Normative References Informative References It could spread across multiple routing domains. They also consume resources on the control plane of the routers affected by the flap. As such, they are detrimental to the overall network performance.
This document describes a mechanism for BGP that would help minimize the negative effects on routing caused by BGP restart. Although the End-of-RIB marker is specified for the purpose of BGP graceful restart, it is noted that the generation of such a marker upon completion of the initial update would be useful for routing convergence in general, and thus the practice is recommended. In addition, it would be beneficial for routing convergence if a BGP speaker can indicate to its peer up-front that it will generate the End-of-RIB marker, regardless of its ability to preserve its forwarding state during BGP restart.
This can be accomplished using the Graceful Restart Capability described in the next section. It can also be used to convey to its peer its intention of generating the End-of-RIB marker upon the completion of its initial routing updates. The remaining bits are reserved and MUST be set to zero by the sender and ignored by the receiver. Restart Time: This is the estimated time in seconds it will take for the BGP session to be re-established after a restart.
This can be used to speed up routing convergence by its peer in case that the BGP speaker does not come back after a restart. When set value 1 , the bit indicates that the forwarding state has been preserved. In that case, the value of the "Restart Time" field advertised by the sender is irrelevant.
If more than one instance of the Graceful Restart Capability is carried in the capability advertisement, the receiver of the advertisement MUST ignore all but the last instance of the Graceful Restart Capability. There are two reasons for doing this. The first is to indicate its intention of generating the End-of-RIB marker upon the completion of its initial routing updates, as doing this would be useful for routing convergence in general.
The second is to indicate its support for a peer which wishes to perform a graceful restart. In the following sections, "Restarting Speaker" refers to a router whose BGP has restarted, and "Receiving Speaker" refers to a router that peers with the restarting speaker. Consider that the Graceful Restart Capability for an address family is advertised by the Restarting Speaker, and is understood by the Receiving Speaker, and a BGP session between them is established.
Unless allowed via configuration, the "Forwarding State" bit for an address family in the capability can be set only if the forwarding state has indeed been preserved for that address family during the restart. Once the session between the Restarting Speaker and the Receiving Speaker is re-established, the Restarting Speaker will receive and process BGP messages from its peers.
It is noted that prior to route selection, the speaker has no routes to advertise to its peers and no routes to update the forwarding state. Once the initial update is complete for an address family including the case that there is no routing update to send , the End-of-RIB marker MUST be sent.
To put an upper bound on the amount of time a router defers its route selection, an implementation MUST support a configurable timer that imposes this upper bound.
The value of this timer should be large enough, so as to provide all the peers of the Restarting Speaker with enough time to send all the routes to the Restarting Speaker. If one wants to apply graceful restart only when the restart is planned as opposed to both planned and unplanned restart , then one way to accomplish this would be to set the Forwarding State bit to 1 after a planned restart, and to 0 in all other cases.
Book Contents Book Contents. Find Matches in This Book. PDF - Complete Book Updated: September 12, BGP Peer Session Templates Peer session templates are used to group and apply the configuration of general BGP session commands to groups of neighbors that share session configuration elements.
Enter your password if prompted. Step 2 configure terminal Example: Device configure terminal Enters global configuration mode. Step 3 router bgp autonomous-system-number Example: Device config router bgp Enters router configuration mode and creates a BGP routing process. Step 4 address-family ipv4 [ unicast multicast vrf vrf-name ] Example: Device config-router address-family ipv4 unicast Specifies the IPv4 address family and enters address family configuration mode.
Step 5 neighbor ip-address remote-as autonomous-system-number Example: Device config-router-af neighbor Step 6 neighbor ip-address activate Example: Device config-router-af neighbor In this example, the internal BGP peer at Step 7 neighbor ip-address ha-mode graceful-restart [ disable ] Example: Device config-router-af neighbor Step 9 show ip bgp neighbors [ ip-address [ received-routes routes advertised-routes paths [ regexp ] dampened-routes flap-statistics received prefix-filter policy [ detail ]]] Example: Device show ip bgp neighbors Examples The following example shows partial output from the show ip bgp neighbors command for the BGP peer at Device show ip bgp neighbors Address tracking is enabled, the RIB does have a route to Figure 1.
Network Topology Showing BGP Neighbors The restart and stale-path timers can be modified only using the global bgp graceful-restart command. Note A BGP peer cannot inherit from a peer policy or session template and be configured as a peer group member at the same.
Step 4 template peer-session session-template-name Example: Device config-router template peer-session S1 Enters session-template configuration mode and creates a peer session template. In this example, a peer session template named S1 is created. Step 6 exit-peer-session Example: Device config-router-stmp exit-peer-session Exits session-template configuration mode and returns to router configuration mode.
Step 7 template peer-session session-template-name Example: Device config-router template peer-session S2 Enters session-template configuration mode and creates a peer session template. In this example, a peer session template named S2 is created. Step 9 exit-peer-session Example: Device config-router-stmp exit-peer-session Exits session-template configuration mode and returns to router configuration mode.
