- Read Me First
- Cisco BGP Overview
- BGP 4
- Configuring a Basic BGP Network
- BGP 4 Soft Configuration
- BGP Support for 4-byte ASN
- IPv6 Routing: Multiprotocol BGP Extensions for IPv6
- IPv6 Routing: Multiprotocol BGP Link-Local Address Peering
- IPv6 Multicast Address Family Support for Multiprotocol BGP
- Configuring Multiprotocol BGP (MP-BGP) Support for CLNS
- BGP IPv6 Admin Distance
- Connecting to a Service Provider Using External BGP
- BGP Route-Map Continue
- BGP Route-Map Continue Support for Outbound Policy
- Removing Private AS Numbers from the AS Path in BGP
- Configuring BGP Neighbor Session Options
- BGP Neighbor Policy
- BGP Dynamic Neighbors
- BGP Support for Next-Hop Address Tracking
- BGP Restart Neighbor Session After Max-Prefix Limit Reached
- BGP Support for Dual AS Configuration for Network AS Migrations
- Configuring Internal BGP Features
- BGP VPLS Auto Discovery Support on Route Reflector
- BGP FlowSpec Route-reflector Support
- BGP Flow Specification Client
- BGP NSF Awareness
- BGP Graceful Restart per Neighbor
- BGP Support for BFD
- IPv6 NSF and Graceful Restart for MP-BGP IPv6 Address Family
- BGP Persistence
- BGP Link Bandwidth
- Border Gateway Protocol Link-State
- iBGP Multipath Load Sharing
- BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN
- Loadsharing IP Packets over More Than Six Parallel Paths
- BGP Policy Accounting
- BGP Policy Accounting Output Interface Accounting
- BGP Cost Community
- BGP Support for IP Prefix Import from Global Table into a VRF Table
- BGP Support for IP Prefix Export from a VRF Table into the Global Table
- BGP per Neighbor SoO Configuration
- Per-VRF Assignment of BGP Router ID
- BGP Next Hop Unchanged
- BGP Support for the L2VPN Address Family
- BGP Event-Based VPN Import
- BGP Best External
- BGP PIC Edge for IP and MPLS-VPN
- Detecting and Mitigating a BGP Slow Peer
- Configuring BGP: RT Constrained Route Distribution
- Configuring a BGP Route Server
- BGP Diverse Path Using a Diverse-Path Route Reflector
- BGP Enhanced Route Refresh
- Configuring BGP Consistency Checker
- BGP—Origin AS Validation
- BGP MIB Support
- BGP 4 MIB Support for Per-Peer Received Routes
- BGP Support for Nonstop Routing (NSR) with Stateful Switchover (SSO) Using L2VPN VPLS
- BGP Support for Nonstop Routing (NSR) with Stateful Switchover (SSO) Using L2VPN VPLS
- BGP NSR Auto Sense
- BGP NSR Support for iBGP Peers
- BGP Graceful Shutdown
- BGP — mVPN BGP sAFI 129 - IPv4
- BGP-MVPN SAFI 129 IPv6
- BFD—BGP Multihop Client Support, cBit (IPv4 and IPv6), and Strict Mode
- BGP Attribute Filter and Enhanced Attribute Error Handling
- BGP Additional Paths
- BGP-Multiple Cluster IDs
- BGP-VPN Distinguisher Attribute
- BGP-RT and VPN Distinguisher Attribute Rewrite Wildcard
- VPLS BGP Signaling
- Multicast VPN BGP Dampening
- BGP—IPv6 NSR
- BGP-VRF-Aware Conditional Advertisement
- BGP—Selective Route Download
- BGP—Support for iBGP Local-AS
- eiBGP Multipath for Non-VRF Interfaces (IPv4/IPv6)
- L3VPN iBGP PE-CE
- BGP NSR Support for MPLS VPNv4 and VPNv6 Inter-AS Option B
- BGP-RTC for Legacy PE
- BGP PBB EVPN Route Reflector Support
- BGP Monitoring Protocol
- VRF Aware BGP Translate-Update
- BGP Support for MTR
- BGP Accumulated IGP
- BGP MVPN Source-AS Extended Community Filtering
- BGP AS-Override Split-Horizon
- BGP Support for Multiple Sourced Paths Per Redistributed Route
- Maintenance Function: BGP Routing Protocol
- Finding Feature Information
- Information About BGP Support for Next-Hop Address Tracking
- Example: Enabling and Disabling BGP Next-Hop Address Tracking
- Example: Adjusting the Delay Interval for BGP Next-Hop Address Tracking
- Examples: Configuring BGP Selective Next-Hop Route Filtering
- Example: Configuring Fast Session Deactivation for a BGP Neighbor
- Example: Configuring Selective Address Tracking for Fast Session Deactivation
BGP Support for Next-Hop Address Tracking
The BGP Support for 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 bestpath calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.
