BGP的消息報文類型

 BGP的消息報文有如下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仍是IBGP）；

Holdtime（表示路由器在收到Keeplive消息等待最長時間，cisco默認爲180s，若是雙方的holdtime時間不同，鄰居不會down，這點與OSPF不一樣，在BGP中將會協商，採用最小的holdtime時間做爲雙方的holdtime時間，通常狀況下，該值爲keepalive的三臺，默認狀況下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，默認狀況下，60s發送一次keepalive消息；

l、notification消息

對錯誤的配置進行響應，具備差錯檢測功能，當檢測到錯誤時，會向對等體發送notification消息，並關閉BGP鏈接；

從中可知，有錯誤類型、錯誤代碼、錯誤子代碼（錯誤緣由）

*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Withdrawn routeslength 表示不可達路由長度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，若是是0表示沒有path attributes字段；

n、route-Refresh消息

要求對等體從新發送路由更新信息，可用clearip bgp * soft in軟清讓路由器從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報文的狀況

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  //再次查看狀況

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