350-401 BGP Practice Questions

Question 1

An enterprise network has two routers, R1 and R2, both running BGP. R1 is an eBGP speaker with ISP1, and R2 is an eBGP speaker with ISP2. Both routers are in the same AS 65000. The engineer wants to ensure that traffic from the enterprise to the Internet prefers the path through ISP1 when both links are up. R1 learns a default route from ISP1, and R2 learns a default route from ISP2. Which BGP attribute should the engineer modify on R1 to influence outbound traffic selection?

Accepted Answer

Set a higher local preference on R1 for the default route learned from ISP1.. Local preference is the BGP attribute used to influence outbound traffic from an AS. It is propagated within the AS and a higher value is preferred. By setting a higher local preference on R1 for the default route learned from ISP1, R1 will prefer that route over the default route from ISP2, ensuring traffic from the enterprise to the Internet exits via ISP1.

Answer

Set a lower MED on R1 for the default route learned from ISP1.

Answer

Prepend AS 65000 multiple times on R2's updates to ISP2.

Answer

Configure a community on R1 to mark the default route as no-export.

Question 2

A network engineer is troubleshooting BGP peering between two routers in different autonomous systems. The peering is established over a directly connected Ethernet link. The engineer notices that the BGP session is flapping every few minutes. The configuration on both routers appears correct, and the IP connectivity is stable. The engineer checks the BGP logs and sees messages like 'BGP-3-NOTIFICATION: sent 4/0 (Hold Timer Expired)'. What is the most likely cause of this issue?

Accepted Answer

The hold timer values are mismatched between the two routers.. The BGP notification 'Hold Timer Expired' indicates that the router did not receive keepalive or update messages within the configured hold time. When hold timers are mismatched between two eBGP peers, the router with the shorter hold timer will expire first, causing the session to reset. Since the IP connectivity is stable and the configuration appears correct, a mismatch in hold timer values is the most direct cause of the flapping session.

Answer

The MTU on the link is mismatched, causing BGP packets to be fragmented.

Answer

The TTL for eBGP is set to 1, and the routers are not directly connected.

Answer

The BGP update timer is set too high, causing delays in sending updates.

Question 3

A network engineer is configuring BGP on a Cisco router that is part of an enterprise network with multiple BGP peers. The router receives routes from two different ISPs. The engineer wants to ensure that only specific prefixes from ISP-A are installed in the routing table, while all other routes from ISP-A are ignored. Additionally, the engineer wants to accept all routes from ISP-B. Which BGP feature should be used on the router for the peering with ISP-A?

Accepted Answer

Apply a distribute list under the BGP neighbor configuration for ISP-A.. A distribute list applied under the BGP neighbor configuration for ISP-A allows the engineer to filter specific prefixes using an access list or prefix list, ensuring only the desired prefixes are installed in the routing table while all others from ISP-A are ignored. This is the correct tool for inbound route filtering on a per-neighbor basis, as it directly controls which routes are accepted into the BGP table and subsequently the routing table.

Answer

Configure a network statement under BGP for the desired prefixes.

Answer

Use the default-information originate command under BGP.

Answer

Apply a route map to the neighbor using the route-map command in the inbound direction.

Question 4

An enterprise has two BGP routers, R1 and R2, both in AS 65000. R1 peers with ISP1 (AS 100) and R2 peers with ISP2 (AS 200). The enterprise advertises a prefix 192.168.0.0/24 to both ISPs. The engineer wants to ensure that traffic from the Internet to this prefix enters the network primarily via R1, and only uses R2 if the link to ISP1 fails. Which BGP attribute should be manipulated on the updates sent to the ISPs?

Accepted Answer

Prepend AS 65000 multiple times on R2's updates to ISP2.. To influence inbound traffic from the Internet, you must manipulate attributes sent to the ISPs. AS path prepending makes a route appear less preferred by artificially lengthening the AS path. By prepending AS 65000 multiple times on R2's updates to ISP2, ISP2 will see a longer AS path for the prefix and prefer the shorter path via ISP1, causing traffic to enter primarily via R1 unless the ISP1 link fails.

Answer

Set a higher MED on R1's updates to ISP1.

Answer

Set a higher local preference on R1 for routes learned from ISP1.

Answer

Use the no-export community on R1's updates to ISP1.

Question 5

A network engineer is configuring BGP on a Cisco router that connects to two ISPs. The router has a default route pointing to each ISP. The engineer wants to load balance outbound traffic across both ISPs. The router receives a default route from both ISPs. Which BGP configuration approach will allow the router to install both default routes in the routing table and load balance traffic?

Accepted Answer

Configure the maximum-paths command under the BGP address family and use the bgp bestpath as-path multipath-relax command.. Option A is correct because the `maximum-paths` command under the BGP address family enables the router to install multiple paths for the same prefix (in this case, the default route 0.0.0.0/0) into the routing table. The `bgp bestpath as-path multipath-relax` command is necessary because the two default routes from different ISPs will have different AS_PATH lengths; this command relaxes the requirement for equal AS_PATH length, allowing the router to consider them as multipath candidates. Together, they allow both default routes to be installed and used for load balancing outbound traffic.

Answer

Configure the network command to advertise the default route from both ISPs.

Answer

Set the local preference to the same value on both default routes.

Answer

Use the redistribute command to redistribute the default routes into BGP.

Question 6

An engineer is configuring BGP on a router that will act as a route reflector to reduce iBGP peering requirements. The router has several iBGP peers. The engineer wants to ensure that the route reflector does not modify the next-hop attribute of routes it reflects to its clients. Which configuration command should the engineer use?

