network-bgp-diagnostics

name network-bgp-diagnostics description Diagnostics-only BGP troubleshooting patterns for neighbor state, route exchange, prefix policy, AS path inspection, and safe evidence collection. Use when a BGP neighbor is down, routes are missing, or prefix policy and AS path need inspection. metadata {"origin":"community"} Network BGP Diagnostics Use this skill when a BGP session is down, flapping, established with missing routes, or advertising unexpected prefixes. The default workflow is read-only evidence collection; policy and reset actions belong in a reviewed change window. When to Use BGP neighbors are stuck in Idle, Connect, Active, OpenSent, or OpenConfirm. A session is Established but expected prefixes are missing. A route-map, prefix-list, max-prefix limit, or AS path policy may be filtering routes. You need before/after evidence for a BGP change. You are reviewing automation that parses BGP summary output. Read-Only Triage Flow Identify the exact neighbor, address family, VRF, and local/remote ASNs. Capture summary state and last reset reason. Prove reachability to the peer source address. Check route policy references before assuming transport failure. Compare advertised, received, and installed routes where the platform supports those commands. show bgp summary show bgp neighbors <peer> show ip route <peer> show tcp brief | include <peer>|:179 show logging | include BGP|<peer> show running-config | section router bgp show ip prefix-list show route-map Use platform-specific address-family commands when the device uses VRFs, IPv6, VPNv4, or EVPN. Do not assume global IPv4 unicast. State Interpretation State First checks Established with prefix count Route exchange is up; inspect policy and table selection Established with zero prefixes Check inbound policy, max-prefix, advertised routes, and AFI/SAFI Active TCP session is not completing; check routing, source, ACLs, and peer reachability Connect TCP connection is in progress; check path and remote listener OpenSent/OpenConfirm TCP works; check ASN, authentication, timers, capabilities, and logs Idle Neighbor may be disabled, missing config, blocked by policy, or backoff timer Transport Checks ping <peer> source <local-source> traceroute <peer> source <local-source> show ip route <peer> show bgp neighbors <peer> | include BGP state|Last reset|Local host|Foreign host If the peer is sourced from a loopback, confirm both directions route to the loopback addresses and that the neighbor config uses the expected update source. Avoid disabling ACLs or firewall policy as a diagnostic shortcut. Read hit counters, logs, and path state first. Route Policy Checks show bgp neighbors <peer> advertised-routes show bgp neighbors <peer> routes show ip prefix-list <name> show route-map <name> show bgp <prefix> Some platforms require additional configuration before received-routes is available. Do not add that configuration during incident triage unless the operator approves the change. AS Path And Prefix Review show bgp regexp _65001_ show bgp regexp ^65001$ show bgp <prefix> show bgp neighbors <peer> advertised-routes | include Network|Path|<prefix> Use AS-path regex carefully. _65001_ matches AS 65001 as a token. Plain 65001 can match longer ASNs or unrelated text. Parser Pattern import re from typing import Any BGP_SUMMARY_RE = re. compile ( r"^(?P<neighbor>\d{1,3}(?:\.\d{1,3}){3})\s+" r"(?P<version>\d+)\s+" r"(?P<remote_as>\d+)\s+" r"(?P<msg_rcvd>\d+)\s+" r"(?P<msg_sent>\d+)\s+" r"(?P<table_version>\d+)\s+" r"(?P<input_queue>\d+)\s+" r"(?P<output_queue>\d+)\s+" r"(?P<uptime>\S+)\s+" r"(?P<state_or_prefixes>\S+)$" , re.M, ) def parse_bgp_summary ( raw: str ) -> list [ dict [ str , Any ]]: rows = [] for match in BGP_SUMMARY_RE.finditer(raw): state_or_prefixes = match .group( "state_or_prefixes" ) if state_or_prefixes.isdigit(): state = "Established" prefixes_received = int (state_or_prefixes) else : state = state_or_prefixes prefixes_received = None rows.append({ "neighbor" : match .group( "neighbor" ), "remote_as" : int ( match .group( "remote_as" )), "state" : state, "prefixes_received" : prefixes_received, "uptime" : match .group( "uptime" ), }) return rows Prefer structured parser output when available, but store raw output with the incident record because BGP summary formats vary by platform and address family. Change-Window Only These actions can affect routing and should not be suggested as automatic diagnostics: Clearing a BGP session. Changing neighbor authentication, timers, update source, route-maps, or prefix-lists. Enabling additional received-route storage. Relaxing firewall, ACL, or control-plane policy. If a reset is approved, prefer the least disruptive soft or route-refresh option supported by the platform and document exactly why it is safe. Anti-Patterns Assuming Active always means the remote side is down. Ignoring VRF, address family, or update-source differences. Using broad AS-path regex without token boundaries. Hard-resetting a peer before reading last reset reason and logs. Treating missing received-routes output as proof that no routes arrived. See Also Skill: cisco-ios-patterns Skill: network-config-validation Skill: network-interface-health