FAQ ON BGP ROUTING# INTERVIEW PREP GUIDE ABOUT BGP ROUTING
🔹 What is BGP ?
BGP (Border Gateway Protocol) is a path-vector routing protocol used to exchange routing information between Autonomous Systems (AS).
🔹 What is an Autonomous System (AS) ?
A network under a single administrative control identified by a unique ASN.
🔹 What is the default port used by BGP ?
TCP port 179.
🔹 What is the difference between eBGP and iBGP ?
eBGP: Between different AS & iBGP: Within the same AS
🔹 What type of routing protocol is BGP ?
Path-vector protocol.
🔹 What is BGP path selection ?
Process of choosing the best path based on attributes.
🔹 What is the first attribute checked in BGP ?
Weight (Cisco proprietary).
🔹 What is Local Preference ?
Defines preferred exit path; higher is better.
🔹 What is AS Path ?
List of AS numbers a route has passed through.
🔹 What is MED (Multi-Exit Discriminator) ?
Suggests preferred entry point into an AS; lower is better.
🔹 What is Next-Hop attribute ?
IP address to reach the destination.
🔹 What is Origin attribute ?
Indicates how route was learned (IGP, EGP, Incomplete).
🔹 What is Community attribute ?
Tag used to apply routing policies.
🔹 What is Atomic Aggregate ?
Indicates route summarization has occurred.
🔹 What is Aggregator attribute ?
Shows router that performed aggregation.
🔹 What is the BGP best path selection order ?
Weight → Local Preference → Locally Originated → AS Path → MED → eBGP over iBGP → IGP metric → Router ID
🔹 What is a BGP neighbor ?
Router configured to exchange BGP routes.
🔹 What is BGP peering ?
Establishing connection between BGP neighbors.
🔹 What is the BGP finite state machine ?
Idle → Connect → Active → OpenSent → OpenConfirm → Established
🔹 When is BGP fully operational ?
In Established state.
🔹 What causes BGP session failure ?
Incorrect ASN, IP, ACL, TTL or Authentication.
🔹 Why iBGP requires full mesh ?
To avoid routing loops.
🔹 What is split-horizon rule in iBGP ?
Routes learned from iBGP are not advertised to another iBGP peer.
🔹 What is Route Reflector ?
Reduces full-mesh requirement.
🔹 What is Route Reflector Client ?
Router receiving reflected routes.
What is Confederation in BGP ?
Divides AS into sub-AS for scalability.
What is route aggregation ?
Combining multiple routes into one.
What is prefix filtering ?
Controlling advertised routes.
What is route dampening ?
Suppresses flapping routes.
What is BGP convergence ?
Time taken to update routing tables.
What is BGP authentication ?
MD5-based security for neighbors.
What is TTL security ?
Protects against spoofing attacks.
What is prefix-list ?
Filters routes based on prefixes.
What is route-map ?
Advanced policy control tool.
What is BGP hijacking ?
Unauthorized route advertisement.
What is Multipath in BGP ?
Allows multiple best paths.
What is BGP Add-Path ?
Advertises multiple paths for same prefix.
What is Graceful Restart ?
Maintains forwarding during restart.
What is BGP PIC (Prefix Independent Convergence) ?
Fast reroute mechanism.
What is EVPN with BGP ?
Used in data center overlays.
Why BGP is used in the Internet ?
Handles large-scale routing efficiently.
What is default administrative distance of BGP ?
eBGP: 20 & iBGP: 200
What is BGP table vs Routing table ?
BGP table: All learned routes
Routing table: Best routes only
What is Next-Hop Self ?
Fixes next-hop issue in iBGP.
What is soft reset ?
Applies policy without restarting session.
How to check BGP neighbors ?
show ip bgp summary.
How to view BGP routes ?
show ip bgp.
Why routes are not advertised ?
Due to filters, policies, or synchronization.
What is synchronization rule ?
BGP routes must exist in IGP (older concept).
How to troubleshoot BGP issues ?
Check : Neighbor state, Reachability, Attributes, Filters
# Troubleshooting BGP problems is always a layered process. You don’t start with routes—you start with the session, then reachability, then attributes, then filters.
1) Neighbor State (Is BGP session even up?)
First confirm whether the BGP peering is Established.
📚 What to check - BGP state: Idle, Connect, Active, OpenSent, OpenConfirm, Established.
- Remote AS number.
- Source / update-source interface.
- TCP 179 connectivity.
- Keepalive/Hold timers.
- Authentication (MD5, if used).
📚 Commands (example on Cisco Systems IOS).
- show ip bgp summary
- show ip bgp neighbors
📚 Healthy output
State/PfxRcd = Established
📚 If NOT Established : -
| State | Action | Meaning |
|---|---|---|
| Idle | Check config, ASN, neighbor IP | BGP not starting |
| Active | Check routing / ping | Cannot reach neighbor |
| Connect | Check firewall / port 179 | TCP not forming |
| Open Sent/Open Confirm | Verify ASN, MD5 password | ASN/Auth mismatch |
2) Reachability (Can routers reach each other?)
BGP runs over TCP. If IP connectivity fails, BGP fails.
📚 What to check
- Can you ping neighbor IP?
- Is there a route to neighbor?
- Correct source interface?
- Loopback peering requires IGP/static route.
📚 Commands - ping "neighbor-ip"
- traceroute "neighbor-ip"
- show ip route "neighbor-ip"
📚 Common issue
If using loopbacks:
- neighbor x.x.x.x update-source loopback0
" But no route to loopback → BGP stuck in Active "
3) Attributes (Routes received but not selected?)
Session is up, but routes are not preferred or not installed in routing table. Now, Check BGP path attributes.
📚 What to check
- Local Preference
- AS Path
- MED
- Next-Hop
- Weight (vendor specific)
- Origin
📚 Commands - show ip bgp
- show ip bgp "prefix"
📚 Typical problems
| Symptom | Root Cause | Fix |
|---|---|---|
| Route in BGP table, not in routing table | Next-hop unreachable | Fix next-hop / IGP |
| Wrong path selected | Local-pref / AS-path | Adjust policy |
| Route not advertised | Missing network statement | Add network / redistribute |
- show ip bgp 10.10.10.0
"Check Next Hop is reachable"
4) Filters (Routes blocked by policy?)
Very common in real networks. BGP is up, routes exist, but filters stop them.
📚 What to check
- Prefix-list
- Route-map
- Distribute-list
- AS-path filter
- Community filter
📚 Commands
- show ip bgp neighbors "ip" received-routes
- show ip bgp neighbors "ip" advertised-routes
- show run | section route-map
- show ip prefix-list
📚 Common issues
| Problem | Cause |
|---|---|
| No routes received | Inbound prefix-list blocking |
| Routes not sent | Outbound route-map |
| Specific prefix | missing Prefix-list deny |
| Only default route received | Provider filter |
📚 Real Troubleshooting Flow (Field Method)
1. show ip bgp summary → Established?
2. If No → Check ping, route, ASN, TCP 179
3. If Yes → show ip bgp → Are routes present?
4. If Yes → show ip route → Installed?
5. If No → Check next-hop
6. If Yes → Traffic wrong path → Check attributes
7. Missing routes → Check filters / route-maps
Golden Rule
- BGP problems are rarely BGP problems.
- 80% are reachability or filters.
