BGP Best Externalとは
BGPは同じプレフィックスに複数の経路があっても、ベストパスを1本だけ選んでピアへ広告します。RFC 4271の9.1.3（Phase 3: Route Dissemination）が定める規則で、iBGPピアへもeBGPピアへも、広告されるのはLoc-RIBに入ったベストパスだけです。
この規則には、境界ルータが自分のeBGP経路より他の境界ルータからのiBGP経路をベストに選んだとき、自分のeBGP経路をどこにも広告しなくなるという帰結があります。AS内の他のルータから見ると、その境界ルータの先にある出口は存在しないのと同じです。
Best Externalは、この場合に「ベストではない外部経路のうち最良のもの（best external）」をiBGPピアへ広告する機能です。IETFのドラフト draft-ietf-idr-best-external（期限切れ、標準化はされていない）が手続きを整理しています。
Given that Internal paths are not re-advertised to Internal peers, it was specified that the best of the external paths, as determined by the path selection tie breaking algorithm, would be advertised to Internal peers.
ドラフトが述べているとおり、初期のBGP-4（RFC 1771）ではiBGPピアへの広告は「外部経路のうち最良のもの」で、RFC 4271で「全体のベストパス」に変わりました。Best Externalは、その古い規則をルートリフレクタやコンフェデレーションの環境でも使えるように整理し直したものです。
なぜ必要になるか：active/backup構成
出口を1つに寄せる運用（active/backup構成）でこの問題が起きます。
| ルータ | 受けている経路 | ベストパス | iBGPピアへの広告 |
|---|---|---|---|
| R1（主出口） | R4からのeBGP（LOCAL_PREF 200を付ける） | 自分のeBGP | する |
| R2（バックアップ出口） | R4からのeBGP（LOCAL_PREF 100）、R1からのiBGP（LOCAL_PREF 200） | R1からのiBGP | しない（ベストがiBGPなので、iBGPピアへ広告する経路が無い） |
| R3（内部） | R1からのiBGPだけ | R1経由 | — |
R2の出口は生きているのに、R3もR1もR2経由の経路を知りません。BGP PICで説明したバックアップパスは「同じプレフィックスを別のネクストホップから受けた経路」なので、PICのバックアップの材料が無くなります。R1-R4リンクが落ちると、R1がR3へ取り消しを送り、R2がベストパスを選び直してR3へ広告し、R3がそれを入れるまで転送が止まります。
R2にadvertise best-externalを入れると、R2は自分のeBGP経路（ベストではないが外部経路のうち最良）をiBGPピアへ広告します。R3とR1はR2経由の経路を受け取り、PICのバックアップとしてFIBに入れられます。ドラフトが挙げる効果は、障害時の接続の早い回復（Faster restoration of connectivity）のほか、ドメイン間のチャーンとブラックホールの削減、iBGPの経路振動の抑止、同じ隣接ASからのMEDの比較の改善です。
ループの条件とアドレスファミリーの制限
Best Externalで広告される経路は、受け取ったルータでは（LOCAL_PREFが低いので）ベストになりません。定常状態では転送に使われず、障害のときだけバックアップとして使われます。ドラフトはこれをAS内の経路選択の一貫性の根拠にしています。
consistency in the path selection process across the AS is still guaranteed since the ingress routers will not choose the best-external route as the best route for a destination in steady state.
ただし障害の瞬間には注意が要ります。バックアップに切り替えたルータのパケットは、ネクストホップ（R2）までIGPの経路をたどります。途中に別のルータがいると、そのルータは自分のBGPテーブルで転送先を決めるので、そのルータのベストパスがまだ元の出口を向いていればパケットは送り返されてループします。ラベル（MPLS）でネクストホップまで運ぶか、この記事の構成のようにバックアップ側の境界ルータへ直結リンクで届くなら、この問題は起きません。
Ciscoのガイドはこの理由から、Best Externalを設定できるアドレスファミリーを限定して説明しています。
| OS | ガイドの記述 | 出典 |
|---|---|---|
| IOS XR | VRF IPv4/IPv6、VPNv4/VPNv6、L2VPN、IPv4/IPv6 labeled-unicast。“Unlabelled best-external is not supported as it may create routing loop” | ASR 9000 Routing Command Reference, IOS XR 6.4.x |
| IOS XE | VPNv4、VPNv6、IPv4 VRF、IPv6 VRF | IOS XE 17.x IP Routing Configuration Guide |
一方、この記事で使うXRd 26.1.1は素のIPv4 unicastのaddress-familyでもadvertise best-externalを受け付け、そのとおりに動きます（STEP 5で確認）。古いコマンドリファレンスの制限は現行のリリースには当てはまりません。MPLS VPN（VPNv4）で使う形は、BGP Best External（MPLS VPN）で解説します。
IOS XRでの設定と確認
| 項目 | 内容 |
|---|---|
advertise best-external | address-family配下。ベストがiBGP経路のとき、最良の外部経路をiBGPピアとRRクライアントへ広告する |
show bgp <プレフィックス> | Best Externalに選ばれた経路にbest-externalが付き、Advertised ... to peersの行が付く。ベストではない経路にNot advertised to any peerと出れば広告されていない |
show bgp neighbors <ピア> advertised-routes | そのピアへ実際に広告した経路 |
show route <プレフィックス> / show cef <プレフィックス> | PICのバックアップが入っているか（BGP backup path / backup） |
実機での検証
XRd 26.1.1のルータ5台で確認します。
- AS 65001はR1・R2が境界、R3が内部（iBGPフルメッシュ、
next-hop-self、OSPF area 0）。R1-R2は直結 - AS 65002のR4はR1・R2の両方にeBGPで接続し、Lo1
192.168.4.0/24とLo2100.63.255.0/24を広告する。戻り（R4 → R5）はR2経由に固定している（R4の入方向でR2からの経路にLOCAL_PREF 200）。障害を入れるR1-R4リンクを戻りが通らないようにして、往路だけを測るため - AS 65003のR5は通信の発信元。R5 → R4の通信はR3 → R1 → R4を通る
