Catalyst 4500 Series Switch Software Configuration Guide, IOS XE 3.6.xE and IOS 15.2(2)Ex

About Cisco IOS NSF-Aware and NSF-Capable Support

Cisco IOS Nonstop Forwarding (NSF) has two primary components:

• NSF-awareness—If neighboring router devices detect that an NSF router can still forward packets when a supervisor engine switchover happens, this capability is referred to as NSF-awareness. Cisco IOS enhancements to the Layer 3 routing protocols (OSPF, BGP, EIGRP and IS-IS) are designed to prevent route-flapping so that the CEF routing table does not time out or the NSF router does not drop routes. An NSF-aware router helps to send routing protocol information to the neighboring NSF router.

Note OSPF Version 2 fast hellos generate false alarm. We recommend that you use Bidirectional Forwarding Detection (BFD) instead.

• NSF-capability—NSF works with SSO to minimize the amount of time that a Layer 3 network is unavailable following a supervisor engine switchover by continuing to forward IP packets. Reconvergence of Layer 3 routing protocols (BGP, EIGRP, OSPF v2, and IS-IS) is transparent to the user and happens automatically in the background. The routing protocols recover routing information from neighbor devices and rebuild the Cisco Express Forwarding (CEF) table.

Note NSF does not support IPv6.

Note NSF- capable devices include Catalyst 4500 series switches, Catalyst 6500 series switches, Cisco 7500 series routers, Cisco 10000 series routers, and Cisco 12000 series routers.

A typical topology for NSF and NSF-aware routers is given below.

Figure 12-1 Topology for NSF and NSF-Capable Switches

Table 12-1 lists the supervisor engines and Catalyst 4500 series switches that support NSF-awareness:

Starting with Cisco IOS Release 12.2(20)EWA, the Catalyst 4500 series switch supported NSF-awareness for the EIGRP, IS-IS, OSPF, and BGP protocols. Starting with Cisco IOS Release 12.2(31)SG, the Catalyst 4500 series switch supported NSF-awareness for the EIGRP-stub in IP Base image for all supervisor engines. NSF-awareness is turned on by default for EIGRP-stub, EIGRP, IS-IS, and OSPF protocols. You need to turn BGP on manually.

If the supervisor engine is configured for BGP (with the graceful-restart command), EIGRP, OSPF, or IS-IS routing protocols, routing updates are automatically sent during the supervisor engine switchover of a neighboring NSF-capable switch (typically a Catalyst 6500 series switch).

Starting with Cisco IOS Release 12.2(31)SG, the Catalyst 4500 series switch supports NSF-capability. Table 12-2 lists the supervisor engines and the associated Catalyst 4500 series switches that are NSF-capable.

NSF with SSO Supervisor Engine Redundancy Overview

Catalyst 4500 series switches support fault resistance by allowing a redundant supervisor engine to take over if the primary supervisor engine fails. NSF works with SSO to minimize the amount of time a network is unavailable to its users following a switchover.

NSF provides these benefits:

• Improved network availability

NSF continues forwarding network traffic and application state information so that user session information is maintained after a switchover.

• Overall network stability

Network stability may be improved with the reduction in the number of route flaps, which were created when routers in the network failed and lost their routing tables.

• Neighboring routers do not detect a link flap

Because the interfaces remain up during a switchover, neighboring routers do not detect a link flap (the link does not go down and come back up).

• Prevents routing flaps

Because SSO continues forwarding network traffic during a switchover, routing flaps are avoided.

• Maintains user sessions established prior to the switchover

Catalyst 4500 series switches also support route processor redundancy (RPR). For information about these redundancy modes, see Chapter10, “Configuring Supervisor Engine Redundancy Using RPR and SSO on Supervisor Engine 6-E and Supervisor Engine 6L-E”

SSO Operation

SSO establishes one of the supervisor engines as active while the other supervisor engine is designated as standby, and then SSO synchronizes information between them. A switchover from the active to the redundant supervisor engine occurs when the active supervisor engine fails, or is removed from the switch, or is manually shut down for maintenance.

