Configuring Bgp Decision Attributes - Cisco WS-CBS3032-DEL Software Configuration Manual

Configuring BGP
Command
Step 3
clear ip bgp {* | address |
peer-group-name} soft out
Step 4 show ip bgp
show ip bgp neighbors

Configuring BGP Decision Attributes

When a BGP speaker receives updates from multiple autonomous systems that describe different paths
to the same destination, it must choose the single best path for reaching that destination. The selected
path is then entered in the BGP routing table and propagated to its neighbors. The decision is based on
the value of attributes that the update contains and other BGP-configurable factors.
When a BGP peer learns two EBGP paths for a prefix from a neighboring autonomous system, it chooses
the best path and inserts that path in the IP routing table. If BGP multipath support is enabled and the
EBGP paths are learned from the same neighboring autonomous systems, instead of a single best path,
multiple paths are entered in the IP routing table. Then, during packet switching, per-packet or
per-destination load-balancing is performed among the multiple paths. The maximum-paths router
configuration command controls the number of paths allowed.
These factors summarize the order in which BGP evaluates the attributes for choosing the best path:
1.
2.
3.
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6.
7.
8.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
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Purpose
(Optional) Perform an outbound soft reset to reset the inbound routing table on the
specified connection. Use this command if route refresh is supported.
• Enter an asterisk (*) to reset all connections.
• Enter an IP address to reset a specific connection.
• Enter a peer group name to reset the peer group.
Verify the reset by reviewing the routing table information and BGP neighbor
information.
If the path specifies a next hop that is inaccessible, drop the update. The BGP next-hop attribute,
automatically determined by the software, is the IP address of the next hop to be used to reach a
destination. For EBGP, this is usually the IP address of the neighbor specified by the neighbor
remote-as router configuration command. You can disable next-hop processing by using route maps
or the neighbor next-hop-self router configuration command.
Prefer the path with the largest weight (a Cisco-proprietary parameter). The weight attribute is local
to the router and not propagated in routing updates. By default, the weight attribute is 32768 for
paths that the router originates and zero for other paths. Routes with the largest weight are preferred.
You can use access lists, route maps, or the neighbor weight router configuration command to set
weights.
Prefer the route with the highest local preference. Local preference is part of the routing update and
is exchanged among routers in the same autonomous system. The default value of the local
preference attribute is 100. You can set local preference by using the bgp default local-preference
router configuration command or by using a route map.
Prefer the route that was originated by BGP running on the local router.
Prefer the route with the shortest autonomous-system path.
Prefer the route with the lowest origin type. An interior route or IGP is lower than a route learned
by EGP, and an EGP-learned route is lower than one of unknown origin or learned in another way.
Prefer the route with the lowest multi-exit discriminator (MED) metric attribute if the neighboring
autonomous system is the same for all routes considered. You can configure the MED by using route
maps or by using the default-metric router configuration command. When an update is sent to an
IBGP peer, the MED is included.
Prefer the external (EBGP) path over the internal (IBGP) path.
Chapter 39 Configuring IP Unicast Routing
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