- 出方向・入方向ポリシーはネイバーごとに別名（中身は
pass）、全ネイバーにsoft-reconfiguration inbound always
障害のあいだのロスは、R5から192.168.4.1と100.63.255.1へ100ミリ秒間隔で流したpingと、R3-R5間のキャプチャーの応答の途切れから求めます（BGP PICと同じ測り方）。
検証の全体像
| STEP | 操作 | 確かめること |
|---|---|---|
| 0 | 初期状態 | R1・R2とも自分のeBGPがベストで、互いに広告している。R3には2本届く |
| 1 | R1の入方向でR4からの経路にLOCAL_PREF 200 | R2のベストがiBGP（R1経由）になり、R2は自分のeBGP経路を広告しなくなる（WITHDRAW）。R3・R1の経路は1本に |
| 2 | R1・R3にPIC（backup 1 install） | 材料が無いのでbackupが付かない |
| 3 | R1-R4リンク断 | バックアップが無いのでロスが出る。R1の取り消し → R2の再選択と広告 → R3が入れるまで |
| 4 | 復旧 | |
| 5 | R2にadvertise best-external | R2がベストでない自分のeBGP経路をiBGPピアへ広告する（UPDATE、best-externalの印）。R3・R1にbackupが付く |
| 6 | R1-R4リンク断（Best Externalあり） | バックアップがあるのでロスなし |
| 7 | 復旧 | |
| 8 | no advertise best-external（最終状態） | R2の広告が消え（WITHDRAW）、バックアップも消える |
以降のshowとpcapの時刻はすべてUTCです（ルータのsyslogと揃えています）。
STEP 0：初期状態
R1・R2とも自分のeBGP経路がベストで、互いにiBGPで広告し合っています。 R2のshow bgpでは、eBGP経路（10.2.4.4）にAdvertised IPv4 Unicast paths to peersが付いてR1・R3へ広告されており、R3には2本届いています。
R2 show bgp 192.168.4.0/24（STEP 0）
RP/0/RP0/CPU0:R2#show bgp 192.168.4.0/24
Wed Sep 16 13:25:22.272 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 4 4
Last Modified: Sep 16 12:47:14.023 for 00:38:08
Paths: (2 available, best #2)
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.0.0.3 10.0.0.1
Path #1: Received by speaker 0
Not advertised to any peer
65002, (received & used)
10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal
Received Path ID 0, Local Path ID 0, version 0
Path #2: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.0.0.3 10.0.0.1
65002, (received & used)
10.2.4.4 from 10.2.4.4 (10.0.0.4)
Origin IGP, metric 0, localpref 100, valid, external, best, group-best
Received Path ID 0, Local Path ID 1, version 4
Origin-AS validity: (disabled)R1 show bgp 192.168.4.0/24（STEP 0）
RP/0/RP0/CPU0:R1#show bgp 192.168.4.0/24
Wed Sep 16 13:24:30.104 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 6 6
Last Modified: Sep 16 12:47:16.023 for 00:37:14
Paths: (2 available, best #2)
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.0.0.3 10.0.0.2
Path #1: Received by speaker 0
Not advertised to any peer
65002, (received & used)
10.0.0.2 (metric 2) from 10.0.0.2 (10.0.0.2)
Origin IGP, metric 0, localpref 100, valid, internal
Received Path ID 0, Local Path ID 0, version 0
Path #2: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.0.0.3 10.0.0.2
65002, (received & used)
10.1.4.4 from 10.1.4.4 (10.0.0.4)
Origin IGP, metric 0, localpref 100, valid, external, best, group-best
Received Path ID 0, Local Path ID 1, version 6
Origin-AS validity: (disabled)R3 show bgp 192.168.4.0/24（STEP 0）
RP/0/RP0/CPU0:R3#show bgp 192.168.4.0/24
Wed Sep 16 13:26:14.746 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 7 7
Last Modified: Sep 16 12:47:18.023 for 00:38:56
Paths: (2 available, best #1)
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
Path #1: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
65002, (received & used)
10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 7
Path #2: Received by speaker 0
Not advertised to any peer
65002, (received & used)
10.0.0.2 (metric 2) from 10.0.0.2 (10.0.0.2)
Origin IGP, metric 0, localpref 100, valid, internal
Received Path ID 0, Local Path ID 0, version 0R5からR4への通信はR3 → R1 → R4を通ります。
R5 traceroute 192.168.4.1（STEP 0）
RP/0/RP0/CPU0:R5#traceroute 192.168.4.1 source 192.168.5.1 probe 3 timeout 1 maxttl 6
Wed Sep 16 13:28:22.320 UTC
Type escape sequence to abort.
