Information Technology Reference
In-Depth Information
Output of
debug ip bgp
for Session Establishment (Continued)
Example 2-5
*Oct 16 16:00:14.802: BGP: 192.168.100.2 OPEN has ROUTE-REFRESH capability(new)
for all address-families
*Oct 16 16:00:14.802: BGP: 192.168.100.2 went from OpenSent to OpenConfirm
*Oct 16 16:00:14.882: BGP: 192.168.100.2 went from OpenConfirm to Established
*Oct 16 16:00:14.882: %BGP-5-ADJCHANGE: neighbor 192.168.100.2 Up
BGP-IGP Routing Exchange
Routing exchange between BGP and an IGP can occur in two directions: from the IGP to
BGP, and from BGP into the IGP. There are two common ways to inject routes from an IGP
into BGP:
•
Using the
redistribute
command
•
Using the
network
command
IGP routes can be dynamically injected into BGP using the
redistribute
command. You
should use proper filtering and summarization whenever you do this to reduce the impact
of IGP instability on BGP. Even with these measures, dynamic redistribution of IGP routes
into BGP is not encouraged because of its dynamic nature and thus its lack of administrative
control.
When you redistribute routes into BGP using the
redistribute
command, only the classful
networks are redistributed by default. To have all routes redistributed individually, you must
disable BGP
auto-summary
(otherwise, an automatic classful summary is created). A new
default behavior was introduced in recent Cisco IOS software releases in which
no auto-
summary
is automatically enabled.
NOTE
The BGP
network
command operates differently from an IGP
network
command in Cisco
IOS software. In most IGP configurations, the
network
command binds a local interface to
a routing protocol and injects the interface address into the IGP. With BGP, the
network
command creates the route in the BGP table only if the route is already present in the IP
routing table. This allows IGP routes to be injected into BGP semistatically. It is semistatic
because the route is injected into BGP only when it already exists in the IP routing table.
Redistribution of BGP routes into an IGP should be used with only a small subset of the
BGP Internet routes or when the number of BGP routes is small. Proper filtering should be
deployed during redistribution to minimize the prefix count in the IGP. Various filtering
techniques are discussed in Chapter 4, “Effective BGP Policy Control.”