In networking devices running SSO, both supervisor engines must be running the same Cisco IOS software version and ROMMON version so that the redundant supervisor engine is always ready to assume control following a fault on the active supervisor engine. SSO switchover also preserves FIB and adjacency entries and can forward Layer 3 traffic after a switchover. Configuration information and data structures are synchronized from the active to the redundant supervisor engine at startup and whenever changes to the active supervisor engine configuration occur. Following an initial synchronization between the two supervisor engines, SSO maintains state information between them, including forwarding information.

During switchover, system control and routing protocol execution is transferred from the active supervisor engine to the redundant supervisor engine.

Note Use the [no] service slave-log configuration command to forward all error messages from the standby supervisor engine to the active engine. By default, this capability is enabled. For details, refer to the Catalyst 4500 Series Switch Cisco IOS System Error Message Guide, Release 12.2(37)SG.

NSF Operation

NSF always runs with SSO and provides redundancy for Layer 3 traffic. NSF is supported by the BGP, OSPF, IS-IS, and EIGRP routing protocols and is supported by Cisco Express Forwarding (CEF) for forwarding. The routing protocols have been enhanced with NSF-capability and awareness, which means that 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 can be configured to use state information that has been synchronized between the active and the redundant supervisor engine to recover route information following a switchover instead of information received from peer devices.

A networking device is NSF-aware if it is running NSF-compatible software. A device is NSF-capable if it has been configured to support NSF; it rebuilds routing information from NSF-aware or NSF-capable neighbors.

Each protocol depends on CEF to continue forwarding packets during switchover while the routing protocols rebuild the Routing Information Base (RIB) tables. After the routing protocols have converged, CEF updates the FIB table and removes stale route entries. CEF then updates the line cards with the new FIB information.

Cisco Express Forwarding

A key element of NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided by Cisco Express Forwarding (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 traffic interruption during the switchover.

During normal NSF operation, CEF on the active supervisor engine synchronizes its current FIB and adjacency databases with the FIB and adjacency databases on the redundant supervisor engine. Upon switchover of the active supervisor engine, the redundant supervisor engine initially has FIB and adjacency databases that are mirror images of those that were current on the active supervisor engine. For platforms with forwarding engines, CEF keeps the forwarding engine on the redundant supervisor engine current with changes that are sent to it by CEF on the active supervisor engine. The forwarding engine can 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 cause prefix-by-prefix updates to CEF, which it uses to update the FIB and adjacency databases. Existing and new entries receive the new version (“epoch”) number, indicating that they have been refreshed. The forwarding information is updated on the forwarding engine during convergence. The supervisor engine signals when the RIB has converged. The software 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

Note Use of the routing protocols require the Enterprise Services Cisco IOS Software image for the Catalyst 4500 series switch.

The routing protocols run only on the active supervisor engine, and they receive routing updates from their neighbor routers. Routing protocols do not run on the standby supervisor engine. Following a switchover, the routing protocols request that the NSF-aware neighbor devices send state information to help rebuild the routing tables. Alternately, the IS-IS protocol can be configured to synchronize state information from the active to the redundant supervisor engine to help rebuild the routing table on the NSF-capable device in environments where neighbor devices are not NSF-aware. NSF supports the BGP, OSPF, IS-IS, and EIGRP protocols.

Note For NSF operation, the routing protocols depend on CEF to continue forwarding packets while the routing protocols rebuild the routing information.

BGP Operation

When an 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 statement that the NSF-capable device has “graceful” restart capability. Graceful restart is the mechanism by which BGP routing peers avoid a routing flap following a switchover. If the BGP peer has received this capability, it is aware that the device sending the message is NSF-capable. Both the NSF-capable router and its BGP peers need to exchange the graceful restart capability in their OPEN messages at the time of session establishment. If both peers do not exchange the graceful restart capability, the session will not be capable of a graceful restart.

If the BGP session is lost during the supervisor engine switchover, the NSF-aware BGP peer marks all the routes associated with the NSF-capable router as stale; however, it continues to use these routes to make forwarding decisions for a set period of time. This functionality prevents packets from being lost while the newly active supervisor engine is waiting for convergence of the routing information with the BGP peers.

After a supervisor engine switchover occurs, the 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 NSF-capable router as having restarted.

At this point, the routing information is exchanged between the two BGP peers. After this exchange is complete, the NSF-capable device uses the routing information to update the RIB and the FIB with the new forwarding information. The NSF-aware device uses the network information to remove stale routes from its BGP table; the BGP protocol then is fully converged.