- Finding Feature Information
- Information About BGP Support for Next-Hop Address Tracking
- How to Configure BGP Support for Next-Hop Address Tracking
- Configuration Examples for BGP Support for Next-Hop Address Tracking
- Additional References
- Feature Information for BGP Support for Next-Hop Address Tracking
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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 table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Information About BGP Support for Next-Hop Address Tracking
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.
BGP Next-Hop Dampening Penalties
If the penalty threshold value is higher than 950, then the delay is calculated as the reuse time using the dampening calculations. The dampening calculations use the following parameters:
-
Penalty
-
Half-life time
-
Reuse time
-
max-suppress-time
The values for the dampening parameters used are a max-suppress-time of 60 seconds, the half-life of 8 seconds, and the reuse-limit of 100.
For example, if the original penalty of 1600 is added, then after 16 seconds it becomes 800, and after 40 seconds, the penalty becomes 100. Hence, for the route update penalty of 1600, a delay of 40 seconds is used to schedule the BGP scanner.
These parameters (penalty threshold and any of the dampening parameters) cannot be modified.
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.
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 device 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 device 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.
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.
Note | Use route map on ASR series devices to set the next hop as BGP peer for the route and apply that route map in outbound direction towards the peer. |
Note | Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported. |
BGP Support for Fast Peering Session Deactivation
- BGP Hold Timer
- BGP Fast Peering Session Deactivation
- Selective Address Tracking for BGP Fast Session Deactivation
BGP Hold Timer
By default, the BGP hold timer is set to run every 180 seconds in Cisco software. This timer value is set as the default to protect the BGP routing process from instability that can be caused by peering sessions with other routing protocols. BGP devices typically carry large routing tables, so frequent session resets are not desirable.
BGP Fast Peering Session Deactivation
BGP fast peering session deactivation improves BGP convergence and response time to adjacency changes with BGP neighbors. This feature is event driven and configured on a per-neighbor basis. When this feature is enabled, BGP will monitor the peering session with the specified neighbor. Adjacency changes are detected and terminated peering sessions are deactivated in between the default or configured BGP scanning interval.
Selective Address Tracking for BGP Fast Session Deactivation
In Cisco IOS XE Release 2.1 and later releases, the BGP Selective Address Tracking feature introduced the use of a route map with BGP fast session deactivation. The route-map keyword and map-name argument are used with the neighbor fall-over BGP neighbor session command to determine if a peering session with a BGP neighbor should be reset when a route to the BGP peer changes. The route map is evaluated against the new route, and if a deny statement is returned, the peer session is reset. The route map is not used for session establishment.
Note | Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported. |
How to Configure BGP Support for Next-Hop Address Tracking
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 “Configuring BGP Route Dampening.”
- Configuring BGP Selective Next-Hop Route Filtering
- Adjusting the Delay Interval for BGP Next-Hop Address Tracking
- Disabling BGP Next-Hop Address Tracking
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. Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported.
For more examples of how to use the bgp nexthop command, see the “Examples: Configuring BGP Selective Next-Hop Route Filtering” section in this module.
1.
enable
2.
configure
terminal
3.
router
bgp
autonomous-system-number
4.
address-family
ipv4
[unicast |
multicast|
vrf
vrf-name]
5.
bgp
nexthop
route-map
map-name
6.
exit
7.
exit
8.
ip
prefix-list
list-name
[seq
seq-value] {deny
network
/
length |
permit
network/length} [ge
ge-value] [le
le-value]
9.
route-map
map-name
[permit |
deny] [sequence-number]
10.
match
ip
address
prefix-list
prefix-list-name
[prefix-list-name...]
