Cisco Nonstop Forwarding

First Published: July 22, 2002

Last Updated: December 4, 2006

Cisco Nonstop Forwarding (NSF) works with the Stateful Switchover (SSO) feature in Cisco IOS
software. 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 continue forwarding IP packets following
a Route Processor (RP) switchover.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach
links to specific feature documentation in this module and to see a list of the releases in which each feature is
supported, use the Feature Information for Cisco Nonstop Forwarding section on page 26.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS
software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An
account on Cisco.com is not required.

Note Throughout this document, the term Route Processor is used to describe the route processing engine
on all networking devices, regardless of the platform designation, unless otherwise noted. For example,
on the Cisco 10000 series Internet router the RP is called the Performance Routing Engine (PRE), on the
Cisco 12000 series Internet router the RP is called the Gigabit Route Processor (GRP), and on the Cisco
7500 series router the RP is called the Route Switch Processor (RSP).

Contents
Prerequisites for Cisco Nonstop Forwarding, page 2
Restrictions for Cisco Nonstop Forwarding, page 2
Information About Cisco Nonstop Forwarding, page 3
How to Implement Cisco NSF, page 9
Configuration Examples, page 19

Americas Headquarters:
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2007 Cisco Systems, Inc. All rights reserved.
 Cisco Nonstop Forwarding
Prerequisites for Cisco Nonstop Forwarding

Additional References, page 25

Feature Information for Cisco Nonstop Forwarding, page 26

Prerequisites for Cisco Nonstop Forwarding

NSF must be configured on a networking device that has been configured for SSO.
On platforms supporting the Route Switch Processor (RSP), and where the CEF switching mode is
configurable, configure distributed CEF (dCEF) switching mode using the ip cef distributed
command.

Restrictions for Cisco Nonstop Forwarding

General Restrictions
The Hot Standby Routing Protocol (HSRP) is not supported with Cisco Nonstop Forwarding with
Stateful Switchover. Do not use HSRP with Cisco Nonstop Forwarding with Stateful Switchover.

BGP NSF
All neighboring devices participating in BGP NSF must be NSF-capable, having been configured
for BGP graceful restart as described in the Configuring and Verifying BGP NSF section.

EIGRP NSF
All neighboring devices participating in EIGRP NSF operation must be NSF-capable or NSF-aware.
An NSF-aware router cannot support two NSF-capable peers performing an NSF restart operation
at the same time. However, both neighbors will reestablish peering sessions after the NSF restart
operation is complete.

OSPF NSF
OSPF NSF for virtual links is not supported.
All OSPF networking devices on the same network segment must be NSF-aware (that is, running an
NSF software image).
OSPF NSF for sham links is not supported.

IS-IS NSF
For IETF IS-IS, all neighboring devices must be running an NSF-aware software image.

Cisco 7200 Series Router

The Cisco 7200 series router has a single CPU; therefore, it cannot support the stateful switchover
in the event of a network processor engine (NPE) fault.
The Cisco 7206 does support NSF and can operate in a peer role with a Cisco 7500, 10000, or 12000
series router running Cisco IOS Release 12.0(23)S. With NSF enabled, an RP switchover on the
Cisco 7500, 10000, or 12000 series router peer should not cause a loss of PPP, ATM, high-level data
link control (HDLC), or Frame Relay sessions, or a loss of any OSPF, BGP, or IS-IS adjacencies
established between the Cisco 7200 and the peer.

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Information About Cisco Nonstop Forwarding

Cisco NSF works with the Stateful Switchover (SSO) feature in Cisco IOS software. 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 continue forwarding IP packets following a route processor (RP)
switchover.
Usually, when a networking device restarts, all routing peers of that device 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 helps to suppress routing flaps in
SSO-enabled devices, thus reducing network instability.
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 assumes control from the failed active RP during a
switchover. The ability of line cards and FPs to remain up through a switchover and to be kept current
with the Forwarding Information Base (FIB) on the active RP is key to Cisco NSF operation.

NSF Dependency on SSO

Cisco NSF always runs together with SSO. This section provides some background information on the
SSO feature.
In specific Cisco networking devices that support dual RPs, SSO establishes one of the RPs as the active
processor while the other RP is designated as the standby processor, and then synchronizes 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.
In networking devices running SSO, both RPs must be running the same configuration so that the
standby RP is always ready to assume control following a fault on the active RP. The configuration
information is synchronized from the active RP to the standby RP at startup and whenever changes to
the active RP configuration occur. Following an initial synchronization between the two processors, SSO
maintains RP state information between them, including forwarding information.
During switchover, system control and routing protocol execution is transferred from the active
processor to the standby processor. The time required by the device to switch over from the active to the
standby processor ranges from just a few seconds to approximately 30 seconds, depending on the
platform.
SSO supported protocols and applications must be high-availability (HA)-aware. A feature or protocol
is HA aware if it maintains, either partially or completely, undisturbed operation through an RP
switchover. For some HA aware protocols and applications, state information is synchronized from the
active to the standby processor. For Cisco NSF, enhancements to the routing protocols (Cisco Express
Forwarding, or CEF; Open Shortest Path First, or OSPF; Border Gateway Protocol, or BGP; and
Intermediate System-to-Intermediate System, or IS-IS) have been made to support the HA features in
SSO.
For more information on SSO, see the How to Implement Cisco NSF section.

