Cisco Globally Resilient IP: Overview and Applications
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Table Of Contents
Cisco Globally Resilient IP Overview and Applications
Enterprise Backbone and Service Provider Core Features
Cisco NSF Routing and Forwarding Operation
Bulk Synchronization During Initialization
SSO-Supported Protocols and Applications
IP Event Dampening Ramifications
IP Event Dampening Application
Border Gateway Protocol Convergence Optimization
BGP Convergence Optimization Benefits
BGP Convergence Optimization Ramifications
BGP Convergence Optimization Application
Enterprise Backbone and Service Provider Core Resiliency
Multicast Subsecond Convergence
Multicast Subsecond Convergence Benefits
Multicast Subsecond Convergence Application
MPLS Fast Reroute for Node Protection
MPLS Fast Reroute for Node Protection Benefits
MPLS Fast Reroute for Node Protection Application
Cisco Globally Resilient IP: Putting It All Together
Multicast Subsecond Convergence
Service Provider—IP Internet Scenario
Service Provider—MPLS Scenario
MPLS Fast Reroute for Node Protection
Cisco Globally Resilient IP Overview and Applications
Version History
Overview
Cisco Globally Resilient IP in Cisco IOS software addresses resiliency as a network-wide challenge that must be solved both at the device level and across the network as a whole, with resiliency built into multiple IP services.
Cisco Globally Resilient IP is a portfolio of technologies in Cisco IOS software that enables network-wide resilience to increase IP network availability. Users can select the best Globally Resilient IP features from this portfolio in order to achieve maximum redundancy and scalability on their networks.
Network application traffic must cross different segments, from the enterprise backbone, enterprise edge, and service provider (SP) edge through the SP core. All segments must be resilient enough to recover quickly from faults and to prevent faults from affecting user activities and network applications. A fault anywhere in the network can result in termination, interruption, or violation of Service Level Agreements (SLAs) for business-critical applications.
With Cisco Globally Resilient IP, network hardware and software work together and enable rapid recovery from disruptions to ensure fault transparency to users and network applications. These features are offered on existing Cisco IOS platforms and currently deployed hardware.
This document describes the following key Cisco Globally Resilient IP features in the initial release, and it also provides high-level examples showing how to deploy these features in different network environments:
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Cisco nonstop forwarding (NSF) with stateful switchover (SSO)
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Cisco nonstop forwarding (NSF) with stateful switchover (SSO) enables continuous packet forwarding during a Route Processor (RP) takeover and route convergence. SSO allows a backup RP to take immediate control from the active RP while maintaining WAN connectivity protocols. The MPLS fast reroute node protection and multicast subsecond convergence features work on the enterprise backbone and the SP core, and IP event dampening and BGP convergence optimization provide routing protocol convergence enhancements to the Cisco Globally Resilient IP network.
Figure 1 provides a high-level look at how Globally Resilient IP features are applied in global network environments.
Figure 1 Cisco IP Technology Solutions for Globally Resilient IP
Edge Resiliency Features
Cisco NSF with SSO enables the fastest time to recovery for IP networks from hardware or software faults in an RP in the industry. Cisco IOS provides zero IP packet loss for an instantaneous RP switchover on a Cisco 12000 series Internet router, eliminating the aggregation router of an SP as a single point of failure. This breakthrough is highly important because a loss of connectivity at the aggregation edge, which often terminates hundreds of customers and thousands of sessions, has a direct impact on customer SLAs.
Network Resiliency Features
Incremental enhancements to routing protocols improve convergence times, reduce network overhead, and increase resiliency end-to-end across the network. Globally Resilient IP has several key network resiliency enhancements, which are described in the following sections:
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IP event dampening improves convergence times and stability throughout the network by isolating faults so that disturbances are not propagated.
New BGP convergence optimization improves convergence time 40 percent for the largest BGP production environments.
Enterprise Backbone and Service Provider Core Features
The multicast subsecond convergence feature enhances the enterprise backbone, and the MPLS fast reroute node protection feature enhances the SP core.
For both enterprise and SP networks, multicast subsecond convergence provides almost instantaneous recovery of multicast paths after unicast routing recovery.
For MPLS traffic engineering environments, MPLS fast reroute now provides a node protection solution to rapidly reroute traffic in case of node (MPLS label switch router) fault, joining existing fast reroute link protection capability, to provide recovery times competitive with SONET.
Cisco Globally Resilient IP
Cisco Globally Resilient IP addresses resiliency as a network-wide challenge that must be solved both at the device level and across the network as a whole, with resiliency built into multiple IP services.
