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BGP Practice Questions

15 original questions covering ENARSI 1.11.a to 1.11.e. Most are exhibit-based, as on the exam: find the fault from the output. Answer every one before opening any explanation. Target: 12 of 15 (80%).

1. Blueprint 1.11.b

Refer to the exhibit. R1 and R2 are iBGP peers between loopbacks. Both loopbacks are reachable. The session stays in Active. What is the cause?

R1# show logging | include BADAUTH
%TCP-6-BADAUTH: No MD5 digest from 10.255.255.2(27811) to 10.255.255.1(179)
%TCP-6-BADAUTH: No MD5 digest from 10.255.255.2(41720) to 10.255.255.1(179)
  • A. Both routers have a password configured, but the passwords differ
  • B. R2 has a password configured for R1, and R1 has none configured for R2
  • C. R1 has a password configured for R2, and R2 has none configured for R1
  • D. R2 is missing update-source Loopback0
Answer: C

R1 is the receiver logging the message, and "No MD5 digest" means the segments from R2 carry no signature while R1 expects one. So R1 has a password and R2 does not. Mismatched passwords (A) give "Invalid MD5 digest" instead. If R2 had the password and R1 did not (B), R1 would not be checking for a digest at all. The source address in the log is R2's loopback, so update-source is working (D).

2. Blueprint 1.11.b

Refer to the exhibit. What happened to the session with 10.3.5.2, and what restores it?

R3# show ip bgp summary | begin Neighbor
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.255.255.1    4        65010     220     214       31    0    0 03:12:40        3
10.3.5.2        4        65002      96      90        0    0    0 00:00:47 Idle (PfxCt)
  • A. The neighbor was shut down administratively; remove neighbor 10.3.5.2 shutdown
  • B. The neighbor sent more prefixes than the configured maximum-prefix limit; raise the limit or add a restart interval, then clear the session
  • C. The neighbor is not activated under the IPv4 address family
  • D. The hold timer expired; lower the keepalive interval
Answer: B

Idle (PfxCt) means the prefix count exceeded neighbor x maximum-prefix. The router tears the session down and stays idle until it is cleared, or until the restart interval runs out if one is configured. An administrative shutdown shows Idle (Admin) (A). A hold timer expiry produces a NOTIFICATION and normal retries, not this flag (D). A neighbor not activated in any family would not have exchanged messages in the first place (C).

3. Blueprint 1.11.b

Refer to the exhibit. R2 is configured with neighbor 10.2.4.2 remote-as 65010. What change fixes the session?

%BGP-3-NOTIFICATION: sent to neighbor 10.2.4.2 2/2 (peer in wrong AS) 2 bytes FDE9
  • A. Change the remote AS on R2 to 65001
  • B. Add neighbor 10.2.4.2 ebgp-multihop 2
  • C. Add bgp asnotation dot
  • D. Change R2's own AS number to 65001
Answer: A

The 2 bytes in the message are the AS number the peer put in its OPEN: hex FDE9 is 65001 (FDE8 is 65000). R2 expects 65010, so R2's remote-as is wrong. The TCP connection and OPEN exchange worked, so TTL is not the problem (B). asdot only changes how numbers are displayed (C). R2's own AS is not what the peer is complaining about; R2 sent this notification about the peer (D).

4. Blueprint 1.11.b

R1 has neighbor 10.1.2.2 timers 30 90. R2 uses the IOS defaults. Which two statements are true once the session is up? (Choose two.)

  • A. The session does not form because the timers do not match
  • B. R2 sends keepalives every 60 seconds and R1 every 30 seconds, with hold times of 180 and 90
  • C. R2 sends keepalives every 30 seconds
  • D. Both routers use a hold time of 180 seconds
  • E. Both routers use a hold time of 90 seconds
Answer: C, E

The hold time is negotiated in the OPEN and both sides use the lower value, 90 seconds. The keepalive interval follows the negotiated hold time at one third, so R2 also sends every 30 seconds. Timer mismatches never prevent a session (A). D uses the higher value, and B describes no negotiation at all.

5. Blueprint 1.11.a

Refer to the exhibit. The IPv6 session to 2001:db8:12::2 never comes up. What is missing?

router bgp 65010
 bgp router-id 10.255.255.1
 no bgp default ipv4-unicast
 neighbor 2001:db8:12::2 remote-as 65020
 !
 address-family ipv6 unicast
  network 2001:db8:10::/48
  • A. ipv6 unicast-routing is the only possible cause
  • B. neighbor 2001:db8:12::2 activate under address-family ipv6 unicast
  • C. neighbor 2001:db8:12::2 update-source Loopback0
  • D. A route-map that sets the IPv6 next hop
Answer: B

With no bgp default ipv4-unicast, the neighbor is not activated in IPv4, and it has not been activated in IPv6 either, so it has no address family to negotiate and stays down. Activating it under the IPv6 family fixes it. Missing ipv6 unicast-routing would also cause problems, but nothing in the exhibit shows it is missing, and "the only possible cause" is wrong (A). The neighbor is the directly connected link address, so update-source is not needed (C). The next-hop route-map is only needed when IPv6 prefixes ride an IPv4 session (D).

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