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Cisco NSF Routing and Forwarding Operation

Cisco NSF is supported by the BGP, EIGRP, OSPF, and IS-IS protocols for routing and by Cisco Express
Forwarding (CEF) for forwarding. Of the routing protocols, BGP, EIGRP, OSPF, and IS-IS 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 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 NSF-aware if it is running NSF-compatible software.
A device is said to be NSF-capable if it has been configured to support NSF; therefore, it would rebuild
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. Once the routing protocols have converged,
CEF updates the FIB table and removes stale route entries. CEF, in turn, updates the line cards with the
new FIB information.
Table 1 lists the routing protocol and CEF support in Cisco NSF.

Table 1 Routing Protocol and CEF Support in Cisco NSF

NSF Support in Cisco IOS Software Release

Protocol Platform 12.0(22)S 12.0(23)S 12.0(24)S 12.2(18)S 12.2(28)SB 12.2(33)SRA
1 1 1 2
BGP Cisco 7200 Yes Yes Yes No No No
Cisco 7304 No No No No Yes No
Cisco 7500 Yes Yes Yes Yes No No
Cisco 7600 No No No No No Yes
Cisco 10000 Yes Yes Yes No Yes No
Cisco 12000 Yes Yes Yes No No No
1 1 1 2
OSPF Cisco 7200 Yes Yes Yes No No No
Cisco 7304 No No No No Yes No
Cisco 7500 Yes Yes Yes Yes No No
Cisco 7600 No No No No No Yes
Cisco 10000 Yes Yes Yes No No No
Cisco 12000 Yes Yes Yes No No No
IS-IS Cisco 7200 Yes1 Yes1 Yes1 No2 No No
Cisco 7304 No No No No Yes No
Cisco 7500 Yes Yes Yes Yes No No
Cisco 7600 No No No No No Yes
Cisco 10000 Yes Yes Yes No Yes No
Cisco 12000 Yes Yes Yes No No No

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Table 1 Routing Protocol and CEF Support in Cisco NSF

NSF Support in Cisco IOS Software Release

Protocol Platform 12.0(22)S 12.0(23)S 12.0(24)S 12.2(18)S 12.2(28)SB 12.2(33)SRA
3
CEF Cisco 7200
Cisco 7304 No No No No Yes No
Cisco 7500 Yes Yes Yes Yes No No
Cisco 7600 No No No No No Yes
Cisco 10000 Yes Yes Yes No No No
Cisco 12000 Yes Yes Yes No No No
2
EIGRP Cisco 7200 No No No Yes No No
Cisco 7304 No No No No Yes No
Cisco 7500 No No No Yes No No
Cisco 7600 No No No No No Yes
Cisco 10000 No No No No No No
Cisco 12000 No No No No No No
1. The Cisco 7200 is a single-route processor system and cannot maintain its forwarding table in the event of a route processor
failure. It cannot perform nonstop forwarding of packets. However, it supports the NSF protocol extensions for BGP, EIGRP,
OSPF, and ISIS. Therefore, it can peer with NSF-capable routers and facilitate the resynchronization of routing information with
such routers.
2. The Cisco 7200 is NSF-aware in the Cisco IOS Release 12.2(18)S.
3. The Cisco 7200 is a single processor device and does not support SSO; therefore, CEF support for NSF does not apply.

Cisco Express Forwarding

A key element of 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 traffic interruption
during the switchover.
During normal 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 it 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 RP
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.

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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 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
standby RP to help rebuild the routing table on the NSF-capable device in environments where neighbor
devices are not NSF-aware.

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 a 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 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 peer(s) need to exchange the Graceful Restart
Capability in their OPEN messages, at the time of session establishment. If both the peers do not
exchange the Graceful Restart Capability, the session will not be graceful restart capable.
If the BGP session is lost during the RP 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 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 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. Once 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. Following that, the BGP protocol is 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 NSF-capable device. This function will
allow interoperability with non-NSF-aware BGP peers (and without NSF functionality), but the BGP
session with non-NSF-aware BGP peers will not be graceful restart capable.

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 will not establish an NSF-capable session with that neighbor. All other neighbors
that have Graceful Restart Capability will continue to have NSF-capable sessions with this NSF-capable
networking device.

