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Routing issue to Softlayer's DNS servers

psross
Dabbler
Posts: 24
Registered: 30-07-2007

Routing issue to Softlayer's DNS servers

I'm seeing a routing issue from Plusnet to the authoritative DNS servers SoftLayer (a hosting company) run for their customers to use (ns1.softlayer.com and ns2.softlayer.com).
The servers use anycast, so packets should be routed to the closest location to the source. This should be the servers in SoftLayer's Amsterdam data centre when accessed from the UK. However, packets from Plusnet are being routed to SoftLayer's Singapore data centre instead:
# traceroute -IAq 1 ns1.softlayer.com
traceroute to ns1.softlayer.com (67.228.254.4), 30 hops max, 60 byte packets
1  192.168.1.1 (192.168.1.1) [AS28513]  0.318 ms
2  lo0-central10.ptn-ag03.plus.net (195.166.128.192) [AS6871]  33.046 ms
3  link3-central10.ptn-gw01.plus.net (84.93.248.196) [AS6871]  28.375 ms
4  xe-8-2-0.ptw-cr01.plus.net (212.159.1.32) [AS6871]  28.420 ms
5  te7-5.br03.ldn01.pccwbtn.net (195.66.224.167) [AS10026/AS4637]  29.992 ms
6  softlayer.te7-5.br02.sin02.pccwbtn.net (63.218.213.174) [AS19710/AS3491]  352.589 ms
7  ae6.dar01.sr03.sng01.networklayer.com (50.97.18.201) [AS36351]  353.139 ms
8  po1.slr01.sr03.sng01.networklayer.com (174.133.118.135) [AS36351]  352.476 m
9  ns1.softlayer.com (67.228.254.4) [AS36351/AS22773]  371.409 ms

This is causing quite a delay (about 350 ms) when performing DNS queries for domains hosted on these servers. Please could someone from Plusnet take a look into this? Or could someone let me know the best way of getting in touch with the Plusnet networks team?
For reference, here's another traceroute from Bytemark in the UK where the route does go to Amsterdam:
# traceroute -IAq 1 ns1.softlayer.com
traceroute to ns1.softlayer.com (67.228.254.4), 30 hops max, 60 byte packets
1  89-16-xxx-xxx.no-reverse-dns-set.bytemark.co.uk (89.16.xxx.xxx) [AS35425]  0.397 ms
2  89-16-188-39.no-reverse-dns-set.bytemark.co.uk (89.16.188.39) [AS35425]  0.623 ms
3  te3-4.cr02.man.bytemark.co.uk (91.223.58.64) [AS35425]  0.745 ms
4  te4-1.cr01.thn.bytemark.co.uk (91.223.58.34) [AS35425]  68.455 ms
5  bbr02.lon01.networklayer.com (195.66.237.183) [AS5459]  7.864 ms
6  ae7.bbr02.tg01.lon01.networklayer.com (50.97.18.207) [AS36351]  7.868 ms
7  ae0.bbr02.eq01.ams02.networklayer.com (50.97.18.211) [AS36351]  12.374 ms
8  ae6.dar01.sr01.ams01.networklayer.com (50.97.18.249) [AS36351]  12.518 ms
9  po1.slr01.sr01.ams01.networklayer.com (159.253.158.135) [AS36351]  12.629 ms
10  ns1.softlayer.com (67.228.254.4) [AS36351/AS22773]  12.496 ms

Thanks,
Phil
4 REPLIES
Plusnet Alumni (retired) orbrey
Plusnet Alumni (retired)
Posts: 10,540
Registered: 18-07-2007

Re: Routing issue to Softlayer's DNS servers

Hi there,
Done a few checks on this and it seems the slowdown is at the sng01 server at the networklayer section - I tried a trace from a virgin connection which ended up around 100ms and went via slr01 and another from uk2 which went via the same server (slr01) - perhaps sng01 is just under a lot of load?
Is this having an effect on your connectivity etc or is it more the increased time delay?
psross
Dabbler
Posts: 24
Registered: 30-07-2007

Re: Routing issue to Softlayer's DNS servers

Hi Matt,
The sng01 routers are in SoftLayer's Singapore data center. The issue is that the route shouldn't be going to Singapore in the first place. The route should be going to the closest data centre to the source, which in this case is SoftLayer's Amsterdam data center (ams01).
SoftLayer use anycast to publish/announce the same IP addresses at various different locations in the world. Other routers are then supposed to be able to choose the best route to one of the locations. It looks like Plusnet's routers are choosing the route to Singapore instead of the preferable Amsterdam route.
I've run a trace on Virgin Media too. I found that this was going via po2.slr01.sr01.wdc01.networklayer.com as the penultimate hop. This is a router in SoftLayer's Washington DC data center (wdc01). Routing to Washington DC is also not the most optimal route, but is better than routing to Singapore.
This routing issue causes a delay when DNS records are queried to translate a host name into an IP address. Querying a DNS record from the server in Singapore will take about 350ms longer than looking up the same record from the server in Amsterdam. This is therefore slowing down browsing to sites that use SoftLayer for their DNS.
Community Veteran
Posts: 5,153
Thanks: 474
Fixes: 19
Registered: 10-06-2010

Re: Routing issue to Softlayer's DNS servers

You could try "gateway hopping" (disconnecting and reconnecting from your router's web interface). Sometimes there is slightly different routing on different gateways. I'm on ptn-ag02 and get the same traceroute to Singapore.
psross
Dabbler
Posts: 24
Registered: 30-07-2007

Re: Routing issue to Softlayer's DNS servers

Thanks for the suggestion.
I have tried running the traceroute on ptn-ag02, ptn-ag03, pcl-ag03 and ptw-ag03 as well as the Plusnet Looking Glass. All route to the Singapore data centre.