Cisco Nexus 7000 Series - Load Sharing and Multipath; Route Aggregation

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Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 11 Configuring Advanced BGP
Information About Advanced BGP
Note The router does not apply a penalty to a resetting BGP peer when route dampening is enabled, even
though the peer reset withdraws the route.
Load Sharing and Multipath
BGP can install multiple equal-cost eBGP or iBGP paths into the routing table to reach the same
destination prefix. Traffic to the destination prefix is then shared across all the installed paths.
The BGP best-path algorithm considers the paths as equal-cost paths if the following attributes are
identical:
• Weight
• Local preference
• AS_path
• Origin code
• Multi-exit discriminator (MED)
• IGP cost to the BGP next hop
BGP selects only one of these multiple paths as the best path and advertises the path to the BGP peers.
Note Paths received from different AS confederations are considered as equal-cost paths if the external
AS_path values and the other attributes are identical.
Note When you configure a route reflector for iBGP multipath, and the route reflector advertises the selected
best path to its peers, the next hop for the path is not modified.
Route Aggregation
You can configure aggregate addresses. Route aggregation simplifies route tables by replacing a number
of more specific addresses with an address that represents all the specific addresses. For example, you
can replace these three more specific addresses, 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24 with one
aggregate address, 10.1.0.0/16.
Aggregate prefixes are present in the BGP route table so that fewer routs are advertised.
Note Cisco NX-OS does not support automatic route aggregation.
Route aggregation can lead to forwarding loops. To avoid this problem, when BGP generates an
advertisement for an aggregate address, it automatically installs a summary discard route for that
aggregate address in the local routing table. BGP sets the administrative distance of the summary discard
to 220 and sets the route type to discard. BGP does not use discard routes for next-hop resolution.