05 Apr

Tata – Airtel domestic peering IRR filtering and OpenDNS latency!

Last month I noticed quite high latency with Cisco’s OpenDNS from my home fibre connection. The provider at home is IAXN (AS134316) which is peering with content folks in Delhi besides transit from Airtel.

ping -c 5
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=51 time=103 ms
64 bytes from icmp_seq=2 ttl=51 time=103 ms
64 bytes from icmp_seq=3 ttl=51 time=103 ms
64 bytes from icmp_seq=4 ttl=51 time=103 ms
64 bytes from icmp_seq=5 ttl=51 time=103 ms
--- ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4005ms
rtt min/avg/max/mdev = 103.377/103.593/103.992/0.418 ms

This is bit on the higher side as from Haryana to Mumbai (OpenDNS locations list here). My ISP is backhauling from Faridabad which is probably 6-8ms away from my city and 2-3ms further to Delhi and from there to Mumbai around 30ms. Thus latency should be around ~40-45ms.

Here’s how forward trace looked like

traceroute to (, 30 hops max, 60 byte packets
 1 (  0.730 ms  0.692 ms  0.809 ms
 2  axntech-dynamic- (  4.904 ms  4.314 ms  4.731 ms
 3 (  6.000 ms  6.414 ms  6.326 ms
 4 (  6.836 ms  7.135 ms  7.047 ms
 5  nsg-static- (  9.344 ms  9.416 ms  9.330 ms
 6 (  62.274 ms (  66.874 ms (  61.297 ms
 7 (  85.789 ms  82.250 ms  79.591 ms
 8 (  110.049 ms (  114.350 ms  113.673 ms
 9 (  112.598 ms (  114.889 ms (  113.415 ms
10 (  125.770 ms  125.056 ms  123.779 ms
11  resolver1.opendns.com (  113.648 ms  115.044 ms  106.066 ms

Forward trace looks fine except that latency jumps as soon as we hit Tata AS4755 backbone. OpenDNS connects with Tata AS4755 inside India and announces their anycast prefixes to them. If the forward trace is logically correct but has high latency, it often reflects the case of bad return path. Thus I requested friends at OpenDNS to share the return path towards me. As expected, it was via Tata AS6453 Singapore.

Here’s what Tata AS4755 Mumbai router had for IAXN prefix:

BGP routing table entry for
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
6453 9498 134316 134316 134316 134316 134316 134316 134316 134316 134316 134316 from (
Origin IGP, localpref 62, valid, internal, best
Community: 4755:44 4755:97 4755:888 4755:2000 4755:3000 4755:47552 6453:50 6453:3000 6453:3400 6453:3402
Originator:, Cluster list:
Last update: Mon Mar 25 15:26:36 2019

Thus what was happening is this:

Forward path: IAXN (AS134316) > Airtel (AS9498) > Tata (AS4755) > OpenDNS (AS36692)

Return path: OpenDNS (AS36692) > Tata (AS4755) > Tata (AS6453) > Airtel (AS9498) > IAXN (AS134316)

While this may seem like a Tata – Airtel routing issue but it wasn’t. I could see some of the prefixes with a direct path as well. Here’s a trace from Tata AS4755 Mumbai PoP to an IP from a different pool of IAXN:

traceroute to (, 15 hops max, 60 byte packets
1 * * *
2 ( 0.911 ms 0.968 ms 0.643 ms
3 ( 1.233 ms 0.821 ms 0.810 ms
4 ( 23.540 ms 23.454 ms 23.367 ms
5 ( 49.175 ms 48.832 ms 49.107 ms
6 ( 48.777 ms ( 49.043 ms ( 54.879 ms
7 ( 60.865 ms 60.540 ms 60.644 ms

This clearly was fine. So why Tata was treating different from The reason for that lies in following:

  • Airtel (AS9498) very likely peers with Tata (AS4755). They do interconnect for sure as we see in traceroutes and my understanding is that it’s based on settlement-free peering for Indian traffic.
  • Airtel (AS9498) buys IP transit from Tata (AS6453) (besides a few others). Tata AS6453 is carrying the routing announcements to other networks in the transit free zone and that confirms that Airtel (at least technically) has a downstream customer relationship here.
  • Tata (AS4755) has IRR based filters on peering but not the Tata (AS6453) for it’s downstream. Hence while Tata rejected the route in India, they did accept that in Singapore PoP.
  • My IP was from prefix and there was no valid route object for it at any of key IRRs like ATLDB, APNIC or RADB. But other prefix did had a valid route object on APNIC.

Now after almost 10 days of it, my ISP has changed the BGP announcement and announcing (which does a valid route object on APNIC). This fixes the routing problem and give me pretty decent latency with OpenDNS:

ping -c 5
PING ( 56 data bytes
64 bytes from icmp_seq=0 ttl=55 time=52.552 ms
64 bytes from icmp_seq=1 ttl=55 time=53.835 ms
64 bytes from icmp_seq=2 ttl=55 time=53.330 ms
64 bytes from icmp_seq=3 ttl=55 time=52.700 ms
64 bytes from icmp_seq=4 ttl=55 time=52.504 ms
--- ping statistics ---
5 packets transmitted, 5 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 52.504/52.984/53.835/0.518 ms

So if you are a network operator and originating prefixes, please do document them in any of the IRRs. You can do that via IRR of your RIR (APNIC, ARIN etc) or a free IRR like ALTDB. If you have downstreams, make sure to create AS SET, add downstreams ASNs in your AS SET and also include that AS SET on peeringdb for the world to see!

Misc Notes

  • Posted strictly in my personal capacity and has nothing to do with my employrer.
  • Thanks for folks from Cisco/OpenDNS for quick replies with relevant data which helped in troubleshooting. 🙂