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EIGRP Operation
EIGRP NSF capabilities are exchanged by EIGRP peers in hello packets. The NSF-capable router
notifies its neighbors that an NSF restart operation has started by setting the restart (RS) bit in a hello
packet. When an NSF-aware router receives notification from an NSF-capable neighbor that an
NSF-restart operation is in progress, the NSF-capable and NSF-aware routers immediately exchange
their topology tables. The NSF-aware router sends an end-of-table (EOT) update packet when the
transmission of its topology table is complete. The NSF-aware router then performs the following
actions to assist the NSF-capable router:
The EIGRP hello hold timer is expired to reduce the time interval set for hello packet generation and
transmission. This allows the NSF-aware router to reply to the NSF-capable router more quickly
reducing the amount of time required for the NSF-capable router to rediscover neighbors and rebuild
the topology table.
The route-hold timer. is started. This timer is used to set the period of time that the NSF-aware router
will hold known routes for the NSF-capable neighbor. This timer is configured with the timers nsf
route-hold command. The default time period is 240 seconds.
The NSF-aware router notes in the peer list that the NSF-capable neighbor is restarting, maintains
adjacency, and holds known routes for the NSF-capable neighbor until the neighbor signals that it
is ready for the NSF-aware router to send its topology table or the route-hold timer expires. If the
route-hold timer expires on the NSF-aware router, the NSF-aware router will discard held routes and
treat the NSF-capable router as a new router joining the network and reestablishing adjacency
accordingly.
The NSF-aware router will continue to send queries to the NSF-capable router which is still in the
process of converging after switchover, effectively extending the time before a stuck-in-active (SIA)
condition can occur.
When the switchover operation is complete, the NSF-capable router notifies its neighbors that it has
reconverged and has received all of their topology tables by sending an EOT update packet to the
assisting routers. The NSF-capable then returns to normal operation. The NSF-aware router will look for
alternate paths (go active) for any routes that are not refreshed by the NSF-capable (restarting router).
The NSF-aware router will then return to normal operation. If all paths are refreshed by the NSF-capable
router, the NSF-aware router will immediately return to normal operation.

Note NSF-aware routers are completely compatible with non-NSF aware or capable neighbors in an EIGRP
network. A non-NSF aware neighbor will ignore NSF capabilities and reset adjacencies and otherwise
maintain the peering sessions normally.

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 NSF-capable router sends an OSPF NSF signal to
neighboring 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 NSF-capable router receives signals
from other routers on the network, it can begin to rebuild its neighbor list.

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Once 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 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 will disable NSF
capabilities for that segment. Other network segments composed entirely of NSF-capable or NSF-aware
routers will continue to provide NSF capabilities.

The OSPF RFC 3623 graceful restart feature allows you to configure IETF NSF in multivendor
networks. For more information, see OSPF RFC 3623 Graceful Restart, Cisco IOS Release
12.2(31)SB2.

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 NSF:
Internet Engineering Task Force (IETF) IS-IS
Cisco IS-IS
If neighbor routers on a network segment are NSF-aware, meaning that neighbor routers are running a
software version that supports the IETF Internet draft for router restartability, they will assist an IETF
NSF router which 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 will
abort 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 standby RP. A benefit of Cisco configuration is that it does not rely on NSF-aware
neighbors.

IETF IS-IS Configuration

Using the IETF IS-IS configuration, as quickly as possible after an RP switchover, the NSF-capable
router sends IS-IS NSF restart requests to neighboring 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 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.

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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 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 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 standby RP. Following a switchover, the newly active RP maintains its adjacencies using the
checkpointed 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 that had adjacencies prior to the switchover to come up. If an
interface does not come up 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 up in a timely fashion.

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 NSF restart. During this time, the new standby RP will
boot up and synchronize its configuration with the active RP. Once this synchronization is completed,
IS-IS adjacency and LSP data is checkpointed to the standby RP; however, a new NSF restart will not be
attempted by IS-IS until the interval time expires. This functionality prevents IS-IS from attempting
back-to-back NSF restarts.

Cisco NSF Benefits

The Cisco NSF feature has several benefits, including the following:
Improved network availabilityNSF continues forwarding network traffic and application state
information so that user session information is maintained after a switchover.
Overall network stabilityNetwork stability may be improved with the reduction in the number of
route flaps that had been created when routers in the network failed and lost their routing tables.
Neighboring routers do not detect link flappingBecause the interfaces remain up across a
switchover, neighboring routers do not detect a link flap (that is, the link does not go down and come
back up).
Prevents routing flapsBecause SSO continues forwarding network traffic in the event of a
switchover, routing flaps are avoided.
No loss of user sessionsUser sessions established prior to the switchover are maintained.

How to Implement Cisco NSF

Configuring and Verifying CEF NSF, page 10
Configuring and Verifying BGP NSF, page 10
Configuring and Verifying EIGRP NSF, page 12

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Configuring and Verifying OSPF NSF, page 14

Configuring and Verifying IS-IS NSF, page 15

Configuring and Verifying CEF NSF

The CEF NSF feature operates by default while the networking device is running in SSO mode. No
configuration is necessary.
The following task explains how to verify that CEF is NSF-capable.

