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Table Of Contents
Configuring Advanced BGP Features
Prerequisites for Configuring Advanced BGP Features
Restrictions for Configuring Advanced BGP Features
Information About Configuring Advanced BGP Features
BGP Support for Next-Hop Address Tracking
Selective BGP Next-Hop Route Filtering
BGP Nonstop Forwarding Awareness
Cisco NSF Routing and Forwarding Operation
Cisco Express Forwarding for NSF
BGP Graceful Restart per Neighbor
How to Configure Advanced BGP Features
Configuring BGP Next-Hop Address Tracking
Disabling BGP Next-Hop Address Tracking
Adjusting the Delay Interval for BGP Next-Hop Address Tracking
Configuring BGP Selective Next-Hop Route Filtering
Configuring BGP Nonstop Forwarding Awareness Using BGP Graceful Restart
Enabling BGP Global NSF Awareness Using BGP Graceful Restart
Configuring BGP NSF Awareness Timers
Enabling and Disabling BGP Graceful Restart Using BGP Peer Session Templates
Enabling BGP Graceful Restart for an Individual BGP Neighbor
Disabling BGP Graceful Restart for a BGP Peer Group
Verifying the Configuration of BGP Nonstop Forwarding Awareness
Configuring BGP Route Dampening
Enabling and Configuring BGP Route Dampening
Monitoring and Maintaining BGP Route Dampening
Decreasing BGP Convergence Time Using BFD
Configuring BFD Session Parameters on the Interface
Configuring BFD Support for BGP
Configuration Examples for Configuring Advanced BGP Features
Enabling and Disabling BGP Next-Hop Address Tracking: Example
Adjusting the Delay Interval for BGP Next-Hop Address Tracking: Example
Configuring BGP Selective Next-Hop Route Filtering: Examples
Enabling BGP Global NSF Awareness Using Graceful Restart: Example
Enabling and Disabling BGP Graceful Restart per Neighbor: Examples
Configuring BGP Route Dampening: Example
Enabling BGP MIB Support: Examples
Feature Information for Configuring Advanced BGP Features
Configuring Advanced BGP Features
First Published: October 31, 2005Last Updated: May 4, 2009This module describes configuration tasks for various advanced Border Gateway Protocol (BGP) features. BGP is an interdomain routing protocol that is designed to provide loop-free routing between organizations. This module contains tasks to configure BGP next-hop address tracking, BGP Nonstop Forwarding (NSF) awareness using the BGP graceful restart capability, route dampening, Bidirectional Forwarding Detection (BFD) support for BGP, and BGP MIB support.
Finding Feature Information
For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Configuring Advanced BGP Features" section.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
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Prerequisites for Configuring Advanced BGP Features
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Prerequisites for Configuring Advanced BGP Features
Before configuring advanced BGP features you should be familiar with the "Cisco BGP Overview" module and the "Configuring a Basic BGP Network" module.
Restrictions for Configuring Advanced BGP Features
A router that runs Cisco IOS XE software can be configured to run only one BGP routing process and to be a member of only one BGP autonomous system. However, a BGP routing process and autonomous system can support multiple address family configurations.
Information About Configuring Advanced BGP Features
To configure the BGP features in this module, you should understand the following concepts:
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BGP Version 4
Border Gateway Protocol (BGP) is an interdomain routing protocol designed to provide loop-free routing between separate routing domains that contain independent routing policies (autonomous systems). The Cisco IOS XE software implementation of BGP version 4 includes multiprotocol extensions to allow BGP to carry routing information for IP multicast routes and multiple Layer 3 protocol address families including IP Version 4 (IPv4), IP Version 6 (IPv6), Virtual Private Networks version 4 (VPNv4), and Connectionless Network Services (CLNS). For more details about configuring a basic BGP network, see the "Configuring a Basic BGP Network" module.
BGP is mainly used to connect a local network to an external network to gain access to the Internet or to connect to other organizations. When connecting to an external organization, external BGP (eBGP) peering sessions are created. For more details about connecting to external BGP peers, see the "Connecting to a Service Provider Using External BGP" module.
Although BGP is referred to as an exterior gateway protocol (EGP) many networks within an organization are becoming so complex that BGP can be used to simplify the internal network used within the organization. BGP peers within the same organization exchange routing information through internal BGP (iBGP) peering sessions. For more details about internal BGP peers, see the "Configuring Internal BGP Features" module of the Cisco IOS XE IP Routing: BGP Configuration Guide, Release 2.3.
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BGP Support for Next-Hop Address Tracking
To configure BGP next-hop address tracking, you should understand the following concepts:
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BGP Next-Hop Address Tracking
The BGP next-hop address tracking feature is enabled by default when a supporting Cisco software image is installed. BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to the BGP routing process as they are updated in the RIB. This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a best-path calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.
Default BGP Scanner Behavior
BGP monitors the next hop of installed routes to verify next-hop reachability and to select, install, and validate the BGP best path. By default, the BGP scanner is used to poll the RIB for this information every 60 seconds. During the 60 second time period between scan cycles, Interior Gateway Protocol (IGP) instability or other network failures can cause black holes and routing loops to temporarily form.
Selective BGP Next-Hop Route Filtering
BGP selective next-hop route filtering was implemented as part of the BGP Selective Address Tracking feature to support BGP next-hop address tracking. Selective next-hop route filtering uses a route map to selectively define routes to help resolve the BGP next hop.
