[ixpmanager] Fwd: Malformed AS_PATH BGP Error Message from CDN Peer
Brian Longwe
brian at lionex.org
Wed Jul 22 16:34:36 IST 2026
Thanks Nick,
I have sent a request to the engineering team "on the other side".
Thanks also Frank - for the validation.
Best regards,
Brian
--
Brian Munyao Longwe
IXP Coordinator | Cell: +265999708883
[image: https://lionex.org] <https://lionex.org>
On Wed, Jul 22, 2026 at 5:18 PM Nick Hilliard <nick at foobar.org> wrote:
> Brian Longwe via ixpmanager wrote on 22/07/2026 16:07:
>
> Strange problem here. We have just turned up a new CDN at our IXP -
> EdgeNext. Our Route Server is refusing to peer with them, citing the error
> "Malformed AS_PATH". Tcpdump shows them having a "my AS" of 123456 - which
> it seems Bird is misunderstanding.
>
>
> Hi Brian,
>
> "AS23456" is the pseudo ASN used for negotiating a BGP session with a
> 32-bit AS number, so this would be expected in a tcpdump packet capture for
> AS139057.
>
> You received a "Malformed AS_PATH" error from the remote side. This is
> usually seen on Cisco and similar style BGP stacks which expect the AS Path
> to include the peer AS as the first ASN in the AS path. This doesn't happen
> at a route server, so all Cisco style customers of yours need to include
> "no bgp enforce-first-as" in the "router bgp" stanza of their
> configuration. Once this is configured, the BGP session should work fine.
>
> Nick
>
>
> Any idea how we can fix this? Is there a skin/neighbor profile that we can
> configure for them?
>
> Here is output from birdc show protocol:
>
> root at rs1:/home/devops# /usr/sbin/birdc -s
> /var/run/bird/bird-rs1-lan1-ipv4.ctl
> BIRD 2.19.1 ready.
> bird> show protocols all pb_0025_as139057
> Name Proto Table State Since Info
> pb_0025_as139057 BGP --- start 2026-07-22 14:45:32 Idle
> Received: Malformed AS_PATH
> Description: AS139057 - CDN EdgeNext
> BGP state: Idle
> Neighbor address: 196.x.y.z
> Neighbor AS: 139057
> Local AS: 37800
> Error wait: 35.054/60
> Last error: Received: Malformed AS_PATH
> Channel ipv4
> State: DOWN
> Table: t_0025_as139057
> Preference: 100
> Input filter: f_import_as139057
> Output filter: f_export_as139057
> Import limit: 1000
> Action: restart
>
> And here is output from tcpdump packet capture:
> 11:38:15.176666 ens18 In IP (tos 0xc0, ttl 1, id 36934, offset 0, flags
> [none], proto TCP (6), length 123)
> 196.60.122.44.49581 > rs1-lan1-ipv4.bgp: Flags [P.], cksum 0x56b2
> (correct), seq 1:72, ack 1, win 5840, options [nop,nop,TS val 41713288 ecr
> 3839994927], length 71: BGP
> Open Message (1), length: 71
> Version 4, my AS 23456, Holdtime 180s, ID 172.16.0.1
> Optional parameters, length: 42
> Option Capabilities Advertisement (2), length: 6
> Multiprotocol Extensions (1), length: 4
> AFI IPv4 (1), SAFI Unicast (1)
> 0x0000: 0001 0001
> Option Capabilities Advertisement (2), length: 2
> Route Refresh (Cisco) (128), length: 0
> Option Capabilities Advertisement (2), length: 2
> Route Refresh (2), length: 0
> Option Capabilities Advertisement (2), length: 16
> Graceful Restart (64), length: 14
> Restart Flags: [none], Restart Time 120s
> AFI IPv4 (1), SAFI Unicast (1), Forwarding state preserved: yes
> AFI IPv6 (2), SAFI Unicast (1), Forwarding state preserved: yes
> AFI VPLS (25), SAFI EVPN (70), Forwarding state preserved: yes
> 0x0000: 0078 0001 0180 0002 0180 0019 4680
> Option Capabilities Advertisement (2), length: 6
> 32-Bit AS Number (65), length: 4
> 4 Byte AS 139057
> 0x0000: 0002 1f31
> 0x0000: 45c0 007b 9046 0000 0106 abce c43c 7a2c E..{.F.......<z,
> 0x0010: c43c 7a03 c1ad 00b3 da49 51e5 b19f 25d5 .<z......IQ...%.