11.
exit
12.
route-map
map-name
[permit |
deny] [sequence-number]
13.
end
14.
show
ip
bgp
[network] [network-mask]
DETAILED STEPS
Example
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 172.17.1.99 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network 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 32768
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.
1.
enable
2.
configure
terminal
3.
router
bgp
autonomous-system-number
4.
address-family
ipv4
[[mdt |
multicast |
tunnel |
unicast [vrf
vrf-name] |
vrf
vrf-name] |
vpnv4 [unicast]]
5.
bgp
nexthop
trigger
delay
delay-timer
6.
end
DETAILED STEPS
Command or Action | Purpose | |
---|---|---|
Step 1 |
enable
Example: Device> enable |
Enables privileged EXEC mode. |
Step 2 |
configure
terminal
Example: Device# configure terminal |
Enters global configuration mode. |
Step 3 |
router
bgp
autonomous-system-number
Example: Device(config)# router bgp 64512 |
Enters router configuration mode to create or configure a BGP routing process. |
Step 4 |
address-family
ipv4
[[mdt |
multicast |
tunnel |
unicast [vrf
vrf-name] |
vrf
vrf-name] |
vpnv4 [unicast]]
Example: Device(config-router)# address-family ipv4 unicast |
Enter address family configuration mode to configure BGP peers to accept address family-specific configurations. |
Step 5 |
bgp
nexthop
trigger
delay
delay-timer
Example: Device(config-router-af)# bgp nexthop trigger delay 20 |
Configures the delay interval between routing table walks for next-hop address tracking. |
Step 6 |
end
Example: Device(config-router-af)# end |
Exits address-family configuration mode, and enters privileged EXEC mode. |
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. Beginning with Cisco IOS Release 12.2(33)SB6, BGP next-hop address tracking is also enabled by default under the VPNv6 address family whenever the next hop is an IPv4 address mapped to an IPv6 next-hop address.
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 nexthopcommand with the trigger and enable keywords.
1.
enable
2.
configure
terminal
3.
router
bgp
autonomous-system-number
4.
address-family
ipv4
[[mdt |
multicast |
tunnel |
unicast [vrf
vrf-name] |
vrf
vrf-name] |
vpnv4 [unicast] |
vpnv6 [unicast]]
5.
no bgp
nexthop
trigger
enable
6.
end
DETAILED STEPS
Command or Action | Purpose | |
---|---|---|
Step 1 |
enable
Example: Device> enable |
Enables privileged EXEC mode. |
Step 2 |
configure
terminal
Example: Device# configure terminal |
Enters global configuration mode. |
Step 3 |
router
bgp
autonomous-system-number
Example: Device(config)# router bgp 64512 |
Enters router configuration mod to create or configure a BGP routing process. |
Step 4 |
address-family
ipv4
[[mdt |
multicast |
tunnel |
unicast [vrf
vrf-name] |
vrf
vrf-name] |
vpnv4 [unicast] |
vpnv6 [unicast]]
Example: Device(config-router)# address-family ipv4 unicast |
Enter address family configuration mode to configure BGP peers to accept address family-specific configurations.
|
Step 5 |
no bgp
nexthop
trigger
enable
Example: Device(config-router-af)# no bgp nexthop trigger enable |
Disables BGP next-hop address tracking.
|
Step 6 |
end
Example: Device(config-router-af)# end |
Exits address-family configuration mode, and enters Privileged EXEC mode. |
Configuring Fast Session Deactivation
The tasks in this section show how to 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 route dampening, see the "Configuring Internal BGP Features" module.
- Configuring Fast Session Deactivation for a BGP Neighbor
- Configuring Selective Address Tracking for Fast Session Deactivation
Configuring Fast Session Deactivation for a BGP Neighbor
Perform this task to establish a peering session with a BGP neighbor and then configure the peering session for fast session deactivation to improve the network convergence time if the peering session is deactivated.
