Anycast routing issue: KDDI (AS2516) and IIJ (AS2497) routing to US West (SJC/LAX) instead of NRT

Questions / Help
1

Hello,

I have an application running primarily in the nrt region.

When accessing it from Nagano, Japan, requests do not appear to reach the Tokyo edge. Instead, traffic is entering Fly.io through US West Coast regions.

I understand that Fly.io uses Anycast routing, but crossing the Pacific Ocean introduces significant additional latency for users located in Japan.

To investigate, I checked https://debug.fly.dev from two different networks:

  • KDDI fixed-line connection (AS2516) → Fly-Region: sjc

  • IIJ mobile connection over NTT Docomo access network (AS2497) → Fly-Region: lax

Results are included below.

debug.fly.dev

KDDI fixed-line (AS2516)

=== Headers ===
Host: debug.fly.dev
Fly-Region: sjc
Sec-Ch-Ua-Mobile: ?0
Accept-Language: ja,en-US;q=0.9,en;q=0.8,ja-JP;q=0.7
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
X-Forwarded-Proto: https
X-Forwarded-Ssl: on
Via: 2 fly.io
Sec-Ch-Ua: "Google Chrome";v="149", "Chromium";v="149", "Not)A;Brand";v="24"
Sec-Fetch-Site: none
Fly-Client-Ip: 121.109.xxx.xxx
Fly-Forwarded-Proto: https
Fly-Forwarded-Ssl: on
Fly-Forwarded-Port: 443
X-Forwarded-Port: 443
X-Request-Start: t=1781744871330046
Accept-Encoding: gzip, deflate, br, zstd
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,\*/\*;q=0.8,application/signed-exchange;v=b3;q=0.7
Sec-Fetch-Mode: navigate
Priority: u=0, i
X-Forwarded-For: 121.109.xxx.xxx, 37.16.xxx.xxx
Fly-Request-Id: 01KVC4A9X1GMJQX5V1RZ6ZX855-sjc
Sec-Ch-Ua-Platform: "macOS"
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/149.0.0.0 Safari/537.36

IIJ mobile / NTT Docomo access network (AS2497)

=== Headers ===
Host: debug.fly.dev
Fly-Forwarded-Proto: https
Accept-Encoding: gzip, deflate, br, zstd
X-Request-Start: t=1781745018095727
X-Forwarded-Proto: https
Fly-Forwarded-Ssl: on
X-Forwarded-Port: 443
Fly-Region: lax
User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 26_5_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) CriOS/149.0.7827.137 Mobile/15E148 Safari/604.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: ja
Priority: u=0, i
Fly-Client-Ip: 2001:240:xxxx:xxxx::xxxx
X-Forwarded-Ssl: on
Fly-Request-Id: 01KVC4ES7FR0DXGWQG9B626GG2-lax
Sec-Fetch-Dest: document
Sec-Fetch-Site: none
Sec-Fetch-Mode: navigate
X-Forwarded-For: 2001:240:xxxx:xxxx::xxxx, 2a09:8280:xxxx:xxxx::xxxx
Fly-Forwarded-Port: 443
Via: 2 fly.io

For comparison, I also tested from OCI Tokyo and Sakura Internet, and those appear to reach nrt as expected.

This suggests that the observed behavior may depend on the originating network.

Traceroute to my-container.xxxx.example.dev (66.241.124.119)

traceroute to my-container.xxxx.example.dev (66.241.124.119), 64 hops max, 40 byte packets
 1  192.168.0.1 (192.168.0.1)  5.723 ms  6.160 ms  4.417 ms
 2  111.87.247.201 (111.87.247.201)  15.193 ms  15.299 ms  16.095 ms
 3  sjkacs001.bb.kddi.ne.jp (27.85.215.133)  19.016 ms
    210.234.253.105 (210.234.253.105)  19.014 ms
    sjkacs001.bb.kddi.ne.jp (27.85.213.117)  17.297 ms
 4  27.80.241.109 (27.80.241.109)  17.518 ms
 5  106.187.13.42 (106.187.13.42)  131.684 ms
    sjegcs001.int-gw.kddi.ne.jp.13.187.106.in-addr.arpa (106.187.13.14)  124.489 ms
 6  ix-pa10.int-gw.kddi.ne.jp (111.87.3.122)  127.518 ms
 7  124.215.192.6 (124.215.192.6)  126.000 ms
 8  be-2201-cs02.9greatoaks.ca.ibone.comcast.net (96.110.36.221)  126.968 ms
 9  be-2313-pe13.9greatoaks.ca.ibone.comcast.net (96.110.40.218)  128.468 ms
10  96-87-8-222-static.hfc.comcastbusiness.net (96.87.8.222)  139.582 ms
11  * * *