The ability to use a route map with the bgp nexthop command allows the configuration of the length of a prefix that applies to the BGP Next_Hop attribute. The route map is used during the BGP bestpath calculation and is applied to the route in the routing table that covers the next-hop attribute for BGP prefixes. If the next-hop route fails the route map evaluation, the next-hop route is marked as unreachable. This command is per address family, so different route maps can be applied for next-hop routes in different address families.
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BGP Next_Hop Attribute
The Next_Hop attribute identifies the next-hop IP address to be used as the BGP next hop to the destination. The router makes a recursive lookup to find the BGP next hop in the routing table. In external BGP (eBGP), the next hop is the IP address of the peer that sent the update. Internal BGP (iBGP) sets the next-hop address to the IP address of the peer that advertised the prefix for routes that originate internally. When any routes to iBGP that are learned from eBGP are advertised, the Next_Hop attribute is unchanged.
A BGP next-hop IP address must be reachable in order for the router to use a BGP route. Reachability information is usually provided by the IGP, and changes in the IGP can influence the forwarding of the next-hop address over a network backbone.
BGP Nonstop Forwarding Awareness
To configure BGP Nonstop Forwarding (NSF) awareness, you should understand the following concepts:
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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.
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 then updates the line cards with the new FIB information.
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Cisco Express Forwarding for NSF
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 for 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
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.
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BGP Graceful Restart for NSF
When an NSF-capable router begins a BGP session with a BGP peer, it sends an OPEN message to the peer. Included in the message is a declaration that the NSF-capable or NSF-aware router 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) (NSF-aware peers) 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 functionality 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.
BGP NSF Awareness
BGP support for NSF requires that neighbor routers are NSF-aware or NSF-capable. NSF awareness in BGP is also enabled by the graceful restart mechanism. A router that is NSF-aware functions like a router that is NSF-capable with one exception: an NSF-aware router is incapable of performing an SSO operation. However, a router that is NSF-aware is capable of maintaining a peering relationship with a NSF-capable neighbor during a NSF SSO operation, as well as holding routes for this neighbor during the SSO operation.
The BGP Nonstop Forwarding Awareness feature provides an NSF-aware router with the capability to detect a neighbor that is undergoing an SSO operation, maintain the peering session with this neighbor, retain known routes, and continue to forward packets for these routes. The deployment of BGP NSF awareness can minimize the affects of route-processor (RP) failure conditions and improve the overall network stability by reducing the amount of resources that are normally required for reestablishing peering with a failed router.
NSF awareness for BGP is not enabled by default. The bgp graceful-restart command is used to globally enable NSF awareness on a router that is running BGP. NSF-aware operations are also transparent to the network operator and BGP peers that do not support NSF capabilities.
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BGP Graceful Restart per Neighbor
The ability to enable or disable BGP graceful restart for every individual BGP neighbor was introduced. Three new methods of configuring BGP graceful restart for BGP peers, in addition to the existing global BGP graceful restart configuration, are now available. Graceful restart can be enabled or disabled for a BGP peer or a BGP peer group using the neighbor ha-mode graceful-restart command, or a BGP peer can inherit a graceful restart configuration from a BGP peer-session template using the ha-mode graceful-restart command.
Although BGP graceful restart is disabled by default, the existing global command enables graceful restart for all BGP neighbors regardless of their capabilities. The ability to enable or disable BGP graceful restart for individual BGP neighbors provides a greater level of control for a network administrator.
When the BGP graceful restart capability is configured for an individual neighbor, each method of configuring graceful restart has the same priority, and the last configuration instance is applied to the neighbor. For example, if global graceful restart is enabled for all BGP neighbors but an individual neighbor is subsequently configured as a member of a peer group for which the graceful restart is disabled, graceful restart is disabled for that neighbor.
The configuration of the restart and stale-path timers is available only with the global bgp graceful-restart command, but the default values are set when the neighbor ha-mode graceful-restart or ha-mode graceful-restart commands are configured. The default values are optimal for most network deployments, and these values should be adjusted only by an experienced network operator.
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. General session commands that are common for neighbors that are configured in different address families can be configured within the same peer session template. Peer session templates are created and configured in peer session configuration mode. Only general session commands can be configured in a peer session template.
General session commands can be configured once in a peer session template and then applied to many neighbors through the direct application of a peer session template or through indirect inheritance from a peer session template. The configuration of peer session templates simplifies the configuration of general session commands that are commonly applied to all neighbors within an autonomous system.
Peer session templates support direct and indirect inheritance. A BGP neighbor can be configured with only one peer session template at a time, and that peer session template can contain only one indirectly inherited peer session template. A BGP neighbor can directly inherit only one session template and can indirectly inherit up to seven additional peer session templates.
Peer session templates support inheritance. A directly applied peer session template can directly or indirectly inherit configurations from up to seven peer session templates. So, a total of eight peer session templates can be applied to a neighbor or neighbor group.
Peer session templates support only general session commands. BGP policy configuration commands that are configured only for a specific address family or NLRI configuration mode are configured with peer policy templates.
For more details about BGP peer session templates, see the "Configuring a Basic BGP Network" module.
To use a BGP peer session template to enable or disable BGP graceful restart, see the section Enabling and Disabling BGP Graceful Restart Using BGP Peer Session Templates.
BGP Route Dampening
Route dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork. A route is considered to be flapping when its availability alternates repeatedly.
For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous system 2, and autonomous system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes unavailable). Under circumstances without route dampening, the eBGP neighbor of autonomous system 1 to autonomous system 2 sends a withdraw message to autonomous system 2. The border router in autonomous system 2, in turn, propagates the withdraw message to autonomous system 3. When the route to network A reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to autonomous system 3. If the route to network A repeatedly becomes unavailable, then available, many withdrawal and advertisement messages are sent. This is a problem in an internetwork connected to the Internet because a route flap in the Internet backbone usually involves many routes.
