接續上個章節，我們進行 BGP 相關實作與封包觀察。
下圖為本實驗的拓樸。
分配自治系統編號 (ASN)：必須為該 BGP 節點分配一個 ASN。在兩層式的 Leaf-and-Spine 架構中，可以利用 auto BGP 功能讓 Cumulus Linux 自動分配 ASN。
Router ID
設定 Neighbor 資訊
指定 Prefixes：明確指定要從此 BGP 節點發布出去的網路前綴
- set:
system:
hostname: spine01
interface:
lo:
ip:
address:
1.1.1.1/32: {}
swp1:
ip:
address:
10.0.0.0/31: {}
router:
bgp:
enable: on
autonomous-system: 65001
router-id: 1.1.1.1
vrf:
default:
router:
bgp:
address-family:
ipv4-unicast:
enable: on
network:
1.1.1.1/32: {}
enable: on
neighbor:
10.0.0.1:
type: numbered
remote-as: external
- set:
system:
hostname: Leaf01
interface:
lo:
ip:
address:
2.2.2.2/32: {}
swp1:
ip:
address:
10.0.0.1/31: {}
router:
bgp:
autonomous-system: 65002
enable: on
router-id: 2.2.2.2
vrf:
default:
router:
bgp:
address-family:
ipv4-unicast:
enable: on
network:
2.2.2.2/32: {}
enable: on
neighbor:
10.0.0.0:
remote-as: external
type: numbered
根據 Spine 和 Leaf 配置，可以看到
自治系統編號 (ASN) 分配
65001。65002。remote-as: external，這符合兩者 ASN 不同（eBGP）的邏輯。Router ID 設定
1.1.1.1，Leaf 為 2.2.2.2
鄰居 (Neighbor) 資訊
10.0.0.1，Leaf 指向 10.0.0.0，指定鄰居的 IP 地址。swp1 介面都配置了同一網段的 IPv4 地址（10.0.0.0/31 與 10.0.0.1/31），且明確標註 type: numbered。路由發布 (Prefix Origination)
address-family ipv4-unicast 下使用 network 指令發布了各自的 Loopback 地址（1.1.1.1/32 與 2.2.2.2/32）。驗證 BGP 鄰居摘要狀態
快速確認連線是否建立。
Leaf01:mgmt:~$ sudo vtysh -c "show ip bgp summary"
IPv4 Unicast Summary (VRF default):
BGP router identifier 2.2.2.2, local AS number 65002 vrf-id 0
BGP table version 2
RIB entries 3, using 672 bytes of memory
Peers 1, using 20 KiB of memory
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd PfxSnt Desc
spine01(10.0.0.0) 4 65001 1754 1754 0 0 0 01:27:42 1 2 N/A
Total number of neighbors 1
State/PfxRcd：這是最重要的欄位。如果顯示 數字（例如 1），表示連線已成功建立（Established），且從該鄰居收到了多少條路由；如果顯示 Active、Connect 或 Idle，則表示連線尚未建立，通常是 IP 不通、ASN 設錯或防火牆阻擋。
查看 BGP 路由表與路徑選擇
如果想確認路由是否正確傳遞，以及 BGP 如何選路：
Leaf01:mgmt:~$ sudo vtysh -c "show ip bgp"
BGP table version is 2, local router ID is 2.2.2.2, vrf id 0
Default local pref 100, local AS 65002
Status codes: s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath, + multipath nhg,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 1.1.1.1/32 10.0.0.0(spine01)
0 0 65001 i
*> 2.2.2.2/32 0.0.0.0(Leaf01) 0 32768 i
Displayed 2 routes and 2 total paths
此指令會列出所有學到的 BGP 路由。路由前方的 * 代表有效（Valid），> 代表這是最佳路徑（Best Path）。可以看到各項屬性如 Weight、Local Preference、AS_Path 等，這些都是 BGP 進行路徑選擇的依據。
查看特定鄰居的詳細資訊
如果需要排除故障（例如查看為什麼能力協商失敗）：
Leaf01:mgmt:~$ sudo vtysh -c "show ip bgp neighbor 10.0.0.0"
BGP neighbor is 10.0.0.0, remote AS 65001, local AS 65002, external link
Local Role: undefined
Remote Role: undefined
Hostname: spine01
BGP version 4, remote router ID 1.1.1.1, local router ID 2.2.2.2
BGP state = Established, up for 01:45:40
Last read 00:00:01, Last write 00:00:01
Hold time is 9 seconds, keepalive interval is 3 seconds
Configured hold time is 9 seconds, keepalive interval is 3 seconds
Configured conditional advertisements interval is 60 seconds
Neighbor capabilities:
4 Byte AS: advertised and received
Extended Message: advertised and received
AddPath:
IPv4 Unicast: RX advertised and received
Long-lived Graceful Restart: advertised and received
Address families by peer:
Route refresh: advertised and received(old & new)
Enhanced Route Refresh: advertised and received
