BGP Attributes

⚡ BGP Path Attributes — Quick Revision Notes

📘 Goal: Fast-track learning & interview-ready reference for the 5 most important BGP attributes.

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🧱 1. AS_PATH (Type Code: 2)

Property	Summary
Classification	Well-known Mandatory
Purpose	Lists all AS numbers the route has passed through → used for loop prevention & best path
Structure	Ordered AS_SEQUENCE, unordered AS_SET (used in aggregation)
Best Path Rule	Prefer shortest AS_PATH
Manipulation	set as-path prepend <asn> → makes path longer, less preferred (controls inbound traffic)
Transitivity	✅ Yes (carried in UPDATEs)
Notes	Loop prevention + Inbound TE tool

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🌉 2. NEXT_HOP (Type Code: 3)

Property	Summary
Classification	Well-known Mandatory
Purpose	Identifies the next router IP to reach the destination prefix
Behavior	eBGP → next-hop = neighbor IP iBGP → next-hop unchanged
Manipulation	neighbor <ip> next-hop-self → make local router next-hop
Requirement	Must be reachable in local routing table
Transitivity	✅ Yes
Notes	Ensures correct forwarding — key for iBGP reachability

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🌐 3. ORIGIN (Type Code: 1)

Property	Summary
Classification	Well-known Mandatory
Purpose	Shows how route entered BGP (source of origination)
Codes	i (IGP – network statement) e (EGP – old protocol) ? (Incomplete – redistribution)
Preference Order	IGP (i) > EGP (e) > Incomplete (?)
Manipulation	set origin igp / set origin incomplete
Transitivity	✅ Yes
Notes	Affects internal preference; used in best-path Step #7

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⚖️ 4. WEIGHT (Cisco Proprietary)

Property	Summary
Classification	Optional Non-Transitive (local only)
Purpose	Determines local outbound route preference (first best-path step)
Range	0–65,535 → higher = more preferred
Default	32,768 (local) / 0 (learned)
Manipulation	set weight <value> or neighbor <ip> weight <value>
Transitivity	❌ No
Notes	Local router only → controls local exit traffic

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🏛️ 5. LOCAL_PREFERENCE (LOCAL_PREF) (Type Code: 5)

Property	Summary
Classification	Well-known Discretionary
Purpose	Defines AS-wide outbound preference (second best-path step)
Default	100
Rule	Higher LOCAL_PREF = more preferred exit
Manipulation	set local-preference <value> or neighbor <ip> local-preference <value>
Propagation	✅ iBGP only (not sent to eBGP)
Notes	Controls outbound routing for the entire AS

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🧠 BGP Best Path Order (Top 5 Steps)

Step	Attribute	Rule
1️⃣	Weight	Higher = better
2️⃣	Local Preference	Higher = better
3️⃣	Locally originated	Preferred over learned
4️⃣	AS_PATH	Shorter = better
5️⃣	Origin	IGP > EGP > Incomplete

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🧾 Quick Summary

Attribute	Influence	Scope	Transitive	Typical Use
⚖️ Weight	Outbound	Local Router	❌ No	Local path control
🏛️ Local Pref	Outbound	Entire AS	✅ Internal only	AS exit preference
🧱 AS_PATH	Inbound	Global	✅ Yes	Loop prevention, inbound TE
🌐 Origin	Internal	Global	✅ Yes	Route source & preference
🌉 Next Hop	Forwarding	Global	✅ Yes	Ensures reachability

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⚙️ Tip:

• Weight & Local Pref → Outbound control
• AS_PATH → Inbound control
• NEXT_HOP → Reachability
• ORIGIN → Preference hint

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🧩 Perfect For:
CCNA/CCNP/ENARSI Learners | Network Engineers | Interview Quick Revision

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