Tracing the route to 192.168.4.1
1 10.3.5.3 7 msec 5 msec 5 msec
2 10.1.3.1 9 msec 8 msec 28 msec
3 10.1.4.4 12 msec * 13 msec STEP 1：R1の入方向でR4からの経路にLOCAL_PREF 200
R2のベストパスがR1からのiBGPになり、R2は自分のeBGP経路を広告しなくなります。
R1 STEP 1 で commit した設定
RP/0/RP0/CPU0:R1#show configuration commit changes last 1
Wed Sep 16 13:28:46.113 UTC
!! Building configuration...
!! IOS XR Configuration 26.1.1
route-policy FROM-R4
set local-preference 200
pass
end-policy
!
endR2のshow bgpでは、ベストがlocalpref 200のR1経由に変わり、自分のeBGP経路はNot advertised to any peerになっています。
R2 show bgp 192.168.4.0/24（STEP 1）
RP/0/RP0/CPU0:R2#show bgp 192.168.4.0/24
Wed Sep 16 13:31:44.925 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 9 9
Last Modified: Sep 16 13:28:47.023 for 00:02:58
Paths: (2 available, best #1)
Not advertised to any peer
Path #1: Received by speaker 0
Not advertised to any peer
65002, (received & used)
10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 200, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 9
Path #2: Received by speaker 0
Not advertised to any peer
65002, (received & used)
10.2.4.4 from 10.2.4.4 (10.0.0.4)
Origin IGP, metric 0, localpref 100, valid, external
Received Path ID 0, Local Path ID 0, version 0
Origin-AS validity: (disabled)キャプチャーでは、R1がLOCAL_PREF 200の経路をUPDATEで送った30ミリ秒後に、R2がR3とR1へWITHDRAW（MP_UNREACH_NLRIで2つのプレフィックス）を送っています。R2の出口は生きていますが、AS内の他のルータからはもう見えません。
STEP 1 の R2-R3 間（R2 → R3 の UPDATE）
$ tshark -r bgp-best-external-step1-r2r3.pcap -t ud -Y 'bgp.type==2'
2 2026-09-16 13:28:47.539877Z 10.0.0.2 → 10.0.0.3 BGP 85 UPDATE MessageR3とR1の経路は1本になります。
R3 show bgp 192.168.4.0/24（STEP 1）
RP/0/RP0/CPU0:R3#show bgp 192.168.4.0/24
Wed Sep 16 13:32:37.238 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 9 9
Last Modified: Sep 16 13:28:47.023 for 00:03:50
Paths: (1 available, best #1)
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
Path #1: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
65002, (received & used)
10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 200, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 9R1 show bgp 192.168.4.0/24（STEP 1）
RP/0/RP0/CPU0:R1#show bgp 192.168.4.0/24
Wed Sep 16 13:30:53.362 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 10 10
Last Modified: Sep 16 13:28:47.023 for 00:02:06
Paths: (2 available, best #1)
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.0.0.3 10.0.0.2
Path #1: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.0.0.3 10.0.0.2
65002
10.1.4.4 from 10.1.4.4 (10.0.0.4)
Origin IGP, metric 0, localpref 200, valid, external, best, group-best
Received Path ID 0, Local Path ID 1, version 10
Origin-AS validity: (disabled)
Path #2: Received by speaker 0
Not advertised to any peer
65002, (received-only)
10.1.4.4 from 10.1.4.4 (10.0.0.4)
Origin IGP, metric 0, localpref 100, valid, external
Received Path ID 0, Local Path ID 0, version 0
Origin-AS validity: (disabled)STEP 2：R1・R3にPIC（backup 1 install）
バックアップにする経路が無いので、PICを設定してもbackupは付きません。 R3のルーティングテーブルにはネクストホップが1つだけで、Number of pic pathsの行も出ません。
R3 STEP 2 で commit した設定
RP/0/RP0/CPU0:R3#show configuration commit changes last 1
Wed Sep 16 13:35:20.308 UTC
!! Building configuration...