Address Family IPv4 Unicast: advertised and received
Hostname Capability: advertised (name: Leaf01,domain name: n/a) received (name: spine01,domain name: n/a)
Graceful Restart Capability: advertised and received
Remote Restart timer is 120 seconds
Address families by peer:
Graceful Restart Capability: advertised and received
Remote Restart timer is 120 seconds
Address families by peer:
none
Graceful restart information:
End-of-RIB send: IPv4 Unicast
End-of-RIB received: IPv4 Unicast
Local GR Mode: Helper*
Remote GR Mode: Helper
R bit: False
N bit: False
Timers:
Configured Restart Time(sec): 120
Received Restart Time(sec): 120
IPv4 Unicast:
F bit: False
End-of-RIB sent: Yes
End-of-RIB sent after update: Yes
End-of-RIB received: Yes
Timers:
Configured Stale Path Time(sec): 360
Message statistics:
Inq depth is 0
Outq depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 3 3
Keepalives: 2109 2109
Route Refresh: 0 0
Capability: 0 0
Total: 2113 2113
Minimum time between advertisement runs is 0 seconds
For address family: IPv4 Unicast
Update group 1, subgroup 1
Packet Queue length 0
Community attribute sent to this neighbor(all)
1 accepted prefixes
Connections established 1; dropped 0
Last reset 01:45:41, Waiting for peer OPEN
External BGP neighbor may be up to 1 hops away.
Local host: 10.0.0.1, Local port: 54890
Foreign host: 10.0.0.0, Foreign port: 179
Nexthop: 10.0.0.1
Nexthop global: fe80::e60:5dff:fef7:1
Nexthop local: fe80::e60:5dff:fef7:1
BGP connection: shared network
BGP Connect Retry Timer in Seconds: 10
Estimated round trip time: 3 ms
Read thread: on Write thread: on FD used: 59
這會顯示該鄰居的詳細狀態、BGP 版本、保持時間（Hold Time）以及各項能力的協商狀況。
使用 NVUE 查看
$ nv show router bgp applied
------------------------------ -----------
enable on
autonomous-system 65002
router-id 2.2.2.2
policy-update-timer 5
graceful-shutdown off
wait-for-install off
graceful-restart
mode helper-only
restart-time 120
path-selection-deferral-time 360
stale-routes-time 360
convergence-wait
time 0
establish-wait-time 0
queue-limit
input 10000
output 10000
Leaf01:mgmt:~$ nv show vrf default router bgp neighbor
AS - Remote Autonomous System, PeerEstablishedTime - Peer established time in
UTC format, UpTime - Last connection reset time in days,hours:min:sec, Afi-Safi
- Address family, PfxSent - Transmitted prefix counter, PfxRcvd - Recieved
prefix counter
Neighbor AS State PeerEstablishedTime UpTime MsgRcvd MsgSent Afi-Safi PfxSent PfxRcvd
-------- ----- ----------- -------------------- ------- ------- ------- ------------ ------- -------
10.0.0.0 65001 established 2026-02-10T03:31:12Z 1:47:49 2155 2155 ipv4-unicast 2 1
Leaf01:mgmt:~$ nv show vrf default router bgp address-family ipv4-unicast route
PathCount - Number of paths present for the prefix, MultipathCount - Number of
paths that are part of the ECMP, DestFlags - * - bestpath-exists, w - fib-wait-
for-install, s - fib-suppress, i - fib-installed, x - fib-install-failed
Prefix PathCount MultipathCount DestFlags
---------- --------- -------------- ---------
1.1.1.1/32 1 1 *
2.2.2.2/32 1 1 *
使用 ping
Leaf01:mgmt:~$ ping 10.0.0.0 -c 2
vrf-wrapper.sh: switching to vrf "default"; use '--no-vrf-switch' to disable
PING 10.0.0.0 (10.0.0.0) 56(84) bytes of data.
