Border Gateway Protocol - CCIE Routing and Switching Exam Certification Guide - page 430

Display of BGP Neighbors Example 9-1

Router A# show ip bgp neighbors

BGP neighbor is 1.1.1.2, remote AS 200, external link
BGP version 4, remote router ID 10.10.10.10
BGP state = Established, up for 00:00:22
Last read 00:00:21, hold time is 180, keepalive interval is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(new)
Address family IPv4 Unicast: advertised and received
Received 3 messages, 0 notifications, 0 in queue
Sent 3 messages, 0 notifications, 0 in queue
Route refresh request: received 0, sent 0
Default minimum time between advertisement runs is 30 seconds

remote router ID 10.10.10.10
BGP state = Established
hold time is 180, keepalive interval is 60 seconds

For address family: IPv4 Unicast
BGP table version 1, neighbor version 1
Index 1, Offset 0, Mask 0x2
0 accepted prefixes consume 0 bytes
Prefix advertised 0, suppressed 0, withdrawn 0
Number of NLRIs in the update sent: max 0, min 0

Connections established 1; dropped 0
Last reset never
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 1.1.1.1, Local port: 11007
Foreign host: 1.1.1.2, Foreign port: 179

BGP Forms of Peering Relationships

BGP can establish two kinds of peering relationships: iBGP and eBGP.

iBGP

iBGP describes the peering between BGP neighbors in the same autonomous system(AS). You
use iBGP in transit autonomous systems. Transit autonomous systems forward traffic from one
AS to another AS. If you do not use iBGP in transit autonomous systems, the eBGP learned
routes are redistributed into an IGP and then redistributed into the BGP process in another
eBGP router. iBGP provides a better way to control the routes for the transit AS. With iBGP,
the external route information (attributes) are forwarded. iBGP is preferred over redistribution
with an IGP because the IGP does not understand AS paths and other BGP attributes.

Another use for iBGP is in large corporations where the IGP networks are too large to be sup-
ported by an IGP, such as Open Shortest Path First (OSPF). Three independent IGPs are in
Figure 9-2: one for the Americas; one for Asia and Australia; and one for Europe, the Middle
East, and Africa. Routes are redistributed into an iBGP core.