Step 10 bgp log-neighbor-changes Example: Device config-router bgp log-neighbor-changes Enables logging of BGP neighbor status changes up or down and neighbor resets. Step 11 neighbor ip-address remote-as autonomous-system-number Example: Device config-router neighbor Step 12 neighbor ip-address inherit peer-session session-template-number Example: Device config-router neighbor Step 13 neighbor ip-address remote-as autonomous-system-number Example: Device config-router neighbor Step 14 neighbor ip-address inherit peer-session session-template-number Example: Device config-router neighbor Step 16 show ip bgp template peer-session [ session-template-number ] Example: Device show ip bgp template peer-session Optional Displays locally configured peer session templates.
Step 17 show ip bgp neighbors [ ip-address [ received-routes routes advertised-routes paths [ regexp ] dampened-routes flap-statistics received prefix-filter policy [ detail ]]] Example: Device show ip bgp neighbors In this example, the peer group named PG1 is created. Step 6 neighbor peer-group-name remote-as autonomous-system-number Example: Device config-router-af neighbor PG1 remote-as Configures peering with a BGP peer group in the specified autonomous system.
Step 7 neighbor peer-group-name ha-mode graceful-restart [ disable ] Example: Device config-router-af neighbor PG1 ha-mode graceful-restart disable Enables the BGP graceful restart capability for a BGP neighbor.
Step 8 neighbor ip-address peer-group peer-group-name Example: Device config-router-af neighbor Step 10 show ip bgp neighbors [ ip-address [ received-routes routes advertised-routes paths [ regexp ] dampened-routes flap-statistics received prefix-filter policy [ detail ]]] Example: Device show ip bgp neighbors Figure 2. Table 1. Was this Document Helpful?
Yes No Feedback. Related Cisco Community Discussions. Step 1. Enables privileged EXEC mode. Step 2. Enters global configuration mode. Step 3. Enters router configuration mode and creates a BGP routing process. This requirement causes a problem while ensuring that "stale" routing information does not leak beyond the perimeter of routers that support these procedures where one or more IBGP routers are not upgraded. The principal motivation for restricting the propagation of "stale" routing information is the reason to prevent it from spreading without limit once it exits the BGP confederation boundary.
VPN deployments are typically topologically constrained, removing this concern. For this reason, an implementation might advertise stale routes over a PE-CE session, when explicitly configured. In that case, if you configure the PE routers to advertise such routes, you must notify the operator of the CE receiving the routes, and the CE must be configured to depreference the routes. When the LLGR receiver mode is enabled or disabled, the session is reset.
This behavior enables the new capability value to be sent to the neighbor. When the advertise-to-non-llgr-neighbor option is enabled or disabled, export policy is reevaluated, and LLGR stale routes might be advertised or withdrawn. When the omit-no-export option is added or removed, the session is reset.
This rest of a session enables LLGR stale routes to be readvertised with or without the no- export community which is added outside of the export policy. To enable the BGP long-lived graceful restart capability at the system or global level and configure its properties:. To enable the BGP long-lived graceful restart capability at the neighbor or peer group level and configure its properties:.
Two new well-known communities are introduced. These new BGP communities can be used in any of the configuration hierarchy levels as other symbolic well-known communities such as no-advertise, no-export, and no-export-subconfed in the community attribute of static route definitions or in a policy-options community definition. The two new communities are as follows:. The Notification message feature does not have any associated configuration parameters.
You can include the llgr-stale and no-llgr options with the community name members statement to associate BGP community information with a static, aggregate, or generated route at the following hierarchy levels:. To configure the BGP long-lived graceful restart communities for use in a routing policy match condition:. The privileges are the same as for protocols bgp. The long-lived-graceful-restart section is visible only for families l2vpn, inet labeled-unicast, inet flow and route-target.
It is prohibited for inet-mvpn, inet6-mvpn and inet-mdt. It is hidden for other families. You can associate the community that you previously defined and a list of address prefixes in a routing policy to selectively accept or reject the routes for long-lived graceful restart for the specified prefixes, as follows:. Two hidden configuration statements are added under the [edit protocols bgp graceful-restart ] hierarchy level for global, group-level, and neighbor group-level configuration.
The disable-notification-flag statement at the [edit protocols bgp graceful-restart] , [edit protocols bgp group group-name graceful-restart] , or [edit protocols bgp group group-name neighbor neighbor-address graceful-restart] hierarchy level disables the transmission of the N flag in the graceful restart capability negotiation.
The disable-notification-extensions statement at the [edit protocols bgp graceful-restart] , [edit protocols bgp group group-name graceful-restart] , or [edit protocols bgp group group-name neighbor neighbor-address graceful-restart] hierarchy level also disables the transmission of the N flag in the graceful restart capability negotiation, but in addition, it disables the new rules for invoking graceful restart receiver mode as specified in the IETF bgp-gr-notification draft, and disables the transmission of the Hard Reset subcode.