64 bytes from 10.0.0.0: icmp_seq=1 ttl=64 time=0.616 ms
64 bytes from 10.0.0.0: icmp_seq=2 ttl=64 time=0.479 ms
--- 10.0.0.0 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1009ms
rtt min/avg/max/mdev = 0.479/0.547/0.616/0.068 ms
spine01
- set:
system:
hostname: spine1
interface:
lo:
ip:
address:
10.0.0.101/32: {}
swp1: {}
swp2: {}
router:
bgp:
enable: on
autonomous-system: 65000
router-id: 10.0.0.101
vrf:
default:
router:
bgp:
enable: on
address-family:
ipv4-unicast:
enable: on
network:
10.0.0.101/32: {} # 發布自己的 Loopback 路由
neighbor:
swp1:
type: unnumbered
remote-as: external # 使用 external 關鍵字簡化 ASN 管理
swp2:
type: unnumbered
remote-as: external
spine02
- set:
system:
hostname: spine2
interface:
lo:
ip:
address:
10.0.0.102/32: {}
swp1: {}
swp2: {}
router:
bgp:
enable: on
autonomous-system: 65000
router-id: 10.0.0.102
vrf:
default:
router:
bgp:
enable: on
address-family:
ipv4-unicast:
enable: on
network:
10.0.0.102/32: {} # 發布自己的 Loopback 路由
neighbor:
swp1:
type: unnumbered
remote-as: external # 使用 external 關鍵字簡化 ASN 管理
swp2:
type: unnumbered
remote-as: external
leaf01
- set:
system:
hostname: leaf1
interface:
lo:
ip:
address:
10.0.0.1/32: {}
swp1: {}
swp2: {}
router:
bgp:
enable: on
autonomous-system: 65001
router-id: 10.0.0.1
vrf:
default:
router:
bgp:
enable: on
address-family:
ipv4-unicast:
enable: on
network:
10.0.0.1/32: {}
neighbor:
swp1:
type: unnumbered
remote-as: external
swp2:
type: unnumbered
remote-as: external
leaf02
- set:
system:
hostname: leaf2
interface:
lo:
ip:
address:
10.0.0.2/32: {}
swp1: {}
swp2: {}
router:
bgp:
enable: on
autonomous-system: 65002
router-id: 10.0.0.2
vrf:
default:
router:
bgp:
enable: on
address-family:
ipv4-unicast:
enable: on
network:
10.0.0.2/32: {}
neighbor:
swp1:
type: unnumbered
remote-as: external
swp2:
type: unnumbered
remote-as: external
確認鄰居連線狀態
spine01:
~$ sudo vtysh -c "show ip bgp summary"
IPv4 Unicast Summary (VRF default):
BGP router identifier 10.0.0.101, local AS number 65000 vrf-id 0
BGP table version 3
RIB entries 5, using 1120 bytes of memory
Peers 2, using 40 KiB of memory
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd PfxSnt Desc
leaf1(swp1) 4 65001 587 587 0 0 0 00:29:06 1 3 N/A
leaf2(swp2) 4 65002 585 584 0 0 0 00:28:59 1 3 N/A
Total number of neighbors 2
leaf2:
$ sudo vtysh -c "show ip bgp summary"
IPv4 Unicast Summary (VRF default):
BGP router identifier 10.0.0.2, local AS number 65002 vrf-id 0
BGP table version 5
RIB entries 7, using 1568 bytes of memory
Peers 2, using 40 KiB of memory
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd PfxSnt Desc
spine2(swp1) 4 65000 621 623 0 0 0 00:30:51 2 4 N/A
spine1(swp2) 4 65000 606 608 0 0 0 00:30:06 2 4 N/A
Total number of neighbors 2
在 State/PfxRcd 欄位應顯示收到的路由數量（數字），若顯示 Active 或 Idle 則表示連線失敗。
查看 BGP 路由表與選路
spine01
$ sudo vtysh -c "show ip bgp"
BGP table version is 3, local router ID is 10.0.0.101, vrf id 0
Default local pref 100, local AS 65000