The route appears to leave Japan and reach the US West Coast before arriving at Fly.io.

Could you please check whether routing from these networks toward the Tokyo (nrt) edge can be improved, or whether additional advertisements/peering toward Japanese networks would help?

Thank you.

Additionally, it appears that AS2519 (ARTERIA Networks) routes correctly to NRT,
which suggests this may be specific to certain upstreams (e.g., KDDI / IIJ).

Additional information. I collected MTR results from a few different networks. Apologies if this is more detail than necessary.

MTR

KDDI (AS2516) → SJC

% sudo mtr --report -bzo "LSD NABVW" hello-container.fly.dev
Password:
Start: 2026-06-18T14:23:19+0900
HOST: jinoMacBook-Air.local       Loss%   Snt Drop   Last   Avg  Best StDev  Wrst
  1. AS???         192.168.0.1     0.0%    10    0    6.3   7.3   5.8   2.6  14.7
  2. AS2516        111.87.247.201  0.0%    10    0   17.8  19.2  17.8   1.5  21.9
  3. AS2516        sjkacs001.bb.k  0.0%    10    0   21.3  20.8  19.2   1.0  22.8
  4. AS2516        27.80.241.109   0.0%    10    0   21.4  25.6  17.3  19.3  80.5
  5. AS2516        106.187.13.6    0.0%    10    0  131.4 131.5 130.7   0.7 133.2
  6. AS2516        ix-pa10.int-gw  0.0%    10    0  132.9 129.9 128.3   1.5 132.9
  7. AS2516        124.215.192.6   0.0%    10    0  136.5 130.8 127.9   2.4 136.5
  8. AS7922        be-2201-cs02.9  0.0%    10    0  130.6 129.5 127.3   1.0 130.9
  9. AS7922        be-2213-pe13.9 10.0%    10    1  130.9 131.3 129.7   1.0 133.2
 10. AS7922        96-87-8-222-st  0.0%    10    0  132.8 149.4 130.8  28.3 201.7
 11. AS40509       ip-66-241-124-  0.0%    10    0  132.5 130.5 128.9   1.0 132.5

IIJ Mobile / NTT Docomo access network (AS2497) → LAX

% sudo mtr --report -bzo "LSD NABVW" hello-container.fly.dev
Start: 2026-06-18T14:26:20+0900
HOST: jinoMacBook-Air.local       Loss%   Snt Drop   Last   Avg  Best StDev  Wrst
  1. AS???         172.20.10.1     0.0%    10    0    5.6   5.9   5.0   0.6   7.0
  2. AS???         ???            100.0    10   10    0.0   0.0   0.0   0.0   0.0
  3. AS2497        tky008kgigw400  0.0%    10    0   54.4  57.4  41.3  10.3  74.7
  4. AS2497        tky008bb50.iij  0.0%    10    0   67.3  63.1  42.4  12.2  82.9
  5. AS2497        tky009bb10.iij 90.0%    10    9   59.1  59.1  59.1   0.0  59.1
  6. AS2497        tky009ix53.iij 90.0%    10    9   57.9  57.9  57.9   0.0  57.9
  7. AS3257        ae6.cr4-tyo1.i  0.0%    10    0   49.2  60.4  40.5  17.7 101.6
  8. AS3257        ae3.cr4-lax1.i  0.0%    10    0  170.6 177.8 154.5  32.5 267.4
  9. AS3257        ip4.gtt.net (2  0.0%    10    0  168.4 181.6 143.0  38.4 272.5
 10. AS40509       ip-66-241-124-  0.0%    10    0  175.2 178.2 155.6  17.8 209.5