Enabling fast session deactivation for a BGP neighbor can significantly improve BGP convergence time. However, unstable IGP peers can still introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP.
1.
enable
2.
configure
terminal
3.
router
bgp
autonomous-system-number
4.
address-family
ipv4
[mdt |
multicast |
tunnel |
unicast [vrf
vrf-name] |
vrf
vrf-name]
5.
neighbor
ip-address
remote-as
autonomous-system-number
6.
neighbor
ip-address
fall-over
7.
end
DETAILED STEPS
Command or Action | Purpose | |
---|---|---|
Step 1 |
enable
Example: Device> enable |
Enables privileged EXEC mode. |
Step 2 |
configure
terminal
Example: Device# configure terminal |
Enters global configuration mode. |
Step 3 |
router
bgp
autonomous-system-number
Example: Device(config)# router bgp 50000 |
Enters router configuration mode to create or configure a BGP routing process. |
Step 4 |
address-family
ipv4
[mdt |
multicast |
tunnel |
unicast [vrf
vrf-name] |
vrf
vrf-name]
Example: Device(config-router)# address-family ipv4 unicast |
Enters address family configuration mode to configure BGP peers to accept address family-specific configurations.
|
Step 5 |
neighbor
ip-address
remote-as
autonomous-system-number
Example: Device(config-router-af)# neighbor 10.0.0.1 remote-as 50000 |
Establishes a peering session with a BGP neighbor. |
Step 6 |
neighbor
ip-address
fall-over
Example: Device(config-router-af)# neighbor 10.0.0.1 fall-over |
Configures the BGP peering to use fast session deactivation.
|
Step 7 |
end
Example: Device(config-router-af)# end |
Exits configuration mode and returns to privileged EXEC mode. |
Configuring Selective Address Tracking for Fast Session Deactivation
Perform this task to configure selective address tracking for fast session deactivation. The optional route-map keyword and map-name argument of the neighbor fall-over command are used to determine if a peering session with a BGP neighbor should be deactivated (reset) when a route to the BGP peer changes. The route map is evaluated against the new route, and if a deny statement is returned, the peer session is reset.
Note | Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported. |
1.
enable
2.
configure
terminal
3.
router
bgp
autonomous-system-number
4.
neighbor
{ip-address|
peer-group-name}
remote-as
autonomous-system-number
5.
neighbor
ip-address
fall-over
[route-map
map-name]
6.
exit
7.
ip
prefix-list
list-name
[seq
seq-value]{deny
network
/
length
|
permit
network
/
length}[ge
ge-value]
[le
le-value]
8.
route-map
map-name
[permit
|
deny][sequence-number]
9.
match
ip
address
prefix-list
prefix-list-name
[prefix-list-name...]
10.
end
DETAILED STEPS
Command or Action | Purpose | |||
---|---|---|---|---|
Step 1 |
enable
Example: Device> enable |
Enables privileged EXEC mode. | ||
Step 2 |
configure
terminal
Example: Device# configure terminal |
Enters global configuration mode. | ||
Step 3 |
router
bgp
autonomous-system-number
Example: Device(config)# router bgp 45000 |
Enters router configuration mode for the specified routing process. | ||
Step 4 |
neighbor
{ip-address|
peer-group-name}
remote-as
autonomous-system-number
Example: Device(config-router)# neighbor 192.168.1.2 remote-as 40000 |
Adds the IP address or peer group name of the neighbor in the specified autonomous system to the IPv4 multiprotocol BGP neighbor table of the local router. | ||
Step 5 |
neighbor
ip-address
fall-over
[route-map
map-name]
Example: Device(config-router)# neighbor 192.168.1.2 fall-over route-map CHECK-NBR |
Applies a route map when a route to the BGP changes. | ||
Step 6 |
exit
Example: Device(config-router)# exit |
Exits router configuration mode and enters global configuration mode. | ||
Step 7 |
ip
prefix-list
list-name
[seq
seq-value]{deny
network
/
length
|
permit
network
/
length}[ge
ge-value]
[le
le-value]
Example: Device(config)# ip prefix-list FILTER28 seq 5 permit 0.0.0.0/0 ge 28 |
Creates a prefix list for BGP next-hop route filtering. | ||
Step 8 |
route-map
map-name
[permit
|
deny][sequence-number]
Example: Device(config)# route-map CHECK-NBR permit 10 |
Configures a route map and enters route-map configuration mode. | ||
Step 9 |
match
ip
address
prefix-list
prefix-list-name
[prefix-list-name...]
