Throughout this chapter, the term router in this document refers to a router in a generic sense or an S9500 series routing switch running routing protocols.

1.1 Introduction to Graceful Restart

1.1.1 Graceful Restart Overview

Graceful Restart ensures the continuity of packet forwarding when a routing protocol restarts.

The mechanism of Graceful Restart works as follows: when the routing protocol on a Graceful Restart capable device restarts, the device can notify its neighbors to temporarily preserve its adjacency with them and the routing information. After the routing protocol restart is finished, the neighbors help the device to synchronize its routing information and to restore it to the state prior to the restart in minimal time. The routing and forwarding remain highly stable during the restart, the packet forwarding paths remain the same, and the whole system can forward IP packets continuously. Hence, it is called “Graceful Restart”.

1.1.2 Basic Mechanism of Graceful Restart

A router with the Graceful Restart feature enabled is called a Graceful Restart capable router. It can perform a Graceful Restart when its routing protocol restarts. Routers that are not Graceful Restart capable will follow the normal restart procedures after a routing protocol restarts.

l GR Restarter: Graceful restarting router, the router whose routing protocol has restarted due to administrator instructions or network failure. It must be Graceful Restart capable.

l GR Helper: The neighbor of the GR Restarter, which helps the GR Restarter to retain the routing information. It must be Graceful Restart capable.

l GR Session: A Graceful Restart session, which is the negotiation between the GR Restarter and the GR Helper. A GR session includes restart notification and communications across restart. Through this session, GR Restarter and GR Helper can know the GR capability of each other.

l GR Time: The time taken for the GR Restarter and the GR Helper to establish a session between them. Upon detection of the down state of a neighbor, the GR Helper will preserve the topology and routing information sent from the GR Restarter for a period as specified by the GR Time.

1.1.3 Graceful Restart Communication Procedure

Configure a device as GR Restarter in a network. This device and its GR Helper must support GR or be GR capable. Thus, when GR Restarter restarts, its GR Helper can know its restart process.

& Note:

l In some cases, GR Restarter and GR Helper can replace with each other.

l If a router is to act as a Graceful Restarter, it must have the ability to preserve the routing information in the routing table (forwarding table). Routers that fail to meet this can only act as a GR Helper.

The communication procedure between the GR Restarter and the GR Helper works as follows:

1) A GR session is established between the GR Restarter and the GR Helper.

Figure 1-1 A GR session is established between the GR Restarter and the GR Helper

As illustrated in Figure 1-1, Router A works as GR Restarter, Router B, Router C and Router D are the GR Helpers of Router A. A GR session is established between the GR Restarter and the GR Helper.

2) GR Restarter restarting

Figure 1-2 Restarting process for the GR Restarter

As illustrated in Figure 1-2. The GR Helper detects that the GR Restarter has restarted its routing protocol and assumes that it will recover within the GR Time. Before the GR Time expires, the GR Helper will neither terminate the session with the GR Restarter nor delete the topology or routing information of the latter.

3) GR Restarter signaling to GR Helper

Figure 1-3 The GR Restarter signals to the GR Helper(s) after restart

As illustrated in Figure 1-3, after the GR Restarter has recovered, it will signal to all its neighbors and will reestablish GR Session.

4) The GR Restarter obtaining topology and routing information from the GR Helper

Figure 1-4 The GR Restarter obtains topology and routing information from the GR Helper

As illustrated in Figure 1-4, the GR Restarter obtains the necessary topology and routing information from all its neighbors through the GR sessions between them and calculates its own routing table based on this information.

1.1.4 Graceful Restart Mechanism for Several Commonly Used Protocols

Graceful restart is currently implemented in BGP, OSPF, IS-IS, and LDP.

I. BGP-based Graceful Restart

1) To establish a session with its peer, a BGP-based GR Restarter needs to send an OPEN message first to its peer including its Graceful Restart Capability.

2) Upon receipt of this message, the peer is aware that the sending router is capable of Graceful Restart. It will use the OPEN messages to exchange the Graceful Restart Capability with the GR Restarter and to establish a GR session with it. If neither party has the Graceful Restart Capability, the session established between them will not be Graceful Restart Capable.

3) The GR session between the GR Restarter and the GR Helper goes down when a BGP restarts. A Graceful Restart Capable GR Helper will mark all routes associated with the GR Restarter as stale. However, during the configured GR Time, it still uses these routes in packet forwarding, ensuring that no packet will be lost when routing information from its peer is recollected.

