Nonstop Forwarding Awareness - Cisco Catalyst 9500 series Configuration Manual

Configuring IP Unicast Routing
A router or Device running Cisco IOS does not select or use an IBGP route unless it has a route available to
the next-hop router and it has received synchronization from an IGP (unless IGP synchronization is disabled).
When multiple routes are available, BGP bases its path selection on attribute values. See the "Configuring
BGP Decision Attributes" section for information about BGP attributes.
BGP Version 4 supports classless interdomain routing (CIDR) so you can reduce the size of your routing
tables by creating aggregate routes, resulting in supernets. CIDR eliminates the concept of network classes
within BGP and supports the advertising of IP prefixes.

Nonstop Forwarding Awareness

The BGP NSF Awareness feature is supported for IPv4 in the Network Advantage license.. To enable this
feature with BGP routing, you need to enable Graceful Restart. When the neighboring router is NSF-capable,
and this feature is enabled, the Layer 3 Device continues to forward packets from the neighboring router
during the interval between the primary Route Processor (RP) in a router failing and the backup RP taking
over, or while the primary RP is manually reloaded for a nondisruptive software upgrade.
For more information, see the "BGP Nonstop Forwarding (NSF) Awareness" section of the Cisco IOS IP
Routing Protocols Configuration Guide, Release 12.4.
Information About BGP Routing
To enable BGP routing, you establish a BGP routing process and define the local network. Because BGP must
completely recognize the relationships with its neighbors, you must also specify a BGP neighbor.
BGP supports two kinds of neighbors: internal and external. Internal neighbors are in the same AS; external
neighbors are in different autonomous systems. External neighbors are usually adjacent to each other and
share a subnet, but internal neighbors can be anywhere in the same AS.
The switch supports the use of private AS numbers, usually assigned by service providers and given to systems
whose routes are not advertised to external neighbors. The private AS numbers are from 64512 to 65535. You
can configure external neighbors to remove private AS numbers from the AS path by using the neighbor
remove-private-as router configuration command. Then when an update is passed to an external neighbor,
if the AS path includes private AS numbers, these numbers are dropped.
If your AS will be passing traffic through it from another AS to a third AS, it is important to be consistent
about the routes it advertises. If BGP advertised a route before all routers in the network had learned about
the route through the IGP, the AS might receive traffic that some routers could not yet route. To prevent this
from happening, BGP must wait until the IGP has propagated information across the AS so that BGP is
synchronized with the IGP. Synchronization is enabled by default. If your AS does not pass traffic from one
AS to another AS, or if all routers in your autonomous systems are running BGP, you can disable
synchronization, which allows your network to carry fewer routes in the IGP and allows BGP to converge
more quickly.
Routing Policy Changes
Routing policies for a peer include all the configurations that might affect inbound or outbound routing table
updates. When you have defined two routers as BGP neighbors, they form a BGP connection and exchange
routing information. If you later change a BGP filter, weight, distance, version, or timer, or make a similar
configuration change, you must reset the BGP sessions so that the configuration changes take effect.
Routing Configuration Guide, Cisco IOS XE Everest 16.6.x (Catalyst 9500 Switches)
Nonstop Forwarding Awareness
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