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Minimizing Flapping
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to network A flaps. The router in autonomous system 2 (where route dampening is enabled) assigns network A a penalty of 1000 and moves it to history state. The router in autonomous system 2 continues to advertise the status of the route to neighbors. The penalties are cumulative. When the route flaps so often that the penalty exceeds a configurable suppress limit, the router stops advertising the route to network A, regardless of how many times it flaps. Thus, the route is dampened.
The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once again advertised. At half of the reuse limit, the dampening information for the route to network A is removed.
Understanding Route Dampening Terms
The following terms are used when describing route dampening:
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The routes external to an autonomous system learned via iBGP are not dampened. This policy prevent the iBGP peers from having a higher penalty for routes external to the autonomous system.
BFD for BGP
Bidirectional Forwarding Detection (BFD) is a detection protocol designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning will be easier, and reconvergence time will be consistent and predictable. The main benefit of implementing BFD for BGP is a marked decrease in reconvergence time.
One caveat exists for BFD; BFD and BGP graceful restart capability cannot both be configured on a router running BGP. If an interface goes down, BFD detects the failure and indicates that the interface cannot be used for traffic forwarding and the BGP session goes down, but graceful restart still allows traffic forwarding on platforms that support NSF even though the BGP session is down, allowing traffic forwarding using the interface that is down. Configuring both BFD and BGP graceful restart for NSF on a router running BGP may result in suboptimal routing.
See also the "Configuring BGP Neighbor Session Options" chapter, the section "Configuring BFD for BGP IPv6 Neighbors."
For more details about BFD, see the "Bidirectional Forwarding Detection" module of the Cisco IOS XE IP Routing: BFD Configuration Guide, Release 2.3.
BGP MIB Support
The Management Information Base (MIB) that supports BGP is the CISCO-BGP4-MIB. In Cisco IOS XE Release 2.1 and later releases, the BGP MIB Support Enhancements feature introduced support in the CISCO-BGP4-MIB for new SNMP notifications. The following sections describe the objects and notifications (traps) that are supported:
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BGP FSM Transition Change Support
The cbgpRouteTable supports BGP Finite State Machine (FSM) transition state changes.
The cbgpFsmStateChange object allows you to configure SNMP notifications (traps) for all FSM transition state changes. This notification contains the following MIB objects:
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The cbgpBackwardTransition object supports all BGP FSM transition state changes. This object is sent each time the FSM moves to either a higher or lower numbered state. This notification contains the following MIB objects:
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The snmp-server enable bgp traps command allows you to enable the traps individually or together with the existing FSM backward transition and established state traps as defined in RFC 1657.
BGP Route Received Route Support
The cbgpRouteTable object supports the total number of routes received by a BGP neighbor. The following MIB object is used to query the CISCO-BGP4-MIB for routes that are learned from individual BGP peers:
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Routes are indexed by the address-family identifier (AFI) or subaddress-family identifier (SAFI). The prefix information displayed in this table can also viewed in the output of the show ip bgp command.
BGP Prefix Threshold Notification Support
The cbgpPrefixMaxThresholdExceed and cbgpPrfefixMaxThresholdClear objects were introduced to allow you to poll for the total number of routes received by a BGP peer.
The cbgpPrefixMaxThresholdExceed object allows you to configure SNMP notifications to be sent when the prefix count for a BGP session has exceeded the configured value. This notification is configured on a per address family basis. The prefix threshold is configured with the neighbor maximum-prefix command. This notification contains the following MIB objects:
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The cbgpPrfefixMaxThresholdClear object allows you to configure SNMP notifications to be sent when the prefix count drops below the clear trap limit. This notification is configured on a per address family basis. This notification contains the following objects:
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Notifications are sent when the prefix count drops below the clear trap limit for an address family under a BGP session after the cbgpPrefixMaxThresholdExceed notification is generated. The clear trap limit is calculated by subtracting 5 percent from the maximum prefix limit value configured with the neighbor maximum-prefix command. This notification will not be generated if the session goes down for any other reason after the cbgpPrefixMaxThresholdExceed is generated.
VPNv4 Unicast Address Family Route Support
The cbgpRouteTable object allows you to configure SNMP GET operations for VPNv4 unicast address-family routes.
The following MIB object allows you to query for multiple BGP capabilities (for example, route refresh, multiprotocol BGP extensions, and graceful restart):
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The following MIB object allows you to query for IPv4 and VPNv4 address family routes:
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Each route is indexed by peer address, prefix, and prefix length. This object indexes BGP routes by the AFI and then by the SAFI. The AFI table is the primary index, and the SAFI table is the secondary index. Each BGP speaker maintains a local Routing Information Base (RIB) for each supported AFI and SAFI combination.
cbgpPeerTable Support
The cbgpPeerTable has been modified to support the enhancements described in this document. The following new table objects are supported in the CISCO-BGP-MIB.my:
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The following table objects are not supported. The status of theses objects is listed as deprecated, and these objects are not operational:
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How to Configure Advanced BGP Features
This section contains the following task groups:
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Configuring BGP Next-Hop Address Tracking
The tasks in this section show how configure BGP next-hop address tracking. BGP next-hop address tracking significantly improves the response time of BGP to next-hop changes in the RIB. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP. For more details about configuring route dampening, see the "Configuring BGP Route Dampening" section.