ARTERIA Networks (AS2519) → NRT (expected behavior)

 % sudo mtr --report -bzo "LSD NABVW" hello-container.fly.dev
Start: 2026-06-18T14:27:20+0900
HOST: jinoMacBook-Air.local       Loss%   Snt Drop   Last   Avg  Best StDev  Wrst
  1. AS???         192.168.11.1    0.0%    10    0   49.1  81.0  46.5  51.8 224.3
  2. AS2519        ngn1hzbiz1.vec  0.0%    10    0   94.1  90.9  33.7  30.7 148.8
  3. AS2519        163.139.152.20  0.0%    10    0  118.6 109.5  43.9  62.0 273.7
  4. AS2519        163.139.152.14 10.0%    10    1   73.1 122.0  56.2  67.5 269.6
  5. AS2519        ae3.core1.niho  0.0%    10    0   77.5 106.1  70.0  54.5 237.4
  6. AS2519        163.139.37.250  0.0%    10    0   71.9 137.5  71.9  72.7 289.8
  7. AS174         hu0-2-0-25.ccr  0.0%    10    0   62.2 117.0  61.3  76.2 309.5
  8. AS174         be2999.agr61.t  0.0%    10    0   81.3  99.4  58.6  23.1 131.0
  9. AS174         154.18.28.90    0.0%    10    0   82.1 107.8  69.1  31.4 169.6
 10. AS40509       ip-66-241-124-  0.0%    10    0  114.6 118.9  78.3  36.2 188.5
2

hi, thanks for the detailed report. we’re looking into ways to improve routing for these and other APAC networks.

by the way: please use --report-wide in mtr in the future, so the full IP/name is shown.

3

@lillian

Thanks for checking and for looking into this.

I’ve rerun the test with --report-wide and posted the updated output.

KDDI (AS2516) → SJC

% sudo mtr --report -bzo "LSD NABVW" --report-wide hello-container.fly.dev
Start: 2026-06-18T16:38:10+0900
HOST: jinoMacBook-Air.local                                                      Loss%   Snt Drop   Last   Avg  Best StDev  Wrst
  1. AS???         192.168.0.1                                                    0.0%    10    0    5.7   6.9   5.4   1.0   8.2
  2. AS2516        111.87.247.201                                                 0.0%    10    0   17.9  18.4  17.0   1.2  21.4
  3. AS2516        sjkacs001.bb.kddi.ne.jp (27.85.215.133)                        0.0%    10    0   19.4  20.5  18.6   1.2  22.7
  4. AS2516        27.80.241.109                                                  0.0%    10    0   19.2  19.6  18.6   0.7  20.6
  5. AS2516        106.187.13.6                                                   0.0%    10    0  132.1 134.5 129.6   7.6 155.8
  6. AS2516        ix-pa10.int-gw.kddi.ne.jp (111.87.3.122)                       0.0%    10    0  128.5 128.5 126.6   0.9 129.7
  7. AS2516        124.215.192.6                                                  0.0%    10    0  129.0 130.0 128.7   0.9 130.9
  8. AS7922        be-2201-cs02.9greatoaks.ca.ibone.comcast.net (96.110.36.221)   0.0%    10    0  127.8 128.4 127.1   1.0 130.6
  9. AS7922        be-2213-pe13.9greatoaks.ca.ibone.comcast.net (96.110.40.214)   0.0%    10    0  131.1 131.3 128.2   1.9 134.3
 10. AS7922        96-87-8-222-static.hfc.comcastbusiness.net (96.87.8.222)       0.0%    10    0  133.2 146.7 129.2  25.4 195.0
 11. AS40509       ip-66-241-124-119.shared.customer.flyio.net (66.241.124.119)   0.0%    10    0  132.2 130.5 128.4   1.5 132.9