!! IOS XR Configuration 26.1.1
route-policy PIC
set path-selection backup 1 install
end-policy
!
router bgp 65001
address-family ipv4 unicast
additional-paths selection route-policy PIC
!
!
endR3 show route 192.168.4.0/24（STEP 2）
RP/0/RP0/CPU0:R3#show route 192.168.4.0/24
Wed Sep 16 13:39:14.906 UTC
Routing entry for 192.168.4.0/24
Known via "bgp 65001", distance 200, metric 0
Tag 65002, type internal
Installed Sep 16 13:28:47.509 for 00:10:27
Routing Descriptor Blocks
10.0.0.1, from 10.0.0.1
Route metric is 0, Wt is 1
No advertising protos. STEP 3：R1-R4リンク断（Best Externalなし）
バックアップが無いので、R2の経路がAS内に広まるまで転送が止まります。 R1のGi0/0/0/2をshutdownすると（R4側も落としています）、R1はR2へWITHDRAWを送り、それを受けたR2は自分のeBGP経路をベストに選び直してR1・R3へ広告します。
R1 show logging（STEP 3）
RP/0/RP0/CPU0:R1#show logging start Sep 16 13:37:54
Wed Sep 16 13:45:42.363 UTC
Time Zone UTC, DST disabled
Syslog logging: enabled (0 messages dropped, 0 flushes, 0 overruns)
Console logging: Disabled
Monitor logging: level debugging, 0 messages logged
Trap logging: level informational, 0 messages logged
Buffer logging: level debugging, 158 messages logged
Log Buffer (2097152 bytes):
RP/0/RP0/CPU0:Sep 16 13:37:54.525 UTC: logger[68497]: %OS-SYSLOG-6-LOG_INFO : informational STEP3-BEGIN
RP/0/RP0/CPU0:Sep 16 13:37:54.918 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11107]: Received disconnect from 10.100.3.1 port 50706:11: disconnected by user
RP/0/RP0/CPU0:Sep 16 13:37:54.918 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11107]: Disconnected from user cisco 10.100.3.1 port 50706
RP/0/RP0/CPU0:Sep 16 13:41:58.111 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11495]: Accepted authentication/pam for cisco from 10.100.3.1 port 51046 ssh2
RP/0/RP0/CPU0:Sep 16 13:42:02.703 UTC: ifmgr[155]: %PKT_INFRA-LINK-5-CHANGED : Interface GigabitEthernet0/0/0/2, changed state to Administratively Down
RP/0/RP0/CPU0:Sep 16 13:42:02.707 UTC: bgp[1084]: %ROUTING-BGP-5-ADJCHANGE : neighbor 10.1.4.4 Down - Admin. shutdown (CEASE notification sent - administrative shutdown) (VRF: default) (AS: 65002)
RP/0/RP0/CPU0:Sep 16 13:42:03.132 UTC: config[68893]: %MGBL-CONFIG-6-DB_COMMIT : Configuration committed by user 'cisco'. Use 'show configuration commit changes 1000000005' to view the changes.
RP/0/RP0/CPU0:Sep 16 13:42:03.327 UTC: config[68893]: %MGBL-SYS-5-CONFIG_I : Configured from console by cisco on vty0 (10.100.3.1)
RP/0/RP0/CPU0:Sep 16 13:42:05.334 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11500]: Received disconnect from 10.100.3.1 port 51046:11: disconnected by user
RP/0/RP0/CPU0:Sep 16 13:42:05.334 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11500]: Disconnected from user cisco 10.100.3.1 port 51046
RP/0/RP0/CPU0:Sep 16 13:45:24.732 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11852]: Accepted authentication/pam for cisco from 10.100.3.1 port 51339 ssh2
RP/0/RP0/CPU0:Sep 16 13:45:39.023 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11868]: Received disconnect from 10.100.3.1 port 51339:11: disconnected by user
RP/0/RP0/CPU0:Sep 16 13:45:39.023 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[11868]: Disconnected from user cisco 10.100.3.1 port 51339
RP/0/RP0/CPU0:Sep 16 13:45:40.116 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[12068]: Accepted authentication/pam for cisco from 10.100.3.1 port 51349 ssh2 STEP 3 の R1-R2 間（R1 → R2 の WITHDRAW と R2 → R1 の UPDATE）
$ tshark -r bgp-best-external-step3-r1r2.pcap -t ud -Y 'bgp.type==2'
9 2026-09-16 13:42:02.905376Z 10.0.0.1 → 10.0.0.2 BGP 85 UPDATE Message
10 2026-09-16 13:42:02.939058Z 10.0.0.2 → 10.0.0.1 BGP 125 UPDATE MessageSTEP 3 の R2-R3 間（R2 → R3 の UPDATE）
$ tshark -r bgp-best-external-step3-r2r3.pcap -t ud -Y 'bgp.type==2'
307 2026-09-16 13:42:02.935153Z 10.0.0.2 → 10.0.0.3 BGP 125 UPDATE Messageリンク断（13:42:02.703）からR2の再広告（13:42:02.935）までは0.23秒ですが、R3がそれを入れて応答が戻ったのは13:42:03.928で、途切れは1.12秒です。100ミリ秒間隔のpingは900発中1発を失いました。
R5 ping 192.168.4.1（STEP 3、100ミリ秒間隔）
RP/0/RP0/CPU0:R5#ping 192.168.4.1 source 192.168.5.1 count 900 interval 100 timeout 1
Wed Sep 16 13:41:44.291 UTC
Type escape sequence to abort.