SP and enterprise customers with large IP networks benefit from increased resiliency in the backbone or core and at the network edge, which is often a single point of failure for network connectivity. Further optimization of IP routing protocols improves efficiency and stability across the network.
The rest of this document describes the features in the following sections in detail by closely examining the various network topologies in which they are deployed:
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Edge Resiliency
An edge access device represents the first hop in a customer network connection. Often, customers have a single circuit between their customer premise router and the edge access router. If an edge router fails, many customer connections become unavailable until the access router recovers. By contrast, core networks include routers configured in dual, mesh configurations, so traffic can take an alternate route if one transmission path is disrupted.
Traditionally, core routers protect against network faults using router redundancy and mesh connections that allow traffic to bypass failed network elements. SSO provides protection for network edge devices with redundant processors that represent a single point of failure in the network design, and where an outage might result in loss of service for customers.
SSO is used in conjunction with Cisco NSF. Cisco NSF allows for the forwarding of data packets to continue along known routes while the routing protocol information is being restored during a switchover. The Cisco NSF and SSO features are described in the following sections:
Cisco Nonstop Forwarding
A component of Cisco Globally Resilient IP is Cisco NSF, which is a complementary feature to the SSO feature in Cisco IOS software. Cisco NSF works with SSO to minimize the amount of time a network is unavailable to its users following a switchover. The main objective of Cisco NSF is to ensure that IP packets continue to be forwarded following an RP switchover.
All routing peers of a device that restarts detect that the device went down and then came back up. This transition results in what is called a routing flap, which could spread across multiple routing domains. Routing flaps caused by routing restarts create routing instabilities, which are detrimental to the overall network performance. Cisco NSF with SSO reduces and, on the Cisco 12000 series Internet router, eliminates packet loss caused by either hardware or software faults on an RP.
Cisco NSF allows for the forwarding of data packets to continue along known routes while the routing protocol information is being restored following a switchover. With Cisco NSF, peer networking devices do not experience routing flaps. Data traffic is forwarded through intelligent line cards or dual forwarding processors (FPs) while the standby RP takes control from a compromised active RP through an RP switchover. The ability of line cards and FPs to remain up through a switchover is key to Cisco NSF operation.
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Cisco NSF Routing and Forwarding Operation
Cisco NSF supports BGP, Open Shortest Path First (OSPF), and Intermediate System-to-Intermediate System (IS-IS) protocols for routing and Cisco Express Forwarding (CEF) for forwarding. Routers running these protocols can detect a switchover and take the necessary actions to continue forwarding network traffic and to recover route information from the peer devices. The IS-IS protocol is also configurable to use state information that has been synchronized between the active and the standby RP to recover route information following a switchover instead of information received from peer devices. In this document, a networking device is said to be Cisco-NSF-aware if it is running Cisco-NSF-compatible software; a device is said to be Cisco-NSF-capable if it has been configured to support Cisco NSF.
Each protocol depends on CEF to continue forwarding packets during switchover while the routing protocols rebuild the routing information base (RIB) tables. Once the routing protocols have converged, CEF updates the forwarding information base (FIB) table and removes stale route entries. CEF, in turn, updates the line cards with the new FIB information.
Cisco Express Forwarding
A key element of Cisco NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided by CEF. CEF maintains the FIB, and uses the FIB information that was current at the time of the switchover to continue forwarding packets during a switchover. This feature reduces any traffic interruption during the switchover.
During normal Cisco NSF operation, CEF on the active RP synchronizes its current FIB and adjacency databases with the FIB and adjacency databases on the standby RP. Upon switchover of the active RP, the standby RP initially has FIB and adjacency databases that are mirror images of those that were current on the active RP. For platforms with intelligent line cards, the line cards will maintain the current forwarding information over a switchover; for platforms with forwarding engines, CEF will keep the forwarding engine on the standby RP current with changes that are sent to it by CEF on the active RP. In this way, the line cards or forwarding engines will be able to continue forwarding after a switchover as soon as the interfaces and a data path are available.
As the routing protocols start to repopulate the RIB on a prefix-by-prefix basis, the updates in turn cause prefix-by-prefix updates to CEF, which CEF then uses to update the FIB and adjacency databases. Existing and new entries will receive the new version ("epoch") number, indicating that they have been refreshed. The forwarding information is updated on the line cards or forwarding engine during convergence. The RIB signals when the routing protocols have converged, and CEF removes all FIB and adjacency entries that have an epoch older than the current switchover epoch. The FIB now represents the newest routing protocol forwarding information.
