability areas per April APRs will keep a copy of the database for Yaz DEA service. If a router is connected to ten areas, for example, it will have to keep a list of ten different databases. The number of areas per ABR depends on many factors, including type of area, number of routes per area, and number of external routes per area. Whenever possible, try not to overload ABR areas over several routers. However, typical designs only require a few routers to serve as multi-card AVRs, and these routers can be upgraded to the latest hardware to support 50 and more as per ABR. Placing an ABR in tens of areas simultaneously is no longer an issue, especially if area topologies are simple. Sometimes lower performance can be tolerated. For this reason, a specific number of areas per APR cannot be recommended. Carefully monitor your ABRs and add extra ABRs to distribute the load if needed. OSPF's Hierarchies Enterprise Network comprises three layers: core, distribution, and access. OSP, however, only allows for two levels of hierarchy: zero and all other areas that are adopted to the backbone via ABRs. How can you apply two-layer OSPF to a three-layer network? Should you place the area borders in the distribution layer or in the core? It depends on the two general principles that exist: separate complexity for full mesh topologies, largescale hub-and-spoke topologies, and highly redundant topologies. With ABRs placearea borders to reducesuboptimating and to increase summarization. OSPF naturally fits when there is a backbone "area zero" and an area of the backbone with one or a few routers, interconnecting the other areas to area zero if you must. In a large network, you can use BGP to connect different OSPF routing levels. A difficult question in the OSPF design is where to put the ABRs: in the core or in the DBut layer. The general design advice is to separate complexity from complexity and to put parts of the network into separate areas. A part of the network might be considered complexhas considerable routing information, such as a full mesh,a large hub and spoke, or a highly redundanttipole, such as a redundant campuses or data center. summarises as much as possible to maintain a reliable and scalable OSPF network. ABRs provide opportunities to support route summarization. creates totally stubby areas. A structured IP addressing scheme needs to align with the areas for effective route summarization. One of the simplest ways to allocate addresses in OSPF is to assign a separate network number for each area. Toad Tuli's tubby areas cannot distinguish one airfare from another in terms of the best route to destinations outside of the area. Unless the abbreviations are graphically asymmetric, it should not matter. Stub areas cannot distinguish among ABRs for destinations that are external to the OSPF domain. Redistributed routes, unless the ABR is geographically far apart, should not matter. OSPF V two for IPV four and osvv three forIPV six are implemented as two Andy independent protocols. This independence means that theoretically, the area structure and ABRs could be entirely different for each of these protocols. However, from a design standpoint, it is often bestto align the area structure and ABS for bothprotocols to reduce operational complexity and east troubleshooting. This approach implies that the Vsix and IPV fourblocks that are assigned to the areas should also be aligned to support memorization of both protocols.

4. Define Area and Domain Summarization

The amount of bandwidth, CPU power, and memory resources that the OSPF routing process uses can be directly affected by a root summarization. Without route summarization, every specific link A is propagated into the OSPF backbone and elsewhere. This will cause unnecessary networkkick and router overhead. When utilising route summarization, only the summarised routes are propagated into the backbone. Summarization prevents every router from having to rerun the ESPF atom, increases the stability of the network, and reduces unnecessary LSA flooding. Also, if a network link fails, the topology changes are not propagated into the backbone and other areas by way of the bone-specific link LSA. Flooding outside the area does not occur. When a type III LSA is received in its area, it is appropriately added to or deleted from the router's routing table, but PDF calculation is not performed. area and domain route summarization. There are many ways to summarise routes in Oospf. The effectiveness of root summarization mechanisms depends on scheme summarization and should be supported in and out of areas at the ABR or ASBR. Some of the ways to summarise routes and otherwise reduce the LSA database size and flooding are to configure area ranges for the OSPfrfcs Configure the area summary address at NSSA Rootfiltering also configures originating default configuration summarization into and out of areas in order to reduce the reachability of information inserted into those areas. to minimise route information that is inserted into the area. Consider the following lines when planning your OSPFinter network: "Configure the network addressing schemes so that the range of subnets that are assigned within an area are contiguous." Create an address space that will split easily as the network grows. If possible, assign subnets according to a simple octet boundary. Plan ahead for the addition of new routers to the OS PF environment. Ensure that new routers are inserted appropriately as Aria backbone or border ABR and ASBR summarization). Two methods of OSPF route summarization are available internal route summarization on the ABRs external route summarization on the Aasbrs If summarization of internal routes is used, all the prefixes from an aria will be passed into the backbone as preintere roots. When summarization is enabled, the ABR intercepts this process and injects a single Type III LSA, which describes the summary route, into the backbone. OSF summarization can also be performed for external routes without summarization. Each route redistributed into OSPF from other protocols is advertised individually with an external LSA. Configuring a summary for external routes will reduce the size of the OSPF LSDB. Summarization of external routes can be done for the type five LSAs before injecting them into the OSPF domain. All redistributed external prefixes for fraternal autonomous systems are passed without summary into the OSPF failure. A summary route to null is created automatically for each summary. Range is also injected into the route table. Designated area design can be used to reduce routing information in an area. Area design requires considering your network topology and dressing when designing OSPF areas. Examine the dressing topology and look for areas of summarization. Minimize the routing information that is advertised into and out of areas and use stub areas where possible; avoid adding too much into areas because it tends to keep growing. Initially, design the network topology and address it with the division of labour in mind. Ideally, network topology and addressing should be designed with division IRS in mind from the start, whereas EIGRP will tolerate more arbitrary network topologies. OSPF requires a cleaner hierarchy with a clearer backbone and area topology. Geographic and functional trees should be considered in determining OSPF area placement. As discussed previously, the routing information that is advertised into and out of areas Keep in mind that anything in the LSA database must be propagated to all routers within the area. In particular, changes are to be propagated, consuming bandwidth and CPU for links and routers within the area. Rapingures and flapping necessitate the most effort because routers must repeatedly propagate changes. Stubby areas, totally stubby areas, and summitry routes not only reduce the size of the LSA database, but also insulate the area from external changes. According to past experience, you should keep routers to the backbone area zero. Some businesses have discovered that more than two routers end up in area zero. A recommended practise is to put only the essential backbone and AARs in two areas with a zero.