If a BGP peer does not support the graceful restart capability, it ignores the graceful restart capability in an OPEN message but establishes a BGP session with the NSF-capable device. This function allows interoperability with non-NSF-aware BGP peers (and without NSF functionality), but the BGP session with non-NSF-aware BGP peers is not capable of a graceful restart.

Note BGP support in NSF requires that neighbor networking devices be NSF-aware; that is, the devices must have the graceful restart capability and advertise that capability in their OPEN message during session establishment. If an NSF-capable router discovers that a particular BGP neighbor does not have graceful restart capability, it does not establish an NSF-capable session with that neighbor. All other neighbors that have graceful restart capability continue to have NSF-capable sessions with this NSF-capable networking device.

OSPF Operation

Note OSPF Version 2 fast hellos generate false alarm. We recommend that you use Bidirectional Forwarding Detection (BFD) instead.

When an OSPF NSF-capable router performs a supervisor engine switchover, it must perform the following tasks in order to resynchronize its link state database with its OSPF neighbors:

• Relearn the available OSPF neighbors on the network without causing a reset of the neighbor relationship
• Reacquire the contents of the link state database for the network

As quickly as possible after a supervisor engine switchover, the NSF-capable router sends an OSPF NSF signal to neighboring NSF-aware devices. Neighbor networking devices recognize this signal as an indicator that the neighbor relationship with this router should not be reset. As the NSF-capable router receives signals from other routers on the network, it can begin to rebuild its neighbor list.

After neighbor relationships are reestablished, the NSF-capable router begins to resynchronize its database with all of its NSF-aware neighbors. At this point, the routing information is exchanged between the OSPF neighbors. Once this exchange is complete, the NSF-capable device uses the routing information to remove stale routes, update the RIB, and update the FIB with the new forwarding information. The OSPF protocols are then fully converged.

Note OSPF support in NSF requires that all neighbor networking devices be NSF-aware. If an NSF-capable router discovers that it has non-NSF -aware neighbors on a particular network segment, it disables NSF capabilities for that segment. Other network segments composed entirely of NSF-capable or NSF-aware routers continue to provide NSF capabilities.

IS-IS Operation

When an IS-IS NSF-capable router performs a supervisor engine switchover, it must perform the following tasks in order to resynchronize its link state database with its IS-IS neighbors:

• Relearn the available IS-IS neighbors on the network without causing a reset of the neighbor relationship
• Reacquire the contents of the link state database for the network

The IS-IS NSF feature offers two options when you configure NSF:

• Internet Engineering Task Force (IETF) IS-IS
• Cisco IS-IS

If neighbor routers on a network segment are running a software version that supports the IETF Internet draft for router restartability, they assist an IETF NSF router that is restarting. 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 operation between peer devices based on a proposed standard.

Note If you configure IETF on the networking device, but neighbor routers are not IETF-compatible, NSF aborts following a switchover.

If the neighbor routers on a network segment are not 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 redundant supervisor engine. An advantage of Cisco configuration is that it does not rely on NSF-aware neighbors.

IETF IS-IS Configuration

As quickly as possible after a supervisor engine switchover, the NSF-capable router sends IS-IS NSF restart requests to neighboring NSF-aware devices using the IETF IS-IS configuration. Neighbor networking devices recognize this restart request as an indicator 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.

After this exchange is complete, the 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 supervisor engine 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 attempts a second NSF restart. During this time, the new redundant supervisor engine boots up and synchronizes its configuration with the active supervisor engine. The IS-IS NSF operation waits for a specified interval to ensure that connections are stable before attempting another restart of IS-IS NSF. This functionality prevents IS-IS from attempting back-to-back NSF restarts with stale information.

Cisco IS-IS Configuration

Using the Cisco configuration option, full adjacency and LSP information is saved, or checkpointed, to the redundant supervisor engine. Following a switchover, the newly active supervisor engine maintains its adjacencies using the check-pointed data, and can quickly rebuild its routing tables.