Sending 900, 100-byte ICMP Echos to 192.168.4.1 timeout is 1 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 99 percent (899/900), round-trip min/avg/max = 8/16/94 msR5 ping 100.63.255.1（STEP 3、100ミリ秒間隔）
RP/0/RP0/CPU0:R5#ping 100.63.255.1 source 192.168.5.1 count 900 interval 100 timeout 1
Wed Sep 16 13:41:48.992 UTC
Type escape sequence to abort.
Sending 900, 100-byte ICMP Echos to 100.63.255.1 timeout is 1 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 99 percent (899/900), round-trip min/avg/max = 9/15/93 msSTEP 4：復旧
両端をno shutdownにしてSTEP 2の状態に戻します。
STEP 5：R2にadvertise best-external
R2はベストではない自分のeBGP経路をiBGPピアへ広告し、R3・R1にバックアップが戻ります。 設定はaddress-family配下の1行です。
R2 STEP 5 で commit した設定
RP/0/RP0/CPU0:R2#show configuration commit changes last 1
Wed Sep 16 13:56:10.659 UTC
!! Building configuration...
!! IOS XR Configuration 26.1.1
router bgp 65001
address-family ipv4 unicast
advertise best-external
!
!
endR2のshow bgpでは、eBGP経路にbest-externalが付き、Advertised IPv4 Unicast paths to peersの行が戻っています。ベストパスはR1経由のままです。
R2 show bgp 192.168.4.0/24（STEP 5）
RP/0/RP0/CPU0:R2#show bgp 192.168.4.0/24
Wed Sep 16 13:59:09.675 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 16 16
Last Modified: Sep 16 13:56:12.023 for 00:02:57
Paths: (2 available, best #1)
Not advertised to any peer
Path #1: Received by speaker 0
Not advertised to any peer
65002, (received & used)
10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 200, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 14
Path #2: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.0.0.3 10.0.0.1
65002, (received & used)
10.2.4.4 from 10.2.4.4 (10.0.0.4)
Origin IGP, metric 0, localpref 100, valid, external, best-external
Received Path ID 0, Local Path ID 2, version 16
Origin-AS validity: (disabled)キャプチャーには、commitの直後にR2がR3へ送ったUPDATE（LOCAL_PREF 100の2プレフィックス）が入っています。
STEP 5 の R2-R3 間（R2 → R3 の UPDATE）
$ tshark -r bgp-best-external-step5-r2r3.pcap -t ud -Y 'bgp.type==2'
2 2026-09-16 13:56:11.824278Z 10.0.0.2 → 10.0.0.3 BGP 125 UPDATE MessageR3はR2経由の経路を受け取り、STEP 2で設定したPICがそれをバックアップにします（backup, add-path、BGP backup path）。R1も同じです。
R3 show bgp 192.168.4.0/24（STEP 5）
RP/0/RP0/CPU0:R3#show bgp 192.168.4.0/24
Wed Sep 16 14:00:02.113 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 19 19
Last Modified: Sep 16 13:56:12.023 for 00:03:50
Paths: (2 available, best #1)
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
Path #1: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
65002, (received & used)
10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 200, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 16
Path #2: Received by speaker 0
Not advertised to any peer
65002, (received & used)
10.0.0.2 (metric 2) from 10.0.0.2 (10.0.0.2)
Origin IGP, metric 0, localpref 100, valid, internal, backup, add-path
Received Path ID 0, Local Path ID 3, version 19R3 show route 192.168.4.0/24（STEP 5）
RP/0/RP0/CPU0:R3#show route 192.168.4.0/24
Wed Sep 16 14:00:04.028 UTC
Routing entry for 192.168.4.0/24
Known via "bgp 65001", distance 200, metric 0
Tag 65002
Number of pic paths 1 , type internal
Installed Sep 16 13:56:11.827 for 00:03:52
Routing Descriptor Blocks
10.0.0.1, from 10.0.0.1
Route metric is 0, Wt is 1
10.0.0.2, from 10.0.0.2, BGP backup path
Route metric is 0, Wt is 1
No advertising protos. R1 show cef 192.168.4.0/24（STEP 5）
RP/0/RP0/CPU0:R1#show cef 192.168.4.0/24
Wed Sep 16 13:58:20.239 UTC
192.168.4.0/24, version 54, internal 0x5000001 0x40 (ptr 0x8851a380) [1], 0x0 (0x0), 0x0 (0x0)