> 0x0020: 8018 16d0 56b2 0000 0101 080a 027c 7e88 ....V........|~.
> 0x0030: e4e1 ac2f ffff ffff ffff ffff ffff ffff .../............
> 0x0040: ffff ffff 0047 0104 5ba0 00b4 ac10 0001 .....G..[.......
> 0x0050: 2a02 0601 0400 0100 0102 0280 0002 0202 *...............
> 0x0060: 0002 1040 0e00 7800 0101 8000 0201 8000 ... at ..x.........
> 0x0070: 1946 8002 0641 0400 021f 31 .F...A....1
> 11:38:15.176686 ens18 Out IP (tos 0xc0, ttl 64, id 0, offset 0, flags
> [DF], proto TCP (6), length 40)
> rs1-lan1-ipv4.bgp > 196.60.122.44.49581: Flags [R], cksum 0x9962
> (correct), seq 2979997141, win 0, length 0
> 0x0000: 45c0 0028 0000 4000 4006 bd67 c43c 7a03 E..(.. at .@..g.<z.
> 0x0010: c43c 7a2c 00b3 c1ad b19f 25d5 0000 0000 .<z,......%.....
> 0x0020: 5004 0000 9962 0000 P....b..
> 11:38:26.177549 ens18 In IP (tos 0xc0, ttl 1, id 37226, offset 0, flags
> [none], proto TCP (6), length 60)
> 196.60.122.44.43812 > rs1-lan1-ipv4.bgp: Flags [S], cksum 0x6704
> (correct), seq 2085855246, win 5840, options [mss 1460,sackOK,TS val
> 41714388 ecr 0,nop,wscale 0], length 0
> 0x0000: 45c0 003c 916a 0000 0106 aae9 c43c 7a2c E..<.j.......<z,
> 0x0010: c43c 7a03 ab24 00b3 7c53 a00e 0000 0000 .<z..$..|S......
> 0x0020: a002 16d0 6704 0000 0204 05b4 0402 080a ....g...........
> 0x0030: 027c 82d4 0000 0000 0103 0300 .|..........
> 11:38:26.177593 ens18 Out IP (tos 0xc0, ttl 255, id 0, offset 0, flags
> [DF], proto TCP (6), length 60)
> rs1-lan1-ipv4.bgp > 196.60.122.44.43812: Flags [S.], cksum 0x7cd7
> (incorrect -> 0x8971), seq 316352219, ack 2085855247, win 65160, options
> [mss 1460,sackOK,TS val 3840005929 ecr 41714388,nop,wscale 7], length 0
> 0x0000: 45c0 003c 0000 4000 ff06 fe52 c43c 7a03 E..<.. at ....R.<z.
> 0x0010: c43c 7a2c 00b3 ab24 12db 26db 7c53 a00f .<z,...$..&.|S..
> 0x0020: a012 fe88 7cd7 0000 0204 05b4 0402 080a ....|...........
> 0x0030: e4e1 d729 027c 82d4 0103 0307 ...).|......
> 11:38:26.179093 ens18 In IP (tos 0xc0, ttl 1, id 37227, offset 0, flags
> [none], proto TCP (6), length 52)
> 196.60.122.44.43812 > rs1-lan1-ipv4.bgp: Flags [.], cksum 0x9ff6
> (correct), seq 1, ack 1, win 5840, options [nop,nop,TS val 41714388 ecr
> 3840005929], length 0
> 0x0000: 45c0 0034 916b 0000 0106 aaf0 c43c 7a2c E..4.k.......<z,
> 0x0010: c43c 7a03 ab24 00b3 7c53 a00f 12db 26dc .<z..$..|S....&.