Routing Protocols
The routing protocols run only on the active RP, and they receive routing updates from their neighbor routers. Routing protocols do not run on the standby RP. Following a switchover, the routing protocols request that the Cisco-NSF-aware neighbor devices send state information to help rebuild the routing tables. Alternately, the IS-IS protocols can be configured to synchronize state information from the active to the standby RP to help rebuild the routing table on the Cisco-NSF-capable device in environments where neighbor devices are not Cisco-NSF aware.
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BGP Operation
When a Cisco-NSF-capable router begins a BGP session with a BGP peer, it sends an OPEN message to the peer. Included in the message is a declaration that the Cisco-NSF-capable device has "graceful restart" capability. Graceful restart is the mechanism by which BGP routing peers avoid a session routing flap following a switchover. If the BGP peer has received this capability, it is aware that the device sending the message is Cisco-NSF-capable.
If the BGP session is lost during the RP switchover, the Cisco-NSF-aware BGP peer marks all the routes associated with the Cisco-NSF-capable router as stale; however, it continues to use these routes to make forwarding decisions for a set period of time. This functionality means that no packets are lost while the newly active RP is waiting for convergence of the routing information with the BGP peers.
After an RP switchover occurs, the Cisco-NSF-capable router reestablishes the session with the BGP peer. In establishing the new session, it sends a new graceful restart message that identifies the Cisco-NSF-capable router as having restarted, as opposed to establishing a new BGP session.
At this point, the routing information is exchanged between the two BGP peers. Once this exchange is complete, the Cisco-NSF-capable device uses the network information to update the RIB and the FIB with the new forwarding information. The Cisco-NSF-aware device uses the network information to remove stale routes from its BGP table. The BGP protocol is then fully converged.
If a BGP peer does not support the graceful restart capability, it will ignore the graceful restart capability in an OPEN message but will establish a BGP session with the Cisco-NSF-capable device. This capability will allow interoperability with non-Cisco-NSF-aware BGP peers (and without Cisco NSF functionality).
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The following example shows NSF for BGP in the running configuration:
OSPF Operation
When an OSPF NSF-capable router performs an RP switchover, it must perform two tasks in order to resynchronize its link-state database with its OSPF neighbors. First, it must relearn the available OSPF neighbors on the network without causing a reset of the neighbor relationship. Second, it must reacquire the contents of the link-state database for the network.
As quickly as possible after an RP switchover, the Cisco-NSF-capable router sends an OSPF NSF signal to neighboring Cisco-NSF-aware devices. Neighbor networking devices recognize this signal as a cue that the neighbor relationship with this router should not be reset. As the Cisco-NSF-capable router receives signals from other routers on the network, it can begin to rebuild its neighbor list.
Once neighbor relationships are reestablished, the Cisco-NSF-capable router begins to resynchronize its database with all of its Cisco-NSF-aware neighbors. At this point, the network information is exchanged between the OSPF neighbors. Once this exchange is complete, the Cisco-NSF-capable device uses the adjacency information to remove stale routes, update the RIB, and update the FIB with the new forwarding information. The OSPF protocols are then fully converged.
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The following example shows NSF for OSPF in the running configuration:
This configuration enables the Cisco NSF feature for OSPF.
IS-IS Operation
When an IS-IS NSF-capable router performs an RP switchover, it must perform two tasks in order to resynchronize its link-state database with its IS-IS neighbors. First, it must relearn the available IS-IS neighbors on the network without causing a reset of the neighbor relationship. Second, it must reacquire the contents of the link-state database for the network.
The IS-IS NSF feature offers two options when configuring Cisco NSF:
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If neighbor routers on a network segment are Cisco-NSF-aware, meaning that neighbor routers are running a software version that supports the IETF Internet draft for router restartability, you can configure IS-IS NSF for Internet Engineering Task Force (IETF). With IETF, neighbor routers provide adjacency and link state information to help rebuild the routing information following a switchover. A benefit of IETF IS-IS configuration is standards-based operation between peer devices.
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If the neighbor routers on a network segment are not Cisco-NSF aware, you must use the Cisco configuration option. The Cisco IS-IS configuration transfers both protocol adjacency and link-state information from the active to the standby RP. A benefit of Cisco configuration is that it does not rely on Cisco-NSF-aware neighbors.
IETF IS-IS Configuration
Using the IETF IS-IS configuration, as quickly as possible after an RP switchover, the Cisco-NSF-capable router sends IS-IS NSF restart requests to neighboring Cisco-NSF-aware devices. Neighbor networking devices recognize this restart request as a cue that the neighbor relationship with this router should not be reset, but that they should initiate database resynchronization with the restarting router. As the restarting router receives restart request responses from routers on the network, it can begin to rebuild its neighbor list.
