BGP Local,Dual-AS済

dual-asの存在理由についてはCCO参照

Autonomous system migration can be necessary when a telecommunications or Internet service provider purchases another network. It is desirable for the provider to be able to integrate the second autonomous system without disrupting existing customer peering arrangements. The amount of configuration required in the customer networks can make this a cumbersome task that is difficult to complete without disrupting service.

Chapter: BGP Support for Dual AS Configuration for Network AS Migrations

あかん例

iosv-1(config)#router bgp 200
iosv-1(config-router)#neighbor 10.1.1.1 remote-as 100
iosv-1(config-router)#network 2.2.2.2 mask 2
*Jun 24 13:44:24.376: %BGP-3-NOTIFICATION: sent to neighbor 10.1.1.1 passive 2/2 (peer in wrong AS) 2 bytes 0001
*Jun 24 13:44:24.378: %BGP-4-MSGDUMP: unsupported or mal-formatted message received from 10.1.1.1:
FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 0039 0104 0001 00B4 0101 0101 1C02 0601
0400 0100 0102 0280 0002 0202 0002 0246 0002 0641 0400 0000 01
*Jun 24 13:44:24.402: %BGP-3-NOTIFICATION: received from neighbor 10.1.1.1 active 2/8 (no supported AFI/SAFI) 3 bytes 0000005

・対向にlocal-asが設定されているときに、大本のAS番号を指定するとえらいこっちゃになるメモ

正しい例

iosv-0(config)#router bgp 100
iosv-0(config-router)#neighbor 10.1.1.2 remote-as 200
iosv-0(config-router)#neighbor 10.1.1.2 local-as 1
iosv-0(config-router)#neighbor 1.1.1.1 mask 255.255.255.255

iosv-0#sh ip bgp 2.2.2.2
BGP routing table entry for 2.2.2.2/32, version 3
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  1 200
    10.1.1.2 from 10.1.1.2 (2.2.2.2)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

iosv-1(config)#router bgp 200
iosv-1(config-router)#neighbor 10.1.1.1 remote-as 1
iosv-1(config-router)#network 2.2.2.2 mask 255.255.255.255

iosv-1#sh ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  1 100
    10.1.1.1 from 10.1.1.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

・iosv-1で設定したlocal-AS 1がそれぞれのパスに含まれている

no-prepend

iosv-0(config-router)#neighbor 10.1.1.2 local-as 1 no-prepend
iosv-0(config-router)#
*Jun 24 08:54:30.050: %BGP-5-NBR_RESET: Neighbor 10.1.1.2 reset (Local AS change)
*Jun 24 08:54:30.076: %BGP-5-ADJCHANGE: neighbor 10.1.1.2 Down Local AS change
*Jun 24 08:54:30.077: %BGP_SESSION-5-ADJCHANGE: neighbor 10.1.1.2 IPv4 Unicast topology base removed from session  Local AS change
*Jun 24 08:54:30.915: %BGP-5-ADJCHANGE: neighbor 10.1.1.2 Up

iosv-0#sh ip bgp 2.2.2.2
BGP routing table entry for 2.2.2.2/32, version 5
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  200
    10.1.1.2 from 10.1.1.2 (2.2.2.2)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

iosv-1#sh ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 5
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  1 100
    10.1.1.1 from 10.1.1.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

・実際にやってみた所感、no-prepend設定後のピアの繋ぎなおしは超一瞬だった
・iosv-0ではlocal-AS 1が消滅し、パスが200のみになっている

replace-as

iosv-0(config-router)#neighbor 10.1.1.2 local-as 1 no-prepend replace-as
iosv-0(config-router)#
*Jun 24 09:03:24.536: %BGP-5-NBR_RESET: Neighbor 10.1.1.2 reset (Local AS change)
*Jun 24 09:03:24.563: %BGP-5-ADJCHANGE: neighbor 10.1.1.2 Down Local AS change
*Jun 24 09:03:24.565: %BGP_SESSION-5-ADJCHANGE: neighbor 10.1.1.2 IPv4 Unicast topology base removed from session  Local AS change
*Jun 24 09:03:25.109: %BGP-5-ADJCHANGE: neighbor 10.1.1.2 Up

iosv-1#sh ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 7
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  1
    10.1.1.1 from 10.1.1.1 (1.1.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

・あ、そっちが消えるのね
・no-prependではlocal-ASが消え、replace-asでは大本のASが消滅

dual-as

iosv-0(config-router)#neighbor 10.1.1.2 local-AS 1 no-prepend replace-as dual-as
iosv-0(config-router)#
*Jun 24 09:11:10.488: %BGP-5-NBR_RESET: Neighbor 10.1.1.2 reset (Local AS change)
*Jun 24 09:11:10.507: %BGP-5-ADJCHANGE: neighbor 10.1.1.2 Down Local AS change
*Jun 24 09:11:10.509: %BGP_SESSION-5-ADJCHANGE: neighbor 10.1.1.2 IPv4 Unicast topology base removed from session  Local AS change
iosv-0(config-router)#
*Jun 24 09:11:11.519: %BGP-3-NOTIFICATION: received from neighbor 10.1.1.2 active 2/2 (peer in wrong AS) 2 bytes 0064
*Jun 24 09:11:11.521: %BGP-5-NBR_RESET: Neighbor 10.1.1.2 active reset (BGP Notification received)
*Jun 24 09:11:11.530: %BGP-5-ADJCHANGE: neighbor 10.1.1.2 active Down BGP Notification received
*Jun 24 09:11:11.531: %BGP_SESSION-5-ADJCHANGE: neighbor 10.1.1.2 IPv4 Unicast topology base removed from session  BGP Notification received

iosv-1(config)#router bgp 200
iosv-1(config-router)#no neighbor 10.1.1.1 remote-as 1
iosv-1(config-router)#
*Jun 24 14:51:08.047: %BGP-3-NOTIFICATION: sent to neighbor 10.1.1.1 6/3 (Peer De-configured) 0 bytes
iosv-1(config-router)#neighbo
*Jun 24 14:51:08.061: %BGP_SESSION-5-ADJCHANGE: neighbor 10.1.1.1 IPv4 Unicast topology base removed from session  Neighbor deleted
*Jun 24 14:51:08.063: %BGP-5-ADJCHANGE: neighbor 10.1.1.1 Down Neighbor deleted
iosv-1(config-router)#neighbor 10.1.1.1 remote-as 100
iosv-1(config-router)#
*Jun 24 14:51:26.710: %BGP-5-ADJCHANGE: neighbor 10.1.1.1 Up

・あかん例で張った伏線がここで活きるなんて
・dual-asを設定することで、大本のASでもlocal-ASでもeBGPピアリングができるようになる
・ちなみに、no-prepend、replace-asを設定しているのでshow ip bgpに変化はない

ちなみに

iosv-0(config-router)#neighbor 10.1.1.2 local-as 1 dual-as
% dual-as not allowed without no-prepend/replace-as for eBGP local-as

・dual-asだけ設定しようとしてもno-prepend,replace-asを入れろと怒られる

参考

【BGP】Local-AS設定（local-as,no-prepend,replace-as,dual-as）動作確認　Cisco

Chapter: BGP Support for Dual AS Configuration for Network AS Migrations