SUMMARY STEPS

1. enable
2. show cef state

DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 show cef state Displays the state of Cisco Express Forwarding on
a networking device.
Example:
Router# configure terminal

Configuring and Verifying BGP NSF

The following task explains how to configure BGP for NSF. Repeat this task on each of the BGP NSF
peer devices.

SUMMARY STEPS

1. enable
2. configure terminal
3. router bgp autonomous-system-number
4. bgp graceful-restart [restart-time seconds | stalepath-time seconds]

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DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 configure terminal Enters global configuration mode.

Example:
Router# configure terminal
Step 3 router bgp autonomous-system-number Enables a BGP routing process, and enters router
configuration mode.
Example:
Router(config)# router bgp 120
Step 4 bgp graceful-restart [restart-time seconds | Enables the BGP graceful restart capability, which
stalepath-time seconds] starts NSF for BGP.

Example:
Router(config-router)# bgp graceful-restart

To verify NSF for BGP, you must check that the graceful restart function is configured on the
SSO-enabled networking device and on the neighbor devices. The following task explains how to
perform this function.

SUMMARY STEPS

1. enable
2. show running-config
3. show ip bgp neighbors [ip-address [advertised-routes | dampened-routes | flap-statistics |
paths [reg-exp] | received prefix-filter | received-routes | routes | policy [detail]]]

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DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 show running-config Displays the contents of the current running
configuration file. Verify that the phrase bgp
graceful-restart appears in the BGP configuration
Example:
Router# show running-config
of the SSO-enabled router.
Repeat this step on each of the BGP neighbors.
Step 3 show ip bgp neighbors [ip-address [advertised-routes | display information about BGP and TCP
dampened-routes | flap-statistics | paths [reg-exp] | connections to neighbors
received prefix-filter | received-routes | routes |
policy [detail]]] On the SSO device and the neighbor device, this
command verifies that the graceful restart function
is shown as both advertised and received, and
Example:
confirms the address families that have the graceful
Router# show ip bgp neighbors
restart capability. If no address families are listed,
then BGP NSF also will not occur.

Configuring and Verifying EIGRP NSF

EIGRP NSF support is enabled by default. Distributed platforms that run a supporting version of Cisco
IOS software can support full NSF capabilities. These routers can perform a restart operation and can
support other NSF capable peers. Single processor platforms that run a supporting version of Cisco IOS
software support only NSF awareness. These routers maintain adjacency and hold known routes for the
NSF-capable neighbor until it signals that it is ready for the NSF-aware router to send its topology table
or the route-hold timer expires.

Note An NSF-aware router must be completely converged with the network before it can assist an
NSF-capable router in an NSF restart operation.

The following task explains how to configure EIGRP for NSF. Repeat this procedure on each of the
EIGRP NSF peer devices.

SUMMARY STEPS

1. enable
2. configure terminal
3. router eigrp as-number
4. nsf [{cisco | ietf} | interface wait seconds | interval minutes | t3 [adjacency | manual seconds]
5. timers nsf converge seconds
6. timers nsf route-hold seconds
7. timers nsf signal seconds

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DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 configure terminal Enters global configuration mode.

Example:
Router# configure terminal
Step 3 router eigrp as-number Enables an EIGRP routing process, and enters
router configuration mode.
Example:
Router(config)# router eigrp 109
Step 4 nsf [{cisco | ietf} | interface wait seconds | Enables EIGRP NSF support on an NSF capable
interval minutes | t3 [adjacency | manual seconds] router.
This command is entered on only NSF-capable
Example: routers. NSF awareness is enabled by default when
Router(config-router)# nsf a supporting version of Cisco IOS software is
installed on a router that supports NSF capability or
NSF awareness.
Step 5 timers nsf converge seconds Adjusts the maximum time that restarting router
will wait for the EOT notification from an
NSF-capable or NSF-aware peer.
Example:
Router(config-router)# timers nsf converge 60
Step 6 timers nsf route-hold seconds Sets the route-hold timer to determine how long an
NSF-aware router that is running EIGRP will hold
routes for an inactive peer.
Example:
Router(config-router)# timers nsf route-hold 120
Step 7 timers nsf signal seconds Adjusts the maximum time for the initial restart
period.
Example:
Router(config-router)# timers nsf signal seconds

To verify NSF for EIGRP, you must check that NSF awareness and/or capability is enabled on the
SSO-enabled networking device and on the neighbor devices. The following task explains how to
perform this function.

SUMMARY STEPS

1. enable
2. show ip protocols

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DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 show ip protocols Displays the parameters and current state of the
active routing protocol process.
Example: Repeat this step on each of the EIGRP neighbors.
Router# show ip protocols

Configuring and Verifying OSPF NSF

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

The following task explains how to configure OSPF for NSF.