4) After the restart, the GR Restarter will reestablish the GR session with its peer and send new GR messages notifying the completion of restart. Routing information can be exchanged between the peers and used by the GR Restarter in creating the new routing table (forwarding table) in place of the stale routing information. Thus the BGP routing convergence is complete.

II. OSPF-based Graceful Restart

After an OSPF-based GR Restarter restarts its OSPF, it needs to perform the following two tasks in order to update its link-state database with its neighbor.

1) To obtain once again effective OSPF adjacency while retaining the original one.

2) To obtain once again link-state database information.

Before the restart, the GR Restarter originates Grace-LSA negotiation GR capability. During the restart, the GR Helper continues to advertise its adjacency with the GR Restarter.

After the restart, the GR Restarter will send an OSPF GR signal to its neighbor so that the adjacency is not reset. In this way, the GR Restarter can restore its adjacency with its neighbor upon receiving the responses from the latter.

Upon the reestablishment of the adjacency, the GR Restarter will update its link-state database with all of its GR Capable neighbors and exchange routing information with them. After that, the GR Restarter will update its own routing table (forwarding table) based on the new routing information and delete the stale routes. In this way, the OSPF routing convergence is complete.

III. IS-IS-based Graceful Restart

After an IS-IS based GR Restarter restarts its IS-IS, it needs to perform the following two tasks in order to update its link state database with its neighbor.

l To obtain once again effective IS-IS adjacency while retaining the original one.

l To obtain once again link-state database information.

After the restart, the GR Restarter will send an IS-IS GR signal to its adjacent GR Helper so that the adjacency is not reset. In this way, the GR Restarter can restore its adjacency with its IS-IS neighbor upon receiving responses from the latter.

After the adjacency has been reestablished, the GR Restarter will update its link-state database with all of its GR Capable IS-IS neighbors and exchange routing information with them. After that, the GR Restarter will update its own routing table (forwarding table) based on the new routing information and delete and the stale routes. In this way, the IS-IS routing convergence is complete.

IV. MPLS LDP-based Graceful Restart

During LDP session establishment, LDP enabled devices need to negotiate their GR Capability. If two parties are both GR capable, the session between them will be GR capable. If at least one party is not GR capable, the session established will not be FT/GR capable. In order to support Graceful Restart, a GR Restarter needs to make a backup of its FEC (forwarding equivalence class) and label information when LDP restarts.

Suppose an LDP session is GR capable:

1) After the GR Restarter has restarted LDP, the GR Helper will detect an improper working state of the related LDP session.

2) Before the Reconnect Timer times out, if the GR Helper receives a session request from the GR Restarter, it will maintain the LSP and label information of the session and restore the session with the GR Restarter; otherwise, it will delete all the LDP and label information associated with the session.

3) After the session has recovered, the GR Restarter and the GR Helper will activate their Neighbor Liveness Timer and Recovery Timer, restore all the LSP information relative to this session, and send to each other the Label Mapping (Downstream Unsolicited) and Label Request (Downstream-on-Demand) Messages.

4) The GR Restarter and the GR Helper will delete the LSP stale upon receipt of the Label Mapping Message, and will delete all the LSP information associated after the Neighbor Liveness Timer and the Recovery Timer have timed out.

To summarize, during the Graceful Restart, the LSP information on the data link layer will be preserved so that the MPLS packets can be forwarded without interruption.

1.2 Configuring Graceful Restart

& Note:

One device can act as both GR Restarter and GR Helper at the same time.

Graceful Restart configuration can be normally achieved by enabling the Graceful Restart Capability. The time parameters can be configured if necessary, but under normal circumstances the defaults should be used.

1.2.1 Configuring BGP-based Graceful Restart

Follow these steps to configure BGP-based GR on the GR Restarter and the GR Helper:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable BGP, and enter its view	bgp as-number	Required Disabled by default
Enable Graceful Restart Capability for BGP	graceful-restart	Required Disabled by default
Configure the maximum time allowed for the peer to reestablish a BGP session	graceful-restart timer restart timer	Optional 150 seconds by default
Configure the maximum time to wait for the End-of-RIB marker	graceful-restart timer wait-for-rib timer	Optional 180 seconds by default
Note: End-of-RIB = End of Router-Information-Base

& Note:

l In general the maximum time allowed for the peer to reestablish a BGP session should be less than the Holdtime carried in the OPEN message. Upon detection of the termination of the session, the GR Helper will wait for a period of time as specified by the GR Time before it reestablishes the BGP session.

l The End-of-RIB marker can be used to indicate that the updated routing information has been sent.

l For the command related to enabling BGP and entering BGP view, refer to BGP Commands in the IP Routing Volume.

