Specifications

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Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
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Chapter 6 Configuring Advanced BGP
Information About Advanced BGP
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.
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 “Configuring BGP Conditional Advertisement” section on page 6-29 for more
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:
• 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.