BGP Path Selection - Technical Notes

🧭 BGP Path Selection – Complete Technical Notes

For Interviewers, Learners, and Network Engineers

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I. Beginner Level – Core Concepts & Attributes

📘 What is BGP Path Selection?

BGP (Border Gateway Protocol) is an Exterior Gateway Protocol (EGP) used to exchange routing information between Autonomous Systems (AS).
Unlike OSPF or EIGRP, BGP is policy-based, not metric-based — it uses multiple path attributes for decision-making.

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🎯 Key Objectives of BGP Path Selection

BGP determines the best route using these five primary attributes:

#	Attribute	Description
1	Weight	Cisco proprietary; higher value preferred
2	Local Preference	Influences routing within the AS; higher preferred
3	AS Path	Fewer AS hops preferred
4	Origin	Route source indicator; IGP > Incomplete
5	MED (Multi-Exit Discriminator)	Lower value preferred

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🧠 Mnemonic for Attribute Order

“We Love Oranges As Oranges Mean Pure Refreshments”

Mnemonic	Attribute	Preference
We	Weight	Higher is better
Love	Local Preference	Higher is better
Oranges	Originated Locally	Locally originated routes preferred
As	AS Path	Shorter is better
Oranges	Origin Code	IGP (i) > Incomplete (?)
Mean	MED	Lower is better
Pure	Path Type	eBGP > iBGP
Refreshments	Router ID	Lower is better

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⚙️ Attribute Highlights

Attribute	Type	Scope	Default	Notes
Weight	Cisco proprietary	Local to router	0 (learned), 32768 (local)	Not advertised to peers
Local Preference	Well-known discretionary	Entire AS	100	Shared within iBGP peers
AS Path	Well-known mandatory	Global	—	Counts AS hops
Origin Code	Well-known mandatory	Global	—	i (IGP) > ? (Incomplete)
MED	Optional non-transitive	Between ASes	—	Lower preferred, not passed beyond neighbor AS

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II. Intermediate to Advanced Level – Technical Criteria & Configuration

🔖 A. BGP Attribute Classification

Type	Description	Examples
Well-Known Mandatory	Must exist in all updates, understood by all vendors	AS Path, Origin
Well-Known Discretionary	Understood by all vendors, may not appear	Local Preference
Optional Transitive	May not be understood, but carried across ASes	Communities
Optional Non-Transitive	Not understood or carried beyond neighbor AS	MED, Weight (Cisco)

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⚡ B. Full BGP Path Selection Criteria (Top–Down)

BGP always follows the same decision order.
If a tie occurs at one level, it continues to the next.

Step	Criteria	Preference Rule
1	Next-Hop Reachability	Must be reachable
2	Highest Weight	Higher wins
3	Highest Local Preference	Higher wins
4	Locally Originated Route	Prefer self-originated
5	Shortest AS Path	Fewer AS hops preferred
6	Lowest Origin Code	IGP (i) > Incomplete (?)
7	Lowest MED	Lower wins
8	eBGP over iBGP	External routes preferred
9	Lowest IGP Metric to Next Hop	Lower metric preferred
10	Oldest Path	More stable route preferred
11	Lowest BGP Router ID	Smaller ID preferred
12	Shortest Cluster List	Used with Route Reflectors
13	Lowest Neighbor IP	Used as final tiebreaker

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🧩 C. Configuration & Attribute Manipulation Rules

Attributes are applied in the direction of control-plane updates.

Attribute	Direction	Scope / Impact	Example Command
Weight	Inbound only	Local router only	set weight 1000
Local Preference	Inbound from eBGP / Outbound to iBGP	Entire AS	set local-preference 200
AS Path	Inbound / Outbound	Makes path appear longer	set as-path prepend 100 100 100
Origin	Inbound / Outbound	Affects route preference	set origin incomplete
MED	Outbound to eBGP	Affects neighbor’s path selection	set metric 50

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🧱 D. Configuration Differences – Cisco IOS XE vs IOS XR

Platform	Policy Mechanism	Matching Method	Implicit Deny Behavior	Notes
Cisco IOS XE (XZ)	Route Map	match ip address <ACL>	Must include a final permit for unmatched routes	Common in classic IOS/IOS-XE
Cisco IOS XR (XR)	Route Policy (RPL)	if destination in <prefix-set>	Must include else pass	Modular and reusable
XR eBGP Default	—	—	Routes blocked unless explicit pass	Policy required for eBGP peers

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🧮 BGP Path Manipulation Summary

Scenario	Attribute to Tune	Direction	Use Case
Prefer local exit within AS	Local Preference	Inbound	Internal traffic control
Prefer one ISP over another	Weight	Inbound	Local outbound path selection
Make path less preferred	AS Path Prepend	Outbound	Influence inbound traffic
Influence neighbor’s choice	MED	Outbound	Encourage entry via certain link
Modify origin type	Origin	Outbound	Adjust route preference

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🧠 Real-World Key Points

• Weight – Local-only, not shared.
• Local Preference – Shared within AS via iBGP.
• AS Path Prepending – Used to influence inbound traffic.
• MED – Only applies between directly connected ASes.
• eBGP > iBGP – Always preferred when all else is equal.
• Router ID – Final tie-breaker.

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III. Quick Revision & Interview Shortcut

Step	Attribute	Preferred
1	Next-Hop Reachability	Must be reachable
2	Weight	Higher
3	Local Preference	Higher
4	Originated Locally	Yes
5	AS Path	Shorter
6	Origin Code	IGP (i)
7	MED	Lower
8	Path Type	eBGP
9	IGP Metric	Lower
10	Age	Older
11	Router ID	Lower
12	Cluster List	Shorter
13	Neighbor IP	Lower

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IV. Essential Interview & Exam Keywords

BGP Decision Process, AS Path Prepend, Local Preference,
Weight vs MED, iBGP vs eBGP, Well-Known Attributes,
Optional Attributes, Route Map, RPL, Policy Control,
Cisco IOS XE, Cisco IOS XR.

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End of Notes – “BGP Path Selection Complete Technical Guide”

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