300-410 Administrative Distance — Timed 10 Questions

Question 1

A network engineer is troubleshooting a BGP route reachability issue. R1 learns the prefix 10.1.1.0/24 via eBGP from R2 with an AD of 20, and via OSPF from R3 with an AD of 110. The engineer notices that R1 installs the OSPF route in the routing table instead of the eBGP route, even though the eBGP route is preferred by default. What is the most likely cause of this behavior?

Accepted Answer

The distance bgp 20 200 200 command is configured under the BGP process, increasing the AD of eBGP routes to 200.. The default administrative distance for eBGP is 20, and for OSPF is 110, so eBGP should be preferred. However, if the distance command is applied to the eBGP neighbor or the BGP process, it can increase the AD of eBGP routes, making them less preferred than OSPF.

Answer

The OSPF route has a lower metric than the eBGP route.

Answer

The OSPF route is an inter-area route, which has a lower AD than intra-area routes.

Answer

The eBGP route is not the best path because the next-hop is unreachable.

Question 2

An engineer is troubleshooting a network where R1 and R2 are running EIGRP, and R2 redistributes a static route for 192.168.1.0/24 into EIGRP. R1 also learns the same prefix via OSPF from R3 with an AD of 110. The engineer observes that R1 prefers the EIGRP external route (AD 170) over the OSPF route. What configuration change would cause this behavior?

Accepted Answer

The engineer applied the distance eigrp 90 100 command under EIGRP, lowering the AD for external routes to 100.. By default, EIGRP external routes have an AD of 170, and OSPF has AD 110, so OSPF should be preferred. If the EIGRP external route is preferred, the AD must have been lowered, likely via the distance command under EIGRP.

Answer

The OSPF route is a type 5 LSA, which has a higher AD than type 3 LSAs.

Answer

The OSPF route has a metric of 20, while the EIGRP route has a metric of 2560.

Answer

The static route was redistributed with a route-map that sets the EIGRP metric to 1.

Question 3

A network engineer is troubleshooting a connectivity issue between two sites. R1 learns the prefix 10.0.0.0/8 via RIP (AD 120) from R2, and also via a directly connected interface on R3. The engineer notices that R1 uses the RIP route instead of the connected route. What is the most likely cause?

Accepted Answer

The RIP route has an AD of 120, but the connected route is for a different subnet mask.. Connected routes have an AD of 0, which is always preferred over any dynamic routing protocol. If the connected route is not used, it likely does not exist for that exact prefix, or the interface is down. However, if the prefix is learned via RIP with AD 120, the connected route should take precedence. The only way RIP is preferred is if the connected route is not present for that prefix.

Answer

The connected interface on R3 has a higher metric than the RIP route.

Answer

The distance rip 0 command was applied under RIP, making RIP routes have AD 0.

Answer

The connected interface is administratively down.

Question 4

An engineer is troubleshooting a routing loop between two routers. R1 and R2 are running both OSPF and EIGRP. R1 learns the prefix 172.16.1.0/24 via OSPF with AD 110 and via EIGRP internal with AD 90. The engineer notices that R1 installs the EIGRP route, but traffic to 172.16.1.0/24 is being dropped. What is the most likely issue?

Accepted Answer

The EIGRP route is a summary route pointing to a null0 interface.. EIGRP internal routes have AD 90, which is lower than OSPF's 110, so EIGRP is preferred. However, if the EIGRP route points to a next-hop that is not reachable or is a backup path, traffic may be dropped. The root cause could be that the EIGRP route is learned via a secondary path that is not optimal or has a next-hop issue.

Answer

The OSPF route has a better metric, but the EIGRP route is preferred due to lower AD.

Answer

The OSPF route has a higher AD because it is a type 5 LSA.

Answer

The EIGRP route is an external route with AD 170.

Question 5

A network engineer is troubleshooting a multi-homed BGP setup. R1 receives the prefix 10.1.1.0/24 from two eBGP peers: R2 (AS 100) and R3 (AS 200). The engineer configures the distance bgp 20 20 20 command on R1 to make all BGP routes have the same AD. However, R1 still prefers the route from R2 over R3. What is the most likely reason?

Accepted Answer

The route from R2 has a higher local preference than the route from R3.. The distance bgp command sets AD for eBGP, iBGP, and local routes. With AD equal, the router uses other BGP path attributes, such as local preference, AS path length, or MED. The most common tie-breaker is local preference (default 100) or the oldest route.

Answer

The route from R2 has a lower MED than the route from R3.

Answer

The route from R2 is the oldest BGP route.

Answer

The AS path for the route from R2 is shorter than that from R3.