Once this exchange is complete, the Cisco-NSF-capable device uses the link-state information to remove stale routes, update the RIB, and update the FIB with the new forwarding information. IS-IS is then fully converged.
The switchover from one RP to the other happens within seconds. IS-IS reestablishes its routing table and resynchronizes with the network within a few additional seconds. At this point, IS-IS waits for a specified interval before it will attempt a second Cisco NSF restart. During this time, the new standby RP will boot up and synchronize its configuration with the active RP. The IS-IS NSF operation waits for a specified interval time to ensure that connections are stable before attempting another restart of IS-IS NSF. This function prevents IS-IS from attempting back-to-back Cisco NSF restarts with stale information.
Cisco IS-IS Configuration
Using the Cisco configuration option, full adjacency and link-state packet (LSP) information is saved, or "checkpointed," to the standby RP. Following a switchover, the newly active RP maintains its adjacencies using the checkpointed data, and can quickly rebuild its routing tables.
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Cisco NSF Benefits
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Cisco NSF Ramifications
To run OSPF, BGP NSF, or IETF IS-IS, all neighboring network devices must be running a Cisco-NSF-aware software image. If you have a mixed environment, you may need to switch to a homogeneous environment to gain the maximum benefit from these protocols.
Cisco NSF Application
Using Cisco NSF with SSO is particularly useful at the network edge. The routers on the network edge are responsible for many WAN connections and often represent a single point of failure. Keeping these connections alive is necessary to protect against hardware or software RP fault.
Figure 2 illustrates how SSO is typically deployed in SP networks. In this example, SSO and Cisco NSF are primarily at the access layer (edge) of the SP network. A fault at this point could result in loss of service for enterprise customers requiring access to the SP network.
Notice that Figure 2 shows that neighbor routers must be NSF-aware in order for NSF to work. If the neighbor routers are not NSF-aware, the connection is broken.
Figure 2 Cisco NSF with SSO Network Deployment—Service Provider Network
For Cisco NSF protocols that require neighboring devices to participate in Cisco NSF, Cisco-NSF-aware software images must be installed on those neighboring distribution layer devices. Additional network availability benefits might be achieved by applying Cisco NSF with SSO features at the core layer of your network; consult your network design engineers to evaluate your specific site requirements.
Additional levels of availability may be gained deploying SSO and Cisco NSF at other points in the network where a single point of failure exists. Figure 3 illustrates an optional deployment strategy that applies SSO and Cisco NSF at the enterprise network access layer. In this example, each access point in the enterprise network represents another single point of failure in the network design. In the event of a switchover or a planned software upgrade, enterprise customer sessions would continue uninterrupted through the network.
Figure 3 Cisco NSF with SSO Network Deployment Option—Enterprise Network
Stateful Switchover
The SSO feature in Cisco Globally Resilient IP works with Cisco NSF in Cisco IOS software on Cisco networking device platforms. The main objective of SSO is to improve the availability of networks constructed with Cisco IOS routers.
Using Cisco NSF with SSO is particularly useful at the network edge. Traditionally, core routers protect against network faults using router redundancy and mesh connections that allow traffic to bypass failed network elements. SSO provides protection for network edge devices with RPs that represent a single point of failure in the network design, and where an outage might result in loss of service for customers.
In specific Cisco networking devices that support dual RPs, SSO takes advantage of RP redundancy to increase network availability. The SSO feature takes advantage of RP redundancy by establishing one of the RPs as the active processor while the other RP is designated as the standby processor, and then synchronizing critical state information between them. Following an initial synchronization between the two processors, SSO dynamically maintains RP state information between them.
A switchover from the active to the standby processor occurs when the active RP fails, is removed from the networking device, or is manually taken down for maintenance.
Figure 2 illustrates how SSO is typically deployed in SP networks. In this example, SSO and Cisco NSF are primarily at the access layer (edge) of the SP network. A fault at this point could result in loss of service for enterprise customers requiring access to the SP network.
For Cisco NSF protocols that require neighboring devices to participate in Cisco NSF, Cisco-NSF-aware software images must be installed on those neighboring distribution layer devices. Additional network availability benefits might be achieved by applying Cisco NSF with SSO features at the core layer of your network; consult your network design engineers to evaluate your specific site requirements.
Additional levels of availability may be gained deploying SSO and Cisco NSF at other points in the network where a single point of failure exists. Figure 3 illustrates an optional deployment strategy that applies SSO and Cisco NSF at the enterprise network access layer. In this example, each access point in the enterprise network represents another single point of failure in the network design. In the event or a switchover or a planned software upgrade, enterprise customer sessions would continue uninterrupted through the network.
