300-410 Route Maps and Route Filtering — 10 Questions

Question 1

A network engineer is troubleshooting a BGP route filtering issue. Router R1 is advertising a prefix 10.1.1.0/24 to its eBGP neighbor R2, but R2 is not receiving it. The engineer checks R1's BGP configuration and sees a route-map named FILTER-OUT applied outbound to the neighbor. The route-map references an ACL that permits 10.1.1.0/24, but the prefix is still not being sent. What is the most likely cause?

Accepted Answer

The route-map is missing a 'permit' statement; the default action is deny.. The route-map must have an explicit permit statement; if the route-map is missing the permit clause, or if the sequence number is incorrect, the implicit deny at the end of the route-map will filter all routes. The ACL permitting the prefix is necessary but not sufficient if the route-map itself does not have a permit action.

Answer

The ACL is using the wrong wildcard mask; it should be 0.0.0.255 instead of 0.0.0.0.

Answer

The neighbor is configured with 'soft-reconfiguration inbound' which blocks outbound updates.

Answer

The route-map is applied inbound instead of outbound on R1.

Question 2

A network engineer is troubleshooting a redistribution issue between OSPF and EIGRP. Router R3 is redistributing OSPF routes into EIGRP, but some OSPF external routes are not appearing in the EIGRP topology table. The engineer checks the redistribute command under EIGRP and sees a route-map named RM-OSPF that uses a prefix-list to match specific prefixes. The missing routes are permitted by the prefix-list. What is the most likely cause?

Accepted Answer

The route-map is missing a 'set metric' command; EIGRP requires a metric for redistributed routes.. The route-map may have a 'set metric' command that is misconfigured, or the route-map may be missing the 'set metric' command entirely, causing EIGRP to reject the route because it requires a metric for redistributed routes. Alternatively, the route-map might have a 'match route-type' that excludes external type-2 routes.

Answer

The prefix-list is using the wrong sequence number and is being overridden by a later deny statement.

Answer

The OSPF routes are type-5 LSAs, which cannot be redistributed into EIGRP.

Answer

The route-map is applied to the OSPF process instead of the EIGRP process.

Question 3

A network engineer is troubleshooting a PBR (Policy-Based Routing) issue on router R5. The engineer configured a route-map to set the next-hop for traffic from a specific source subnet. The route-map is applied to the incoming interface, but traffic from the source subnet is still being forwarded using the regular routing table. The engineer verifies that the ACL matches the traffic correctly. What is the most likely cause?

Accepted Answer

The route-map is missing a 'set ip next-hop' command, or the next-hop is not reachable.. PBR route-maps must have at least one 'match' and one 'set' statement. If the route-map has a 'match ip address' but no 'set ip next-hop' (or the next-hop is unreachable), PBR will not apply and traffic will use the routing table. Also, the route-map must be applied to the correct interface and direction.

Answer

The route-map is applied outbound instead of inbound on the interface.

Answer

The ACL is using a standard ACL, which cannot match source subnet correctly.

Answer

The route-map has a 'set default interface' command that overrides the next-hop.

Question 4

A network engineer is troubleshooting a route filtering problem with prefix-lists. Router R6 is using a prefix-list to filter routes from a BGP neighbor. The prefix-list is configured to permit only 192.168.0.0/16 and 192.168.1.0/24, but routes with prefix 192.168.2.0/24 are also being accepted. The engineer checks the prefix-list configuration and sees only two permit statements. What is the most likely cause?

Accepted Answer

The prefix-list is not applied to the BGP neighbor; the neighbor is using a different filter or no filter.. Prefix-lists have an implicit deny at the end, but if the prefix-list is not applied correctly (e.g., to the neighbor inbound direction), or if the prefix-list has a 'ge' or 'le' operator that is too broad, it could permit more than intended. However, the most common issue is that the prefix-list is not applied at all, or the neighbor is using a different filter.

Answer

The prefix-list has an implicit permit at the end for all routes.

Answer

The prefix-list is using 'ge 24' which permits any prefix with a mask >= 24, including 192.168.2.0/24.

Answer

The BGP neighbor is configured with 'soft-reconfiguration inbound' which overrides prefix-list filtering.

Question 5

A network engineer is troubleshooting a redistribution loop between OSPF and EIGRP. Router R7 is redistributing EIGRP routes into OSPF, and also redistributing OSPF routes into EIGRP. The engineer notices that some OSPF routes are appearing in the EIGRP topology table with a higher metric than expected, causing suboptimal routing. What is the most likely cause?

Accepted Answer

Routes are being re-redistributed due to missing route tags; a route-map with 'set tag' should be used to prevent loops.. Redistribution loops can cause routes to be re-injected with different metrics. The most common fix is to use route-maps with tags to prevent re-redistribution. Without proper tagging, routes can loop between the two protocols.

Answer

The OSPF process has a 'default-information originate' command that is injecting a default route into EIGRP.

Answer

The EIGRP process has a 'variance' command that is causing unequal-cost load balancing.

