BGP — path vector, AS, policy routing — Hinglish notes

4.3.17 · Coding › Computer Networks

BGP KYA hai?

OSPF/RIP se alag protocol KYU? Intra-domain protocols (OSPF, RIP) ek single metric (hops, link cost) optimize karte hain aur maante hain ki sab cooperate karte hain. Internet par yeh toot jaata hai: ek transit provider aapke traffic ko free mein carry karne se mana kar sakta hai chahe woh shortest path ho. BGP ko policy ("main yeh route nahin karunga") encode karni hoti hai, ~1M prefixes tak scale karna hota hai, aur untrusting parties ke beech loops prevent karne hote hain. Yeh fundamentally alag kaam hai.

Path Vector: KYU aur KAISE (derive karo)

Hum ASes ke across ek loop-free routing protocol banana chahte hain. Aaiye ise scratch se build karte hain.

Steel-man the fix: Woh minimum extra info kya hai jo loops ko khatam kar de? Agar har advertisement mein un saare ASes ki poori list hoti jo route se guzarte hain, toh koi bhi router jo list mein apna khud ka AS number dekhe samajh leta: "ise accept karna loop create karega" → reject karo.

Toh ek advertisement kuch aisa dikhta hai:

$prefix 8.8.8.0/24 AS_PATH = [A S_{2}, A S_{5}, A S_{15169}]$

right-to-left padho: prefix AS15169 mein originate hua, AS5 ko pass kiya gaya, phir AS2 ko.

Figure — BGP — path vector, AS, policy routing

Policy Routing: BGP KYU shortest-path nahin hai

Formula Gao–Rexford export rules (the "valley-free" policy)

Ek AS, neighbor type $A$ se seekha hua route neighbor type $B$ ko tab hi advertise (export) karta hai jab allowed ho:

Route kahan se seekha	Customer ko export	Peer ko export	Provider ko export
Customer	✅	✅	✅
Peer	✅	❌	❌
Provider	✅	❌	❌

KYU: Tum apne paying customers ko khushi se har route bataaoge (tum ise carry karke paisa kamaoge). Tum un do parties ke beech traffic carry karne mein paisa/effort nahin lagaoge jo tumhe nahin dete — isliye peer- aur provider-learned routes sirf customers ko diye jaate hain.

Yeh valley-free paths produce karta hai: ek path zero ya zyada baar upar jaata hai (customer→provider), phir optionally ek peer hop, phir neeche (provider→customer). Yeh kabhi "down ke baad up" nahin hota — iska matlab hoga ek AS free transit de raha hai. Isliye shortest AS path aksar choose nahin hota — policy distance ko override kar deti hai.

BGP Decision Process (route actually kaise pick hota hai)

Ek hi prefix ke liye multiple routes mein se, BGP ek pick karta hai, is priority order mein:

Worked Example: poora policy decision

Flashcards

AS_PATH attribute mein kya hota hai?

Un AS numbers ki ordered list jo ek route traverse karta hai; origin se right-to-left padho.

BGP (path vector) loops kaise prevent karta hai?

Ek router koi bhi woh advertisement reject karta hai jiske AS_PATH mein uska apna ASN already hota hai.

eBGP par route re-advertise karte waqt ek AS, AS_PATH ke saath kya karta hai?

Apna ASN front mein prepend karta hai.

BGP, distance vector ki jagah path vector kyun use karta hai?

Poora AS path carry karne se instant loop detection milta hai aur count-to-infinity se bacha jaata hai.

BGP best-path selection mein PEHLA tie-breaker kya hai?

Highest LOCAL_PREF (ek policy knob), shortest path nahin.

AS_PATH length, decision process mein kahan matter karta hai?

Step 2 — sirf LOCAL_PREF ke baad.

Gao–Rexford: PEER se seekha hua route kise export kiya ja sakta hai?

Sirf customers ko (peers ya providers ko nahin).

Gao–Rexford: CUSTOMER se seekha hua route kise export kiya ja sakta hai?

Sabko (customers, peers, providers).

"Valley-free" path kya hota hai?

Up-hops (cust→prov) ka path, phir ≤1 peer hop, phir down-hops (prov→cust); down ke baad kabhi up nahin.

eBGP aur iBGP mein kya fark hai?

eBGP alag ASes ke routers ke beech chalta hai; iBGP externally-learned routes ko ek AS ke andar distribute karta hai.

Hot-potato routing kya hai?

Sabse kam IGP cost wala egress choose karna — traffic ko apne AS se jitna jaldi ho sake bahar dump karo.

LOCAL_PREF ke zariye provider routes par customer routes kyun prefer karte hain?

Tum customer traffic carry karke paisa kamaate ho lekin provider transit ke liye dete ho, isliye customer sasta hota hai.

Recall Feynman: 12-saal ke bachche ko samjhao

Socho bahut saare mohalle hain, har ek alag family ka. Shahar bhar khat bhejna ho toh padosi-se-padosi haath karna padta hai. Har family lifaafe ki route list par apna naam stamp karti hai. Agar koi family list mein pehle se apna naam dekhe, toh samajh jaati hai khat circles mein ja raha hai aur use phenk deti hai — koi loop nahin! Aur families sirf shortest route nahin chunti: kuch padosi paisa lete hain (providers), kuch free dost hain (peers), aur kuch paying customers hain. Ek family hamesha paying customer ko khat dena prefer karti hai (isko paisa milta hai) us route ki jagah jo use paisa cost kare — chahe us customer ka path lamba hi kyun na ho. Yahi BGP hai: route by kaun paisa deta hai, na ki by kaun sabse paas hai.

Connections

OSPF and link-state routing — intra-domain, single metric, full trust (contrast).
Distance Vector and Count-to-Infinity — woh problem jo path vector solve karta hai.
Autonomous Systems and ASN allocation
IP addressing and CIDR prefixes — woh cheez jo BGP actually advertise karta hai.
Internet topology — Tier-1, transit, peering — policy ke peeche ki economics.
Hot-potato vs Cold-potato routing
Route flap damping and BGP convergence