Skip to content
Log in Sign up My account

ENARSI 1.11 Troubleshoot BGP

BGP is the largest single objective in the ENARSI Layer 3 domain (35% of the exam), and the verb is troubleshoot. ENCOR 3.2.c only asked you to bring up eBGP between directly connected neighbors. ENARSI assumes you can do that and asks why a session sits in Active, why a prefix is in the BGP table without a >, why a reflected route never reaches a client, or why traffic leaves through the wrong ISP. Expect exhibits of show ip bgp, show ip bgp summary, show ip bgp neighbors and syslog messages, plus ticket-style simulations where several faults hide behind one complaint.

You probably run BGP in production. The exam still catches experienced engineers on the defaults (TTL 1 for eBGP, next hop unchanged on iBGP, MED compared only between paths from the same AS, weight local to one router), on the direction rules (local preference set inbound, prepend set outbound) and on the order of the best-path algorithm. Spend your time there.

Objective Verb Page
1.11.a Address families (IPv4, IPv6); 1.11.b neighbor operation, states and timers, authentication, update-source, multihop, peer groups Troubleshoot Sessions, States and Address Families
1.11.b Next hop, 4-byte AS, private AS, route refresh, synchronization Troubleshoot Next Hop, AS Numbers and Route Exchange
1.11.c Path preference: attributes and the best-path algorithm Troubleshoot Path Attributes and Best Path
1.11.d Route reflector: single RR, cluster ID, originator ID Troubleshoot Route Reflectors
1.11.e Policies: inbound and outbound filtering, path manipulation Troubleshoot Filtering and Path Manipulation

Time budget: week 18 of the study plan. Spend the first two days on sessions and next hop (the two pages where most "the session is down" and "the route is not best" faults live), one day on best path, one day on route reflectors, and the rest of the week on policies and the full set of break/fix tickets.

Module lab topology

Every page in this module uses the same five IOSv routers, so the labs build on each other. The baseline configuration is on the Sessions page. Ten break/fix tickets are spread over the five pages: build the baseline once, save it with copy run flash:baseline.cfg, and return to it between tickets.

graph LR
  subgraph AS65010["AS 65010, OSPF area 0 inside"]
    R1["R1 route reflector<br/>Lo0 10.255.255.1<br/>Lo10 192.0.2.1/24"]
    R2["R2 RR client<br/>Lo0 10.255.255.2"]
    R3["R3 RR client<br/>Lo0 10.255.255.3"]
  end
  R1 -- "Gi0/0 10.1.2.0/30 Gi0/0" --- R2
  R1 -- "Gi0/1 10.1.3.0/30 Gi0/0" --- R3
  R2 -- "Gi0/1 10.2.4.0/30 eBGP" --- R4["R4 ISP-A AS 65001<br/>203.0.113.0/24<br/>198.51.100.0/24"]
  R3 -- "Gi0/1 10.3.5.0/30 eBGP" --- R5["R5 ISP-B AS 65002<br/>203.0.113.0/24"]
Ticket Page Fault area
1 to 4 Sessions Authentication, remote AS, TCP 179 blocked, IPv6 network mismatch
5 and 6 Next hop and AS numbers Next hop unreachable, synchronization
7 Best path RIB failure caused by a better administrative distance
8 Route reflectors Route reflector client configured on the wrong router
9 and 10 Policies Route-map implicit deny, AS-path prepend loop

When you finish, take the BGP practice questions. Aim for 80% or better before moving on.