1.2.2 Configuring OSPF-based Graceful Restart

I. Configuring OSPF-based GR Restarter

Follow these steps to configure the OSPF-based GR Restarter:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable OSPF and enter OSPF view	ospf [ process-id \| router-id router-id \| vpn-instance instance-name ] *	Required Disabled by default
Enable the use of link-local signaling	enable link-local-signaling	Required Disabled by default
Enable out-of-band resynchronization	enable out-of-band-resynchronization	Required Disabled by default
Enable GR for OSPF	graceful-restart	Required Disabled by default

II. Configuring OSPF-based GR Helper

Follow these steps to configure OSPF-based GR Helper:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable OSPF, and enter OSPF view	ospf [ process-id \| router-id router-id \| vpn-instance instance-name ] *	Required Disabled by default
Enable the use of link-local signaling	enable link-local-signaling	Required Disabled by default
Enable out-of-band resynchronization	enable out-of-band-resynchronization	Required Disabled by default
Configure for which OSPF neighbors the current router can serve as a GR Helper	graceful-restart help { acl-number \| prefix prefix-list }	Optional The router can server as a GR Helper for any OSPF neighbor by default.

& Note:

For the command related to enabling OSPF and entering OSPF view, refer to OSPF Commands in the IP Routing Volume.

III. Restarting OSPF Graceful Restart

You can perform the following configuration on a routing switch to restart the GR process for OSPF protocol, but ensure that the routing switch has been enabled with the following capabilities first:

l LLS (link local signaling)

l OOB (out of band resynchronization)

Following the step to restart the OSPF GR process:

To do…	Use the command…	Remarks
Restart OSPF Graceful Restart	reset ospf [ process-id ] process graceful-restart	Required Available in user view

To do…

Use the command…

Remarks

Restart OSPF Graceful Restart

reset ospf [ process-id ] process graceful-restart

Required

Available in user view

1.2.3 Configuring IS-IS-based Graceful Restart

An IS-IS restart may cause the termination of the adjacency between a restarting switch and its neighbors, resulting in transient network disconnection.

IS-IS-based Graceful Restart can help to solve this problem. A switch enabled with IS-IS-based Graceful Restart can notify its neighbors its restarting state to allow its neighbors to reestablish the adjacency without terminating the connections with it.

The IS-IS-based Graceful Restart provides the following enhancements:

l In the event of an ISIS restart, a Graceful Restart Capable switch will resend connection requests to its neighbors instead of terminating their neighboring relationship.

l Graceful Restart has minimized the disruption on network traffic caused by the waiting occurred due to data synchronization before LSP packets generation.

l For a switch starts for the first time, in LSP packets overload bit is set to achieve database synchronization, which ensures no loopback is created in a network.

Graceful Restart Interval is configured as the Remaining Lifetime contained in the IS-IS Hello PDU so that the adjacency between a GR Restarter and its neighbors can be maintained across restart.

Setting the suppress-advertisement (SA) bit is to suppress the advertisement of the adjacency to the restarting switch in the Hello PDU across restart. In this way, the neighbors of the GR Restarter will not advertise the adjacency within the specified period until the completion of data synchronization between the GR Restarter and its GR Helpers. This feature helps to effectively avoid the possible blackholes across GR restart due to the sending or receiving of LSP.

Follow these steps to configure GR on the GR Restarter GR Helper respectively:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Enable IS-IS, and enter IS-IS view.	isis [ process-id ] [ vpn-instance vpn-instance-name ]	Required Disabled by default
Enable Graceful Restart Capability for IS-IS	graceful-restart	Required Disabled by default
Set the Graceful Restart interval	graceful-restart interval interval-value	Optional 300 seconds by default
Configure to set the SA (Suppress-Advertisement) bit during restart	graceful-restart suppress-sa	Optional By default, the SA bit is clear.

& Note:

For the command related to enabling IS-IS and entering IS-IS view, refer to IS-IS Commands in the IP Routing Volume.