> 0x0020: 8010 16d0 9ff6 0000 0101 080a 027c 82d4 .............|..
> 0x0030: e4e1 d729 ...)
> 11:38:26.179093 ens18 In IP (tos 0xc0, ttl 1, id 37228, offset 0, flags
> [none], proto TCP (6), length 52)
> 196.60.122.44.43812 > rs1-lan1-ipv4.bgp: Flags [.], cksum 0x9ff6
> (correct), seq 1, ack 1, win 5840, options [nop,nop,TS val 41714388 ecr
> 3840005929], length 0
> 0x0000: 45c0 0034 916c 0000 0106 aaef c43c 7a2c E..4.l.......<z,
> 0x0010: c43c 7a03 ab24 00b3 7c53 a00f 12db 26dc .<z..$..|S....&.
> 0x0020: 8010 16d0 9ff6 0000 0101 080a 027c 82d4 .............|..
> 0x0030: e4e1 d729 ...)
> 11:38:26.179093 ens18 In IP (tos 0xc0, ttl 1, id 37229, offset 0, flags
> [none], proto TCP (6), length 123)
> 196.60.122.44.43812 > rs1-lan1-ipv4.bgp: Flags [P.], cksum 0xeb7e
> (correct), seq 1:72, ack 1, win 5840, options [nop,nop,TS val 41714388 ecr
> 3840005929], length 71: BGP
> Open Message (1), length: 71
> Version 4, my AS 23456, Holdtime 180s, ID 172.16.0.1
> Optional parameters, length: 42
> Option Capabilities Advertisement (2), length: 6
> Multiprotocol Extensions (1), length: 4
> AFI IPv4 (1), SAFI Unicast (1)
> 0x0000: 0001 0001
> Option Capabilities Advertisement (2), length: 2
> Route Refresh (Cisco) (128), length: 0
> Option Capabilities Advertisement (2), length: 2
> Route Refresh (2), length: 0
> Option Capabilities Advertisement (2), length: 16
> Graceful Restart (64), length: 14
> Restart Flags: [none], Restart Time 120s
> AFI IPv4 (1), SAFI Unicast (1), Forwarding state preserved: yes
> AFI IPv6 (2), SAFI Unicast (1), Forwarding state preserved: yes
> AFI VPLS (25), SAFI EVPN (70), Forwarding state preserved: yes
> 0x0000: 0078 0001 0180 0002 0180 0019 4680
> Option Capabilities Advertisement (2), length: 6
> 32-Bit AS Number (65), length: 4
> 4 Byte AS 139057
> 0x0000: 0002 1f31
> 0x0000: 45c0 007b 916d 0000 0106 aaa7 c43c 7a2c E..{.m.......<z,
> 0x0010: c43c 7a03 ab24 00b3 7c53 a00f 12db 26dc .<z..$..|S....&.
> 0x0020: 8018 16d0 eb7e 0000 0101 080a 027c 82d4 .....~.......|..
> 0x0030: e4e1 d729 ffff ffff ffff ffff ffff ffff ...)............
> 0x0040: ffff ffff 0047 0104 5ba0 00b4 ac10 0001 .....G..[.......
> 0x0050: 2a02 0601 0400 0100 0102 0280 0002 0202 *...............
> 0x0060: 0002 1040 0e00 7800 0101 8000 0201 8000 ... at ..x.........
> 0x0070: 1946 8002 0641 0400 021f 31 .F...A....1
>
>
> --
> Brian Munyao Longwe
> IXP Coordinator | Cell: +265999708883
> [image: https://lionex.org] <https://lionex.org>
>
>
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