Home BGP Deep Dive: Path Attributes, Route Selection, Communities, Confederation และ Route Reflector BGP Deep Dive: Path Attributes, Route Selection, Communities, Confederation และ Route Reflector
BGP Deep Dive: Path Attributes, Route Selection, Communities, Confederation และ Route Reflector
BGP (Border Gateway Protocol) เป็น routing protocol ที่ขับเคลื่อน internet ทั้งหมด Path Attributes กำหนดลักษณะของ routes, Route Selection Algorithm เลือก best path จากหลาย paths, Communities ใช้ tag routes สำหรับ policy, Confederation แบ่ง AS ใหญ่เป็น sub-AS เพื่อ scalability และ Route Reflector ลด iBGP full-mesh requirement
BGP เชื่อมต่อ 70,000+ Autonomous Systems บน internet: ทุก ISP, cloud provider, enterprise ที่มี public IP ต้องใช้ BGP การเข้าใจ BGP อย่างลึกซึ้ง ทั้ง path attributes, route selection process, community-based policy และ scaling techniques เป็นสิ่งจำเป็นสำหรับ network engineers ที่ทำงานกับ service providers หรือ large enterprises
BGP Path Attributes
Attribute
Type
Description
Selection Priority
Weight
Cisco-specific (local)
Higher = preferred (default 0, locally originated = 32768)
1st (highest priority)
Local Preference
Well-known, discretionary
Higher = preferred (default 100) — propagated within iBGP
2nd
Locally Originated
–
Prefer routes originated by local router (network/redistribute)
3rd
AS Path
Well-known, mandatory
Shorter AS path = preferred (list of ASes traversed)
4th
Origin
Well-known, mandatory
IGP (i) > EGP (e) > Incomplete (?)
5th
MED (Multi-Exit Discriminator)
Optional, non-transitive
Lower = preferred — suggest entry point to neighboring AS
6th
eBGP over iBGP
–
Prefer eBGP-learned routes over iBGP
7th
IGP Metric
–
Lowest IGP metric to next-hop = preferred
8th
Router ID
–
Lowest Router ID = tiebreaker
9th (last resort)
Route Selection Algorithm (Detail)
Step
Check
Action
0. Valid?
Next-hop reachable? Synchronized?
Invalid routes eliminated first
1. Weight
Highest weight wins
Cisco local only — use for local preference override
2. Local Pref
Highest local preference wins
Primary tool สำหรับ inbound traffic engineering
3. Locally Originated
Prefer locally originated routes
network/redistribute/aggregate
4. AS Path Length
Shortest AS path wins
Can be manipulated with AS path prepending
5. Origin
IGP > EGP > Incomplete
Rarely decisive
6. MED
Lowest MED wins (same neighbor AS only)
Suggest preferred entry to neighbor
7. Path Type
eBGP > iBGP
Prefer externally-learned routes
8. IGP Metric
Lowest IGP cost to next-hop
Hot-potato routing (exit as quickly as possible)
9. Tiebreakers
Oldest route, lowest Router ID, shortest cluster-list
Final tiebreaker
BGP Communities
Community Type
Format
Use
Standard Community
ASN:Value (e.g., 65000:100)
Tag routes สำหรับ policy (local pref, prepend, filter)
Extended Community
Type:ASN:Value (8 bytes)
Route Target (VPN), Site of Origin, OSPF route type
Large Community
ASN:Value1:Value2 (12 bytes)
สำหรับ 4-byte ASN (32-bit AS numbers)
Well-Known: NO_EXPORT
65535:65281
ไม่ advertise ออกนอก AS (keep within confederation)
Well-Known: NO_ADVERTISE
65535:65282
ไม่ advertise ให้ใครเลย (keep local)
Well-Known: NO_EXPORT_SUBCONFED
65535:65283
ไม่ advertise ออกนอก sub-AS (confederation)
Community-Based Policy Examples
Community
Meaning
Action at ISP
ISP:100
Customer route — full transit
Set local-pref 150, advertise to all peers
ISP:200
Peer route
Set local-pref 100, advertise to customers only
ISP:300
Transit route (upstream)
Set local-pref 80, advertise to customers only
ISP:666
Blackhole route
Set next-hop to null → drop traffic (DDoS mitigation)
ISP:1000
Do not advertise to peer X
Filter out when advertising to specific peer
ISP:2000
Prepend 1x to all peers
AS path prepend 1 time (make path less preferred)
Route Reflector
Feature
รายละเอียด
Problem
iBGP requires full mesh: N routers = N(N-1)/2 sessions (100 routers = 4,950 sessions)
Solution
Route Reflector (RR) reflects routes → clients only peer with RR (not full mesh)
RR Rules
Client → RR → reflect to all clients + non-clients | Non-client → RR → reflect to clients only
Cluster ID
Identifies RR cluster → prevent loops (routes with own cluster-ID = discard)
Originator ID
Original router that injected route → prevent loops
Redundancy
Deploy 2 RRs per cluster (redundant — both reflect same routes)
Hierarchy
RR hierarchy: RR → sub-RR → clients (for very large networks)
Placement
ไม่ต้องอยู่ใน data path (out-of-band RR) — dedicated VM/container ได้
Confederation
Feature
รายละเอียด
Problem
Same as RR — iBGP full mesh ไม่ scale
Solution
แบ่ง AS ใหญ่เป็น sub-AS → eBGP between sub-AS (no full mesh needed)
External View
ภายนอกเห็นเป็น single AS (confederation AS stripped ก่อน advertise ออก)
Confederation ASN
ใช้ private ASN (64512-65535) สำหรับ sub-AS ภายใน
vs Route Reflector
Confederation: more config complexity, better path diversity | RR: simpler, potential suboptimal routing
Use Case
Very large SPs ที่มีหลาย regions (แต่ละ region = sub-AS)
Traffic Engineering with BGP
Direction
Tool
How
Outbound (from your AS)
Local Preference
Set higher local-pref สำหรับ preferred exit path
Outbound
Weight (Cisco)
Per-router preference (local only, not propagated)
Inbound (to your AS)
AS Path Prepending
Make path longer → less preferred by neighbors
Inbound
MED
Suggest entry point to neighbor (lower = preferred)
Inbound
Communities
Ask ISP to adjust local-pref/prepend via community tags
Blackhole
Blackhole Community
Send /32 with blackhole community → ISP null-routes (DDoS mitigation)
ทิ้งท้าย: BGP = The Protocol That Runs the Internet
BGP Deep Dive Path Attributes: Weight → Local Pref → AS Path → Origin → MED → eBGP/iBGP → IGP Metric → Router ID Route Selection: 9-step algorithm → deterministic best path selection Communities: standard (ASN:Value), extended (RT), large (4-byte ASN) → policy tagging Route Reflector: eliminate iBGP full mesh → clients peer with RR only → 2 RRs per cluster Confederation: split AS into sub-AS → eBGP between sub-AS → external sees single AS Traffic Engineering: outbound (local-pref, weight) | inbound (AS prepend, MED, communities) Key: BGP is simple concept (path vector) but complex operation — mastering attributes and policy is essential
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