1.2.4 Configuring MPLS LDP-based Graceful Restart

I. Configuring MPLS LDP-based Graceful Restart

Follow these steps to configure GR on the GR Restarter:

To do…	Use the command…	Remarks
Enter system view	system-view	—
Configure the LSR ID	mpls lsr-id lsr-id	Required No MPLS LSR ID is configured by default.
Enable MPLS, and enter MPLS view	mpls	Required Disabled by default.
Return to system view	quit	—
Enable LDP, and enter MPLS LDP view	mpls ldp	Required Disabled by default
Enable Graceful Restart Capability for MPLS LDP	graceful-restart	Required Disabled by default
Configure the FT Reconnect Timer	graceful-restart timer reconnect timer	Optional 300 seconds by default
Configure the Neighbor Liveness Timer	graceful-restart timer neighbor-liveness timer	Optional 120 seconds by default
Configure the LDP Recovery Timer	graceful-restart timer recovery timer	Optional 300 seconds by default

& Note:

In the MPLS LDP GR process, the GR Helper compares the locally configured LDP Neighbor Liveness Timer with the FT Reconnect Timer configured on the remote GR Restarter, and takes the smaller one as the local FT Reconnect Timer; the GR Helper compares the locally configured LDP Recovery Timer with the Recovery Timer configured on the remote GR Restarter and takes the smaller one as the local Recovery Timer.

II. Restarting MPLS LDP

After configuring parameters for MPLS LDP-based GR, you need to restart MPLS LDP to validate the configurations.

To do…	Use the command…	Remarks
Restart MPLS LDP	reset mpls ldp [ all \| peer ldp-peer \| vpn-instance ldp-vpn-instance [ peer ldp-peer ] ]	Required Available in user view

To do…

Use the command…

Remarks

Restart MPLS LDP

reset mpls ldp [ all | peer ldp-peer | vpn-instance ldp-vpn-instance [ peer ldp-peer ] ]

Required

Available in user view

& Note:

For related description on the reset mpls ldp command, refer to MPLS Basics Commands in the MPLS VPN Volume.

III. Gracefully restarting MPLS LDP

When testing MPLS LDP GR in case main/backup failover is not needed, you can gracefully restart MPLS LDP. Do not perform this operation in normal cases.

Follow these steps to gracefully restart MPLS LDP:

To do…	Use the command…	Remarks
Gracefully restart MPLS LDP	graceful-restart mpls ldp	Required Available in user view

To do…

Use the command…

Remarks

Gracefully restart MPLS LDP

graceful-restart mpls ldp

Required

Available in user view

1.3 Displaying and Maintaining Graceful Restart

To do…	Use the command…	Remarks
Display the Graceful Restart state for IS-IS	display isis graceful-restart status [ level-1 \| level-2 ] [ process-id \| vpn-instance vpn-instance-name ]	Available in any view

1.4 Graceful Restart Configuration Examples

1.4.1 IS-IS-based Graceful Restart Configuration Example

I. Network requirements

Switch A, Switch B, and Switch C belong to the same domain and use IS-IS as their routing protocol, as illustrated in Figure 1-5.

II. Network diagram

Figure 1-5 IS-IS-based Graceful Restart configuration example

III. Configuration procedure

1) Configure IP addresses of the interfaces on each switch and configure IS-IS

Follow Figure 1-5 to configure the IP address and subnet mask of each interface on the switch. The configuration procedure is omitted.

Configure the switches as connected through IS-IS, ensuring that Switch A, Switch B and Switch C can communicate on the network layer and dynamic route update can be implemented among them with IS-IS. The configuration procedure is omitted here.

2) Configure IS-IS Graceful Restart

# Enable IS-IS Graceful Restart on Switch A and configure the Graceful Restart Interval.

<SysnameA> system-view

[SysnameA] interface vlan-interface 100

[SysnameA-Vlan-interface100] ip address 10.0.1.1 255.255.255.0

[SysnameA-Vlan-interface100] isis enable 1

[SysnameA-Vlan-interface100] quit

[SysnameA] router id 1.1.1.1

[SysnameA] isis 1

[SysnameA-isis-1] graceful-restart

[SysnameA-isis-1] graceful-restart interval 150

[SysnameA-isis-1] return

The configurations for Switch B and Switch C are similar, therefore are omitted here.

3) Verify the configuration

The adjacency between Switch A and Switch B and Switch C is established respectively and routing information can be exchanged among them. IS-IS on Switch A has restarted, which puts Switch A into a restart state. Switch A sends connection requests to its neighbors through the Graceful Restart mechanism to synchronize the database. Its restart status can be displayed using the display isis graceful-restart status command.