The Hard Reset subcode is continued to be observed when received in a Notify or a Cease message. To disable the transmission of N flags and to disable rules for triggering graceful restart at the global or system-wide level:. To disable the transmission of N flags and to disable rules for triggering graceful restart at the group level:. To disable the transmission of N flags and to disable rules for triggering graceful restart at the neighbor or peer level:.
You can also configure the BGP long-lived graceful restarter mode negotiation mechanism for a particular address family instead of configuring this capability for all address families in a system, logical system, or routing instance. To enable BGP LLGR for a specific address family, include the graceful-restart long-lived restarter stale-time interval statement at one of the following hierarchy levels.
Each routing table is identified by the protocol family or address family indicator AFI and a subsequent address family identifier SAFI. The AFI parameter can be one of the l2vpn inet route-target protocols and the SAFI parameter can be either of the flow labeled-unicast protocols for inet family and one of the auto-discovery-mspw auto-discovery-only signaling protcols for L2VPN family.. The stanzas in the per-family graceful-restart long-lived restarter configuration section enables LLGR restarter mode negotiation for BGP globally, or for a group or neighbor.
The values are inherited by groups from the global configuration, and by neighbors from the group configuration. The disable attribute is used to override configuration inherited from a higher level. A hidden enable attribute can be used to override an inherited disable attribute. Configuring graceful-restart long-lived restarter at the neighbor level when it is not configured at the containing group level or globally causes an internal group to be split.
When LLGR restarter is enabled or disabled for a family or the stale- time is changed, the session is reset so that the new capability can be sent to the neighbor.
The value is a simple integer giving the number of seconds by default, but it can also be specified using the following notation:. The specified number of days is multiplied by , the number of hours by and the number of minutes by 60; these are added to the seconds to get the total.
A combined format of days and hours, in different time period units, such as 1d36h are permitted, as long as the specified total does not exceed the maximum stale time.
The hours and minutes are optional. The two notations can be combined, for example, 2w1d specifies two weeks, one day, twelve hours and two seconds seconds. Note that the CLI requires double-quotes around a value like this with spaces in it. Expressed in this notation, the maximum stale time is 27w5d 27 weeks, 5 days, 4 hours, 20 minutes and 15 seconds. While the show configuration command displays the actually configured values, when the associated timers are displayed in run-time show commands such as show bgp neighbor , the values are normalized, such as 1d36h becoming 2d The full rules for displaying normalized LLGR times depend on the clear bgp neighbor neighbor-address gracefully command configuration.
To configure the BGP long-lived graceful restart characteristics per-address family and per-subsequent address family at the global level for a logical system or a routing instance:.
To configure the BGP long-lived graceful restart characteristics per-address family and per-subsequent address family at the BGP group level for a logical system or a routing instance:. To configure the BGP long-lived graceful restart characteristics per-address family and per-subsequent address family at the BGP neighbor group level for a logical system or a routing instance:.
After a BGP session goes down and before the session is reestablished, stale routes might be retained for up to two consecutive periods, controlled by the restart time and long-lived stale time parameters, respectively. During the first period routing modifications are prevented but with potential null-route filtering of traffic.
During the second period, possible null-route filtering of traffic might be reduced but routing changes ares visible throughout the network. In your network environment, the setting of the relevant parameters for a particular application must consider the tradeoffs, the network dynamics and potential failure scenarios. If necessary, the first period can be bypassed either by local configuration or by setting the restart time in the graceful restart capability to zero, not listing the address family indicators AFI and a subsequent address family identifiers SAFI in that capability.
The setting of the F bit and the "Forwarding State" bit of the accompanying GR capability depends in part on deployment considerations. The F bit can be interpreted to indicate the helper router needs to flush associated routes if the bit is left clear.
An important scenario in which LLGR is used is for routes that are more similar to configuration than to traditional routing hop-by-hop forwarding instead of tunnel-based routing. For such routes, it might be useful to always set the F bit, regardless of other considerations. Similarly, for control-plane-only entities such as dedicated route reflectors, that do not participate in the forwarding plane, it is preferred that the F bit be always set.
Overall, the guideline to be adopted is that if loss of state on the restarting router can reasonably be expected to cause a forwarding loop or null route, the F bit must be set judiciously, depending on whether state has been retained.
You can determine whether the F bit needs to be set or not, based on your deployment needs and configured settings. In such a scenario, the network operator configuring their PE to advertise such routes must notify the operator of the CE receiving the routes, and the CE must be configured to depreference the routes.
You can specify the Forwarding State bit, which is a BGP configuration option that can be defined at the global, group and neighbor levels, for any logical system or routing instance. To specify the Forwarding State bit at the global, BGP group, or BGP neighbor level, include the forwarding-state-bit as-rr-client from-fib statement at the [edit protocols bgp graceful-restart] , [edit protocols bgp group-group-name graceful-restart] , or [edit protocols bgp group-group-name neighbor neighbor-address graceful-restart] hierarchy level.
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