SUMMARY STEPS

1. enable
2. configure terminal
3. router ospf process-id [vrf vpn-name]
4. nsf [{cisco | ietf} | interface wait seconds | interval minutes | t3 [adjacency | manual seconds]

DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 configure terminal Enters global configuration mode.

Example:
Router# configure terminal

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Command Purpose
Step 3 router ospf process-id [vrf vpn-name] Enables an OSPF routing process, and places the
router in router configuration mode.
Example:
Router(config)# router ospf 12
Step 4 nsf [{cisco | ietf} | interface wait seconds | Enables EIGRP NSF support on an NSF capable
interval minutes | t3 [adjacency | manual seconds] router.
This command is entered on only NSF-capable
Example: routers. NSF awareness is enabled by default when
Router(config-router)# nsf a supporting version of Cisco IOS software is
installed on a router that supports NSF capability or
NSF awareness.

The following task explains how to verify OSPF for NSF.

SUMMARY STEPS

1. enable
2. show running-config
3. show ip ospf [process-id]

DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 show running-config Displays the contents of the current running
configuration file.
Example:
Router# show running-config
Step 3 show ip ospf [process-id] Displays general information about OSPF routing
processes.
Example:
Router# show ip ospf

Configuring and Verifying IS-IS NSF

The following task describe how to configure NSF for IS-IS.

SUMMARY STEPS

1. enable
2. configure terminal

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3. router isis area-tag

4. nsf [{cisco | ietf} | interface wait seconds | interval minutes | t3 [adjacency | manual seconds]
5. nsf interval minutes
6. nsf t3 {manual seconds | adjacency}
7. nsf interface wait seconds

DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 configure terminal Enters global configuration mode.

Example:
Router# configure terminal
Step 3 router isis area-tag Enables the IS-IS routing protocol to specify an
IS-IS process, and places the router in router
configuration mode.
Example:
Router(config)# router isis cisco1
Step 4 nsf [{cisco | ietf} | interface wait seconds | Enables NSF operation for IS-IS.
interval minutes | t3 [adjacency | manual seconds]
Enter the ietf keyword to enable IS-IS in
homogeneous network where adjacencies with
Example: networking devices supporting IETF draft-based
Router(config-router)# nsf ietf restartability is guaranteed.
Enter the cisco keyword to run IS-IS in
heterogeneous networks that might not have
adjacencies with NSF-aware networking devices.
Step 5 nsf interval minutes Configures the minimum time between Cisco NSF
restart attempts.
Example:
Router(config-router)# nsf interval 2
Step 6 nsf t3 {manual seconds | adjacency} Specifies the methodology used to determine how
long IETF Cisco NSF will wait for the link-state
packet (LSP) database to synchronize before
Example:
Router(config-router)# nsf t3 manual 40
generating overloaded link-state information for
itself and flooding that information out to its
neighbors.
Step 7 nsf interface wait seconds Specifies how long a Cisco NSF restart will wait
for all interfaces with IS-IS adjacencies to come up
before completing the restart.
Example:
Router(config-router)# nsf interface wait 15

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How to Implement Cisco NSF

To verify NSF for IS-IS, you must check that the NSF function is configured on the SSO-enabled
networking device. The following task describes how to verify NSF for IS-IS.

SUMMARY STEPS

1. enable
2. show running-config
3. show isis nsf

DETAILED STEPS

Command Purpose
Step 1 enable Enables privileged EXEC mode.
Enter your password if prompted.
Example:
Router> enable
Step 2 show running-config Displays the contents of the current running
configuration file.
Example:
Router# show running-config
Step 3 show isis nsf Displays current state information regarding IS-IS
NSF.
Example:
Router# show isis nsf

Troubleshooting Tips
To troubleshoot the NSF feature, use the following commands in privileged EXEC mode, as needed:

Command Purpose
Router# debug eigrp nsf Displays notifications and information about NSF
events for an EIGRP routing process.
Router# debug ip eigrp notifications Displays information and notifications for an
EIGRP routing process. This output includes NSF
notifications and events.
Router# debug isis nsf [detail] Displays information about the IS-IS state during a
Cisco NSF restart.
Router# debug ospf nsf [detail] Displays debugging messages related to OSPF
Cisco NSF commands.
Router# show cef nsf Displays the current NSF state of CEF on both the
active and standby RPs.
Router# show cef state Displays the state of CEF on a networking device.
Router# show clns neighbors Display both end-system (ES) and intermediate
system (IS) neighbors.

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Command Purpose
Router# show ip bgp Displays entries in the BGP routing table.
Router# show ip bgp neighbor Displays information about the TCP and BGP
connections to neighbor devices.
Router# show ip cef Displays entries in the FIB that are unresolved, or
displays a FIB summary.
Router# show ip ospf Displays general information about OSPF routing
processes.
Router# show ip ospf neighbor [detail] Displays OSPF-neighbor information on a
per-interface basis.
Router# show ip protocols Displays the parameters and current state of the
active routing protocol process. The status of
EIGRP NSF configuration and support is displayed
in the output.
Router# show isis database [detail] Displays the IS-IS link-state database.
Router# show isis nsf Displays the current state information regarding
IS-IS Cisco NSF.

