Managing Routing Policy Changes - Cisco WS-C3750-48PS-S Software Configuration Manual

Configuring BGP
Anything other than state = established means that the peers are not running. The remote router ID is
the highest IP address on that router (or the highest loopback interface). Each time the table is updated
with new information, the table version number increments. A table version number that continually
increments means that a route is flapping, causing continual routing updates.
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 specify where to send updates.
For detailed descriptions of BGP configuration, refer to the "IP Routing Protocols" chapter in the Cisco
IOS IP Configuration Guide, Release 12.2. For details about specific commands, refer to the Cisco IOS
IP Command Reference, Volume 2 of 3: Routing Protocols, Release 12.2. See
Commands in Cisco IOS Release 12.2(20)SE,"
supported by the switch.

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 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 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.
Table 34-10
Table 34-10 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 3750 Switch Software Configuration Guide
34-46
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
Chapter 34
for a list of BGP commands that are visible but not
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).
Configuring IP Unicast Routing
Appendix C, "Unsupported
78-16180-02