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Disabling BGP Next-Hop Address Tracking
Perform this task to disable BGP next-hop address tracking. BGP next-hop address tracking is enabled by default under the IPv4 and VPNv4 address families. Disabling next hop address tracking may be useful if you the network has unstable IGP peers and route dampening is not resolving the stability issues. To reenable BGP next-hop address tracking, use the bgp nexthop command with the trigger and enable keywords.
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Adjusting the Delay Interval for BGP Next-Hop Address Tracking
Perform this task to adjust the delay interval between routing table walks for BGP next-hop address tracking.
You can increase the performance of this feature by tuning the delay interval between full routing table walks to match the tuning parameters for the Interior Gateway protocol (IGP). The default delay interval is 5 seconds. This value is optimal for a fast-tuned IGP. In the case of an IGP that converges more slowly, you can change the delay interval to 20 seconds or more, depending on the IGP convergence time.
BGP next-hop address tracking significantly improves the response time of BGP to next-hop changes in the RIB. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP.
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Configuring BGP Selective Next-Hop Route Filtering
Perform this task to configure selective next-hop route filtering using a route map to filter potential next-hop routes. This task uses prefix lists and route maps to match IP addresses or source protocols and can be used to avoid aggregate addresses and BGP prefixes being considered as next-hop routes.
For more examples of how to use the bgp nexthop command, see the "Configuring BGP Selective Next-Hop Route Filtering: Examples" section.
Restrictions
Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported.
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Step 1
enable
Example:Router> enable
Enables privileged EXEC mode.
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configure terminal
Example:Router# configure terminal
Enters global configuration mode.
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router bgp autonomous-system-number
Example:Router(config)# router bgp 45000
Enters router configuration mode and creates a BGP routing process.
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address-family ipv4 [unicast | multicast | vrf vrf-name]
Example:Router(config-router)# address-family ipv4 unicast
Specifies the IPv4 address family and enters address family configuration mode.
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bgp nexthop route-map map-name
Example:Router(config-router-af)# bgp nexthop route-map CHECK-NEXTHOP
Permits a route map to selectively define routes to help resolve the BGP next hop.
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exit
Example:Router(config-router-af)# exit
Exits address family configuration mode and enters router configuration mode.
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exit
Example:Router(config-router)# exit
Exits router configuration mode and enters global configuration mode.
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ip prefix-list list-name [seq seq-value] {deny network/length | permit network/length} [ge ge-value] [le le-value]
Example:Router(config)# ip prefix-list FILTER25 seq 5 permit 0.0.0.0/0 le 25
Creates a prefix list for BGP next-hop route filtering.
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route-map map-name [permit | deny] [sequence-number]
Example:Router(config)# route-map CHECK-NEXTHOP deny 10
Configures a route map and enters route map configuration mode.
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match ip address prefix-list prefix-list-name [prefix-list-name...]
Example:Router(config-route-map)# match ip address prefix-list FILTER25
Matches the IP addresses in the specified prefix list.
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Step 11
exit
Example:Router(config-route-map)# exit
Exits route map configuration mode and enters global configuration mode.
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route-map map-name [permit | deny] [sequence-number]
Example:Router(config)# route-map CHECK-NEXTHOP permit 20
Configures a route map and enters route map configuration mode.
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end
Example:Router(config-route-map)# end
Exits route map configuration mode and returns to privileged EXEC mode.
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show ip bgp [network] [network-mask]
Example:Router# show ip bgp
Displays the entries in the BGP routing table.
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Examples
The following example from the show ip bgp command shows the next-hop addresses for each route:
BGP table version is 7, local router ID is 172.17.1.99Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incompleteNetwork Next Hop Metric LocPrf Weight Path* 10.1.1.0/24 192.168.1.2 0 0 40000 i* 10.2.2.0/24 192.168.3.2 0 0 50000 i*> 172.16.1.0/24 0.0.0.0 0 32768 i*> 172.17.1.0/24 0.0.0.0 0 32768Configuring BGP Nonstop Forwarding Awareness Using BGP Graceful Restart
The tasks in this section show how configure BGP Nonstop Forwarding (NSF) awareness using the BGP graceful restart capability. The first task enables BGP NSF globally for all BGP neighbors and suggests a few troubleshooting options. The second task describes how to adjust the BGP graceful restart timers although the default settings are optimal for most network deployments. The next three tasks demonstrate how to enable or disable BGP graceful restart for individual BGP neighbors including peer session templates and peer groups. The final task verifies the local and peer router configuration of BGP NSF.
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Enabling BGP Global NSF Awareness Using BGP Graceful Restart
Perform this task to enable BGP NSF awareness globally for all BGP neighbors. BGP NSF awareness is part of the graceful restart mechanism and BGP NSF awareness is enabled by issuing the bgp graceful-restart command in router configuration mode. BGP NSF awareness allows NSF-aware routers to support NSF-capable routers during an SSO operation. NSF-awareness is not enabled by default and should be configured on all neighbors that participate in BGP NSF.
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Restrictions
Configuring both BFD and BGP graceful restart for NSF on a router running BGP may result in suboptimal routing. For more details, see the "BFD for BGP" section.
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Troubleshooting Tips
To troubleshoot the NSF feature, use the following commands in privileged EXEC mode, as needed:
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What to Do Next
If the bgp graceful-restart command has been issued after the BGP session has been established, you must reset by issuing the clear ip bgp * command or by reloading the router before graceful restart capabilities will be exchanged. For more information about resetting BGP sessions and using the clear ip bgp command, see the "Configuring a Basic BGP Network" module.