Answer

The route-map applied to redistribution is missing a 'match tag' statement to filter out redistributed routes.

Question 6

A network engineer is troubleshooting a BGP route-map that is supposed to set a community value on routes from a specific neighbor. The engineer configures a route-map with 'set community 100:100' and applies it inbound to the neighbor. After the configuration, the engineer checks the BGP table on the local router and sees that the routes do not have the community set. What is the most likely cause?

Accepted Answer

The neighbor is missing the 'send-community' command under the BGP neighbor configuration.. By default, BGP does not send or receive communities unless the 'neighbor send-community' command is configured. Even if the route-map sets the community, the router will not attach it to the routes unless the neighbor is configured to send communities.

Answer

The route-map is applied outbound instead of inbound.

Answer

The community value 100:100 is reserved and cannot be used.

Answer

The route-map has a 'match community' statement that is filtering the routes before the set command.

Question 7

A network engineer is troubleshooting a route filtering issue using distribute-lists under OSPF. Router R8 has a distribute-list out applied to the OSPF process to filter routes being advertised to a specific neighbor. The distribute-list references an ACL that denies a specific prefix, but the prefix is still being advertised to the neighbor. What is the most likely cause?

Accepted Answer

The distribute-list is applied to the OSPF process, not to the specific neighbor; use 'neighbor distribute-list' instead.. Distribute-lists under OSPF can be applied in or out, but the 'out' direction filters routes being advertised out of the OSPF process to all neighbors. However, if the distribute-list is applied to the OSPF process (not to a specific interface), it filters routes from the routing table into OSPF, not to a specific neighbor. To filter per neighbor, a distribute-list must be applied to the neighbor under the OSPF process using 'neighbor x.x.x.x distribute-list'.

Answer

The ACL is using the wrong wildcard mask; it should deny the exact prefix with 0.0.0.0 mask.

Answer

The distribute-list is applied inbound instead of outbound.

Answer

The OSPF process has 'default-information originate' that overrides the distribute-list.

Question 8

A network engineer is troubleshooting a PBR route-map that is supposed to set the next-hop for traffic from a specific source to a different next-hop. The route-map is applied to the incoming interface, and the ACL matches the traffic. However, the engineer notices that the traffic is still being forwarded using the routing table. The engineer checks the route-map and sees that the 'set ip next-hop' command points to an IP address that is not reachable. What is the most likely result?

Accepted Answer

PBR will fall back to the routing table and forward the packet normally.. If the next-hop in a PBR set command is not reachable (i.e., not in the routing table), PBR will fall back to the routing table for forwarding. PBR does not drop the packet; it uses the default routing decision.

Answer

The packet will be dropped because PBR cannot forward it.

Answer

The router will send an ICMP unreachable message to the source.

Answer

The route-map will be ignored and all traffic will be dropped.

Question 9

A network engineer is troubleshooting a BGP route-map that is supposed to prepend AS-path to routes from a specific neighbor. The engineer configures a route-map with 'set as-path prepend 65001' and applies it outbound to the neighbor. After the configuration, the engineer checks the BGP table on the neighbor and sees that the AS-path does not include the prepended AS. What is the most likely cause?

Accepted Answer

The route-map is missing a 'match' statement; without it, the set command is not applied to any routes.. The 'set as-path prepend' command must be used with 'ip as-path access-list' or in a route-map, but the route-map must be applied in the correct direction. If the route-map is applied outbound, it modifies the AS-path before sending. However, if the neighbor is an eBGP neighbor, the router will not prepend its own AS unless the AS is different from the neighbor's AS. Also, the route-map must have a 'match' statement that matches the routes.

Answer

The AS 65001 is the same as the neighbor's AS, so BGP ignores the prepend.

Answer

The route-map is applied inbound instead of outbound.

Answer

The neighbor has 'soft-reconfiguration inbound' which prevents AS-path changes.

Question 10

A network engineer runs the following command on Router R1:

R1# show route-map
route-map FILTER_OSPF, permit, sequence 10
  Match clauses:
    ip address prefix-list OSPF_ROUTES
  Set clauses:
  Policy routing matches: 0 packets, 0 bytes
route-map FILTER_OSPF, deny, sequence 20
  Match clauses:
  Set clauses:
  Policy routing matches: 0 packets, 0 bytes

Based on this output, which statement is correct?

Accepted Answer

The route-map will permit all routes that match the prefix-list OSPF_ROUTES and deny all others.. The route-map FILTER_OSPF has two sequences: sequence 10 permits routes matching prefix-list OSPF_ROUTES, and sequence 20 denies all other routes (since no match clause). The match counters show 0 packets/bytes, indicating no traffic has been matched yet, but this is a route-map used for route filtering, not policy routing. The correct answer is that the route-map will deny all routes not matching the prefix-list.

Answer

The route-map will permit all routes because sequence 20 has no match clause.

Answer

The route-map is used for policy-based routing and has matched 0 packets.

Answer

The route-map will deny only routes matching the prefix-list OSPF_ROUTES.