Updated Sep 16 13:56:11.847
local adjacency to GigabitEthernet0/0/0/2
Prefix Len 24, traffic index 0, precedence n/a, priority 4
gateway array (0x8938b008) reference count 2, flags 0x102010, source rib (7), 0 backups
[1 type 3 flags 0x48501 (0x8942df60) ext 0x0 (0x0) (collapsed)]
LW-LDI[type=0, refc=0, ptr=0x0, sh-ldi=0x0]
gateway array update type-time 1 Sep 16 13:56:11.831
LDI Update time Sep 16 13:56:11.847
via 10.0.0.2/32, 3 dependencies, recursive, backup [flags 0x6100]
path-idx 0 NHID 0x0 [0x8851b0f0 0x0]
next hop 10.0.0.2/32 via 10.0.0.2/32
via 10.1.4.4/32, 2 dependencies, recursive, bgp-ext [flags 0x6020]
path-idx 1 NHID 0x0 [0x8851b470 0x0], Internal 0x8a9b80a0
next hop 10.1.4.4/32 via 10.1.4.4/32
Weight distribution:
slot 0, weight 1, normalized_weight 1, class 0
Load distribution: 0 (refcount 1)
Hash OK Interface Address
0 Y GigabitEthernet0/0/0/2 10.1.4.4 STEP 6：R1-R4リンク断（Best Externalあり）
同じ障害で、今度はロスがありません。 pingは両宛先とも900発すべて応答があり、キャプチャー上の応答の途切れも最大0.21秒です。
R5 ping 192.168.4.1（STEP 6、100ミリ秒間隔）
RP/0/RP0/CPU0:R5#ping 192.168.4.1 source 192.168.5.1 count 900 interval 100 timeout 1
Wed Sep 16 14:03:22.827 UTC
Type escape sequence to abort.
Sending 900, 100-byte ICMP Echos to 192.168.4.1 timeout is 1 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 100 percent (900/900), round-trip min/avg/max = 9/17/173 msR5 ping 100.63.255.1（STEP 6、100ミリ秒間隔）
RP/0/RP0/CPU0:R5#ping 100.63.255.1 source 192.168.5.1 count 900 interval 100 timeout 1
Wed Sep 16 14:03:27.390 UTC
Type escape sequence to abort.
Sending 900, 100-byte ICMP Echos to 100.63.255.1 timeout is 1 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 100 percent (900/900), round-trip min/avg/max = 8/17/136 msR1のインタフェースが14:03:40.850にダウンすると、その56ミリ秒後（14:03:40.906）にはR5からのパケットがR1-R2間に現れます。R1のFIBが、Best Externalで受け取っていたR2経由の経路（バックアップ）へ切り替えたためです。STEP 3ではこの経路がそもそも無く、R2の再広告を待っていました。
R1 show logging（STEP 6）
RP/0/RP0/CPU0:R1#show logging start Sep 16 13:58:43
Wed Sep 16 14:07:18.756 UTC
Time Zone UTC, DST disabled
Syslog logging: enabled (0 messages dropped, 0 flushes, 0 overruns)
Console logging: Disabled
Monitor logging: level debugging, 0 messages logged
Trap logging: level informational, 0 messages logged
Buffer logging: level debugging, 223 messages logged
Log Buffer (2097152 bytes):
RP/0/RP0/CPU0:Sep 16 13:58:43.926 UTC: logger[67558]: %OS-SYSLOG-6-LOG_INFO : informational STEP6-BEGIN
RP/0/RP0/CPU0:Sep 16 13:58:44.330 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[14263]: Received disconnect from 10.100.3.1 port 52989:11: disconnected by user
RP/0/RP0/CPU0:Sep 16 13:58:44.330 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[14263]: Disconnected from user cisco 10.100.3.1 port 52989
RP/0/RP0/CPU0:Sep 16 14:03:37.108 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[14723]: Accepted authentication/pam for cisco from 10.100.3.1 port 53249 ssh2
RP/0/RP0/CPU0:Sep 16 14:03:40.850 UTC: ifmgr[155]: %PKT_INFRA-LINK-5-CHANGED : Interface GigabitEthernet0/0/0/2, changed state to Administratively Down
RP/0/RP0/CPU0:Sep 16 14:03:40.851 UTC: bgp[1084]: %ROUTING-BGP-5-ADJCHANGE : neighbor 10.1.4.4 Down - Admin. shutdown (CEASE notification sent - administrative shutdown) (VRF: default) (AS: 65002)
RP/0/RP0/CPU0:Sep 16 14:03:41.152 UTC: config[68025]: %MGBL-CONFIG-6-DB_COMMIT : Configuration committed by user 'cisco'. Use 'show configuration commit changes 1000000007' to view the changes.