Synchronization
In networking devices running SSO, the configuration is synchronized so that the standby RP is always ready to assume control if the active RP fails.
To achieve the benefits of SSO, synchronize the configuration information from the active RP to the standby RP at startup and whenever changes to the active RP configuration occur. This synchronization occurs in two separate phases:
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Bulk Synchronization During Initialization
When a system with SSO is initialized, the active RP performs a chassis discovery (discovery of the number and type of line cards and fabric cards, if available, in the system) and parses the startup configuration file. The active RP then synchronizes this data to the standby RP and instructs the standby RP to complete its initialization. This method ensures that both RPs contain the same configuration information.
Incremental Synchronization
After both RPs are fully initialized, any further changes to the running configuration or active RP states are synchronized to the standby RP as they occur. Active RP states are updated as a result of processing protocol information, external events (such as the interface becoming up or down), or user configuration commands (using command-line interface [CLI] commands or Simple Network Management Protocol [SNMP]) or other internal events.
Switchover Operation
During switchover, system control and routing protocol execution are transferred from the active to the standby RP. Switchover may be due to a manual operation (CLI-invoked) or to a software- or hardware-initiated operation (hardware or software fault induced).
SSO-Supported Protocols and Applications
SSO-supported line protocols and applications must be SSO-aware. A feature or protocol is SSO-aware if it maintains, either partially or completely, undisturbed operation through an RP switchover. State information for high-availability-aware protocols and applications (such as PPP, Frame Relay, ATM, and SNMP) is synchronized from active to standby to achieve stateful switchover for those protocols and applications.
The dynamically created state of SSO-unaware protocols and applications is lost on switchover and must be reinitialized and restarted on switchover.
SSO-aware applications are either platform-independent, such as in the case of line protocols (Frame Relay, ATM, and PPP) or platform-dependent (such as line card drivers). Enhancements to the routing protocols (CEF, OSPF, and BGP) have been made in the SSO feature to prevent loss of peer adjacency through a switchover. These enhancements are platform-independent.
Line Protocols
SSO-aware line protocols synchronize session state information between the active and standby RPs to keep session information current for a particular interface. In the event of a switchover, session information need not be renegotiated with the peer. During a switchover, SSO-aware protocols also check the line card state to learn if it matches the session state information. SSO-aware protocols use the line card interface to exchange messages with network peers in an effort to maintain network connectivity.
SSO is supported in ATM, Frame Relay, PPP, and high-level data link control (HDLC).
Line Card Drivers
Platform-specific line card device drivers are bundled with the Cisco IOS software image for SSO and are correct for a specific image, meaning they are designed to be SSO-aware.
Line cards used with the SSO feature periodically generate status events that are forwarded to the active RP. Information includes the line up or down status, and the alarm status. This information helps SSO support bulk synchronization after standby RP initialization and support state reconciliation and verification after a switchover.
Line cards used with the SSO feature have the following requirements:
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The standby RP communicates only with the active RP, never with the line cards. This function helps to ensure that the active and standby RP always have the same information.
SSO Benefits
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SSO Ramifications
SSO need not be configured on neighboring devices. However, Cisco NSF requires that all neighboring network devices be running a Cisco-NSF-aware software image. If you have a mixed environment, you may need to switch to a homogeneous environment to run any of these protocols.
SSO Application
The use of the Cisco Globally Resilient IP SSO feature is critical for routers on the network edge. The routers on the network edge are responsible for many WAN connections and are often a single point of failure. Keeping these connections alive is necessary to protect against hardware or software RP fault and to support Cisco NSF.
On the Cisco 12000 series Internet router, Cisco NSF provides zero packet loss for the edge and the core. It provides protection from hardware or software faults and prevents route flaps from occurring between participating neighbor routers. This feature allows peering relationships to be reestablished, and it allows transparent route convergence to occur.
Figure 4 and Figure 5 illustrate how Cisco NSF with SSO work together to provide resilient routing. SSO enables zero interruption to Layer 2 sessions and packet forwarding. The active RP synchronizes information with the standby RP. The standby RP immediately takes control when the active RP is compromised in some way, at which point the standby RP becomes the active RP.
Figure 4 Resilient Routing: Stateful Switchover Before a Switchover Occurs
Figure 5 Resilient Routing: Stateful Switchover After a Switchover—Standby Is Now Active
The following example shows SSO in the running configuration:
Network-Wide Resiliency
Resilient routing allows routing protocols to recover quickly from a network fault such as a failed router or circuit. The goal is to recover instantaneously from faults without network disruption. Service providers cannot afford to have real-time applications and business services wait for the network to fix itself using existing methods.
