1.11 Troubleshoot BGP (Internal and External; unicast and VRF-Lite)
📘CCNP Enterprise – ENARSI (300-410)
1. Why Route Reflector Is Needed
The iBGP Full-Mesh Problem
In Internal BGP (iBGP):
- Every router must form a BGP neighbor relationship with every other iBGP router
- This is called a full-mesh
- If there are N routers, total sessions =
N × (N − 1) / 2
Problem:
- Large networks need hundreds or thousands of BGP sessions
- High:
- CPU usage
- Memory usage
- Configuration complexity
- Difficult to troubleshoot
Solution:
👉 Route Reflector (RR)
2. What Is a Route Reflector?
A Route Reflector is a special iBGP router that:
- Reduces the need for full-mesh iBGP
- Allows one router to reflect (forward) BGP routes to other iBGP routers
Key Idea:
Not all routers need to peer with each other
They only peer with the Route Reflector
3. Basic Route Reflector Architecture
Components
- Route Reflector (RR)
- Central router
- Reflects routes
- RR Clients
- Routers that peer only with the RR
- Do NOT peer with each other
- Non-Clients
- iBGP peers of RR that are not clients
- Less common in simple designs
4. Route Reflector Terminology (Exam Important)
| Term | Meaning |
|---|---|
| Route Reflector (RR) | Router that forwards iBGP routes |
| RR Client | Router that receives reflected routes |
| Cluster | Group consisting of RR and its clients |
| Cluster ID | Unique ID to prevent routing loops |
5. How Route Reflection Works
Normal iBGP Rule:
An iBGP router does NOT advertise routes learned from one iBGP peer to another iBGP peer
Route Reflector Changes This Rule
The RR can advertise iBGP-learned routes, based on rules.
6. Route Advertisement Rules (VERY IMPORTANT FOR EXAM)
Rule 1: Route from RR Client
If RR learns a route from a client, it advertises it to:
- Other clients
- Non-clients
Rule 2: Route from Non-Client
If RR learns a route from a non-client, it advertises it to:
- Clients only
- ❌ Not to other non-clients
Rule 3: Route from eBGP
If RR learns a route from eBGP, it advertises it to:
- All clients
- All non-clients
Summary Table
| Learned From | Advertised To |
|---|---|
| RR Client | Clients + Non-Clients |
| Non-Client | Clients only |
| eBGP | Everyone |
7. Cluster and Cluster ID
What Is a Cluster?
- A cluster is a group of:
- One Route Reflector
- Its RR clients
Cluster ID:
- Used to prevent routing loops
- Similar to OSPF area ID concept
- Can be:
- Manually configured
- Or automatically derived from router ID
BGP Attribute Used:
- CLUSTER_LIST
If a route comes back to the same cluster, it is discarded
8. Why Route Reflector Does NOT Break Loop Prevention
BGP uses:
- AS_PATH
- Prevents loops between ASes
- ORIGINATOR_ID
- Prevents loops inside iBGP
- Identifies original router that created the route
- CLUSTER_LIST
- Prevents loops between route reflectors
These attributes ensure loop-free routing
9. Route Reflector Configuration (Conceptual)
Command names are not required in your request, but concept understanding is essential.
Key configuration concepts:
- RR is configured using:
neighbor x.x.x.x route-reflector-client
- Applied on the RR only
- Clients do NOT need special configuration
10. Route Reflector and Best Path Selection
Important points:
- RR does NOT modify BGP attributes
- Best path selection follows standard BGP rules
- RR simply forwards the best path it selects
11. Route Reflector in Troubleshooting (Exam Focus)
Common Issues
1. Routes Not Reaching Clients
Possible causes:
- Client not configured correctly
- Wrong cluster ID
- Neighbor not marked as RR client
2. Partial Routing Table
- Some routes visible on RR
- Missing on clients
- Indicates reflection rule violation
3. Routing Loops (Rare)
- Misconfigured cluster IDs
- Incorrect ORIGINATOR_ID handling
12. Advantages of Route Reflector
- Eliminates full-mesh iBGP
- Scales to large networks
- Reduces:
- CPU load
- Memory usage
- Configuration effort
13. Disadvantages (Know for Exam)
- RR becomes critical point
- Misconfiguration affects many routers
- Sub-optimal routing paths may occur
- Troubleshooting is more complex than full-mesh
14. Route Reflector vs Full-Mesh (Quick Comparison)
| Feature | Full-Mesh iBGP | Route Reflector |
|---|---|---|
| Scalability | Poor | Excellent |
| Complexity | High | Lower |
| Redundancy | High | Depends on design |
| Troubleshooting | Easier | Moderate |
15. VRF-Lite and Route Reflector
- Route Reflector works inside each VRF
- Separate BGP tables per VRF
- RR configuration is VRF-aware
- Reflection rules remain the same
16. Exam Key Points to Remember (Must Memorize)
✔ Purpose: Reduce iBGP full-mesh
✔ Used in iBGP only
✔ RR Clients peer only with RR
✔ Advertisement rules are fixed and strict
✔ Uses:
- ORIGINATOR_ID
- CLUSTER_LIST
✔ Does not change BGP best-path logic
✔ Important for large enterprise networks
17. What Is EXCLUDED (As Per Your Topic)
❌ Multiple Route Reflectors
❌ Confederations
❌ Dynamic Peers
(You do NOT need to study them for this sub-topic)