Accepted Answer

Configure 'neighbor next-hop-unchanged' under the BGP address family for the route reflector clients.. Option A is correct because the 'neighbor next-hop-unchanged' command under the BGP address family instructs the route reflector to preserve the original next-hop attribute when reflecting routes to its clients. By default, a route reflector may modify the next-hop to its own address, but this command overrides that behavior, ensuring the next-hop remains as received from the non-client iBGP peer. This is essential in designs where clients must see the original next-hop for optimal path selection or to avoid unnecessary routing hops.

Answer

Configure 'no bgp next-hop-self' under the BGP address family for the route reflector clients.

Answer

Configure 'bgp route-reflector' under the BGP address family.

Answer

Configure 'neighbor next-hop-self' on the route reflector for its clients.

Question 7

A network engineer is troubleshooting a BGP issue where a router is not installing a specific prefix in its routing table, even though the prefix is present in the BGP table. The engineer runs 'show ip bgp 10.0.0.0/24' and sees that the route is valid but not best. The BGP table shows that the route has a higher local preference than the current best path, but the AS_PATH is longer. What is the most likely reason the route is not being selected as best?

Accepted Answer

The route with higher local preference has a lower weight than the current best path.. BGP selects the best path based on a sequence of comparison steps. Local preference is checked before AS_PATH length, so a higher local preference should normally win. However, weight is the very first criterion in the BGP best-path selection algorithm. If the current best path has a higher weight than the route with higher local preference, weight overrides local preference, making the higher-local-preference route not best.

Answer

The route with higher local preference has a higher MED value.

Answer

The route with higher local preference is not synchronized with IGP.

Answer

The route with higher local preference was learned from an eBGP peer, while the current best path is from an iBGP peer.

Question 8

An enterprise network uses BGP to connect to two ISPs. The router has a BGP configuration that includes the command 'bgp bestpath med missing-as-worst'. The engineer notices that routes from ISP-A that do not have the MED attribute are being preferred over routes from ISP-B that have a MED of 50. What is the effect of the 'bgp bestpath med missing-as-worst' command?

Accepted Answer

It causes routes without the MED attribute to be treated as having the highest MED value, making them less preferred.. The 'bgp bestpath med missing-as-worst' command instructs the router to treat routes that lack the MED (Multi-Exit Discriminator) attribute as if they have the highest possible MED value (effectively infinity). Since a higher MED value is less preferred in BGP path selection, routes without MED become the least preferred among paths from the same AS. In this scenario, routes from ISP-A missing MED are treated as having a very high MED, making them less preferred than routes from ISP-B with a MED of 50, which contradicts the observed behavior, but the command's intended effect is to make missing-MED routes less preferred.

Answer

It causes routes without the MED attribute to be treated as having a MED value of 0, making them more preferred.

Answer

It causes routes with the MED attribute to be ignored during path selection.

Answer

It causes the router to add a MED value of 0 to all routes that are missing MED.

Question 9

A network engineer is configuring BGP on a router that will be used for BGP route summarization. The router receives multiple more-specific prefixes from its eBGP peers. The engineer wants to advertise a summary route to the iBGP peers without advertising the more-specific routes. Which command should the engineer use to suppress the more-specific routes while still installing them in the local routing table?

Accepted Answer

Use the 'aggregate-address' command with the 'summary-only' keyword.. The 'aggregate-address' command with the 'summary-only' keyword in BGP creates a summary route from more-specific prefixes and suppresses the advertisement of those more-specific routes to BGP peers, while still keeping them in the local routing table. This meets the requirement of advertising only the summary to iBGP peers without removing the more-specific routes from the router's own RIB.

Answer

Use the 'network' command to advertise the summary route.

Answer

Use the 'summary-address' command under the BGP address family.

Answer

Use the 'redistribute' command to inject the summary route into BGP.

Question 10

A network engineer runs the following command on Router R1:

R1# show bgp summary
BGP router identifier 10.0.0.1, local AS number 65001
BGP table version is 14, main routing table version 14
4 network entries using 1152 bytes of memory
4 path entries using 320 bytes of memory
2/1 BGP path/bestpath attribute entries using 560 bytes of memory
0 BGP route reflector client to client reflections
2 BGP community entries using 80 bytes of memory

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.0.1.2        4        65002    2345    2346       14    0    0 00:12:34        3
10.0.1.3        4        65003    1234    1235       14    0    0 00:08:21        2
10.0.1.4        4        65004     567     568       14    0    0 00:05:45        0

Based on this output, what can be concluded?

Accepted Answer

Neighbor 10.0.1.4 is not advertising any prefixes to R1.. The 'State/PfxRcd' column shows the number of prefixes received from each neighbor. Neighbor 10.0.1.4 has a value of 0, meaning it has not advertised any prefixes to R1. The session is established (Up/Down shows 00:05:45), so the neighbor is reachable and the BGP session is up, but it is not sending any prefixes.

Answer

Neighbor 10.0.1.4 is in the 'Active' state because it received 0 prefixes.

Answer

Neighbor 10.0.1.4 has sent 0 prefixes to R1.

Answer

The BGP session with 10.0.1.4 is down.

BGP

350-401 BGP — Practice Questions

What is the default value of the BGP 'weight' attribute for routes learned from a neighbor?

Which BGP attribute is used to indicate the degree of preference for a route within an AS and is propagated to all iBGP peers?

In BGP best path selection, which of the following is compared first?

Drag and drop the steps of BGP session establishment between eBGP peers into the correct order, from first to last.

Drag and drop the steps of BGP best path selection process into the correct order, from first to last.

Drag and drop the steps of configuring an iBGP route reflector cluster into the correct order, from first to last.

All 350-401 Objectives

BGP