Example: Device(config-route-map)# match ip address prefix-list FILTER28 |
Matches the IP addresses in the specified prefix list.
| ||
Step 10 |
end
Example: Device(config-route-map)# end |
Exits configuration mode and returns to privileged EXEC mode. |
Configuration Examples for BGP Support for Next-Hop Address Tracking
Example: Enabling and Disabling BGP Next-Hop Address Tracking
In the following example, next-hop address tracking is disabled under the IPv4 address family session:
router bgp 50000 address-family ipv4 unicast no bgp nexthop trigger enable
Example: Adjusting the Delay Interval for BGP Next-Hop Address Tracking
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 50000 address-family ipv4 unicast bgp nexthop trigger delay 20
Examples: Configuring BGP Selective Next-Hop Route Filtering
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 45000 address-family ipv4 unicast bgp nexthop route-map CHECK-BGP exit exit route-map CHECK-BGP deny 10 match source-protocol bgp 1 exit route-map CHECK-BGP permit 20 end
The 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 45000 address-family ipv4 unicast bgp nexthop route-map CHECK-BGP25 exit exit ip prefix-list FILTER25 seq 5 permit 0.0.0.0/0 le 25 route-map CHECK-BGP25 deny 10 match ip address prefix-list FILTER25 exit route-map CHECK-BGP25 deny 20 match source-protocol bgp 1 exit route-map CHECK-BGP25 permit 30 end
Example: Configuring Fast Session Deactivation for a BGP Neighbor
In the following example, the BGP routing process is configured on device A and device B to monitor and use fast peering session deactivation for the neighbor session between the two devices. Although fast peering session deactivation is not required at both devices in the neighbor session, it will help the BGP networks in both autonomous systems to converge faster if the neighbor session is deactivated.
Device A
router bgp 40000 neighbor 192.168.1.1 remote-as 45000 neighbor 192.168.1.1 fall-over end
Device B
router bgp 45000 neighbor 192.168.1.2 remote-as 40000 neighbor 192.168.1.2 fall-over end
Example: Configuring Selective Address Tracking for Fast Session Deactivation
The following example shows how to configure the BGP peering session to be reset if a route with a prefix of /28 or a more specific route to a peer destination is no longer available:
router bgp 45000 neighbor 192.168.1.2 remote-as 40000 neighbor 192.168.1.2 fall-over route-map CHECK-NBR exit ip prefix-list FILTER28 seq 5 permit 0.0.0.0/0 ge 28 route-map CHECK-NBR permit 10 match ip address prefix-list FILTER28 end
Additional References
Related Documents
Related Topic |
Document Title |
---|---|
Cisco IOS commands |
|
BGP commands |
Technical Assistance
Description |
Link |
---|---|
The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
Feature Information for BGP Support for Next-Hop Address Tracking
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Feature Name |
Releases |
Feature Information |
---|---|---|
BGP Support for Next-Hop Address Tracking |
The BGP Support for Next-Hop Address Tracking feature is enabled by default when a supporting Cisco IOS 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. The following command was introduced in this feature: bgp nexthop. |
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BGP Selective Address Tracking |
The 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. The following commands were modified by this feature: bgp nexthop, neighbor fall-over. |
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BGP Support for Fast Peering Session Deactivation |
The BGP Support for Fast Peering Session Deactivation feature introduced an event-driven notification system that allows a Border Gateway Protocol (BGP) process to monitor BGP peering sessions on a per-neighbor basis. This feature improves the response time of BGP to adjacency changes by allowing BGP to detect an adjacency change and deactivate the terminated session in between standard BGP scanning intervals. Enabling this feature improves overall BGP convergence. The following command was modified by this feature: neighbor fall-over. |