5. OSPF Full and Partial Mesh

OSPF (full and partial Mesh, full, and partial mesh topologies are often complex and typically implemented in nooks that demand high throughput and optimal routing, such as core networks. Full mesh networks expedite the growth of interconnecting links as you increase the number of routers and therefore pose a specific scaling challenge. Full-mesh topologies can have multiple routers with many links and therefore routing information. Flooding is the main concern. They mimic mesh groups. Filtering two more routers is known as flooding routers. The other routers only send LSAs to flood routers. A network of two routers requires a single interconnection. A full mesh of six routers requires 15 interconnections, and so on, increasing the number of interconnections following the formula for one router via two interconnections. Flooding routing information through a full mesh topology has the aim of ensuring that each router receives at least one copy of new information from each neighbor. In large full-mesh or partial-mesh OSPF domains, you should deploy techniques to reduce the amount of routing information flooding. ISIS provides a mechanism to counter mesh flooding called "mesh groups." This mechanism is not available in OSPF, but its technique can be mimicked by reducing the flooding in a fish network by manual date.To base-filter configuration, pick a subset of two or more routers in a network that will flood the LSAs to all other routers. Configure all other routers to filter LSA advertisements for all but the selected subset of routers. As a result, the chosen routers will behave similarly to how a doctor behaves in a shared land. As database filtering is a manual technique, it is very error prone.Be careful not to block LSAs on the wrong adjacencies. Another scalability mechanism that is appropriate for full mesh networks is flood reduction. Flood reduction eliminates the need for a periodic refresh of the same LS. Periodic refreshes provide recovery from bugs and glitches in the OSPF implementation. Flood reduction removes this benefit. OSPF Hub and Spoke Design In an OSPF hub's design, any change at one spoke site is passed up the link to the areaear hub and is then replicated at each of the spark sites. These actions can place a heavy burden on the hub. Router change flooding is a problem that is encountered in these designs. Every router within an area receives the same initial Although router B can only reach router C through router A, it still receives all data from router C. Root formation changes flood all links in the area. Keep speaking in spoke areas. If there is redistribution at the spokes, make them completely stubby; make the area run less stubby; fewer spokes in an area will generate less flooding. Redundancy and less information can be summarised with fewer spokes. A separate subinterface is needed for each spoke. Stub areas minimise the amount of information within the area. You should always configure the areas as narrowly as possible. Stubby areas are better than stub areas as a spoke site must redistribute Hutru to OSPF, making it a not-sostubby area. Keep in mind that total NSSAs are also possible. Limiting the number of spokes reduces the flooding at the hub. However, smaller areas allow for less summarization in the backbone. Each spoke requires a subinterference on the hub router. Typical hub and spoke topologies have a single or redundant hub-spoke connection, with multiple hubs serving as go-through points. Many network engineers prefer the use of distance vector routing protocols such as EIGRP in hub-and-spoke networks because distance protocols feature a natural topology hiding behind the hub. OSPF Hub and Spoke A placement hub and spoke topology is typically deployed in a situation where multiple branch offices are linked to headquarters. Connections between a hub and spokes are WAN connections, which are typically less capable than land connections and therefore a common source of routing changes that need to be propagated through the network. The backbone area is extremely important in OSPF. Typical design approaches keep areas low, small, and highly stable. To prevent Wan link flapping from affecting core stability, you will typically use the hub router as an ABR between core area zero and one or multiple spoke areas. For this design, you may need to employ a high-end hubrouter that can serve as an ABR for multiple areas. You can also extend area zero down to spokecrowds, which now act as ABRs between the hub and spoke Wan and their branch lands. With this design, you reduce the pressure on the hub route. The caveat is that all the WAN connections are now in the backbone area. Wan link flapping will produce many routing and update events that can destabilise the core. This design is viable for topology. Small cores or unreliable van links define the number of areas in OSPF's hub-and-spoke design. As the number of remote sites rises, you have to start breaking the network into multiples. The number of frauds per area depends on a couple of factors. If.