The following tips may help you troubleshoot the device.

The system displays FIB errors.

Use the show cef state command to verify that distributed CEF switching is enabled on your platform.
To enable distributed CEF, enter the ip cef distributed command in global configuration mode on the
active RP.

Note For Cisco 10000 series Internet routers and Cisco 12000 series Internet routers, distributed CEF
is always enabled and is not configurable.

Cannot determine if an OSPF neighbor is NSF-aware.

To verify whether an OSPF neighbor device is NSF aware and if NSF is operating between them, use the
show ip ospf neighbor detail command.

EIGRP adjacencies reset too often.

Neighbor adajacencies are maintained during NSF switchover operations. If adjacencies between
NSF-capable and NSF-aware neighbors are being reset too often, the route-hold timers may need to be
adjusted. The show ip eigrp neighbor detail command can be used to help determine if the route-hold
timer value should be set to a longer time period. The output will display the time that adjacency is
established with specific neighbors. This time will tell you if adjacencies are being maintained or reset
and when the last time that specific neighbors have been restarted.

The system loses, or appears to lose, adjacencies with network peers following a stateful switchover.
Use the show clns neighbors detail command to find any neighbors that do not have NSF capable and
make sure that they are running NSF-aware images.
Additionally, for IS-IS, the standby RP must be stable for 5 minutes (which is the default) before another
restart can be initiated. Use the nsf interval command to reset the restart period.

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Configuration Examples

Configuration Examples
This section provides the following configuration examples:
Verifying that CEF Is NSF Capable: Example, page 19
Configuring BGP NSF: Example, page 19
Configuring BGP NSF Neighbor Device: Example, page 20
Verifying BGP NSF: Example, page 20
Configuring EIGRP NSF Converge Timer: Example, page 20
Configuring EIGRP NSF Route-Hold Timer: Example, page 21
Configuring EIGRP NSF Signal Timer: Example, page 21
Disabling EIGRP NSF Support: Example, page 22
Verifying EIGRP NSF: Example, page 21
Configuring OSPF NSF: Example, page 22
Verifying OSPF NSF: Example, page 22
Configuring IS-IS NSF: Example, page 22
Verifying IS-IS NSF: Example, page 23

Verifying that CEF Is NSF Capable: Example

The following example is used to verify that CEF is NSF capable.
Router# show cef state

CEF Status [RP]

CEF enabled/running
dCEF enabled/running
CEF switching enabled/running
CEF default capabilities:
Always FIB switching: yes
Default CEF switching: yes
Default dCEF switching: yes
Update HWIDB counters: no
Drop multicast packets: no
CEF NSF capable: yes
IPC delayed func on SSO: no
RRP state:
I am standby RRP: no
My logical slot: 0
RF PeerComm: no

Configuring BGP NSF: Example

The following example configures BGP NSF on a networking device.
Router# configure terminal
Router(config)# router bgp 590
Router(config-router)# bgp graceful-restart

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Configuration Examples

Configuring BGP NSF Neighbor Device: Example

The following example configures BGP NSF on a neighbor router. All devices supporting BGP NSF
must be NSF-aware, meaning that these devices recognize and advertise graceful restart capability.
Router# configure terminal
Router(config)# router bgp 770
Router(config-router)# bgp graceful-restart

Verifying BGP NSF: Example

Verify that bgp graceful-restart appears in the BGP configuration of the SSO-enabled router by
entering the show running-config command.
Router# show running-config

.
.
.
router bgp 120
.
.
.
bgp graceful-restart
neighbor 10.2.2.2 remote-as 300

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, then BGP NSF also will not occur:
Router# show ip bgp neighbors x.x.x.x

BGP neighbor is 192.168.2.2, remote AS YY, external link

BGP version 4, remote router ID 192.168.2.2
BGP state = Established, up for 00:01:18
Last read 00:00:17, hold time is 180, keepalive interval is 60 seconds
Neighbor capabilities:
Route refresh:advertised and received(new)
Address family IPv4 Unicast:advertised and received
Address famiiy IPv4 Multicast:advertised and received
Graceful Restart Capabilty:advertised and received
Remote Restart timer is 120 seconds
Address families preserved by peer:
IPv4 Unicast, IPv4 Multicast
Received 1539 messages, 0 notifications, 0 in queue
Sent 1544 messages, 0 notifications, 0 in queue
Default minimum time between advertisement runs is 30 seconds

Configuring EIGRP NSF Converge Timer: Example

The timers nsf converge command is used to adjust the maximum time that restarting router will wait
for the EOT notification from an NSF-capable or NSF-aware peer. The following example sets the
converge timer to 1 minute.
Router# configure terminal
Router(config)# router eigrp 101
Router(config-router)# timers nsf converge 60