Question 6

An engineer is troubleshooting a route redistribution issue between OSPF and EIGRP. R1 runs both protocols and redistributes OSPF into EIGRP. The engineer notices that OSPF routes redistributed into EIGRP have an AD of 170, but some routes from OSPF are not being redistributed. What is the most likely cause?

Accepted Answer

The OSPF routes are not in the routing table because they are overridden by a static route with AD 1.. Redistribution into EIGRP requires matching routes; by default, only routes in the routing table are redistributed. If OSPF routes are not in the routing table because they are overridden by another protocol with lower AD, they will not be redistributed.

Answer

The OSPF routes have a higher metric than the EIGRP routes.

Answer

The redistribute ospf 1 metric 10000 command is missing.

Answer

The OSPF routes are external type 2, which are not redistributed by default.

Question 7

A network engineer is troubleshooting a situation where R1 has two routes to 10.0.0.0/8: one via OSPF (AD 110) and one via RIP (AD 120). The engineer wants R1 to prefer the RIP route. After configuring the distance 80 0.0.0.0 255.255.255.255 under the RIP process, the RIP route is still not preferred. What is the most likely reason?

Accepted Answer

The OSPF process has the distance ospf external 70 command configured, lowering the AD of OSPF external routes to 70.. The distance command under RIP can set AD for specific routes. However, if the OSPF route has a lower AD than the new RIP AD, it will still be preferred. The engineer set RIP AD to 80, but OSPF AD is 110, so RIP should be preferred. If it is not, the OSPF AD may have been lowered or the distance command was applied incorrectly.

Answer

The OSPF route has an AD of 110, but the RIP route's AD was set to 80, so RIP should be preferred. The issue is that the distance command was applied to the wrong prefix.

Answer

The RIP route is not in the routing table because it is suppressed by a distribute-list.

Answer

The distance command under RIP only affects internal RIP routes, not redistributed routes.

Question 8

An engineer is troubleshooting a network where R1 and R2 are running iBGP, and R1 learns the prefix 192.168.1.0/24 from R2 with an AD of 200. R1 also learns the same prefix via OSPF from R3 with AD 110. The engineer notices that R1 uses the iBGP route. What configuration change would cause this?

Accepted Answer

The engineer applied the distance 150 0.0.0.0 255.255.255.255 under the OSPF process, raising OSPF AD to 150.. iBGP routes have an AD of 200 by default, which is higher than OSPF's 110, so OSPF should be preferred. If iBGP is preferred, the AD of iBGP must have been lowered or the OSPF AD raised.

Answer

The engineer applied the distance bgp 20 200 200 command, which lowers eBGP AD but not iBGP.

Answer

The OSPF route is a type 5 LSA, which has a higher AD than type 3 LSAs.

Question 9

A network engineer is troubleshooting a route selection issue between two routing protocols. R1 learns the prefix 10.0.0.0/8 via both IS-IS (AD 115) and OSPF (AD 110). The engineer wants R1 to prefer the IS-IS route. After configuring the distance 105 ip 10.0.0.0 0.255.255.255 under the IS-IS process, the IS-IS route is still not preferred. What is the most likely cause?

Accepted Answer

The IS-IS route is a level 2 route, which has a default AD of 115, but the distance command only affects level 1 routes.. The distance command under IS-IS can set AD for specific routes. With the new AD of 105, IS-IS should be preferred over OSPF (AD 110). If it is not, either the command was not applied correctly (e.g., wrong ACL) or the OSPF AD was also lowered.

Answer

The OSPF route has a lower metric than the IS-IS route.

Answer

The distance command was applied under the OSPF process instead of IS-IS.

Answer

The IS-IS route is redistributed from another protocol, so it has a higher default AD of 115.

Question 10

A network engineer runs the following command on Router R1:

R1# show ip route 10.1.1.0
Routing entry for 10.1.1.0/24
  Known via "eigrp 100", distance 170, metric 30720
  Redistributing via eigrp 100
  Last update from 192.168.1.2 on GigabitEthernet0/0, 00:00:05 ago
  Routing Descriptor Blocks:
  * 192.168.1.2, from 192.168.1.2, 00:00:05 ago, via GigabitEthernet0/0
      Route metric is 30720, traffic share count is 1

Based on this output, which statement is correct?

Accepted Answer

The route 10.1.1.0/24 is an EIGRP external route, likely redistributed.. The output shows an EIGRP route with an administrative distance of 170, which is the default distance for EIGRP external routes. This indicates the route was redistributed into EIGRP from another protocol or process. The metric of 30720 is the default for a redistributed connected route.

Answer

The route 10.1.1.0/24 is an EIGRP internal route.

Answer

The route 10.1.1.0/24 is learned via OSPF and redistributed into EIGRP.

Answer

The administrative distance of 170 is non-default and must have been manually configured.