6. OSPF Convergence

In some networks the default reaction time ofthe routing protocol is not fast enough. Understanding influencers' OSPF convergence will help you improve it. Network convergence is the time needed for the network to respond to events. It is the time it takes for traffic to be rerouted to a more optimal path when a node or link fails or when a new link or node appears. Traffic is not rerouted until data structures such as the Fib and adjacency have been adjusted to reflect the new state of the network. In order for this to happen, network devices need toexperience the following steps detect the event loss or additionof a link or neighbour needs to be detected. It can be done through a combination of layer one, layer two, and layer three detection mechanisms such as carrier detection, routing protocol, hello timersand BFD propagate the event routing protocol update mechanisms that are used to forward the information about the TOPOLY change from neighbour to neighbor. Process the event. The information needs to be entered into the appropriate protocol data structures, and the routing algorithm needs to be invoked to calculate updated best parts for the new topology. Update forwarding data structures with the results of the routing algorithm calculation to be entered into the dataplane packet forwarding data structure. At this point, the network converged. The first step is very dependent on the type of failure and the combination of layer one and layer three protocols that are deployed. The second and third steps are most specific to OPF, and tuning the associated parameters can greatly improve OSPF convergence times. The fourth step is not routing protocol-specific, but it depends on the hardware platform and the mechanisms that are involved in programming the data plane. Data structures for bidirectional forwarding detection for OSPF In environments where outers running OSPF need to detect network changes rapidly, you need to rely on external protocols such as BFD to achieve subsecond convergence. BFD subsecond detection of a link failure using frequent link hills the CPU impact were compared to using the routing protocol's fast hellos." Most platforms support some BF decoding at the data plane. You need to configure OSPF to be informed of detected changes. You also need to enable BFD support in OSPF either globally or via the Opera interface. One of the significant factors in routing convergence is the detection of link or node failure. FD is a technology that uses FastLayer2 link hellos to detect failed or one-way links and enable sudden event detection. The CPU impact of BFD is less than the CPU impact of routing-plane Fast Hellos because some of the processing is shifted to the data plane rather than the control plane on distributed platforms. Cisco testing has shown a minor 2% CPU increase above baseline, supporting 100 concurrent BFD sessions. BFD is an independent protocol, and to tie it to the selected routing protocol, you can configure BFD support for OS either via the routing protocol configuration or per specific interface. Network convergence requires all affected routers to process the network event. Understanding OSPF event propagation enables you to optimise protocol behaviour and improve convergence time. The OSPFexponential back cough algorithm is depicted in the figure. It is assumed that every second that an event happens causes a new version of an LSA to be generated with the default timers. After ms, the initial will be generated. Afterward, there will be a five-second wait between SIS and LSAs. The OSPF specification requires a fixed delay when the router generates a LSA, a similar LSA with the same link state ID type and originating router, and possibly updated content. The Cisco OSPF implementation supports an exponential backward algorithm to dynamically calculate delay. Following the initial event, there is rapid Sage narration. Repeated events increase the delay exponentially, which prevents overloading the OSPF topology. Changes are advertised with a LSA flooding propagation delay that is equivalent to the sum of the LSA request delay, LSA arrival delay, and LSAP processing delay. The original OSPF specification, which required that the generation of similar LSAs with the same linkstate IDD type, origin, and router ID but possibly updated content be delayed for a fixed interval that defaulted to 1 second, To optimise this behavior, Cisco implemented an exponential backoff algorithm to dynamically calculate the delay before generating a similar LSA. The initial Bachoff timers arelow, which enables quicker converse. If successive events are generated for the same LSA, the backoff timer has increased control. The delay start interval defines the initial delay. To generate NLSE, this timer can be set at a very low value, such as one ms or even zero ms. Setting this timer to a low value will help improve convergence because initial LSAs for new events will generate it as quickly as possible. The default value is zero ms holdinterval the moon time to elapse beforeflooding an updated instance of an LSA. This value is used as an incremental value. Initially, the whole time between successive LSAs is set to be equal to this configured value. Each time that a new version of an LSA is generated, the whole time between LSAs is doubled until the maximum interval value is reached, at