[ixpmanager] Fwd: Malformed AS_PATH BGP Error Message from CDN Peer
Salvador Bertenbreiter
salvador at peruix.net
Fri Jul 24 18:09:23 IST 2026
Hi Brian,
Based on our experience with previous cases involving malformed AS_PATH
messages on the customer side, the issue has typically been caused by the
absence of the no bgp enforce-first-as command, or the equivalent
configuration depending on the router vendor, as Nick mentioned.
Best regards,
Salvador
El El vie, 24 jul. 2026 a la(s) 10:49, Brian Longwe via ixpmanager <
ixpmanager at inex.ie> escribió:
> Hi,
>
> I've been in touch with the CDN and they seem adamant about not changing
> anything on their side, still getting "Malformed AS_PATH" - is there
> nothing we can do about this?
> [image: image.png]
>
> --
> Brian Munyao Longwe
> IXP Coordinator | Cell: +265999708883
> [image: https://lionex.org] <https://lionex.org>
>
>
> On Wed, Jul 22, 2026 at 5:34 PM Brian Longwe <brian at lionex.org> wrote:
>
>> 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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