Note Following a switchover, Cisco IS-IS NSF has complete neighbor adjacency and LSP information; however, it must wait for all interfaces to come on line that had adjacencies prior to the switchover. If an interface does not come on line within the allocated interface wait time, the routes learned from these neighbor devices are not considered in routing table recalculation. IS-IS NSF provides a command to extend the wait time for interfaces that, for whatever reason, do not come on line in a timely fashion.

The switchover from one supervisor engine 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 attempts a second NSF restart. During this time, the new redundant supervisor engine boots up and synchronizes its configuration with the active supervisor engine. After this synchronization is completed, IS-IS adjacency and LSP data is check-pointed to the redundant supervisor engine; however, a new NSF restart is not attempted by IS-IS until the interval time expires. This functionality prevents IS-IS from attempting back-to-back NSF restarts.

EIGRP Operation

When an EIGRP NSF-capable router initially re-boots after an NSF restart, it has no neighbor and its topology table is empty. The router is notified by the redundant (now active) supervisor engine when it needs to bring up the interfaces, reacquire neighbors, and rebuild the topology and routing tables. The restarting router and its peers must accomplish these tasks without interrupting the data traffic directed toward the restarting router. EIGRP peer routers maintain the routes learned from the restarting router and continue forwarding traffic through the NSF restart process.

To prevent an adjacency reset by the neighbors, the restarting router uses a new Restart (RS) bit in the EIGRP packet header to indicate a restart. The RS bit is set in the hello packets and in the initial INIT update packets during the NSF restart period. The RS bit in the hello packets allows the neighbors to be quickly notified of the NSF restart. Without seeing the RS bit, the neighbor can only detect an adjacency reset by receiving an INIT update or by the expiration of the hello hold timer. Without the RS bit, a neighbor does not know if the adjacency reset should be handled using NSF or the normal startup method.

When the neighbor receives the restart indication, either by receiving the hello packet or the INIT packet, it recognizes the restarting peer in its peer list and maintains the adjacency with the restarting router. The neighbor then sends it topology table to the restarting router with the RS bit set in the first update packet indicating that it is NSF-aware and is helping out the restarting router. The neighbor does not set the RS bit in their hello packets, unless it is also a NSF restarting neighbor.

Note A router may be NSF-aware but may not be helping the NSF restarting neighbor because booting from a cold start.

If at least one of the peer routers is NSF-aware, the restarting router then receives updates and rebuilds its database. The restarting router must then find out if it had converged so that it can notify the routing information base (RIB). Each NSF-aware router is required to send an end of table (EOT) marker in the last update packet to indicate the end of the table content. The restarting router knows it has converged when it receives the EOT marker. The restarting router can then begin sending updates.

An NSF-aware peer knows when the restarting router had converged when it receives an EOT indication from the restarting router. The peer then scans its topology table to search for the routes with the restarted neighbor as the source. The peer compares the route timestamp with the restart event timestamp to determine if the route is still available. The peer then goes active to find alternate paths for the routes that are no longer available through the restarted router.

When the restarting router has received all EOT indications from its neighbors or when the NSF converge timer expires, EIGRP notifies the RIB of convergence. EIGRP waits for the RIB convergence signal and then floods its topology table to all awaiting NSF-aware peers.

NSF Guidelines and Restrictions

NSF with SSO has these restrictions:

• With aggressive protocol timers (such as, when the default exceeds the timer value), upon switchover, the protocol software running on the new active supervisor engine might not initialize in time to send "hello" packets to its neighboring switches or routers. If the protocol takes longer time to initialize because of other CPU-demanding tasks, then the protocol encounters state transitions and causes a loss in traffic on the order of seconds. We recommend that you do not configure aggressive timers in conjunction with SSO/NSF.
• For NSF operation, you must have SSO configured on the device.
• NSF with SSO supports IP Version 4 traffic and protocols only; NSF with SSO does not support IPv6 traffic.
• The Virtual Redundancy Routing Protocols (VRRP) is not SSO-aware, meaning state information is not maintained between the active and standby supervisor engine during normal operation. VRRP and SSO can coexist but both features work independently. Traffic that relies on VRRP may switch to the VRRP standby in the event of a supervisor engine switchover.
• All neighboring devices participating in BGP NSF must be NSF-capable and configured for BGP graceful restart.
• OSPF NSF for virtual links is not supported.
• All OSPF networking devices on the same network segment must be NSF-aware (running an NSF software image).
• For IETF IS-IS, all neighboring devices must be running an NSF-aware software image.