Status codes: s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath, + multipath nhg,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.1/32 swp1 0 0 65001 i
*> 10.0.0.2/32 swp2 0 0 65002 i
*> 10.0.0.101/32 0.0.0.0(spine1) 0 32768 i
Displayed 3 routes and 3 total paths
leaf02
$ sudo vtysh -c "show ip bgp"
BGP table version is 5, local router ID is 10.0.0.2, vrf id 0
Default local pref 100, local AS 65002
Status codes: s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath, + multipath nhg,
i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.0.0.1/32 swp1 0 65000 65001 i
*= swp2 0 65000 65001 i
*> 10.0.0.2/32 0.0.0.0(leaf2) 0 32768 i
*> 10.0.0.101/32 swp2 0 0 65000 i
*> 10.0.0.102/32 swp1 0 0 65000 i
Displayed 4 routes and 5 total paths
確認是否有從鄰居學到的路由，即 lookback 都被學習到。*> 代表最佳路徑選擇。
驗證系統路由表
leaf02
$ sudo vtysh -c "show ip route"
Codes: K - kernel route, C - connected, S - static, R - RIP,
O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
T - Table, A - Babel, D - SHARP, F - PBR, f - OpenFabric,
Z - FRR,
> - selected route, * - FIB route, q - queued, r - rejected, b - backup
t - trapped, o - offload failure
B>* 10.0.0.1/32 [20/0] via fe80::e49:82ff:fef7:1, swp1, weight 1, 00:43:14
* via fe80::e92:7eff:fe37:2, swp2, weight 1, 00:43:14
C>* 10.0.0.2/32 is directly connected, lo, 00:44:56
B>* 10.0.0.101/32 [20/0] via fe80::e92:7eff:fe37:2, swp2, weight 1, 00:43:14
B>* 10.0.0.102/32 [20/0] via fe80::e49:82ff:fef7:1, swp1, weight 1, 00:43:59
確認 BGP 學到的路由是否已成功學習到路由表中，BGP 會標示為 B 路由。
透過 ping 確認連通
leaf2 ping leaf1
$ ping 10.0.0.1 -c 2
vrf-wrapper.sh: switching to vrf "default"; use '--no-vrf-switch' to disable
PING 10.0.0.1 (10.0.0.1) 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_seq=1 ttl=63 time=1.49 ms
64 bytes from 10.0.0.1: icmp_seq=2 ttl=63 time=1.16 ms
--- 10.0.0.1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1003ms
rtt min/avg/max/mdev = 1.155/1.324/1.493/0.169 ms
目標是觀察 BGP Messages 類型，來看如何建立與維護 Peer 關係時使用的訊息。
下圖是 BGP Header，當中紅色區塊是下面類型共同的。
Marker (16 bytes)
主要用於 同步 (Synchronization) 與 認證 (Authentication)。它幫助接收端路由器在 TCP 資料流中定位 BGP 訊息的起始位置。如果沒有使用特殊的認證機制，此欄位通常會被填滿全為「1」的位元。
Length (2 bytes)
指示該 BGP 訊息的 總位元組長度。此長度包含了標頭本身（19 bytes）加上後續所有的資料負載 (Payload)。由於標頭固定為 19 bytes，因此訊息最小長度為 19 (例如 Keepalive 訊息)，最大長度通常限制在 4096 bytes。
Type (1 byte)
用來區分此訊息屬於哪一種 BGP 訊息類型。常見類型值：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Marker (16 bytes) +
| (用於同步與認證，通常為全 1) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (2 bytes) | Type (1 byte) |
| (含標頭總長，最小 29) | (Open = 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | My Autonomous System | Hold Time |
| (BGP 版本) | (本機的 AS 號碼) | (保持時間) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
| (通常為 Router ID) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len | |
| (選用參數長度) | Optional Parameters (variable) |
+-+-+-+-+-+-+-+-+ |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
封包 314，會和對方交換 Version、My AS、Hold Time、BGP Identifier 等訊息。
