Specifications

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6-3
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 6 Configuring Advanced BGP
Information About Advanced BGP
Route Policies and Resetting BGP Sessions
You can associate a route policy to a BGP peer. Route policies use route maps to control or modify the
routes that BGP recognizes. You can configure a route policy for inbound or outbound route updates.
The route policies can match on different criteria, such as a prefix or AS_path attribute, and selectively
accept or deny the routes. Route policies can also modify the path attributes.
When you change a route policy applied to a BGP peer, you must reset the BGP sessions for that peer.
Cisco NX-OS supports the following three mechanisms to reset BGP peering sessions:
• Hard reset—A hard reset tears down the specified peering sessions, including the TCP connection,
and deletes routes coming from the specified peer. This option interrupts packet flow through the
BGP network. Hard reset is disabled by default.
• Soft reconfiguration inbound—A soft reconfiguration inbound triggers routing updates for the
specified peer without resetting the session. You can use this option if you change an inbound route
policy. Soft reconfiguration inbound saves a copy of all routes received from the peer before
processing the routes through the inbound route policy. If you change the inbound route policy,
Cisco NX-OS passes these stored routes through the modified inbound route policy to update the
route table without tearing down existing peering sessions. Soft reconfiguration inbound can use
significant memory resources to store the unfiltered BGP routes. Soft reconfiguration inbound is
disabled by default.
• Route Refresh—A route refresh updates the inbound routing tables dynamically by sending route
refresh requests to supporting peers when you change an inbound route policy. The remote BGP peer
responds with a new copy of its routes that the local BGP speaker processes with the modified route
policy. Cisco NX-OS automatically sends an outbound route refresh of prefixes to the peer.
• BGP peers advertise the route refresh capability as part of the BGP capability negotiation when
establishing the BGP peer session. Route refresh is the preferred option and enabled by default.
Note BGP also uses route maps for route redistribution, route aggregation, route dampening, and other
features. See Chapter 13, “Configuring Route Policy Manager,” for more information on route maps.
eBGP
External BGP (eBGP) allows you to connect BGP peers from different autonomous systems to exchange
routing updates. Connecting to external networks enables traffic from your network to be forwarded to
other networks and across the Internet.
You should use loopback interfaces for establishing eBGP peering sessions because loopback interfaces
are less susceptible to interface flapping. An interface flap occurs when the interface is administratively
brought up or down because of a failure or maintenance issue. See the “Configuring eBGP” section on
page 6-22 for information on multihop, fast external failovers, and limiting the size of the AS-path
attribute.
iBGP
Internal BGP (iBGP) allows you to connect BGP peers within the same autonomous system. You can use
iBGP for multihomed BGP networks (networks that have more than one connection to the same external
autonomous system).
Figure 6-1 shows an iBGP network within a larger BGP network.