China-Direction Line Routing, Explained: CN2 GIA, AS4837, CMI and BGP
The real differences between the four China-direction line types, what the premium buys, and a four-step method to verify any provider claim with your own mea
# What this hub answers
Plain-language explanations of the four China-direction line types, what they really cost, and how to verify them — no mystique, no marketing adjectives. Buy the line before you buy the box: two servers in the same datacenter can feel an order of magnitude apart, and the price difference between "same datacenter, different price" is exactly the egress line mix.
## Quick decision block
- Must stay stable at evening peak and budget allows it → **CN2 GIA**
- Budget first, audience mostly on China Unicom → **AS4837** (Unicom standard/optimized)
- Audience mostly on China Mobile → **CMI** (China Mobile International)
- Overseas audience, or balanced multi-carrier inside China → **BGP multi-line**
## The four lines in one table
| Line | AS | Positioning | Evening peak | Best for |
|---|---|---|---|---|
| CN2 GIA | AS4809 | China Telecom premium international exit, dedicated high-quality backbone both directions | Steadiest, smallest degradation | Trade sites, gaming, remote work — anything latency-sensitive |
| CN2 GT / Telecom 163 | AS4134 | Standard Telecom exit | Obvious congestion 20:00–23:00, loss rises | Peak-insensitive workloads, budget first |
| AS4837 | AS4837 | Unicom standard backbone | Peak pressure lower than Telecom 163 | Unicom-heavy audience, value first |
| CMI | AS58453 / AS9808 | China Mobile International | Low latency for Mobile users; may detour for Telecom/Unicom | Mobile-heavy audience |
| BGP multi-line | multiple | Datacenter peers with several carriers, dynamic routing | Depends on the upstream mix | Standard shape for CN datacenters, balanced multi-carrier |
## Return path is the path that matters
Ping measures the round trip, but the bulk of a page load travels server → you, i.e. the return path. Many cheap lines send your packets over a good outbound and bring the payload home over congested Telecom 163 — the classic "ping looks fine, pages open slowly". A traceroute from your laptop only shows the outbound direction; to see the return you need a reverse MTR from the provider's test IP, or a third-party route tracer (NextTrace / besttrace).
Ask one question before buying: **what is the return path for each of the three carriers?** "CN2-optimized" without that answer means plan for the worst case.
## Verify a provider's line claim in four steps
1. **Get a test IP.** Serious providers publish Looking Glass / test IPs; run them through our speed test nodes first.
2. **Read the AS numbers.** AS4809 or 59.43.x in the path = CN2; AS4837 = Unicom; AS58453/AS9808 = Mobile family. AS numbers do not lie.
3. **Test across time slots.** At least one round at evening peak (20:00–23:00) and one off-peak; the degradation delta is the line's true grade.
4. **Test all three carriers.** One Telecom machine proves nothing about Unicom or Mobile behaviour.
## Why evening congestion happens
International exit capacity is scarce and expands on yearly cycles, while demand peaks every day at the same hours — a scheduled supply/demand imbalance. Standard exits (163) fill first: loss, retransmits, staircase latency. Premium exits (CN2 GIA) carry lower user density and scheduling priority, so they degrade far less. Corollary: every line is worse at peak than at 3 a.m.; the only honest question is *how much worse*, and only time-slot measurements answer it. Be suspicious of "9.9 yuan CN2": it is either CN2 GT sold as GIA, or oversubscription that shows its face at peak.
## CN2 GIA is not an insurance policy
GIA guarantees a better path and higher scheduling priority — not immunity. Submarine cable faults and upstream incidents still cause waves; recovery is simply faster and peak degradation usually much smaller. Magnitudes vary by region and hour, so always self-test with the provider's IP at evening peak before ordering. Your own numbers beat anyone's marketing copy.
## Three contract clauses to read
- **Availability promise:** "network reachable" or "end-to-end experience"? Almost everyone offers the first; the second is where the value is.
- **Line wording:** does the contract state the return path per carrier? If it only says "CN2 optimized", assume the worst case.
- **Compensation mechanics:** what evidence is accepted (your own MTR?), how do you claim, cash or credit? A claim process complex enough is a claim process that does not exist.
## Workload × line cheat sheet
| Workload | Recommended combo | Why |
|---|---|---|
| Trade site visited from CN | HK/SG + CN2 GIA or CMI | Enquiry hours are daytime; support visits hit evening peak |
| Game server | JP/HK + tri-carrier optimized | Jitter hurts more than latency; stability first |
| API called from CN | HK + CN2 GIA | Small requests, high frequency — RTT-dominated |
| Download / distribution | SG/US-West + big bandwidth + CDN | Throughput-dominated |