Configuring BGP NSF Awareness Timers
Perform this task to adjust the BGP graceful restart timers. There are two BGP graceful restart timers that can be configured. The optional restart-time keyword and seconds argument determine how long peer routers will wait to delete stale routes before a BGP open message is received. The default value is 120 seconds. The optional stalepath-time keyword and seconds argument determine how long a router will wait before deleting stale routes after an end of record (EOR) message is received from the restarting router. The default value is 360 seconds.
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SUMMARY STEPS
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DETAILED STEPS
Step 1
enable
Example:Router> enable
Enables privileged EXEC mode.
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Step 2
configure terminal
Example:Router# configure terminal
Enters global configuration mode.
Step 3
router bgp autonomous-system-number
Example:Router(config)# router bgp 45000
Enters router configuration mode and creates a BGP routing process.
Step 4
bgp graceful-restart [restart-time seconds]
Example:Router(config-router)# bgp graceful-restart restart-time 130
Enables the BGP graceful restart capability and BGP NSF awareness.
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Step 5
bgp graceful-restart [stalepath-time seconds]
Example:Router(config-router)# bgp graceful-restart stalepath-time 350
Enables the BGP graceful restart capability and BGP NSF awareness.
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Step 6
Router(config-router)# end
Example:Router(config-router)# end
Exits router configuration mode and returns to privileged EXEC mode.
What to Do Next
If the bgp graceful-restart command has been issued after the BGP session has been established, you must reset the peer sessions by issuing the clear ip bgp * command or by reloading the router before graceful restart capabilities will be exchanged. For more information about resetting BGP sessions and using the clear ip bgp command, see the "Configuring a Basic BGP Network" module.
Enabling and Disabling BGP Graceful Restart Using BGP Peer Session Templates
Perform this task to enable and disable BGP graceful restart for BGP neighbors using peer session templates. In this task, a BGP peer session template is created, and BGP graceful restart is enabled. A second peer session template is created, and this template is configured to disable BGP graceful restart.
In this example, the configuration is performed at Router B in Figure 1 and two external BGP neighbors—at Router A and Router E in Figure 1—are identified. The first BGP peer at Router A is configured to inherit the first peer session template that enables BGP graceful restart, whereas the second BGP peer at Router E inherits the second template that disables BGP graceful restart. Using the optional show ip bgp neighbors command, the status of the BGP graceful restart capability is verified for each BGP neighbor configured in this task.
Figure 1 Network Topology Showing BGP Neighbors
The restart and stale-path timers can be modified only using the global bgp graceful-restart command as shown in the "Configuring BGP NSF Awareness Timers" section. The restart and stale-path timers are set to the default values when BGP graceful restart is enabled for BGP neighbors using peer session templates.
Restrictions
A BGP peer cannot inherit from a peer policy or session template and be configured as a peer group member at the same. BGP templates and BGP peer groups are mutually exclusive.
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Examples
The following example shows partial output from the show ip bgp neighbors command for the BGP peer at 192.168.1.2 (Router A in Figure 1). Graceful restart is shown as enabled. Note the default values for the restart and stale-path timers. These timers can only be set using the bgp graceful-restart command.
Router# show ip bgp neighbors 192.168.1.2BGP neighbor is 192.168.1.2, remote AS 40000, external linkInherits from template S1 for session parametersBGP version 4, remote router ID 192.168.1.2BGP state = Established, up for 00:02:11Last read 00:00:23, last write 00:00:27, hold time is 180, keepalive intervalsNeighbor sessions:1 active, is multisession capableNeighbor capabilities:Route refresh: advertised and received(new)Address family IPv4 Unicast: advertised and receivedGraceful Restart Capability: advertisedMultisession Capability: advertised and received!Address tracking is enabled, the RIB does have a route to 192.168.1.2Connections established 1; dropped 0Last reset neverTransport(tcp) path-mtu-discovery is enabledGraceful-Restart is enabled, restart-time 120 seconds, stalepath-time 360 secsConnection state is ESTAB, I/O status: 1, unread input bytes: 0The following example shows partial output from the show ip bgp neighbors command for the BGP peer at 192.168.3.2 (Router E in Figure 1). Graceful restart is shown as disabled.
Router# show ip bgp neighbors 192.168.3.2BGP neighbor is 192.168.3.2, remote AS 50000, external linkInherits from template S2 for session parametersBGP version 4, remote router ID 192.168.3.2BGP state = Established, up for 00:01:41Last read 00:00:45, last write 00:00:45, hold time is 180, keepalive intervalsNeighbor sessions:1 active, is multisession capableNeighbor capabilities:Route refresh: advertised and received(new)Address family IPv4 Unicast: advertised and received!Address tracking is enabled, the RIB does have a route to 192.168.3.2Connections established 1; dropped 0Last reset neverTransport(tcp) path-mtu-discovery is enabledGraceful-Restart is disabledConnection state is ESTAB, I/O status: 1, unread input bytes: 0Enabling BGP Graceful Restart for an Individual BGP Neighbor
Perform this task on Router B in Figure 1 to enable BGP graceful restart on the internal BGP peer at Router C in Figure 1. Under address family IPv4, the neighbor at Router C is identified, and BGP graceful restart is enabled for the neighbor at Router C with the IP address 172.21.1.2. To verify that BGP graceful restart is enabled, the optional show ip bgp neighbors command is used.
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Examples
The following example shows partial output from the show ip bgp neighbors command for the BGP peer at 172.21.1.2. Graceful restart is shown as enabled. Note the default values for the restart and stale-path timers. These timers can be set using only the global bgp graceful-restart command.