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Configuration Examples

Configuring EIGRP NSF Route-Hold Timer: Example

The timers nsf route-hold command is used to set the maximum period of time that an NSF-aware
router will hold known routes for an NSF-capable neighbor during a switchover operation. The following
example sets the route-hold timer to 2 minutes.
Router# configure terminal
Router(config)# router eigrp 101
Router(config-router)# timers nsf route-hold 120

Configuring EIGRP NSF Signal Timer: Example

The timers nsf signal command is used to adjust the maximum time for the initial restart period. The
following example sets the signal timer to 10 seconds.
Router# configure terminal
Router(config)# router eigrp 101
Router(config-router)# timers nsf signal 10

Verifying EIGRP NSF: Example

Verify that EIGRP NSF support is present in the installed Cisco IOS software image by entering the
show ip protocols command. EIGRP NSF-aware route hold timer is... is displayed in the output when
either NSF awareness or capability is supported by the router. This line displays the default or
user-defined value for the route-hold timer. EIGRP NSF... is displayed in the output only when the
NSF capability is supported by the router. This line will also print disabled or enabled depending on
the status of the EIGRP NSF feature.
Router# show ip protocols

Routing Protocol is "eigrp 100"

Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 100
EIGRP NSF-aware route hold timer is 240s
EIGRP NSF enabled
NSF signal timer is 20s
NSF converge timer is 120s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
10.4.9.0/24
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170

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Configuration Examples

Disabling EIGRP NSF Support: Example

EIGRP NSF capability is enabled by default on distributed platforms that run a supporting version of
Cisco IOS software. The nsf command used to enable or disable the EIGRP NSF capability. The
following example disables NSF capability:
Router# configure terminal
Router(config)# router eigrp 101
Router(config-router)# no nsf

Configuring OSPF NSF: Example

The following example configures OSPF NSF on a networking device:
Router# configure terminal
Router(config)# router ospf 400
Router(config-router)# nsf

Verifying OSPF NSF: Example

To verify NSF for OSPF, you must check that the NSF function is configured on the SSO-enabled
networking device. Verify that nsf appears in the OSPF configuration of the SSO-enabled device by
entering the show running-config command:
Router# show running-config

router ospf 120

log-adjacency-changes
nsf
network 192.168.20.0 0.0.0.255 area 0
network 192.168.30.0 0.0.0.255 area 1
network 192.168.40.0 0.0.0.255 area 2

Next, use the show ip ospf command to verify that NSF is enabled on the device.
Router> show ip ospf

Routing Process "ospf 1" with ID 192.168.2.1 and Domain ID 0.0.0.1

Supports only single TOS(TOS0) routes
Supports opaque LSA
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x0
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
External flood list length 0
Non-Stop Forwarding enabled, last NSF restart 00:02:06 ago (took 44 secs)
Area BACKBONE(0)
Number of interfaces in this area is 1 (0 loopback)
Area has no authentication
SPF algorithm executed 3 times

Configuring IS-IS NSF: Example

The following example configures Cisco proprietary IS-IS NSF operation on a networking device:

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Configuration Examples

Router# configure terminal

Router(config)# router isis
Router(config-router)# nsf cisco

The following example configures IS-IS NSF for IETF operation on a networking device:
Router# configure terminal
Router(config)# router isis
Router(config-router)# nsf ietf

Verifying IS-IS NSF: Example

Verify that nsf appears in the IS-IS configuration of the SSO-enabled device by entering the show
running-config command. The display will show either Cisco IS-IS or IETF IS-IS configuration. The
following example indicates that the device uses the Cisco implementation of IS-IS NSF:
Router# show running-config

router isis
nsf cisco
If the NSF configuration is set to cisco, use the show isis nsf command to verify that NSF is enabled on
the device. Using the Cisco configuration, the display output will be different on the active and
standby RPs. The following example shows output for the Cisco configuration on the active RP. In this
example, note the presence of the phrase NSF restart enabled:
Router# show isis nsf

NSF is ENABLED, mode 'cisco'

RP is ACTIVE, standby ready, bulk sync complete

NSF interval timer expired (NSF restart enabled)
Checkpointing enabled, no errors
Local state:ACTIVE, Peer state:STANDBY HOT, Mode:SSO

The following example shows sample output for the Cisco configuration on the standby RP. In this
example, note the presence of the phrase NSF restart enabled:
Router# show isis nsf

NSF enabled, mode 'cisco'

RP is STANDBY, chkpt msg receive count:ADJ 2, LSP 7
NSF interval timer notification received (NSF restart enabled)
Checkpointing enabled, no errors
Local state:STANDBY HOT, Peer state:ACTIVE, Mode:SSO

The following example shows sample output for the IETF IS-IS configuration on the networking device:
Router# show isis nsf

