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ENARSI Question Bank, Part 1 (Layer 3 and VPN)

55 original questions covering ENARSI 300-410 v1.1 domains 1.0 Layer 3 Technologies and 2.0 VPN Technologies. ENARSI is a troubleshooting exam, so most questions give you an exhibit of show output, debug output or configuration and ask you to find the fault or predict what the router will do. Read every line of the exhibit before you look at the options. Work through all of them before opening any explanation. Target: 44 of 55 (80%).

Exam trap

On ENARSI the exhibit usually contains one detail that matters and several that look suspicious but are perfectly valid. Practise saying why each "odd" line is or is not the fault.

1.0 Layer 3 Technologies

1. Blueprint 1.1

Refer to the exhibit. R1 learns 10.50.0.0/24 from EIGRP and OSPF and also has a static route for it. The EIGRP neighbor 10.1.12.2 fails, but the OSPF neighbor stays up and still advertises the prefix. Which route does R1 install?

ip route 10.50.0.0 255.255.255.0 10.1.14.4 120
!
router eigrp 100
 network 10.1.12.0 0.0.0.3
!
router ospf 1
 network 10.1.13.0 0.0.0.3 area 0

R1# show ip route 10.50.0.0
Routing entry for 10.50.0.0/24
  Known via "eigrp 100", distance 90, metric 3072, type internal
  Last update from 10.1.12.2 on GigabitEthernet0/1, 00:41:10 ago
  Routing Descriptor Blocks:
  * 10.1.12.2, from 10.1.12.2, 00:41:10 ago, via GigabitEthernet0/1
  • A. The static route through 10.1.14.4, because static routes always win
  • B. The static route, because 120 is lower than the EIGRP external distance of 170
  • C. The OSPF route through 10.1.13.3, with administrative distance 110
  • D. No route, until the EIGRP neighbor recovers
Answer: C

The static route has been configured as a floating static with administrative distance 120. With EIGRP gone, the remaining candidates are OSPF (110) and the static (120), so OSPF wins. The static route is only installed if both EIGRP and OSPF lose the prefix. A ignores the configured distance. B compares against EIGRP external, but the EIGRP route was internal and it no longer exists anyway.

2. Blueprint 1.1

Match each route source to its default administrative distance on a Cisco IOS XE router.

Route source Default AD
EIGRP summary route ?
eBGP ?
EIGRP internal ?
OSPF ?
IS-IS ?
RIP ?
EIGRP external ?
iBGP ?
Answer

EIGRP summary: 5. eBGP: 20. EIGRP internal: 90. OSPF: 110. IS-IS: 115. RIP: 120. EIGRP external: 170. iBGP: 200. Connected is 0 and static is 1. A common exam trap is assuming iBGP is trusted more than an IGP; at 200 it is trusted less than almost everything.

3. Blueprint 1.2

Refer to the exhibit. The engineer wants every OSPF route except those in 10.10.0.0/16 to be redistributed into EIGRP. After the change, R2's EIGRP neighbors receive no external routes at all. What is the cause?

ip prefix-list LAB seq 5 permit 10.10.0.0/16 le 24
!
route-map OSPF-TO-EIGRP deny 10
 match ip address prefix-list LAB
!
router eigrp 100
 network 10.2.0.0 0.0.255.255
 redistribute ospf 1 metric 10000 100 255 1 1500 route-map OSPF-TO-EIGRP
  • A. The prefix-list should use deny instead of permit
  • B. The route map has no permit sequence, so every route falls through to the implicit deny
  • C. A seed metric cannot be combined with a route map on the redistribute command
  • D. The le 24 keyword stops the prefix-list from matching anything
Answer: B

A route map ends with an implicit deny. Sequence 10 denies the LAB prefixes, and every other route matches no sequence and is denied as well. Adding route-map OSPF-TO-EIGRP permit 20 with no match statement fixes it. In a route map, the prefix-list permit only means "matches", and the route map sequence decides whether the route is permitted or denied, so A would break the logic. C is false; the metric and the route map work together.

4. Blueprint 1.2

Which two prefixes are matched by ip prefix-list NETS seq 5 permit 172.16.0.0/16 ge 20 le 24? (Choose two.)

  • A. 172.16.32.0/20
  • B. 172.16.0.0/16
  • C. 172.16.5.0/24
  • D. 172.16.10.128/25
  • E. 172.17.4.0/22
Answer: A, C

The first 16 bits must match 172.16, and the prefix length must be from 20 to 24 inclusive. 172.16.32.0/20 and 172.16.5.0/24 both qualify. B has length 16, which is below ge 20. D has length 25, which is above le 24. E is in 172.17, so the first 16 bits do not match.

5. Blueprint 1.2

Refer to the exhibit. The route map is applied to redistribution. Which route is redistributed with metric 50?

ip prefix-list NETS seq 5 permit 10.2.2.0/24
ip prefix-list NETS seq 10 permit 10.3.3.0/24
!
route-map RM permit 10
 match tag 100 200
 match ip address prefix-list NETS
 set metric 50
route-map RM permit 20
  • A. 10.1.1.0/24 with tag 100
  • B. 10.3.3.0/24 with no tag
  • C. All three, because sequence 20 permits everything
  • D. 10.2.2.0/24 with tag 200
Answer: D

Values listed on the same match line are ORed (tag 100 or tag 200). Different match lines are ANDed. Only 10.2.2.0/24 carries an accepted tag and is in the prefix-list. A has the right tag but is not in NETS. B is in NETS but has no tag. Sequence 20 does permit the other routes, so they are still redistributed, but with the default seed metric, not metric 50.

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