Figure 6 shows Globally Resilient IP features that enhance fast network convergence.
Figure 6 Fast Network Convergence
The Cisco Globally Resilient IP features described in the following sections enhance network recovery, allowing instant recovery on network systems that use Cisco Globally Resilient IP:
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IP Event Dampening
Currently, a router with an unstable data link ("link flap") may remove itself and return to service several times in a matter of seconds, requiring all other routers to rebuild their routing tables with each event. IP event dampening enables a router experiencing link flap to selectively remove itself from network routing tables until a return to data-link stability is ensured. This function ensures that only stable circuits and connections remain active. In order for IP event dampening to work, an alternate path must be provided in case of a fault.
A Cisco router with IP event dampening takes the affected interface out of operation until it resumes normal behavior. The Cisco router first waits a predefined period of time to see if the data link remains stable before removing itself from service. After the router removes itself from service, it waits another predefined period of time after the data link is restored to ensure that the link is stable before returning to service. Administrators may set the thresholds at which the router removes itself from and returns itself to service.
Figure 7 provides a comparison of packet loss information on routers that have the IP event dampening feature and those routers that do not.
Figure 7 IP Event Dampening
IP Event Dampening Benefits
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IP Event Dampening Ramifications
The following protocols will work with IP event dampening:
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IP Event Dampening Application
The IP event dampening Cisco Globally Resilient IP feature is necessary because service providers cannot afford to have real-time applications and business services wait long periods of time for the network to fix itself after a fault.
The following example shows IP event dampening in the running configuration:
Border Gateway Protocol Convergence Optimization
BGP is the routing protocol used to communicate among all the global Internet networks. Because of its importance in the Internet, BGP is a major focus for scaling and convergence work.
As an Internet routing table grows, SPs and large enterprise customers are noticing a dramatic increase in the length of time BGP takes to converge. Networks that once converged in 10 or 15 minutes may now take up to 1 hour and even longer in extreme situations.
Several BGP enhancements have been made to improve convergence and basic scaling properties. These enhancements include using peer groups, configuring TCP path Maximum Transmission Unit (MTU) discovery, and creating larger input queues.
BGP Convergence Optimization Benefits
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BGP Convergence Optimization Ramifications
A new algorithm to improve BGP convergence needs approximately 15 MB of available memory for performing caching and formatting of BGP update messages. These memory requirements should not be a problem for most high-end routers. If a router does not have enough memory, the router will use the old algorithm to send BGP update messages.
BGP Convergence Optimization Application
This Cisco Globally Resilient IP feature is necessary because service providers cannot afford to have real-time applications and business services wait long periods of time for the network to fix itself after a fault.
This feature does not require configuration; it is included with the Cisco IOS 12.0(22)S software release. However, TCP MTU discovery needs to be enabled. The following example enables TCP path MTU discovery for the router:
Figure 8 shows how this feature works.
Figure 8 BGP Boot Convergence Time
Enterprise Backbone and Service Provider Core Resiliency
SP and enterprise customers with large IP networks benefit from increased resiliency in the backbone and core and at the network edge. Further optimization of IP routing protocols through Globally Resilient IP features improves efficiency and stability across the network.
In Cisco Globally Resilient IP, the multicast subsecond convergence feature enhances the enterprise backbone, and the MPLS fast reroute node protection feature enhances the SP core.
Multicast Subsecond Convergence
The Multicast Subsecond Convergence feature in Cisco IOS Release 12.0(22)S comprises a comprehensive set of features and protocol enhancements that provide for improved scalability and convergence in multicast-based services. This feature set provides for the ability to scale to larger services levels and to recover multicast forwarding after service failure in subsecond time frames.
Multicast subsecond convergence allows you to send Protocol Independent Multicast (PIM) router-query messages (PIM hellos) every few milliseconds. In previous releases, you could send the PIM hellos every few seconds. By enabling a router to send PIM hello messages more often, this feature allows the router to discover unresponsive neighbors more quickly. As a result, the router can implement failover or recovery procedures more efficiently.
The Multicast Subsecond Convergence feature set enhances both enterprise and service provider network backbones by providing almost instantaneous recovery of multicast paths after unicast routing recovery. Forwarding performance is unaffected by this new feature and is comparable to previous releases of Cisco IOS software.
Because PIM relies on the unicast routing table to calculate its Reverse Path Forwarding (RPF) when a change in the network topology occurs, unicast protocols first need to calculate options for the best paths for traffic, and then multicast can determine the best path.