# Restart Switch A.

<SysnameA> reset isis all 1

Warning : Reset ISIS process? [Y/N]:y

# Check the Graceful Restart status of IS-IS on Switch A.

<SysnameA> display isis graceful-restart status

Restart information for IS-IS(1)

--------------------------------------------------------------------

IS-IS(1) Level-1 Restart Status

Restart Interval: 150

SA Bit Supported

Total Number of Interfaces = 1

Restart Status: RESTARTING

Number of LSPs Awaited: 3

T3 Timer Status:

Remaining Time: 239

T2 Timer Status:

Remaining Time: 59

IS-IS(1) Level-2 Restart Status

Restart Interval: 150

SA Bit Supported

Total Number of Interfaces = 1

Restart Status: RESTARTING

Number of LSPs Awaited: 3

T3 Timer Status:

Remaining Time: 239

T2 Timer Status:

Remaining Time: 59

1.4.2 OSPF-based Graceful Restart Configuration Example

I. Network requirements

l Switch A, Switch B and Switch C belong to the same autonomous system and the same OSPF domain. They are connected through OSPF.

l Switch A acts as the GR Restarter, and Switch B and Switch C are the GR Helpers and remain OOB synchronized with Switch A through GR mechanism.

II. Network diagram

Figure 1-6 OSPF-based Graceful Restart configuration example

III. Configuration procedure

1) Configure Switch A.

<SysnameA> system-view

[SysnameA] interface vlan-interface 100

[SysnameA-Vlan-interface100] ip address 192.1.1.1 255.255.255.0

[SysnameA-Vlan-interface100] quit

[SysnameA] router id 1.1.1.1

[SysnameA] ospf 100

[SysnameA-ospf-100] enable link-local-signaling

[SysnameA-ospf-100] enable out-of-band-resynchronization

[SysnameA-ospf-100] graceful-restart

[SysnameA-ospf-100] area 0

[SysnameA-ospf-100-area-0.0.0.0] network 192.1.1.0 0.0.0.255

[SysnameA-ospf-100-area-0.0.0.0] return

2) Configure Switch B.

<SysnameB> system-view

[SysnameB] acl number 2000

[SysnameB-acl-basic-2000] rule 10 permit source 192.1.1.1 0.0.0.0

[SysnameB-acl-basic-2000] quit

[SysnameB] interface vlan-interface 100

[SysnameB-Vlan-interface100] ip address 192.1.2.1 255.255.255.0

[SysnameB-Vlan-interface100] ospf dr-priority 0

[SysnameB-Vlan-interface100] quit

[SysnameB] router id 2.2.2.2

[SysnameB] ospf 100

[SysnameB-ospf-100] enable link-local-signaling

[SysnameB-ospf-100] enable out-of-band-resynchronization

[SysnameB-ospf-100] graceful-restart

[SysnameB-ospf-100] area 0

[SysnameB-ospf-100-area-0.0.0.0] network 192.1.2.0 0.0.0.255

3) Configure Switch C.

<SysnameC> system-view

[SysnameC] acl number 2000

[SysnameC-acl-basic-2000] rule 10 permit source 192.1.1.1 0.0.0.0

[SysnameC-acl-basic-2000] quit

[SysnameC] interface vlan-interface 100

[SysnameC-Vlan-interface100] ip address 192.1.3.1 255.255.255.0

[SysnameC-Vlan-interface100] ospf dr-priority 2

[SysnameC-Vlan-interface100] quit

[SysnameC] router id 3.3.3.3

[SysnameC] ospf process 100

[SysnameC-ospf-100] enable link-local-signaling

[SysnameC-ospf-100] enable out-of-band-resynchronization

[SysnameC-ospf-100] graceful-restart

[SysnameC-ospf-100] area 0

[SysnameC-ospf-100-area-0.0.0.0] network 192.1.3.0 0.0.0.255

H3C S9500 Operation Manual-Release2132[V2.03]-08 System Volume

1.1 Introduction to Graceful Restart

1.1.2 Basic Mechanism of Graceful Restart

I. BGP-based Graceful Restart

II. OSPF-based Graceful Restart

III. IS-IS-based Graceful Restart

1.2 Configuring Graceful Restart

1.2.2 Configuring OSPF-based Graceful Restart

I. Configuring OSPF-based GR Restarter

II. Configuring OSPF-based GR Helper

III. Restarting OSPF Graceful Restart

I. Configuring MPLS LDP-based Graceful Restart

II. Restarting MPLS LDP

III. Gracefully restarting MPLS LDP

1.3 Displaying and Maintaining Graceful Restart

1.4 Graceful Restart Configuration Examples

I. Network requirements

II. Network diagram

III. Configuration procedure

I. Network requirements

II. Network diagram

III. Configuration procedure

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