BGP Attributes - Technical Notes

📘 BGP Path Attributes: Comprehensive Technical Notes

Audience: Interviewers • Network Engineers • Learners • Revision Notes
Purpose: A one-stop, technically complete, copyright-free guide to BGP Path Attributes (PAs) for understanding, reviewing, and mastering BGP path selection.

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🧩 I. Prerequisites & Overview

Before diving into BGP Path Attributes, ensure the following concepts are clear:

• BGP Complexity:
  Border Gateway Protocol (BGP) is among the most complex routing protocols used in networking.

• Routing Basics:
  Understanding of IP packet routing and route advertisement.

• BGP Peering Knowledge:
  Basic knowledge of EBGP (External BGP) and IBGP (Internal BGP) sessions.

• IBGP vs. EBGP Behavior:

  • EBGP Update: Routes learned from an external peer can be sent to all other peers.
  • IBGP Update: Routes learned from an internal peer can be advertised to external peers, but not to other internal peers (loop prevention rule).

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🧠 II. What Are Path Attributes (PAs)?

Path Attributes are parameters within a BGP Update message that describe the characteristics of advertised routes.

• Location:
  Found inside BGP Update messages, which carry new prefixes or withdraw old ones.

• Structure:
  • Prefixes (routes) sit at the bottom of the update.
  • Path Attributes sit above them, defining properties of those routes.
• Purpose:
  1. Best Path Selection: Helps BGP decide the most optimal route when multiple paths exist.
  2. Loop Prevention: Detects if a route re-enters the same AS.
  3. Route Classification: Tags routes for filtering, redistribution, or policy decisions (e.g., BGP Communities).
  4. Informative Use: Certain attributes (e.g., Atomic Aggregate) signal route summarization.

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🧱 III. Categories of Path Attributes

Each Path Attribute falls into one of four key categories:

Category	Well-Known/Optional	Mandatory/Discretionary	Transitivity	Description & Rule	Examples
Well-Known Mandatory	Well-Known	Mandatory	Always	Must be recognized and included by every BGP device.	Next Hop, AS Path, Origin
Well-Known Discretionary	Well-Known	Discretionary	Always	Understood by all BGP routers but optional for use.	Local Preference, Atomic Aggregate
Optional Transitive	Optional	Discretionary	Transitive	May not be understood; if unknown, still passed along to other ASes.	Aggregator, Community
Optional Non-Transitive	Optional	Discretionary	Non-Transitive	May not be understood; not shared outside the AS.	MED, Originator ID, Cluster List

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🧮 IV. BGP Best Path Selection Process (BPSP)

BGP uses a sequential decision algorithm to determine the best route among multiple available ones.
The steps below follow a common mnemonic: N-W-L-O-A-O-M-N-I-O-R-I.

#	Attribute / Rule	Preference	Key Details
1️⃣	Next Hop Reachability (N)	Must be reachable	If the next hop is unreachable, the route is unusable (remains in the BGP table but not in RIB).
2️⃣	Weight (W)	Highest wins	Cisco-proprietary; locally significant; not carried in updates (default 0).
3️⃣	Local Preference (L)	Highest wins	Influences routing inside the AS; default value 100; non-transitive.
4️⃣	Locally Originated (O)	Local routes preferred	Routes injected via network or redistribute commands are preferred.
5️⃣	AS Path Length (A)	Shortest wins	Fewer AS hops preferred; can be lengthened using AS Path Prepending.
6️⃣	Origin Code (O)	IGP > Incomplete	IGP (i) = from network command; Incomplete (?) = from redistribution.
7️⃣	MED (M)	Lowest wins	Suggests preferred entry point into AS; default 0; compared among same external AS.
8️⃣	EBGP over IBGP (N/E)	EBGP preferred	External paths take precedence over internal.
9️⃣	IGP Metric (I)	Lowest wins	Applies to IBGP-learned routes — smaller IGP cost to next hop wins.
🔟	Oldest EBGP Route (O/D)	Oldest wins	If multiple EBGP routes exist for the same prefix.
11️⃣	BGP Router ID (R)	Lowest wins	Used as a tie-breaker.
12️⃣	Neighbor IP Address	Lowest wins	Final tie-breaker to ensure deterministic selection.

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⚙️ V. Key Path Attributes & Manipulation

🔸 A. Weight (Cisco Proprietary)

• Purpose: Influences local path selection (highest wins).
• Scope: Local to the router; not shared with peers.
• Example:

  route-map SET_WEIGHT permit 10
   set weight 200
  neighbor 10.1.1.2 route-map SET_WEIGHT in
  

🔸 B. Local Preference

• Purpose: Controls routing within the AS (highest preferred).
• Scope: Non-transitive (not advertised to other ASes).
• Default: 100.
• Example:

  route-map SET_LOCAL_PREF permit 10
   set local-preference 200
  neighbor 10.1.1.2 route-map SET_LOCAL_PREF in
  

🔸 C. AS Path Prepending

• Purpose: Artificially lengthen the AS Path to make a route less preferred by external peers.
• Best Practice: Only prepend your own AS number.
• Example:

  route-map PREPEND-AS permit 10
   set as-path prepend 65001 65001 65001
  neighbor 203.0.113.1 route-map PREPEND-AS out
  

🔸 D. Multi Exit Discriminator (MED)

• Purpose: Suggests preferred inbound path for external ASes (lower wins).
• Default: 0.
• Comparison: Only compared if routes come from the same external AS (can be overridden).
• Example:

  route-map SET_MED permit 10
   set metric 50
  neighbor 203.0.113.2 route-map SET_MED out
  

E. Origin Code

• Purpose: Indicates how the route entered BGP.
• Values:
  • i – From the network command (IGP-originated).
  • ? – From redistribution (incomplete). Preference: IGP-originated (i) preferred over incomplete (?).

🔸 F. BGP Communities

Purpose: Tag routes for policy decisions or filtering. Types: - Standard (16-bit) - Extended (32-bit)

Common Well-Known Communities:

no-advertise → Do not advertise further. no-export → Do not advertise outside local AS (IBGP only). local-AS → Keep route within local AS.

🔁 VI. Other Essential Concepts

1. Route Policy Application

• Updating route policies (e.g., route maps) does not immediately affect existing routes in the BGP table.

2. Route Refresh

• Used to reapply new policies by refreshing route advertisements.

clear ip bgp * in     # Request inbound updates (refresh inbound)
clear ip bgp * out    # Resend outbound updates (refresh outbound)

3. Redistribution Caution

• Avoid redistributing BGP routes into IGPs (like OSPF/EIGRP) due to scalability limits — BGP handles far larger routing tables than IGPs.

🧾 Summary: Key Takeaways

Concept	Purpose	Scope	Preference Rule
Weight	Local path choice	Local router	Highest wins
Local Preference	AS-internal routing	Within AS	Highest wins
AS Path	Loop prevention / external routing	Inter-AS	Shortest wins
Origin Code	Route source info	Inter-AS	IGP > Incomplete
MED	External entry preference	Between AS	Lowest wins
Community	Route classification	Local/Global	Policy dependent

🏁 Final Notes

• BGP Path Attributes are core to route decision-making in modern networks.
• Mastery of each attribute — its type, scope, and preference order — is essential for network design, troubleshooting, and optimization.
• Understanding how PAs interact ensures predictable, policy-driven routing behavior across both internal and external BGP topologies.

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