RP/0/RP0/CPU0:Sep 16 14:03:41.251 UTC: config[68025]: %MGBL-SYS-5-CONFIG_I : Configured from console by cisco on vty0 (10.100.3.1)
RP/0/RP0/CPU0:Sep 16 14:03:43.149 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[14728]: Received disconnect from 10.100.3.1 port 53249:11: disconnected by user
RP/0/RP0/CPU0:Sep 16 14:03:43.149 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[14728]: Disconnected from user cisco 10.100.3.1 port 53249
RP/0/RP0/CPU0:Sep 16 14:07:02.413 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[15036]: Accepted authentication/pam for cisco from 10.100.3.1 port 53412 ssh2
RP/0/RP0/CPU0:Sep 16 14:07:15.625 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[15041]: Received disconnect from 10.100.3.1 port 53412:11: disconnected by user
RP/0/RP0/CPU0:Sep 16 14:07:15.625 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[15041]: Disconnected from user cisco 10.100.3.1 port 53412
RP/0/RP0/CPU0:Sep 16 14:07:16.452 UTC: ssh_syslog_proxy[1188]: %SECURITY-SSHD_SYSLOG_PRX-6-INFO_GENERAL : sshd[15223]: Accepted authentication/pam for cisco from 10.100.3.1 port 53421 ssh2 STEP 6 の R1-R2 間（R5 からの Echo request。R1 が R2 へ迂回させたもの）
$ tshark -r bgp-best-external-step6-r1r2.pcap -t ud -Y 'icmp.type==8'
5 2026-09-16 14:03:40.906320Z 192.168.5.1 → 100.63.255.1 ICMP 114 Echo (ping) request id=0x3973, seq=124/31744, ttl=253
6 2026-09-16 14:03:40.908027Z 192.168.5.1 → 100.63.255.1 ICMP 114 Echo (ping) request id=0x3973, seq=124/31744, ttl=252
7 2026-09-16 14:03:40.909325Z 192.168.5.1 → 100.63.255.1 ICMP 114 Echo (ping) request id=0x3973, seq=124/31744, ttl=251
8 2026-09-16 14:03:40.911463Z 192.168.5.1 → 100.63.255.1 ICMP 114 Echo (ping) request id=0x3973, seq=124/31744, ttl=250
9 2026-09-16 14:03:40.912680Z 192.168.5.1 → 100.63.255.1 ICMP 114 Echo (ping) request id=0x3973, seq=124/31744, ttl=249
10 2026-09-16 14:03:40.914636Z 192.168.5.1 → 100.63.255.1 ICMP 114 Echo (ping) request id=0x3973, seq=124/31744, ttl=248
<snip>（以下 18 行）STEP 7：復旧
両端をno shutdownにしてSTEP 5の状態に戻します。
STEP 8：no advertise best-external（最終状態）
設定を外すと、R2はR3・R1へWITHDRAWを送り、R3のバックアップも消えてSTEP 2の状態に戻ります。
R2 STEP 8 で commit した設定
RP/0/RP0/CPU0:R2#show configuration commit changes last 1
Wed Sep 16 14:17:47.153 UTC
!! Building configuration...
!! IOS XR Configuration 26.1.1
router bgp 65001
address-family ipv4 unicast
no advertise best-external
!
!
endSTEP 8 の R2-R3 間（R2 → R3 の WITHDRAW）
$ tshark -r bgp-best-external-step8-r2r3.pcap -t ud -Y 'bgp.type==2'
3 2026-09-16 14:17:48.424068Z 10.0.0.2 → 10.0.0.3 BGP 85 UPDATE MessageR3 show bgp 192.168.4.0/24（STEP 8）
RP/0/RP0/CPU0:R3#show bgp 192.168.4.0/24
Wed Sep 16 14:21:38.105 UTC
BGP routing table entry for 192.168.4.0/24
Versions:
Process bRIB/RIB SendTblVer
Speaker 25 25
Last Modified: Sep 16 14:17:48.023 for 00:03:50
Paths: (1 available, best #1)
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
Path #1: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
10.3.5.5
65002, (received & used)
10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
Origin IGP, metric 0, localpref 200, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 24検証Configおよびshow結果
各STEPで5台すべてから、次のファイルをルータごとに分けて取得しています。検証Configは..._run.txtです（最終状態はSTEP 8のもの）。
| ファイル | 内容 |
|---|---|
..._show.txt | show bgp summary / show bgp / show route bgp / show bgp <プレフィックス>（192.168.1.0/24〜192.168.5.0/24と100.63.255.0/24）/ show route summary / show routeとshow cef（192.168.4.0/24・100.63.255.0/24・192.168.5.0/24・10.0.0.1/32・10.0.0.2/32）/ show bgp neighbors / show bgp process / show rpl route-policy / ピアごとのadvertised-routes / routes / received routes。R1〜R3はshow ospf neighbor / show route ospfも |
..._log.txt | そのSTEPの範囲だけに絞ったshow logging。各STEPの開始時にlogmsgでマーカーを入れ、その時刻をshow logging startに指定して取得したもの |
..._run.txt | そのSTEP時点のshow running-config（＝そのSTEPの検証Config） |
