1.11 Troubleshoot BGP (Internal and External; unicast and VRF-Lite)
📘CCNP Enterprise – ENARSI (300-410)
1. What Is an Address Family in BGP?
In BGP, an address family tells the router what type of routes it should exchange with a neighbor.
Think of it as a container that holds routing information for:
- IPv4 networks
- IPv6 networks
- VPN routes (VRF-Lite)
- Multicast routes
For this exam section, we focus on:
- IPv4 unicast
- IPv6 unicast
2. Why Address Families Are Important in BGP
Modern networks do not use only one type of IP:
- IPv4 is still widely used
- IPv6 is growing fast
- Some networks use both at the same time
BGP address families allow:
- Running IPv4 and IPv6 BGP simultaneously
- Applying separate policies for IPv4 and IPv6
- Supporting VRF-Lite and MPLS VPNs
- Better control and scalability
👉 For the exam:
You must understand how BGP behaves differently inside each address family.
3. Legacy BGP vs Address Family BGP
Legacy BGP (Old Method)
- Only IPv4 supported
- Neighbor configuration automatically exchanged IPv4 routes
- Limited scalability
Address Family BGP (Modern Method)
- Supports IPv4, IPv6, VPNv4, VPNv6, etc.
- Routes are exchanged only if explicitly activated
- Required knowledge for ENARSI
⚠️ Exam Tip:
If a neighbor is configured but not activated under the address family, no routes will be exchanged.
4. Common BGP Address Families for ENARSI
| Address Family | Purpose |
|---|---|
| IPv4 Unicast | Standard IPv4 routing |
| IPv6 Unicast | Standard IPv6 routing |
| VPNv4 / VPNv6 | MPLS VPN (VRF-Lite concepts) |
For this subtopic, focus on:
- IPv4 Unicast
- IPv6 Unicast
5. IPv4 Address Family in BGP
What Is IPv4 Unicast?
- Exchanges IPv4 routes
- Most common BGP usage
- Used in enterprise and ISP networks
Key Characteristics
- Routes use IPv4 prefixes
- Uses IPv4 next-hop addresses
- Policies (route-maps, filters) apply per address family
Important Exam Concept
Even if a BGP neighbor is defined globally:
- IPv4 routes are exchanged only if activated under IPv4 address family
6. IPv6 Address Family in BGP
What Is IPv6 Unicast?
- Exchanges IPv6 routes
- Required for modern networks
- Completely separate from IPv4
Key Characteristics
- Uses IPv6 prefixes
- Uses IPv6 next-hop
- IPv6 BGP can run:
- Over IPv6 transport
- Over IPv4 transport
⚠️ Exam Trap:
- IPv6 BGP does not automatically exchange routes just because IPv4 BGP is working.
7. BGP Neighbor Activation (Critical Exam Topic)
Global Neighbor Configuration
Defines:
- Neighbor IP address
- Remote AS number
But this does NOT mean routes will flow.
Address Family Activation
Routes are exchanged only after:
- Neighbor is activated under the address family
Common Issue (Exam Favorite)
- BGP session is UP
- No routes are exchanged
- Cause: Neighbor not activated under IPv4 or IPv6 address family
8. BGP Address Family Configuration Structure (Conceptual)
BGP configuration has two levels:
1. Global BGP Configuration
- Router ID
- Neighbor AS
- Authentication
- Timers
2. Address Family Configuration
- Activate neighbors
- Advertise networks
- Apply route-maps
- Configure next-hop behavior
👉 All routing logic happens inside the address family
9. Address Families and Route Advertisement
Network Advertisement Rules
A route is advertised in BGP only if:
- The network exists in the routing table
- It is configured under the correct address family
IPv4 Example Logic
- IPv4 routes → IPv4 address family only
IPv6 Example Logic
- IPv6 routes → IPv6 address family only
⚠️ Exam Warning:
- IPv6 routes configured under IPv4 AF will not work
10. Address Families and BGP Policies
Each address family has independent policies:
- Route-maps
- Prefix-lists
- Distribute-lists
- Weight, Local Preference, MED
Why This Matters
- You can allow a route in IPv4 but block it in IPv6
- Policies do not automatically apply across families
👉 Exam Question Pattern:
“Why is a route allowed in IPv4 but missing in IPv6?”
Answer:
- Policy not applied or misconfigured in IPv6 address family
11. Address Families and Next-Hop Behavior
IPv4
- Next hop must be reachable via IPv4 routing
- iBGP does not change next hop by default
IPv6
- Next hop must be reachable via IPv6 routing
- IPv6 next hop may require manual configuration
⚠️ Troubleshooting Tip:
- Routes exist in BGP table but not in routing table → next hop unreachable
12. Address Families and VRF-Lite (High-Value Exam Area)
What Is VRF-Lite?
- Multiple routing tables on one router
- Used for network segmentation
- No MPLS required
BGP with VRF-Lite
Each VRF:
- Has its own address family
- Has separate BGP routing tables
- Uses separate neighbors and policies
Key Exam Concept
Routes in one VRF:
- Are NOT visible in another VRF
- Even if the address family is the same
13. Common BGP Address Family Problems (Exam Scenarios)
1. Neighbor Up but No Routes
Possible Causes:
- Neighbor not activated under AF
- Wrong address family
- Policy blocking routes
2. IPv4 Works but IPv6 Fails
Possible Causes:
- IPv6 AF not configured
- IPv6 neighbor missing
- IPv6 next hop unreachable
3. Routes in BGP Table but Not Routing Table
Possible Causes:
- Next hop unreachable
- Administrative distance
- VRF mismatch
4. Routes Missing in VRF
Possible Causes:
- Wrong VRF AF
- Route leaking not configured
- Incorrect RD/RT (conceptual understanding)
14. Verification Commands (Conceptual Knowledge)
You should recognize what these commands verify:
- Show BGP neighbors per address family
- Show BGP IPv4 routes
- Show BGP IPv6 routes
- Show routing table per VRF
👉 Exam Focus:
Understand what each command checks, not memorizing syntax.
15. Key Differences: IPv4 vs IPv6 Address Families
| Feature | IPv4 AF | IPv6 AF |
|---|---|---|
| Address Type | IPv4 | IPv6 |
| Next Hop | IPv4 | IPv6 |
| Configuration | AF-specific | AF-specific |
| Policies | Independent | Independent |
| Auto Activation | No | No |
16. Must-Remember Exam Points (Very Important)
✔ BGP uses address families to exchange routes
✔ Neighbors must be activated per address family
✔ IPv4 and IPv6 BGP are completely separate
✔ Policies are applied inside address families
✔ VRF-Lite uses separate address families per VRF
✔ BGP session UP ≠ routes being exchanged
17. Simple One-Line Summary for Students
BGP address families decide what type of routes (IPv4 or IPv6) are exchanged, and routes will only work if the neighbor, networks, and policies are configured inside the correct address family.
