BGP Troubleshooting Methodology (with commands)

1. Verify Physical & IGP Reachability

   1. Can you ping the neighbor?

   2. Is the interface up?

      1. Cisco: show ip interface brief

      2. Nokia: show router interface

   3. Is the route to the neighbor present?

      1. Cisco: show ip route <neighbor-ip>

      2. Nokia: show router route-table <neighbor-ip>

2. Check BGP Session State

   1. Cisco: show ip bgp summary

   2. Nokia: show router bgp summary

   3. States you may see:

      1. Idle/Active --> Session not starting --> Check reachability / TCP 179.

      2. OpenSent/OpenConfirm --> AS mismatch or authentication issue.

      3. Established --> Session is up

3. Verify Configuration

   1. Cisco: show running-config | section bgp

   2. Nokia: show router bgp

   3. Check:

      1. Neighbor IP is correct (physical or loopback)

      2. update-source configured if using loopback (Cisco)

      3. AS numbers match remote side.

4. Check Routes (RIB / BGP Table)

   1. Show learned routes

      1. Cisco: show ip bgp neighbors <ip> received-routes

      2. Nokia: show router bgp routes neighbor <ip> received

   2. Show advertised routes

      1. Cisco: show ip bgp neighbors <ip> advertised routes

      2. Nokia: show router bgp routes neighbor <ip> advertised

   3. Show BGP table

      1. Cisco: show ip bgp

      2. Nokia: show router bgp routes

   4. Check if routes made it into RIB

      1. Cisco: show ip route

      2. Nokia: show router route-table

5. Check Path Attributes

   1. Cisco: show ip bgp <prefix>

   2. Nokia: show router bgp routes <prefix>

   3. Look at:

      1. Next-hop --> Reachable?

      2. AS-PATH --> Matches expected?

      3. Local Pref, MED, communities --> causing policy rejection?

6. Check Policies / Filters

   1. Cisco: show running-config | section route-map

   2. Nokia: show router policy

   3. Confirm export/import policies are not rejecting everything:

      1. Cisco: show ip bgp neighbors <ip> advertised-routes

      2. Nokia: show router bgp routes neighbor <ip> advertised

7. Logs / Debug

   1. Cisco:

      1. debug ip bgp

      2. show logging | includ BGp

   2. Nokia:

      1. tools dump router bgp events

      2. show log log-id <id>

   3. Use debugs with care in production - HIGH CPU

BGP Checklist (with commands)

1. Neighbor stuck Idle/active

   1. Cisco:

      1. ping <neighbor ip>

      2. show ip bgp summary

   2. Nokia:

      1. ping <neighbor ip>

      2. show router bgp summary

   3. if unreachable --> check IGP/static route, ACL, TCP/179

2. neighbor stuck OpenSent/OpenConfirm

   1. Cisco: show run | section bgp

   2. Nokia: show router bgp

   3. AS number mismatch, auth key mismatch.

3. Established but no routes

   1. Cisco: show ip bgp neighbors <ip> advertised-routes

   2. Nokia: show router bgp routes neighbor <ip> advertised

   3. Likely export/import policy issue

4. Routes received but not in RIB

   1. Cisco:

      1. show ip bgp <prefix>

      2. show ip route <prefix>

   2. Nokia:

      1. show router bgp routes <prefix>

      2. show router route-table <prefix>

   3. Usually next-hot not reachable

5. Routes in RIB but not used

   1. Cisco: show ip rotue prefix (look at AD, pref value)

   2. Nokia: show router route-table <perfix>

   3. Another protocol (IGP/Static) has better admin distance

Common Mistakes / Pitfalls

• Wrong neighbor IP --> using loopback without update-source (Cisco) or not binding toloopback (Nokia)

• AS mismatch --> local/remote AS numbers don't match.

• Authentication mismatch --> MD5 keys don't match.

• Next-hop unreachable --> especially in iBGP when next-hop not changed

• Policies blocking rotues --> export/import filters accidentally deny everything

• Forget network command (Cisco) or route not in RIB

• Not activating AFI/SAFI --> Cisco XR / Nokia need explicit address-family ipv4

Quick Fix Recipes

• Neighbor Active/idle

  • Ping neighbor IP, check IGP reachability, ACLs/firewall for TCP/179

• AS Mismatch:

  • Compare config:

    • Cisco: show run | section bgp

    • Nokia: show router bgp

• No routes exchanged

  • Check policies:

    • Cisco: show ip bgp neighbors <ip> advertised-rotues

    • Nokia: show router bgp routes neighbor <ip> advertised

• Routes received but not installed

  • Check next-hot reachability

    • Cisco: show ip route <next-hop>

    • Nokia: show router route-table <next-hop>

• Debugging sessoin setup

  • Cisco: debug ip bgp

  • Nokia: tools dump router bgp events