# BGP and Internet Routing

BGP is how the internet holds itself together: about 80,000 autonomous systems trading reachability information, mostly on trust. It is also the protocol behind the outages you read about in the news.

## The model

An **Autonomous System (AS)** is a network under one administrative policy, identified by an ASN. BGP exchanges **prefixes** with attributes, and each AS applies policy about what it will accept and what it will advertise.

| Property | Value |
|---|---|
| Transport | TCP 179 |
| Type | Path-vector, exterior gateway protocol |
| Peering | eBGP (different AS), iBGP (same AS) |
| Convergence | Slow by design — policy, not speed |
| Typical internet table | Just under a million IPv4 prefixes |

A path-vector carries the **AS path** so loops can be detected: if a router sees its own ASN in the path, it rejects the route.

## What BGP actually decides

BGP does not choose the shortest path by distance. It applies a policy ladder, roughly:

1. Highest **weight** (Cisco-local, not advertised)
2. Highest **local preference** (how much this AS likes this exit)
3. **Locally originated** (network/redistribute) beats learned
4. Shortest **AS path**
5. Lowest **origin** type (IGP < EGP < incomplete)
6. Lowest **MED** (a hint to the neighbouring AS)
7. Prefer **eBGP** over iBGP
8. Lowest **IGP metric** to the next hop
9. Lowest **router ID**

Most real traffic engineering happens at steps 2 and 6.

## eBGP vs iBGP, and why iBGP needs a full mesh or reflectors

- **eBGP**: neighbours in different ASes, TTL 1 by default, next hop rewritten.
- **iBGP**: neighbours in the same AS, no next-hop rewrite, and **routes learned from one iBGP peer are not re-advertised to another** — hence full mesh, route reflectors, or confederations.

That split-horizon rule is the single most confusing thing about iBGP, and the reason a new router "has no routes" while its session is up.

## The failure modes that make the news

| Failure | What happens | Recent-scale example class |
|---|---|---|
| **Prefix hijack** | An AS advertises someone else's prefix; the internet believes the more specific route | National outages, crypto theft |
| **Route leak** | A customer re-advertises a provider's routes, pulling traffic through a small network | Global slowdowns from a single misconfiguration |
| **BGP session drop** | Peer resets, routes withdrawn, traffic falls to a backup or a blackhole | Regional outages |
| **Max-prefix exceeded** | Peer sends more prefixes than configured; session is torn down by design | "It worked until it scaled" |
| **RPKI not deployed** | Nobody cryptographically verifies who may originate a prefix | Hijacks propagate |

**RPKI** (Resource Public Key Infrastructure) signs route origin authorisations, and **ROV** (route origin validation) lets a network drop invalid announcements. Deployment is partial, which is why hijacks still work.

## What a practitioner needs to be able to do

```bash
show ip bgp summary                 # session state, prefixes received
show ip bgp 203.0.113.0/24          # attributes and best path
show ip bgp neighbors 198.51.100.1 advertised-routes
show ip route bgp
```

```bash
# Looking glasses and public data
whois -h whois.radb.net 203.0.113.10
curl -s https://stat.ripe.net/data/prefix-overview/data.json?resource=203.0.113.0/24
```

- Explain why traffic leaves via one ISP and returns via another.
- Read an AS path and tell where a route was learned.
- Know that "the internet is down" usually means "our announcement changed".

## Where it goes wrong

- **Advertising a prefix you do not own.** A typo in a route-map takes down someone else's service, and sometimes your own.
- **No max-prefix limit.** A misbehaving peer sends a full table and exhausts memory on the router.
- **Filtering only one direction.** You filter what you send, not what you accept, and inherit a hijack.
- **Assuming BGP converges fast.** It is designed for policy stability; failover can take tens of seconds, which is why fast-external-failover and BFD exist.
- **Asymmetric return paths.** Outbound via ISP A, inbound via ISP B. Works, until the stateful firewall in the middle sees half a conversation.
- **Changing local preference to "fix" performance** without measuring. Traffic moves; latency does not improve; the change stays forever.
- **Forgetting IPv6.** A dual-stack network with no IPv6 BGP policy has a very large implicit "permit any" on v6.

## Teaching hook

**The postal treaty analogy.** Each country's postal service tells its neighbours "we can deliver to these postcodes". Nobody verifies the claim; if a small country claims a postcode it does not serve, mail goes there and disappears. RPKI is the notarised version of the claim, and most of the world has not adopted it yet.

Demo: `traceroute` to a major service from two vantage points (or use a public looking glass) and have learners read the AS path. Then show a route leak write-up and map it onto the same path view.

## Related Notes

- [[Routing/Routing-Fundamentals.md]] — where BGP sits among protocols
- [[Routing/OSPF-Fundamentals.md]] — the IGP that resolves BGP next hops
- [[Routing/Route-Selection-And-Metrics.md]] — combining policy ladders
- [[Security/DDoS-And-Availability-Attacks.md]] — BGP's role in mitigation