Bgp Additional Paths; Ibgp Multipath Load Sharing - Cisco ASR 9000 Series Routing Configuration Manual

BGP Additional Paths

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.
BGP add path enables add path advertisement in an iBGP network and advertises the following types of paths
for a prefix:
• Backup paths—to enable fast convergence and connectivity restoration.
• Group-best paths—to resolve route oscillation.
• All paths—to emulate an iBGP full-mesh.

iBGP Multipath Load Sharing

When a Border Gateway Protocol (BGP) speaking router that has no local policy configured, receives multiple
network layer reachability information (NLRI) from the internal BGP (iBGP) for the same destination, the
router will choose one iBGP path as the best path. The best path is then installed in the IP routing table of the
router.
The iBGP Multipath Load Sharing feature enables the BGP speaking router to select multiple iBGP paths as
the best paths to a destination. The best paths or multipaths are then installed in the IP routing table of the
router.
When there are multiple border BGP routers having reachability information heard over eBGP, if no local
policy is applied, the border routers will choose their eBGP paths as best. They advertise that bestpath inside
the ISP network. For a core router, there can be multiple paths to the same destination, but it will select only
one path as best and use that path for forwarding. iBGP multipath load sharing adds the ability to enable load
sharing among multiple equi-distant paths.
Configuring multiple iBGP best paths enables a router to evenly share the traffic destined for a particular site.
The iBGP Multipath Load Sharing feature functions similarly in a Multiprotocol Label Switching (MPLS)
Virtual Private Network (VPN) with a service provider backbone.
For multiple paths to the same destination to be considered as multipaths, the following criteria must be met:
• All attributes must be the same. The attributes include weight, local preference, autonomous system
• The next hop router for each multipath must be different.
Even if the criteria are met and multiple paths are considered multipaths, the BGP speaking router will still
designate one of the multipaths as the best path and advertise this best path to its neighbors.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
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Add path is not be supported with MDT, tunnel, and L2VPN address families and eBGP
Note
peerings.
path (entire attribute and not just length), origin code, Multi Exit Discriminator (MED), and Interior
Gateway Protocol (iGP) distance.
Implementing BGP