Bgp Conditional Advertisement; Bgp Next-Hop Address Tracking - Cisco Nexus 7000 Series Configuration Manual

Information About Advanced BGP
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .

BGP Conditional Advertisement

BGP conditional advertisement allows you to configure BGP to advertise or withdraw a route based on
whether or not a prefix exists in the BGP table. This feature is useful, for example, in multihomed
networks, in which you want BGP to advertise some prefixes to one of the providers only if information
from the other provider is not present.
Consider an example network with three BGP autonomous systems: AS1, AS2, and AS3, where AS1 and
AS3 connect to the Internet and to AS2. Without conditional advertisement, AS2 propagates all routes
to both AS1 and AS3. With conditional advertisement, you can configure AS2 to advertise certain routes
to AS3 only if routes from AS1 do not exist (if for example, the link to AS1 fails).
BGP conditional advertisement adds an exist or not-exist test to each route that matches the configured
route map. See the
information.

BGP Next-Hop Address Tracking

BGP monitors the next-hop address of installed routes to verify next-hop reachability and to select,
install, and validate the BGP best path. BGP next-hop address tracking speeds up this next-hop
reachability test by triggering the verification process when routes change in the RIB that may affect
BGP next-hop reachability.
BGP receives notifications from the RIB when next-hop information changes (event-driven
notifications). BGP is notified when any of the following events occurs:
•
•
•
•
•
•
•
•
Reachability and recursed metric events trigger a best-path recalculation.
Note
Event notifications from the RIB are classified as critical and noncritical. Notifications for critical and
noncritical events are sent in separate batches. However, a noncritical event is sent with the critical
events if the noncritical event is pending and there is a request to read the critical events.
•
•
See the
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
11-8
"Configuring BGP Conditional Advertisement" section on page 11-30
Next hop becomes unreachable.
Next hop becomes reachable.
Fully recursed IGP metric to the next hop changes.
First hop IP address or first hop interface changes.
Next hop becomes connected.
Next hop becomes unconnected.
Next hop becomes a local address.
Next hop becomes a nonlocal address.
Critical events are related to next-hop reachability, such as the loss of next hops resulting in a
switchover to a different path. A change in the IGP metric for a next hop resulting in a switchover
to a different path can also be considered a critical event.
Non-critical events are related to next hops being added without affecting the best path or changing
the IGP metric to a single next hop.
"Configuring BGP Next-Hop Address Tracking" section on page 11-23
Chapter 11 Configuring Advanced BGP
for more
for more information.
OL-20002-02