Router# show ip bgp neighbors 172.21.1.2BGP neighbor is 172.21.1.2, remote AS 45000, internal linkBGP version 4, remote router ID 172.22.1.1BGP state = Established, up for 00:01:01Last read 00:00:02, last write 00:00:07, hold time is 180, keepalive intervalsNeighbor sessions:1 active, is multisession capableNeighbor capabilities:Route refresh: advertised and received(new)Address family IPv4 Unicast: advertised and receivedGraceful Restart Capability: advertisedMultisession Capability: advertised and received!Address tracking is enabled, the RIB does have a route to 172.21.1.2Connections established 1; dropped 0Last reset neverTransport(tcp) path-mtu-discovery is enabledGraceful-Restart is enabled, restart-time 120 seconds, stalepath-time 360 secsConnection state is ESTAB, I/O status: 1, unread input bytes: 0Disabling BGP Graceful Restart for a BGP Peer Group
Perform this task to disable BGP graceful restart for a BGP peer group. In this task, a BGP peer group is created and graceful restart is disabled for the peer group. A BGP neighbor, 172.16.1.2 at Router D in Figure 1, is then identified and added as a peer group member and inherits the configuration associated with the peer group, which, in this example, disables BGP graceful restart.
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Examples
The following example shows partial output from the show ip bgp neighbors command for the BGP peer at 172.16.1.2. Graceful restart is shown as disabled. Note the default values for the restart and stale-path timers. These timers can be set using only the global bgp graceful-restart command.
Router# show ip bgp neighbors 172.16.1.2BGP neighbor is 172.16.1.2, remote AS 45000, internal linkMember of peer-group PG1 for session parametersBGP version 4, remote router ID 0.0.0.0BGP state = IdleNeighbor sessions:0 active, is multisession capable!Address tracking is enabled, the RIB does have a route to 172.16.1.2Connections established 0; dropped 0Last reset neverTransport(tcp) path-mtu-discovery is enabledGraceful-Restart is disabledVerifying the Configuration of BGP Nonstop Forwarding Awareness
Use the following steps to verify the local configuration of BGP NSF awareness on a router and to verify the configuration of NSF awareness on peer routers in a BGP network.
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Step 1
Enables privileged EXEC mode. Enter your password if prompted.
Router> enableStep 2
Displays the running configuration on the local router. The output will display the configuration of the bgp graceful-restart command in the BGP section. Repeat this command on all BGP neighbor routers to verify that all BGP peers are configured for BGP NSF awareness. In this example, BGP graceful restart is enabled globally and the external neighbor at 192.168.1.2 is configured to be a BGP peer and will have the BGP graceful restart capability enabled.
Router# show running-config...router bgp 45000bgp router-id 172.17.1.99bgp log-neighbor-changesbgp graceful-restart restart-time 130bgp graceful-restart stalepath-time 350bgp graceful-restarttimers bgp 70 120neighbor 192.168.1.2 remote-as 40000neighbor 192.168.1.2 activate...Step 3
Displays information about TCP and BGP connections to neighbors. "Graceful Restart Capability: advertised" will be displayed for each neighbor that has exchanged graceful restart capabilities with this router. In Cisco IOS XE Release 2.1 or later releases, the ability to enable or disable the BGP graceful restart capability for an individual BGP neighbor, peer group or peer session template was introduced and output was added to this command to show the BGP graceful restart status.
Configuring BGP Route Dampening
The tasks in this section configure and monitor BGP route dampening. Route dampening is designed to minimize the propagation of flapping routes across an internetwork. A route is considered to be flapping when its availability alternates repeatedly.
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Enabling and Configuring BGP Route Dampening
Perform this task to enable and configure BGP route dampening.
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Monitoring and Maintaining BGP Route Dampening
Perform the steps in this task as required to monitor and maintain BGP route dampening.
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Step 1
Enables privileged EXEC mode. Enter your password if prompted.
Router> enableStep 2
Use this command to monitor the flaps of all the paths that are flapping. The statistics will be deleted once the route is not suppressed and is stable for at least one half-life.
Router# show ip bgp flap-statisticsBGP table version is 10, local router ID is 172.17.232.182Status codes: s suppressed, d damped, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incompleteNetwork From Flaps Duration Reuse Path*d 10.0.0.0 172.17.232.177 4 00:13:31 00:18:10 100*d 10.2.0.0 172.17.232.177 4 00:02:45 00:28:20 100Step 3
Use this command to clear the accumulated penalty for routes that are received on a router that has BGP dampening enabled. If no arguments or keywords are specified, flap statistics are cleared for all routes. Flap statistics are also cleared when the peer is stable for the half-life time period. After the BGP flap statistics are cleared, the route is less likely to be dampened.
Router# clear ip bgp flap-statistics 172.17.232.177Step 4
Use this command to monitor the flaps of all the paths that are flapping. The statistics will be deleted once the route is not suppressed and is stable for at least one half-life.
Router# show ip bgp dampened-pathsBGP table version is 10, local router ID is 172.29.232.182Status codes: s suppressed, d damped, h history, * valid, > best, i - internalOrigin codes: i - IGP, e - EGP, ? - incompleteNetwork From Reuse Path*d 10.0.0.0 172.16.232.177 00:18:4 100 ?*d 10.2.0.0 172.16.232.177 00:28:5 100 ?Step 5
Use this command to clear stored route dampening information. If no keywords or arguments are entered, route dampening information for the entire routing table is cleared. The following example clears route dampening information for VPNv4 address family prefixes from network 192.168.10.0/24, and unsuppresses its suppressed routes.