Border Gateway Protocol - OPEN Message
Marker: ffffffffffffffffffffffffffffffff
Length: 107
Type: OPEN Message (1) # 用於開啟會話並建立鄰居關係
Version: 4
My AS: 65002
Hold Time: 9
BGP Identifier: 10.0.0.2
Optional Parameters Length: 78
Optional Parameters
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 6
Capability: Multiprotocol extensions capability
Type: Multiprotocol extensions capability (1)
Length: 4
AFI: IPv4 (1)
Reserved: 00
SAFI: Unicast (1)
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 8
Capability: Extended Next Hop Encoding
Type: Extended Next Hop Encoding (5)
Length: 6
AFI: IPv4 (1)
SAFI: Unicast (1)
Next hop AFI: IPv6 (2)
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 2
Capability: Route Refresh Capability (Cisco)
Type: Route Refresh Capability (Cisco) (128)
Length: 0
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 2
Capability: Route refresh capability
Type: Route refresh capability (2)
Length: 0
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 2
Capability: Enhanced route refresh capability
Type: Enhanced route refresh capability (70)
Length: 0
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 6
Capability: Support for 4-octet AS number capability
Type: Support for 4-octet AS number capability (65)
Length: 4
AS Number: 65002
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 2
Capability: BGP-Extended Message
Type: BGP-Extended Message (6)
Length: 0
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 6
Capability: Support for Additional Paths
Type: Support for Additional Paths (69)
Length: 4
AFI: IPv4 (1)
SAFI: Unicast (1)
Send/Receive: Receive (1)
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 9
Capability: FQDN Capability
Type: FQDN Capability (73)
Length: 7
Hostname Length: 5
Hostname: leaf2
Domain Name Length: 0
Domain Name:
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 4
Capability: Graceful Restart capability
Type: Graceful Restart capability (64)
Length: 2
Restart Timers: 0x0078
0... .... .... .... = Restart state: No
.0.. .... .... .... = Graceful notification: No
.... 0000 0111 1000 = Time: 120
Optional Parameter: Capability
Parameter Type: Capability (2)
Parameter Length: 9
Capability: Long-Lived Graceful Restart (LLGR) Capability
Type: Long-Lived Graceful Restart (LLGR) Capability (71)
Length: 7
Unknown: 00010180000000
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Marker (16 bytes) +
| (用於同步與認證，通常為全 1) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (2 bytes) | Type (1 byte) |
| (固定值為 19) | (Keepalive=4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
週期性發送，用來向鄰居確認自己仍活著且連線正常。發送頻率再 cumulus 是 3 秒。
Border Gateway Protocol - KEEPALIVE Message
Marker: ffffffffffffffffffffffffffffffff
Length: 19
Type: KEEPALIVE Message (4)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Marker (16 bytes) +
| (用於同步與認證，通常為全 1) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (2 bytes) | Type (1 byte) |
