BGP Best External
The BGP Best External feature provides the network with a backup external route to avoid loss of connectivity of the primary external route. The BGP Best External feature advertises the most preferred route among those received from external neighbors as a backup route. This feature is beneficial in active-backup topologies, where service providers use routing policies that cause a border router to choose a path received over an Interior Border Gateway Protocol (iBGP) session (of another border router) as the best path for a prefix even if it has an Exterior Border Gateway Protocol (eBGP) learned path. This active-backup topology defines one exit or egress point for the prefix in the autonomous system and uses the other points as backups if the primary link or eBGP peering is unavailable. The policy causes the border router to hide the paths learned over its eBGP sessions from the autonomous system because it does not advertise any path for such prefixes. To cope with this situation, some devices advertise one externally learned path called the best external path.
Prerequisites for BGP Best External
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The Bidirectional Forwarding Detection (BFD) protocol must be enabled to quickly detect link failures.
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Ensure that the BGP and the Multiprotocol Label Switching (MPLS) network is up and running with the customer site connected to the provider site by more than one path (multihomed).
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The backup path must have a unique next hop that is not the same as the next hop of the best path.
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BGP must support lossless switchover between operational paths.
Restrictions for BGP Best External
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The BGP Best External feature will not install a backup path if BGP Multipath is installed and a multipath exists in the BGP table. One of the multipaths automatically acts as a backup for the other paths.
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The BGP Best External feature is not supported with the following features: - MPLS VPN Carrier Supporting Carrier
- MPLS VPN Inter-Autonomous Systems, option B
- MPLS VPN Per Virtual Routing and Forwarding (VRF) Label
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The BGP Best External feature cannot be configured with Multicast or L2VPN VRF address families.
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The BGP Best External feature cannot be configured on a route reflector, unless it is running Cisco IOS XE Release 3.4S or later.
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The BGP Best External feature does not support NSF/SSO. However, ISSU is supported if both Route Processors have the BGP Best External feature configured.
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The BGP Best External feature can only be configured on VPNv4, VPNv6, IPv4 VRF, and IPv6 VRF address families.
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When you configure the BGP Best External feature using the bgp advertise-best-external command, you need not enable the BGP PIC feature with the bgp additional-paths install command. The BGP PIC feature is automatically enabled by the BGP Best External feature.
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When you configure the BGP Best External feature, it will override the functionality of the "MPLS VPN--BGP Local Convergence" feature. However, you do not have to remove the protection local-prefixes command from the configuration.