BGP的消息報文有以下5種報文
open消息
keepalive消息
update消息
notifacation消息
route-refresh消息
j、Open消息
主要包含的內容：
BGP版本、AS號（可用其判斷是EBGP還是IBGP）；
Holdtime（表示路由器在收到Keeplive消息等待最長時間，cisco默認爲180s，如果雙方的holdtime時間不一樣，鄰居不會down，這點與OSPF不同，在BGP中將會協商，採用最小的holdtime時間作爲雙方的holdtime時間，一般情況下，該值爲keepalive的三臺，默認情況下keepalive時間爲60s）；
BGP標識符（router-id）；
可選項optionalpareameter（包含多協議支持的地址簇等）；
k、Keepalive消息
是響應Open消息，用於保持鄰居關係，沒有攜帶什麼消息及數據，默認情況下，60s發送一次keepalive消息；
l、notification消息
對錯誤的配置進行響應，具有差錯檢測功能，當檢測到錯誤時，會向對等體發送notification消息，並關閉BGP連接；
從中可知，有錯誤類型、錯誤代碼、錯誤子代碼（錯誤原因）
*Dec 7 11:35:38.747: %BGP_SESSION-5-ADJCHANGE:neighbor 12.1.1.1 IPv4 Unicast topology base removed from session BGP Notificationsent
m、Update消息
用於對等體之間通告路由、已撤消的前輟路由、NLRI（網絡可達性消息）、路徑屬性值；
Withdrawn routeslength 表示不可達路由長度
Total pathattribute length 總路徑屬性長度，如果是0表示沒有path attributes字段；
n、route-Refresh消息
要求對等體重新發送路由更新信息，可用clearip bgp * soft in軟清讓路由器重新發送路由更新信息；
Address familyidentifier AFI: 地址族標識
Subsequesnt addressfamily identifier（SAFI）子地址族標識
R2#show ip bgp neighbors //用此命令可查看refresh報文的情況
BGP neighbor is4.4.4.4, remote AS 100, internal link
BGP version 4, remote router ID 4.4.4.4
BGP state = Established, up for 00:11:33
Last read 00:00:40, last write 00:00:08, holdtime is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable(disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised andreceived
Address family IPv4 Unicast: advertised andreceived
Enhanced Refresh Capability: advertised andreceived
Multisession Capability:
Stateful switchover support enabled: NO forsession 1
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 4 1
Keepalives: 14 14
Route Refresh: 0 0
Total: 19 16
在上面可知，route refresh的報文數爲零，執行命令後
R2#clear ip bgp* soft in
R2#show ip bgpneighbors //再次查看情況
BGP neighbor is4.4.4.4, remote AS 100, internal link
BGP version 4, remote router ID 4.4.4.4
BGP state = Established, up for 00:11:33
Last read 00:00:40, last write 00:00:08, holdtime is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable(disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised andreceived
Address family IPv4 Unicast: advertised andreceived
Enhanced Refresh Capability: advertised andreceived
Multisession Capability:
Stateful switchover support enabled: NO forsession 1
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 5 1
Keepalives: 24 25
Route Refresh: 1 0
Total: 33 31