| (含標頭總長，變動值) | (Update=2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Withdrawn Routes Length (2 bytes) |
+-------------------------------------+-------------------------+
| Withdrawn Routes (variable) | (要撤銷的路由列表)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total Path Attribute Length (2 bytes) |
+-----------------------------------------+---------------------+
| Path Attributes (variable) | (路徑屬性，如 AS-Path)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Layer Reachability Information (variable) |
| (NLRI，即目的地網路位址列表) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Update 訊息可以同時執行兩件事：
* 撤銷舊路由： 告訴鄰居哪些路不能走了。
* 宣布新路由： 告訴鄰居去哪些地方要經過哪個 Next Hop，且具備哪些屬性。
以下顯示封包 323 部分內容
Border Gateway Protocol - UPDATE Message
Marker: ffffffffffffffffffffffffffffffff
Length: 91
Type: UPDATE Message (2)
Withdrawn Routes Length: 0 # 沒有路由要被刪除
Total Path Attribute Length: 68
Path attributes
Path Attribute - MP_REACH_NLRI
Flags: 0x90, Optional, Extended-Length, Non-transitive, Complete
Type Code: MP_REACH_NLRI (14)
Length: 42
Address family identifier (AFI): IPv4 (1)
Subsequent address family identifier (SAFI): Unicast (1)
Next hop: IPv6=fe80::e37:3fff:feb5:2 Link-local=fe80::e37:3fff:feb5:2 # 流量下一步該送往哪個 IP 介面
IPv6 Address: fe80::e37:3fff:feb5:2
Link-local Address: fe80::e37:3fff:feb5:2
Number of Subnetwork points of attachment (SNPA): 0
Network Layer Reachability Information (NLRI)
Path Attribute - ORIGIN: IGP # 標示此路由是由內部手動或協定產生
Flags: 0x40, Transitive, Well-known, Complete
Type Code: ORIGIN (1)
Length: 1
Origin: IGP (0)
Path Attribute - AS_PATH: 65002 65000 65001 # 路由經過的歷程，用來防止環路與選路
Flags: 0x50, Transitive, Extended-Length, Well-known, Complete
Type Code: AS_PATH (2)
Length: 14
AS Path segment: 65002 65000 65001
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Marker (16 bytes) +
| (用於同步與認證，通常為全 1) |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (2 bytes) | Type (1 byte) |
| (含標頭總長，最小 21) | (Notification=3) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code (1)| Error Subcode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+
| |
| Data (variable，用於診斷錯誤的數據) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
BGP 是一種路徑向量路由演算法 (path-vector routing algorithm)，主要用於在不同的自治系統 (Autonomous Systems, AS) 之間交換路由資訊，其設計目標是確定到達特定目的地的最佳路徑，同時確保網路中不會出現路由迴圈。不像鏈路狀態路由協議那樣包含網路的完整拓撲。BGP 中 AS_Path 用作迴路防止機制。
BGP peer 是什麼 ? 為了交換路由資訊而建立起鄰居關係或*會話（Session）*的 BGP 路由器。會有三大要素
下表是 BGP Numbered 與 BGP Unnumbered 的比較
| 特性 | BGP Numbered | BGP Unnumbered |
|---|---|---|
| 接口 IP 分配 | 每個互聯接口都需要 IPv4 地址 | 接口不需要 IPv4 地址（僅需 Loopback） |
| 鄰居定義方式 | 指定鄰居的 IP 地址 | 指定接口名稱 (如 swp1) |
| 擴展性 | 規模越大，IP 地址管理越痛苦 | 極佳，適合大規模數據中心 (Clos 拓撲) |
| 地址族支持 | IPv4 或 IPv6 | 可在 IPv6 承載上交換 IPv4/IPv6 路由 |
| 主要協議基礎 | 標準 BGP | IPv6 Link-Local + RFC 5549/8950 |
如果需要連接外部防火牆、舊款交換機或服務商，考慮使用 BGP Numbered，因為這些第三方設備往往不支持透過 IPv6 Link-Local 來交換 IPv4 路由。
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