which point this value will be used until the network stabilizes. The default value is 50 ms. Max interval defines the maximum time that can elapse before flooding an updated instance of an LSA. Once the exponential backoff algorithm reaches this value, it will stop increasing for the whole time and use the max interval timer as a fixed interval between newly generated LSAs. The default value is 5000 ms. What are the optimal values? Dishonestly tuning the timers too aggressively could result in excessive CPU load during network convergence, especially when the network is unstable for a period. Lower the values gradually from their defaults and observe router behavior. To determine the optimal values for your network when you adjust SPF LSAT throttling timers, you may also need to adjust the LSA arrival timer. NSAs that are received at a higher frequency than the value of this timer will be discarded. To prevent Ras from dropping valid LSAs, you should make sure that the LSA arrival timer is configured to a lower or equal hold interval timer. Otherwise, a neighbour would be allowed to send an updated LSA sooner than the other would be willing to accept it. With the USPF LSA throttle timers set at 10 ms for the air start interval and 50 ms for LSA, the initial LSA is generated after ten ms. The next LSA will be generated after the LS says hold for 500 ms. The next LSA is generated after two times 500 equals 10 ms. The next LSA is generated after four times 500, or 20 ms. The next LSA is generated after eight times 500 and 40 mists. The next one would be generated up to 16 times 500, or 80 ms, but because the max interval is set at 50 ms, the LSA is generated. From this point onward, a 50 MS weight will be applied to six LSAs until the network stabilises and the timers are reset. OSPF Flood Reduction: by design, OSPF requires unchanged LSAs to be refreshed every 1,802 seconds, or they expire after 3600 seconds. Periodically refreshed LSAs can reduce unnecessary overhead. in the largest possible networks. Unchanged LSAs are, by default, purely refreshed every 30 minutes in stable environments. This introduces unnecessary overhead. OSPF flood reduction eliminates periodic refreshing of unchanged LSAs. OEF flood reduction is defined in RFC 4136. LSAs who are asked do not age. Bit set. It is useful in fully meshed topologies. OSPF flood reduction is configured per interface. OSPF flood reduction is enabled only on stable ic refreshing of Periodic refreshes provide recovery from bugs and glitches, which ensure the robustness of OSPF. The OSPF flood reduction feature works by reducing the necessary refreshing and flooding of all known and unchanged information as defined in RFC 4136. FAIS that is configured with flood reduction advertisers LSAs with the Do Not Age bit set Asalt LSAs do not need to be refreshed unless there is a network change that is detected. The hiatus is achieved in full-mesh topologies by reducing the number of regenerated LS says. You can configure OSPF flood reduction only on a per-interface basis, but make sure you enable OSPF flood reduction only in stable environments. A periodic refresh enables the OSPF mechanism to recover from bugs and glitches, which ensures the robustness of the system. OSPF Database Overload Protection The OSPF Linkstate Database Overload Protection feature allows you to limit the number of non-susgenerated LSAs and protect the OSPF process. Excessive LSAs that are generated by other routers in the OSPF domain can substantially drain the CPU and memory resources of the router. Database overload protection protects the router from receiving many LSAs. Too many LSAs are possibly a result of misconfiguration on the remote router. The router keeps the number of received LSAs. The maximum and threshold values are configurable. When other OSPF routers in the network have been misconfigured, they may generate a high volume of LSAs to redistribute large numbers of prefixes. This protectionism prevents routers from receiving many LSAs and therefore experiencing CPU and memory shortages. When the Ospflinkstate database overload protection feature is enabled, the router keeps the number of received LSAs that it has received until the configured threshold number of LSAs is reached. An error message is logged when the configured maximum number of LSAs is exceeded, and the router sends a notification if the count of received LSAs is still higher than the configured maximum. After a minute, the OSPF process takes down all adjacencies and clears the OSPF database. In this agnostic state, all OSPF packets that are received on any interface that belongs to this OSPF process are ignored, and no OSPF packets are generated on any of these interfaces. The OSP process remains in the ignore state for the time that it is configured by the ignore time keyword of the maxlsaom. Each time the OSPF process goes into an ignore state, a counter is incremented. if this counter exceeds the number of accounts that are configured by the ignore count keyword. The OSPF process remains stably in the same Agnostate, and manual intervention is required to bring the OSPF process out of the Agnostate. When the OSPFprocess remains in the normal state of operation for the amount of time specified by the reset time keyword, the Agnostate counter is reset to zero.

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