IIJ Mobile / NTT Docomo access network (AS2497) → LAX

% sudo mtr --report -bzo "LSD NABVW" --report-wide hello-container.fly.dev
Start: 2026-06-18T16:38:58+0900
HOST: jinoMacBook-Air.local                                                      Loss%   Snt Drop   Last   Avg  Best StDev  Wrst
  1. AS???         172.20.10.1                                                    0.0%    10    0    5.9  16.2   5.7  23.1  80.2
  2. AS???         ???                                                           100.0    10   10    0.0   0.0   0.0   0.0   0.0
  3. AS2497        tky008kgigw400.iij.net (133.142.191.49)                        0.0%    10    0  627.2 176.7  39.0 191.1 627.2
  4. AS2497        tky008bb50.iij.net (133.142.191.33)                            0.0%    10    0  522.3 160.9  41.0 147.1 522.3
  5. AS2497        tky009bb10.iij.net (58.138.80.194)                            90.0%    10    9   56.9  56.9  56.9   0.0  56.9
  6. AS2497        tky009ix53.iij.net (58.138.112.98)                            90.0%    10    9  224.7 224.7 224.7   0.0 224.7
  7. AS3257        ae6.cr4-tyo1.ip4.gtt.net (67.220.149.221)                      0.0%    10    0  208.3 127.6  35.7 142.4 500.7
  8. AS3257        ae3.cr4-lax1.ip4.gtt.net (89.149.128.174)                      0.0%    10    0  221.4 230.7 153.2 117.9 547.8
  9. AS3257        ip4.gtt.net (209.120.140.218)                                  0.0%    10    0  150.0 214.2 150.0 100.2 435.5
 10. AS40509       ip-66-241-124-119.shared.customer.flyio.net (66.241.124.119)   0.0%    10    0  187.0 280.5 151.1 205.5 827.4

ARTERIA Networks (AS2519) → NRT (expected behavior)

% sudo mtr --report -bzo "LSD NABVW" --report-wide hello-container.fly.dev
Start: 2026-06-18T16:39:40+0900
HOST: jinoMacBook-Air.local                                                      Loss%   Snt Drop   Last   Avg  Best StDev  Wrst
  1. AS???         192.168.11.1                                                   0.0%    10    0   43.3  65.3  43.3  11.1  87.9
  2. AS2519        ngn1hzbiz1.vectant.ne.jp (163.139.100.73)                     10.0%    10    1   71.9 112.3  71.9  72.5 302.8
  3. AS2519        163.139.152.205                                               10.0%    10    1  104.8 108.8  79.5  35.8 197.9
  4. AS2519        163.139.152.145                                               10.0%    10    1  109.4 122.9  76.3  55.7 267.1
  5. AS2519        ae3.core1.nihonbashi.vectant.ne.jp (163.139.131.37)            0.0%    10    0   87.8 109.3  80.8  25.5 165.7
  6. AS2519        163.139.37.250                                                30.0%    10    3   67.1 114.6  59.3  59.2 238.0
  7. AS174         hu0-2-0-25.ccr81.tyo01.atlas.cogentco.com (154.18.0.1)         0.0%    10    0   76.2 119.9  57.4  71.9 315.7
  8. AS174         be2999.agr61.tyo01.atlas.cogentco.com (154.54.89.182)          0.0%    10    0   92.8 112.9  74.7  36.7 209.0
  9. AS174         154.18.28.90                                                  20.0%    10    2   98.3 109.5  80.3  27.7 169.4
 10. AS40509       ip-66-241-124-119.shared.customer.flyio.net (66.241.124.119)  10.0%    10    1   94.9  97.5  82.0  12.6 122.2
4

I understand that this routing issue is a complex challenge that cannot be resolved overnight. While Fly.io’s current routing behavior didn’t fit my use case, I sincerely hope the network environment for users in Japan and across the APAC region will continue to improve in the future.

5

sorry to hear the current routing isn’t suitable for your use case. I can provide an update: early this week we ordered a better edge solution for our Tokyo and Singapore PoPs, and are waiting for it to be set up by the provider.