Managing Routing Policy Changes - Cisco Catalyst 3550 series Software Configuration Manual

Configuring BGP
For exterior protocols, a reference to an IP network from the network router configuration command
controls only which networks are advertised. This is in contrast to Interior Gateway Protocols (IGPs),
such as IGRP, which also use the network command to determine where to send updates.
For detailed descriptions of BGP configuration, refer to the "Configuring BGP" chapter in the Cisco IOS
IP and IP Routing Configuration Guide. For details about specific commands, refer to the Cisco IOS IP
and IP Routing Command Reference for Release 12.1. For a list of BGP commands that are visible but
not supported by the switch, see
12.1(19)EA1."

Managing Routing Policy Changes

Routing policies for a peer include all the configurations that might affect inbound or outbound routing
table updates. When you have defined two routers as BGP neighbors, they form a BGP connection and
exchange routing information. If you later change a BGP filter, weight, distance, version, or timer, or
make a similar configuration change, you must reset the BGP sessions so that the configuration changes
take effect.
There are two types of reset, hard reset and soft reset. Cisco IOS software 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.
•
•
A soft inbound reset enables 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.
Table 31-12
Table 31-12 Advantages and Disadvantages of Hard and Soft Resets
Type of Reset
hard reset
outbound soft reset
dynamic inbound soft reset Does not clear the BGP session and cache.
Catalyst 3550 Multilayer Switch Software Configuration Guide
31-50
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.
lists the advantages and disadvantages hard reset and soft reset.
Advantages
No memory overhead.
No configuration, no storing of routing table
updates.
Does not require storing of routing table updates
and has no memory overhead.
Appendix C, "Unsupported CLI Commands in Cisco IOS Release
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 releases).
Chapter 31 Configuring IP Unicast Routing
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