Bgp Decision Attributes - Cisco Catalyst 9500 series Configuration Manual

BGP Decision Attributes

There are two types of reset, hard reset and soft reset. Cisco IOS Releases 12.1 and later support a soft reset
without any prior configuration. To use a soft reset without preconfiguration, both BGP peers must support
the soft route refresh capability, which is advertised in the OPEN message sent when the peers establish a
TCP session. A soft reset allows the dynamic exchange of route refresh requests and routing information
between BGP routers and the subsequent re-advertisement of the respective outbound routing table.
• When soft reset generates inbound updates from a neighbor, it is called dynamic inbound soft reset.
• When soft reset sends a set of updates to a neighbor, it is called outbound soft reset.
A soft inbound reset causes the new inbound policy to take effect. A soft outbound reset causes the new local
outbound policy to take effect without resetting the BGP session. As a new set of updates is sent during
outbound policy reset, a new inbound policy can also take effect.
The table given below lists the advantages and disadvantages hard reset and soft reset.
Table 13: Advantages and Disadvantages of Hard and Soft Resets
Type of Reset
Hard reset
Outbound soft reset
Dynamic inbound soft
reset
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. When chosen, the selected
path is entered into 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 AS, 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 installed 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 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 that is going to be used to reach
a destination. For EBGP, this is usually the IP address of the neighbor specified by the neighbor remote-as
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Advantages
No memory overhead
No configuration, no storing of routing
table updates
Does not clear the BGP session and cache
Does not require storing of routing table
updates and has no memory overhead
Configuring IP Unicast Routing
Disadvantages
The prefixes in the BGP, IP, and FIB
tables provided by the neighbor are lost.
Not recommended.
Does not reset inbound routing table
updates.
Both BGP routers must support the route
refresh capability (in Cisco IOS Release
12.1 and later).