Bgp Additional Paths; Bgp Local Label Retention - Cisco NCS 6000 Series Configuration Manual

BGP Local Label Retention

• On the primary PE—advertises the best path for a prefix to both its internal and external peers
• On the backup PE—advertises the best path selected for a prefix to the external peers and advertises the
The advertise best-external command enables the advertisement of the best–external path in global address
family configuration mode.
BGP Local Label Retention
When a primary PE-CE link fails, BGP withdraws the route corresponding to the primary path along with its
local label and programs the backup path in the Routing Information Base (RIB) and the Forwarding Information
Base (FIB), by default.
However, until all the internal peers of the primary PE reconverge to use the backup path as the new bestpath,
the traffic continues to be forwarded to the primary PE with the local label that was allocated for the primary
path. Hence the previously allocated local label for the primary path must be retained on the primary PE for
some configurable time after the reconvergence. BGP Local Label Retention feature enables the retention of
the local label for a specified period. If no time is specified, the local lable is retained for a default value of
five minutes.
The retain local-label command enables the retention of the local label until the network is converged.
BGP Over GRE Interfaces
Cisco IOS XR software provides the capability to run Border Gateway Protocol (BGP) over Generic Routing
Encapsulation (GRE) tunnel interfaces.
GRE protocol transports packets of one protocol over another protocol by means of encapsulation. Service
Providers can provide IP services between their networks that are connected together by a public network
using GRE encapsulation to carry data securely over the public network.
The packet that needs to be transported is first encapsulated in a GRE header, which is further encapsulated
in another protocol like IPv4 or IPv6 and then forwarded to the destination.
The Cisco IOS XR software GRE implementation is compliant with GRE encapsulation defined in RFC 2784.
Key and Sequence numbering as defined in RFC 2890 is not supported in Cisco IOS XR software GRE
implementation. To be backward compliant with RFC 1701, Cisco IOS XR software transmits GRE packets
with Reserved0 field set to zero. A receiver that is compliant with RFC 1701 treats key present, sequence
number, and strict source route as zero and do not expect key and sequence number. The Cisco IOS XR
software discards a GRE packet with any of the bits in Reserved0 field set.
Command Line Interface (CLI) Consistency for BGP Commands
TheBorder Gateway Protocol (BGP) commands use disable keyword to disable a feature. The keyword
inheritance-disable disables the inheritance of the feature properties from the parent level.

BGP Additional Paths

The Border Gateway Protocol (BGP) Additional Paths feature modifies the BGP protocol machinery for a
BGP speaker to be able to send multiple paths for a prefix. This gives 'path diversity' in the network. The add
path enables BGP prefix independent convergence (PIC) at the edge routers.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
40
best–external path selected for that prefix to the internal peers
Implementing BGP