Configuring SSO

You must configure SSO in order to use NSF with any supported protocol.

To configure SSO, perform this task:

Note The sso keyword is supported in Cisco IOS Release 12.2(20)EWA and later releases.

This example shows how to configure the system for SSO and display the redundancy state:

Configuring CEF NSF

The CEF NSF feature operates by default while the networking device is running in SSO mode. No configuration is necessary.

Verifying CEF NSF

To verify that CEF is NSF-capable, enter the show cef state command:

Configuring BGP NSF

Note You must configure BGP graceful restart on all peer devices participating in BGP NSF.

To configure BGP for NSF, perform this task (repeat this procedure on each of the BGP NSF peer devices):

Verifying BGP NSF

To verify BGP NSF, you must check that BGP graceful restart is configured on the SSO-enabled networking device and on the neighbor devices. To verify, follow these steps:

Step 1 Verify that “bgp graceful-restart” appears in the BGP configuration of the SSO-enabled switch by entering the show running-config command:

Step 2 Repeat Step 1 on each of the BGP neighbors.

Step 3 On the SSO device and the neighbor device, verify that the graceful restart function is shown as both advertised and received, and confirm the address families that have the graceful restart capability. If no address families are listed, BGP NSF does not occur either:

Configuring OSPF NSF

Note All peer devices participating in OSPF NSF must be made OSPF NSF-aware, which happens automatically when you install an NSF software image on the device.

To configure OSPF NSF, perform this task:

Verifying OSPF NSF

To verify OSPF NSF, you must check that the NSF function is configured on the SSO-enabled networking device. To verify OSPF NSF, follow these steps:

Step 1 Verify that ‘nsf’ appears in the OSPF configuration of the SSO-enabled device by entering the show running-config command:

Step 2 Enter the show ip ospf command to verify that NSF is enabled on the device:

Configuring IS-IS NSF

To configure IS-IS NSF, perform this task:

Verifying IS-IS NSF

To verify IS-IS NSF, you must check that the NSF function is configured on the SSO-enabled networking device. To verify IS-IS NSF, follow these steps:

Step 1 Verify that “nsf” appears in the IS-IS configuration of the SSO-enabled device by entering the
show running-config command. The display shows either the Cisco IS-IS or the IETF IS-IS configuration. The following display indicates that the device uses the Cisco implementation of IS-IS NSF:

Step 2 If the NSF configuration is set to cisco, enter the show isis nsf command to verify that NSF is enabled on the device. Using the Cisco configuration, the display output differs on the active and redundant RPs. The following display shows sample output for the Cisco configuration on the active RP. In this example, note the presence of “NSF restart enabled”:

The following display shows sample output for the Cisco configuration on the standby RP. In this example, note the presence of “NSF restart enabled”:

Step 3 If the NSF configuration is set to ietf, enter the show isis nsf command to verify that NSF is enabled on the device. The following display shows sample output for the IETF IS-IS configuration on the networking device:

Configuring EIGRP NSF

To configure EIGRP NSF, perform this task:

Verifying EIGRP NSF

To verify EIGRP NSF, you must check that the NSF function is configured on the SSO-enabled networking device. To verify EIGRP NSF, follow these steps:

Step 1 Verify that “nsf” appears in the EIGRP configuration of the SSO-enabled device by entering the
show running-config command:

Step 2 Enter the show ip protocols command to verify that NSF is enabled on the device:

Enhanced High System Availability

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-stateful-switchover.html

NSF - Graceful Restart (GR) and Non Stop Routing (NSR) for IS-IS

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-nonstop-forwarding.html

NSF - OSPF

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-nonstop-forwarding.html

NSF/SSO (Nonstop Forwarding with Stateful Switchover)

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-nonstop-forwarding.html

SSO - HDLC

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-stateful-switchover.html

SSO - HSRP

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-stateful-switchover.html

SSO - Multilink PPP (MLP)

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-stateful-switchover.html

SSO - PPP

http://www.cisco.com/en/US/docs/ios-xml/ios/ha/configuration/xe-3s/ha-config-stateful-switchover.html