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PBR, VRF-Lite and BFD Practice Questions

15 original questions covering ENARSI 1.6 to 1.8. Most are exhibit-based, as on the real exam: find the fault from the output. Answer every one before opening any explanation. Target: 12 of 15 (80%).

1. Blueprint 1.6

Refer to the exhibit. R1 Gi0/0 faces the LAN. Which statement describes how R1 forwards a packet from 10.4.10.25 to 10.255.255.5?

ip access-list extended STAFF
 permit ip 10.4.10.0 0.0.0.255 any
ip access-list extended ALL-LAN
 permit ip 10.4.0.0 0.0.255.255 any
!
route-map LAN-PBR deny 10
 match ip address STAFF
route-map LAN-PBR permit 20
 match ip address ALL-LAN
 set ip next-hop 10.1.3.2
!
interface GigabitEthernet0/0
 ip policy route-map LAN-PBR
  • A. It is dropped, because it matches a deny sequence
  • B. It is forwarded using the normal routing table
  • C. It is forwarded to 10.1.3.2, because it also matches sequence 20
  • D. It is dropped by the implicit deny at the end of the route map
Answer: B

The packet matches sequence 10, which is a deny. In PBR a deny result means "do not policy route", so the packet is routed normally. Processing stops at the first matching sequence, so sequence 20 is never evaluated for it (C). PBR never drops a packet because of a deny or the implicit deny (A, D); to drop traffic you need set interface Null0 in a permit sequence.

2. Blueprint 1.6

Refer to the exhibit. Guest traffic to 172.16.40.10 should be sent to 10.1.3.2. It still goes to 10.1.2.2. Why?

R1# show running-config | section route-map
route-map GUEST permit 10
 match ip address GUEST-SRC
 set ip default next-hop 10.1.3.2

R1# show ip route 172.16.40.10
Routing entry for 172.16.0.0/16
  Known via "ospf 1", distance 110, metric 3, type intra area
  Last update from 10.1.2.2 on GigabitEthernet0/1, 00:12:44 ago
  Routing Descriptor Blocks:
  * 10.1.2.2, from 10.255.255.2, 00:12:44 ago, via GigabitEthernet0/1
  • A. set ip default next-hop is used only when the routing table has no explicit route, and 172.16.0.0/16 is an explicit route
  • B. The next hop 10.1.3.2 must be configured with set ip next-hop recursive
  • C. PBR cannot override OSPF routes, only static routes
  • D. The route map needs a match length clause
Answer: A

set ip default next-hop is consulted after the RIB lookup and only if the lookup finds nothing more specific than a default route. The OSPF /16 is an explicit match, so it wins. set ip next-hop 10.1.3.2 would apply before the lookup. PBR can override any route source with set ip next-hop (C). recursive is for next hops that are not directly connected (B).

3. Blueprint 1.6

Refer to the exhibit. GigabitEthernet0/2, the interface toward 10.1.3.2, goes down. What happens to packets from 10.4.20.0/24 that arrive on Gi0/0?

route-map GUEST-PBR permit 10
 match ip address GUEST-SRC
 set ip next-hop 10.1.3.2
 set interface Null0
!
interface GigabitEthernet0/0
 ip policy route-map GUEST-PBR
  • A. They are routed normally using the routing table
  • B. They are dropped
  • C. They are queued until 10.1.3.2 is reachable
  • D. They are sent out every interface in the routing table to the destination
Answer: B

When several set clauses are present, set ip next-hop is tried first and set interface second. With the next hop unreachable, the router uses the next usable clause, which is Null0, so the guest traffic is discarded. This is a common way to make guest traffic "fail closed" instead of falling back to the corporate path. Without the Null0 clause, the answer would be A.

4. Blueprint 1.6

Refer to the exhibit. R3 at 10.1.3.2 is up and answers pings from R1. Guest users are nevertheless routed through R2. What is the most likely cause?

R1# show route-map GUEST-PBR
route-map GUEST-PBR, permit, sequence 10
  Match clauses:
    ip address (access-lists): GUEST-SRC
  Set clauses:
    ip next-hop verify-availability 10.1.3.2 10 track 10  [down]
  Policy routing matches: 0 packets, 0 bytes

R1# show track 10
Track 10
  IP SLA 10 reachability
  Reachability is Down
    1 change, last change 01:02:17
  Latest operation return code: Unknown
  • A. The ACL GUEST-SRC matches the wrong subnet
  • B. 10.1.3.2 is not directly connected
  • C. The policy is applied to the wrong interface
  • D. IP SLA operation 10 is not scheduled
Answer: D

The track is down and the IP SLA return code is "Unknown", which means the operation has never run. An IP SLA operation needs ip sla schedule 10 life forever start-time now. While the tracked next hop is down, PBR falls back to normal routing, which sends guest traffic through R2. The ACL and the interface (A, C) would affect the match counter, but the dominant clue is the track state with no SLA results. A directly connected next hop that fails would give a return code of Timeout, not Unknown (B).

5. Blueprint 1.6

An engineer configures PBR on R1 to send traffic from 10.4.20.0/24 to 10.1.3.2 and applies it with ip policy route-map GUEST-PBR on Gi0/0, the LAN interface. To test, the engineer runs traceroute 10.255.255.5 source GigabitEthernet0/0 on R1, and the path uses 10.1.2.2. Which two statements are true? (Choose two.)

  • A. The test does not prove anything, because router-generated traffic is not processed by an interface policy
  • B. PBR must be applied outbound on the WAN interface
  • C. Testing from a host or router in 10.4.20.0/24 would show whether the policy works
  • D. ip local policy route-map GUEST-PBR is needed for guest hosts' traffic to be policy routed
  • E. The Gi0/0 address is in 10.4.20.0/24, so the policy must have failed
Answer: A, C

The traceroute is generated by R1 itself, so it does not arrive on Gi0/0 and the interface policy never sees it, whatever its source address (E). The test has to come from behind Gi0/0. ip local policy (D) is only for router-generated traffic, not for transit traffic from hosts. PBR is always applied inbound (B).

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