Multicast subsecond convergence allows multicast protocol calculations to finish almost immediately after the unicast calculations are completed. As a result, multicast traffic forwarding is restored substantially faster after a topology change.
The scalability enhancements improve on the efficiency of handling increases (or decreases) in service users (receivers) and service load (sources or content). Scalability enhancements in this release include the following:
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The scalability enhancements provide the following benefits:
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Multicast subsecond convergence provides the ability to trigger a check of RPF changes for mroute states. This check is triggered by unicast routing changes. By performing a triggered RPF check, users can set the periodic RPF check to a relatively high value (for example, 10 seconds) and still fail over quickly.
The triggered RPF check enhancement reduces the time needed for service to be restored after disruption, such as for single service events (for example, in a situation with one source and one receiver) or as the service scales along any parameter (for example, many sources, many receivers, and many interfaces). This enhancement decreases in time-to-converge PIM (mroute), IGMP, and MSDP (SA cache) states.
Multicast Subsecond Convergence Benefits
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Multicast Subsecond Convergence Application
The multicast subsecond convergence feature is used in financial, entertainment, and interactive gaming applications.
MPLS Fast Reroute for Node Protection
The MPLS fast reroute for node protection feature in Cisco Globally Resilient IP delivers intelligent path protection to the network core for MPLS-based Virtual Private Networks (VPNs). The feature allows users to configure a backup tunnel to the next next hop, which allows protection against link and node failure. MPLS fast reroute ensures that MPLS traffic reaches its destination despite link or node fault in the network core. When a router in the label-switching path is aware of data link or node problems at the next-hop destination, the router (known as the point of local repair [PLR]) stacks additional labels onto MPLS packets to reroute the traffic around the routing fault.
If the problem is in the data link, the PLR provides line protection by redirecting the traffic to the original next-hop destination through a different path. If there is a fault at the next hop, the PLR provides node protection by bypassing that hop altogether and routing the traffic to the next hop in the original path. In either case, when the MPLS traffic returns to the node on the original path, the fast reroute label is discarded and the original MPLS label is restored.
In addition to rerouting the traffic, the PLR router informs the head-end router of problems in the path, so that future traffic originating at the head-end router will use a different tunnel. The PLR also informs the headend router when an offending link or node is repaired, so the original MPLS tunnel can be assigned again to new traffic.
MPLS Fast Reroute for Node Protection Benefits
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MPLS Fast Reroute for Node Protection Application
Figure 9 shows how the MPLS fast reroute for node protection feature works.
Figure 9 MPLS Fast Reroute for Node Protection
The following is an example of MPLS fast reroute configuration:
Cisco Globally Resilient IP: Putting It All Together
This section provides examples of scenarios in which Cisco Globally Resilient IP features can be deployed. Users can select the Cisco Globally Resilient IP features that will work best in their network.
Figure 10 provides an overall view of example SP and enterprise scenarios in which customers may use Globally Resilient IP features. In this illustration, Service Provider A has both an IP/Internet core and an MPLS core and provides service to various enterprise networks.
Figure 10 Enterprise and Service Provider—Overall View
Enterprise Scenario
The enterprise deployment scenario describes customers that require the highest level of reliability and redundancy in the data network for applications such as real-time market data feeds. Even the shortest outage could cost millions of dollars. Their network must be designed for subsecond convergence and minimal latency. The applications they use typically have IP multicast requirements.
The lead customers in this market segment range from global finance companies and institutions such as NASDAQ and Merrill Lynch to global energy and commodity trading companies such as Shell. Figure 11 shows an enterprise deployment scenario.
Figure 11 Enterprise Scenario
The enterprise scenario shown in Figure 11 uses the Globally Resilient IP features Cisco NSF with SSO, IP event dampening, and multicast subsecond convergence. Figure 12 shows the enterprise scenario with these features, and the following subsections describe these features as they are used in this scenario.
Figure 12 Enterprise Scenario with Globally Resilient IP Features
Cisco NSF with SSO
Cisco NSF with SSO is configured on the edge routers that provide a Frame Relay WAN data traffic infrastructure between the enterprise core and Cisco NSF with SSO-aware routers. These edge routers, running on the Cisco IOS 7500 and Cisco 7200 platforms, are located both in the national and the international WANs. Cisco NSF with SSO-aware routers are located one layer away from the Cisco NSF with SSO-capable routers at the edge. In the release of the Globally Resilient IP features, the Cisco NSF with SSO-aware routers must all be running Cisco IOS Release 12.0(22)S software. Enabling Cisco NSF with SSO on edge routers is important for providing redundancy throughout the enterprise network.