..._ping.txt / ..._ping2.txt | R5から192.168.4.1と100.63.255.1へのpingとtraceroute。障害を入れるSTEP（3・6）は障害の前から100ミリ秒間隔で900発を宛先ごとに流したもの（_ping.txtが192.168.4.1、_ping2.txtが100.63.255.1）。それ以外は50発 |
..._trace.txt | show bgp trace neighbor / show bgp trace error |
..._commit.cfg | そのSTEPで実際にcommitした設定だけ。 設定を変えたルータの分のみ |
STEP 0：初期状態
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | - |
| R2 | show | log | run | - | trace | - |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | - |
| R5 | show | log | run | ping | trace | - |
STEP 1：R1の入方向でR4からの経路にLOCAL_PREF 200
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | cfg |
| R2 | show | log | run | - | trace | - |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | - |
| R5 | show | log | run | ping | trace | - |
STEP 2：R1・R3にPIC（backup 1 install）
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | cfg |
| R2 | show | log | run | - | trace | - |
| R3 | show | log | run | - | trace | cfg |
| R4 | show | log | run | - | trace | - |
| R5 | show | log | run | ping | trace | - |
STEP 3：R1-R4リンク断（Best Externalなし）
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | cfg |
| R2 | show | log | run | - | trace | - |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | cfg |
| R5 | show | log | run | ping / ping2 | trace | - |
STEP 4：復旧
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | cfg |
| R2 | show | log | run | - | trace | - |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | cfg |
| R5 | show | log | run | ping | trace | - |
STEP 5：R2にadvertise best-external
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | - |
| R2 | show | log | run | - | trace | cfg |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | - |
| R5 | show | log | run | ping | trace | - |
STEP 6：R1-R4リンク断（Best Externalあり）
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | cfg |
| R2 | show | log | run | - | trace | - |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | cfg |
| R5 | show | log | run | ping / ping2 | trace | - |
STEP 7：復旧
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | cfg |
| R2 | show | log | run | - | trace | - |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | cfg |
| R5 | show | log | run | ping | trace | - |
STEP 8：no advertise best-external（最終状態）
| ルータ | show出力 | syslog | running-config | ping | trace | commit |
|---|---|---|---|---|---|---|
| R1 | show | log | run | - | trace | - |
| R2 | show | log | run | - | trace | cfg |
| R3 | show | log | run | - | trace | - |
| R4 | show | log | run | - | trace | - |
| R5 | show | log | run | ping | trace | - |
パケットキャプチャーはSTEPごとに、R3-R5間（R5のping。応答の途切れの計測に使ったもの）・R1-R2間・R2-R3間で取得しています。
| STEP | R3-R5間 | R1-R2間 | R2-R3間 |
|---|---|---|---|
| 0 | pcap | pcap | pcap |
| 1 | pcap | pcap | pcap |
| 2 | pcap | pcap | pcap |
| 3 | pcap | pcap | pcap |
| 4 | pcap | pcap | pcap |
| 5 | pcap | pcap | pcap |
| 6 | pcap | pcap | pcap |
| 7 | pcap | pcap | pcap |
| 8 | pcap | pcap | pcap |
参考
| 資料 | タイトル | 概要 |
|---|---|---|
| RFC 4271 | A Border Gateway Protocol 4 (BGP-4) | 9.1.3 広告するのはベストパスだけ |
| draft-ietf-idr-best-external-05 | Advertisement of the best external route in BGP | Best Externalの手続きと効果（期限切れのドラフト） |
| Cisco ASR 9000 Routing Command Reference, IOS XR 6.4.x | advertise best-external | 当時のガイドが挙げるアドレスファミリーと、ラベル無しでループしうる理由（XRd 26.1.1ではIPv4 unicastでも設定できる） |
| Cisco IOS XE 17.x IP Routing Configuration Guide | BGP Best External | active/backup構成の説明と、VPNv4 / VRF限定の制限 |
関連記事
- BGP（Border Gateway Protocol）
- BGPメッセージ
- BGPネイバーの状態遷移
- iBGPとeBGPの違い
- BGP next-hop-self
- BGPルートリフレクタ
- BGPパス属性とベストパス選択
- BGP ORIGIN属性
- BGP AS_PATH属性
- BGP NEXT_HOP属性
- BGP LOCAL_PREF属性
- BGP MED属性
- BGP WEIGHT属性
- BGP COMMUNITY属性
- BGP経路集約（aggregate-address）
- MP-BGP（マルチプロトコル拡張）
- BGP Extended Community（拡張コミュニティ）
- BGPマルチパス（maximum-paths）
- BGPの4バイトAS番号（RFC 6793）
- BGPのプライベートAS番号の除去（remove-private-AS）
- BGPのADD-PATH（RFC 7911）
- BGP PIC（Prefix Independent Convergence）
- BGP Best External（ベストでない外部経路の広告）