Router# clear ip bgp vpnv4 unicast dampening 192.168.10.0 255.255.255.0
Decreasing BGP Convergence Time Using BFD
You start a BFD process by configuring BFD on the interface. When the BFD process is started, no entries are created in the adjacency database, in other words, no BFD control packets are sent or received. The adjacency creation takes places once you have configured BFD support for the applicable routing protocols. The first two tasks must be configured to implement BFD support for BGP to reduce the BGP convergence time. The third task is an optional task to help monitor or troubleshoot BFD.
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Configuring BFD Session Parameters on the Interface
The steps in this procedure show how to configure BFD on the interface by setting the baseline BFD session parameters on an interface. Repeat the steps in this procedure for each interface over which you want to run BFD sessions to BFD neighbors.
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Configuring BFD Support for BGP
Perform this task to configure BFD support for BGP, so that BGP is a registered protocol with BFD and will receive forwarding path detection failure messages from BFD.
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What to Do Next
For more details about BFD, see the "Bidirectional Forwarding Detection" chapter of the Cisco IOS XE IP Routing: BFD Configuration Guide, Release 2.3.
Enabling BGP MIB Support
SNMP notifications can be configured on the router and GET operations can be performed from an external management station only after BGP SNMP support is enabled. Perform this task on a router to configure SNMP notifications for the BGP MIB.
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Configuration Examples for Configuring Advanced BGP Features
This section contains the following examples:
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Enabling and Disabling BGP Next-Hop Address Tracking: Example
In the following example, next-hop address tracking is disabled under the IPv4 address family session:
router bgp 50000address-family ipv4 unicastno bgp nexthop trigger enableAdjusting the Delay Interval for BGP Next-Hop Address Tracking: Example
In the following example, the delay interval for next-hop tracking is configured to occur every 20 seconds under the IPv4 address family session:
router bgp 50000address-family ipv4 unicastbgp nexthop trigger delay 20Configuring BGP Selective Next-Hop Route Filtering: Examples
The following example shows how to configure BGP selective next-hop route filtering to avoid using a BGP prefix as the next-hop route. If the most specific route that covers the next hop is a BGP route, then the BGP route will be marked as unreachable. The next hop must be an IGP or static route.
router bgp 45000address-family ipv4 unicastbgp nexthop route-map CHECK-BGPexitexitroute-map CHECK-BGP deny 10match source-protocol bgp 1exitroute-map CHECK-BGP permit 20endThe following example shows how to configure BGP selective next-hop route filtering to avoid using a BGP prefix as the next-hop route and to ensure that the prefix is more specific than /25.
router bgp 45000address-family ipv4 unicastbgp nexthop route-map CHECK-BGP25exitexitip prefix-list FILTER25 seq 5 permit 0.0.0.0/0 le 25route-map CHECK-BGP25 deny 10match ip address prefix-list FILTER25exitroute-map CHECK-BGP25 deny 20match source-protocol bgp 1exitroute-map CHECK-BGP25 permit 30endEnabling BGP Global NSF Awareness Using Graceful Restart: Example
The following example enables BGP NSF awareness globally on all BGP neighbors. The restart time is set to 130 seconds and the stale path time is set to 350 seconds. The configuration of these timers is optional and the preconfigured default values are optimal for most network deployments.
configure terminalrouter bgp 45000bgp graceful-restartbgp graceful-restart restart-time 130bgp graceful-restart stalepath-time 350endEnabling and Disabling BGP Graceful Restart per Neighbor: Examples
The ability to enable or disable the BGP graceful restart capability for an individual BGP neighbor, peer group, or peer session template was introduced. The following example is configured on Router B in Figure 2 and enables the BGP graceful restart capability for the BGP peer session template named S1 and disables the BGP graceful restart capability for the BGP peer session template named S2. The external BGP neighbor at Router A in Figure 2 (192.168.1.2) inherits peer session template S1, and the BGP graceful restart capability is enabled for this neighbor. Another external BGP neighbor at Router E in Figure 2 (192.168.3.2) is configured with the BGP graceful restart capability disabled after inheriting peer session template S2.
Figure 2 Network Topology Showing BGP Neighbors for BGP Graceful Restart
The BGP graceful restart capability is enabled for an individual internal BGP neighbor, 172.21.1.2 at Router C in Figure 2, whereas the BGP graceful restart is disabled for the BGP neighbor 172.16.1.2 at Router D in Figure 2 because it is a member of the peer group PG1. The disabling of BGP graceful restart is configured for all members of the peer group, PG1. The restart and stale-path timers are modified and the BGP sessions are reset.
router bgp 45000template peer-session S1remote-as 40000ha-mode graceful-restartexit-peer-sessiontemplate peer-session S2remote-as 50000ha-mode graceful-restart disableexit-peer-sessionbgp log-neighbor-changesbgp graceful-restart restart-time 150bgp graceful-restart stalepath-time 400address-family ipv4 unicastneighbor PG1 peer-groupneighbor PG1 remote-as 45000neighbor PG1 ha-mode graceful-restart disableneighbor 172.16.1.2 peer-group PG1neighbor 172.21.1.2 remote-as 45000neighbor 172.21.1.2 activateneighbor 172.21.1.2 ha-mode graceful-restartneighbor 192.168.1.2 remote-as 40000neighbor 192.168.1.2 inherit peer-session S1neighbor 192.168.3.2 remote-as 50000neighbor 192.168.3.2 inherit peer-session S2endclear ip bgp *To demonstrate how the last configuration instance of the BGP graceful restart capability is applied, the following example initially enables the BGP graceful restart capability globally for all BGP neighbors. A BGP peer group, PG2, is configured with the BGP graceful restart capability disabled. An individual external BGP neighbor, 192.168.1.2 at Router A in Figure 2, is then configured to be a member of the peer group, PG2. The last graceful restart configuration instance is applied, and, in this case, the neighbor, 192.168.1.2, inherits the configuration instance from the peer group PG2 and the BGP graceful restart capability is disabled for this neighbor.