IP Event Dampening
IP event dampening is provided on the edge routers running Cisco NSF with SSO, through the Frame Relay connection, and on the Cisco NSF with SSO-aware routers. This feature is provided throughout the WAN data traffic infrastructure because these links are more likely to fail than are the links within the core. All WAN links to the Cisco NSF with SSO aware routers are dual-connected for redundancy.
Multicast Subsecond Convergence
Multicast subsecond convergence is used throughout the entire core and distribution layer in order to disseminate information to all clients. For example, an enterprise network such as a large bank in which clients want to access the latest information on interest rates. Multicast subsecond convergence enabled on the enterprise core allows all clients to receive the latest information as soon as it is available.
Service Provider—IP Internet Scenario
The SP IP Internet deployment scenario describes Tier 1 ISPs that offer a variety of services such as multiple Quality of Service (QoS) classes based on applications and customers and highly available and scalable VPN solutions. They have specific requirements for QoS and IP multicast.
Customers deploying these kinds of services include Sprint, France Telecom, AT&T, UUNet, and Telecom Italia.
Figure 13 shows a possible SP IP Internet deployment scenario.
Figure 13 SP IP Internet Scenario
The SP IP Internet scenario shown in Figure 13 uses the Globally Resilient IP features Cisco NSF with SSO, IP event dampening, and BGP convergence optimization. Figure 14 shows the SP IP Internet scenario with these features, and the following subsections describe these features as they are used in this scenario.
Figure 14 SP IP Internet Scenario with Globally Resilient IP Features
Cisco NSF with SSO
Cisco NSF with SSO is configured on the edge routers that provide connections to various enterprise networks. These edge routers can be running the Cisco 7500, Cisco 10000, or Cisco 12000 platform. The connections are made using protocols including Frame Relay, ATM, HDLC, and PPP, and the enterprises all use Cisco NSF with SSO-aware routers. In the release of the Globally Resilient IP features, the Cisco NSF with SSO-aware routers must all be running Cisco IOS Release 12.0(22)S software. Enabling Cisco NSF with SSO on edge routers is important for providing redundancy throughout the SP IP Internet network.
IP Event Dampening
The IP event dampening feature is enabled on the links throughout the core and on the Packet over SONET (POS) links from the core to the distribution layer. This feature is provided throughout the WAN data traffic infrastructure.
BGP Convergence Optimization
BGP convergence optimization is enabled on the two BGP route reflector (RR) routers connected to the core using a single connection. One BGP RR is running on the Cisco 7500 platform and the other is running on the Cisco 7200 platform. Each of these two routers has many BGP peers in the autonomous system. The BGP convergence optimization feature allows BGP information to be provided to all of the peers of these routers in an efficient manner.
Service Provider—MPLS Scenario
The SP MPLS scenario describes Tier 1 ISPs offering advanced services that rely on MPLS technology. MPLS VPNs are the most widely deployed MPLS technology, and several customers use traffic engineering (TE) to optimize network utilization. Integration of QoS features with MPLS VPNs and MPLS TE allows for service offerings such as virtual leased line and toll bypass for voice. These customers have specific requirements for advanced MPLS features and QoS.
Customers that deploy the SP MPLS scenario include AT&T, Bell Canada, Deutsche Telecom, Global One, NTT, and SITA.
Figure 15 shows an SP MPLS deployment scenario.
Figure 15 SP MPLS Scenario
The SP MPLS scenario shown in Figure 15 uses BGP convergence optimization and MPLS fast reroute for node protection. Figure 16 shows the SP MPLS scenario with these features, and the following subsections describe these features as they are used in this scenario.
Figure 16 SP MPLS Scenario with Globally Resilient IP Features
BGP Convergence Optimization
In this scenario, BGP convergence optimization is provided on the edge routers, which may be running on the Cisco 7500, Cisco 10000, or Cisco 12000 platform. It is also used on the two BGP RR routers connected to the core, one running on the Cisco 7500 platform and the other running on the Cisco 7200 platform. Each of these two routers has many BGP peers in the autonomous system. The BGP convergence optimization feature allows BGP information to be provided to all of the peers of these routers in an efficient manner.
MPLS Fast Reroute for Node Protection
The MPLS fast reroute for node protection feature is implemented on every router in the SP MPLS core (except for the two BGP RR routers, which only perform route redistribution and are not in the MPLS forwarding path). This feature defines an alternate path in case of a single point of failure (SPOF). The feature sets up a tunnel that bypasses the SPOF and goes on to the next hop.
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