Frequently Asked Questions of BGP
Got questions? We've got the answers quick, clear, and hassle-free as per mentioned FAQ
FAQ Execution
BGP (Border Gateway Protocol) is a path-vector routing protocol used to exchange routing information between Autonomous Systems (AS).
A network under a single administrative control identified by a unique ASN.
TCP port 179.
eBGP: Between different AS & iBGP: Within the same AS
Path-vector protocol.
Process of choosing the best path based on attributes.
Weight (Cisco proprietary).
Defines preferred exit path; higher is better.
List of AS numbers a route has passed through.
Suggests preferred entry point into an AS; lower is better.
IP address to reach the destination.
Indicates how route was learned (IGP, EGP, Incomplete).
Tag used to apply routing policies.
Indicates route summarization has occurred.
Shows router that performed aggregation.
Weight → Local Preference → Locally Originated → AS Path → MED → eBGP over iBGP → IGP metric → Router ID
Router configured to exchange BGP routes.
Establishing connection between BGP neighbors.
Idle → Connect → Active → OpenSent → OpenConfirm → Established
In Established state.
Incorrect ASN, IP, ACL, TTL or Authentication.
To avoid routing loops.
Routes learned from iBGP are not advertised to another iBGP peer.
Reduces full-mesh requirement.
Router receiving reflected routes.
Divides AS into sub-AS for scalability.
Combining multiple routes into one.
Controlling advertised routes.
Suppresses flapping routes.
Time taken to update routing tables.
MD5-based security for neighbors.
Protects against spoofing attacks.
Filters routes based on prefixes.
Advanced policy control tool.
Unauthorized route advertisement.
Allows multiple best paths.
Advertises multiple paths for same prefix.
Maintains forwarding during restart.
Fast reroute mechanism.
Used in data center overlays.
Handles large-scale routing efficiently.
eBGP: 20 & iBGP: 200
BGP table: All learned routes
Routing table: Best routes only
Fixes next-hop issue in iBGP.
Applies policy without restarting session.
show ip bgp summary.
show ip bgp.
Due to filters, policies, or synchronization.
BGP routes must exist in IGP (older concept).
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.📚 How 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.