NSF is ENABLED, mode IETF

NSF pdb state:Inactive
NSF L1 active interfaces:0
NSF L1 active LSPs:0
NSF interfaces awaiting L1 CSNP:0
Awaiting L1 LSPs:
NSF L2 active interfaces:0
NSF L2 active LSPs:0
NSF interfaces awaiting L2 CSNP:0
Awaiting L2 LSPs:
Interface:Serial3/0/2
NSF L1 Restart state:Running
NSF p2p Restart retransmissions:0

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Configuration Examples

Maximum L1 NSF Restart retransmissions:3

L1 NSF ACK requested:FALSE
L1 NSF CSNP requested:FALSE
NSF L2 Restart state:Running
NSF p2p Restart retransmissions:0
Maximum L2 NSF Restart retransmissions:3
L2 NSF ACK requested:FALSE
Interface:GigabitEthernet2/0/0
NSF L1 Restart state:Running
NSF L1 Restart retransmissions:0
Maximum L1 NSF Restart retransmissions:3
L1 NSF ACK requested:FALSE
L1 NSF CSNP requested:FALSE
NSF L2 Restart state:Running
NSF L2 Restart retransmissions:0
Maximum L2 NSF Restart retransmissions:3
L2 NSF ACK requested:FALSE
L2 NSF CSNP requested:FALSE
Interface:Loopback1
NSF L1 Restart state:Running
NSF L1 Restart retransmissions:0
Maximum L1 NSF Restart retransmissions:3
L1 NSF ACK requested:FALSE
L1 NSF CSNP requested:FALSE
NSF L2 Restart state:Running
NSF L2 Restart retransmissions:0
Maximum L2 NSF Restart retransmissions:3
L2 NSF ACK requested:FALSE
L2 NSF CSNP requested:FALSE

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 Cisco Nonstop Forwarding
Additional References

Additional References
The following sections provide references related to the Cisco Nonstop Forwarding feature.

Related Documents
Related Topic Document Title
High availability commands: complete command Cisco IOS High Availability Command Reference
syntax, command mode, defaults, usage guidelines,
and examples
Stateful switchover Stateful Switchover, Cisco IOS feature module

Standards
Standard Title
No new or modified standards are supported by this
feature, and support for existing standards has not been
modified by this feature.

MIBs
MIB MIBs Link
To locate and download MIBs for selected platforms, Cisco IOS
releases, and feature sets, use Cisco MIB Locator found at the
following URL:
http://www.cisco.com/go/mibs

RFCs
RFC Title
RFC 3623 Graceful OSPF Restart
RFC 3847 Restart Signaling for Intermediate System to Intermediate System
(IS-IS)
RFC 4781 Graceful Restart Mechanism for BGP

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Feature Information for Cisco Nonstop Forwarding

Technical Assistance
Description Link
The Cisco Support website provides extensive online http://www.cisco.com/techsupport
resources, including documentation and tools for
troubleshooting and resolving technical issues with
Cisco products and technologies. Access to most tools
on the Cisco Support website requires a Cisco.com user
ID and password. If you have a valid service contract
but do not have a user ID or password, you can register
on Cisco.com.

Feature Information for Cisco Nonstop Forwarding

Table 2 lists the features in this module and provides links to specific configuration information. Only
features that were introduced or modified in Cisco IOS Release 12.0(22)S or a later release appear in the
table.
Use Cisco Feature Navigator to find information about platform support and software image support.
Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images
support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to
http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 2 Cisco Nonstop Forwarding Feature History

Release Modification
12.0(22)S This feature was introduced.
12.0(23)S Support was added for 1xGE and 3xGE line cards on the
12.0(24)S Support was added for the following line cards on the
Engine 1
2-port OC-12/STM-4c DPT
Engine 2
1-port OC-48/STM-16c DPT
8-port OC-3/STM-1c ATM
IP Service Engine (ISE)
4-port OC-3c/STM-1c POS/SDH ISE
8-port OC-3c/STM-1c POS/SDH ISE
16-port OC-3c/STM-1c POS/SDH ISE
4-port OC-12c/STM-4c POS/SDH ISE
1-port OC-48c/STM-16c POS/SDH ISE
4-port channelized OC-12/STM-4 (DS3/E3, OC-3c/STM-1c)
POS/SDH ISE
1-port channelized OC-48/STM-16 (DS3/E3, OC-3c/STM-1c)
POS/SDH ISE

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Feature Information for Cisco Nonstop Forwarding

12.2(18)S This feature was integrated into Cisco IOS Release 12.2(18)S. Support
was added for EIGRP.
12.2(20)S Support for the Cisco 7304 router was added.
12.2(28)SB This feature was integrated into Cisco IOS Release 12.2(28)SB.
12.2(33)SRA This feature was integrated into Cisco IOS Release 12.2(33)SRA.
12.2(31)SB2 The following features were added:
OSPF RFC 3623 Graceful Restart

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Feature Information for Cisco Nonstop Forwarding

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