router bgp 45000bgp log-neighbor-changesbgp graceful-restartaddress-family ipv4 unicastneighbor PG2 peer-groupneighbor PG2 remote-as 40000neighbor PG2 ha-mode graceful-restart disableneighbor 192.168.1.2 peer-group PG2endclear ip bgp *Configuring BGP Route Dampening: Example
The following example configures BGP dampening to be applied to prefixes filtered through the route-map named ACCOUNTING:
ip prefix-list FINANCE permit 10.0.0.0/8!route-map ACCOUNTINGmatch ip address ip prefix-list FINANCEset dampening 15 750 2000 60exitrouter bgp 50000address-family ipv4bgp dampening route-map ACCOUNTINGendEnabling BGP MIB Support: Examples
The following example enables SNMP support for all supported BGP events:
Router(config)# snmp-server enable traps bgpThe following verification example shows that SNMP support for BGP is enabled and shown the running-config file:
Router# show run | include snmp-serversnmp-server enable traps bgpWhere to Go Next
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Additional References
Related Documents
BGP commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples
Overview of Cisco BGP conceptual information with links to all the individual BGP modules
"Cisco BGP Overview" module of the Cisco IOS XE IP Routing: BGP Configuration Guide, Release 2.3.
Conceptual and configuration details for basic BGP tasks.
"Configuring a Basic BGP Network" module of the Cisco IOS XE IP Routing: BGP Configuration Guide, Release 2.3.
Information about connecting to external BGP peers.
"Connecting to a Service Provider Using External BGP" module of the Cisco IOS XE IP Routing: BGP Configuration Guide, Release 2.3.
Information about internal BGP peers.
"Configuring Internal BGP Features" module of the Cisco IOS XE IP Routing: BGP Configuration Guide, Release 2.3.
Information about BFD.
"Bidirectional Forwarding Detection" module of the Cisco IOS XE IP Routing: BGP Configuration Guide, Release 2.3.
Information about SNMP and SNMP operations.
"Configuring SNMP Support" section of the Cisco IOS XE Network Management Configuration Guide, Release 2.3.
Cisco IOS master command list, all releases
Standards
MIBs
RFCs
Technical Assistance
Feature Information for Configuring Advanced BGP Features
Table 1 lists the features in this module and provides links to specific configuration information.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE 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.
Note
Table 1 Feature Information for Configuring Advanced BGP Features
BGP Convergence Optimization
Cisco IOS XE
Release 2.1This feature was introduced on the Cisco ASR 1000 Series Routers.
BGP Graceful Restart per Neighbor
Cisco IOS XE
Release 2.1The BGP Graceful Restart per Neighbor feature enables or disables the BGP graceful restart capability for an individual BGP neighbor, including using peer session templates and BGP peer groups.
This feature was introduced on the Cisco ASR 1000 Series Aggregation Services Routers.
The following sections provide information about this feature:
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The following commands were introduced or modified by this feature: ha-mode graceful-restart, neighbor ha-mode graceful-restart, show ip bgp neighbors.
BGP MIB Support Enhancements
Cisco IOS XE
Release 2.1The BGP MIB Support Enhancements feature introduced support in the CISCO-BGP4-MIB for new SNMP notifications.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
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The following command was introduced in this feature: snmp-server enable traps bgp.
BGP Nonstop Forwarding (NSF) Awareness
Cisco IOS XE
Release 2.1Nonstop Forwarding (NSF) awareness allows a router to assist NSF-capable neighbors to continue forwarding packets during a Stateful Switchover (SSO) operation. The BGP Nonstop Forwarding Awareness feature allows an NSF-aware router that is running BGP to forward packets along routes that are already known for a router that is performing an SSO operation. This capability allows the BGP peers of the failing router to retain the routing information that is advertised by the failing router and continue to use this information until the failed router has returned to normal operating behavior and is able to exchange routing information. The peering session is maintained throughout the entire NSF operation.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
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The following commands were introduced or modified by this feature: bgp graceful-restart, show ip bgp, show ip bgp neighbors.
BGP Selective Address Tracking
Cisco IOS XE
Release 2.1The BGP Selective Address Tracking feature introduces the use of a route map for next-hop route filtering and fast session deactivation. Selective next-hop filtering uses a route map to selectively define routes to help resolve the BGP next hop, or a route map can be used to determine if a peering session with a BGP neighbor should be reset when a route to the BGP peer changes.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
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The following commands were modified by this feature: bgp nexthop, neighbor fall-over.
BGP Support for BFD
Cisco IOS XE
Release 2.1Bidirectional Forwarding Detection (BFD) is a detection protocol designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning will be easier, and reconvergence time will be consistent and predictable. The main benefit of implementing BFD for BGP is a significantly faster reconvergence time.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
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The following commands were introduced or modified by this feature: bfd, neighbor fall-over, show bfd neighbors, show ip bgp neighbors.
BGP Support for Next-Hop Address Tracking
Cisco IOS XE
Release 2.1The BGP Support for Next-Hop Address Tracking feature is enabled by default when a supporting Cisco IOS XE software image is installed. BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to the BGP routing process as they are updated in the RIB. This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a bestpath calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.
This feature was introduced on the Cisco ASR 1000 Series Routers.
The following sections provide information about this feature:
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The following command was introduced in this feature: bgp nexthop.
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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