350-401 BGP — 20 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.

Question 11

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast 192.168.1.0
BGP routing table entry for 192.168.1.0/24, version 10
Paths: (2 available, best #2, table default)
  Advertised to update-groups:
     1
  Refresh Epoch 1
  Local
    10.0.0.2 from 10.0.1.2 (10.0.0.2)
      Origin IGP, metric 0, localpref 100, valid, internal
      rx pathid: 0, tx pathid: 0x0
  Local, (received & used)
    10.0.0.3 from 10.0.1.3 (10.0.0.3)
      Origin IGP, metric 0, localpref 200, valid, internal, best
      rx pathid: 0, tx pathid: 0x0

Based on this output, what can be concluded?

Accepted Answer

The path from 10.0.1.3 is preferred because of a higher local preference.. The output shows two BGP paths for 192.168.1.0/24, both with Origin IGP and metric 0. The path from 10.0.1.3 has a local preference of 200, while the path from 10.0.1.2 has a local preference of 100. BGP selects the path with the highest local preference as the best path, making the path from 10.0.1.3 the best (marked as 'best' in the output).

Answer

The path from 10.0.1.2 is the best path because it is received first.

Answer

Both paths are from external BGP peers.

Answer

The path from 10.0.1.2 is marked as best because it has a lower metric.

Question 12

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast summary | include 10.0.1.5
10.0.1.5        4        65005    3456    3457       15    0    0 00:15:22        5

Based on this output, what can be concluded?

Accepted Answer

The BGP session with 10.0.1.5 is up and has received 5 prefixes.. The 'show bgp ipv4 unicast summary' output displays BGP neighbor status. The column 'Up/Down' shows '00:15:22', indicating the session has been established for 15 minutes and 22 seconds, not down. The last column shows '5', which under the 'PfxRcd' (Prefixes Received) column indicates the number of prefixes received from the neighbor. Therefore, the session is up and has received 5 prefixes.

Answer

The BGP session with 10.0.1.5 has been down for 15 minutes and 22 seconds.

Answer

The BGP session with 10.0.1.5 is in the 'Active' state.

Answer

The BGP session with 10.0.1.5 has sent 5 prefixes.

Question 13

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast 10.10.10.0
BGP routing table entry for 10.10.10.0/24, version 20
Paths: (2 available, best #1, table default)
  Advertised to update-groups:
     1
  Refresh Epoch 1
  65050 65100
    10.0.1.2 from 10.0.1.2 (10.0.0.2)
      Origin IGP, metric 0, localpref 100, weight 0, valid, external, best
      rx pathid: 0, tx pathid: 0x0
  65050 65100 65200
    10.0.1.3 from 10.0.1.3 (10.0.0.3)
      Origin IGP, metric 0, localpref 100, weight 0, valid, external
      rx pathid: 0, tx pathid: 0x0

Based on this output, what can be concluded?

Accepted Answer

Path #1 is the best path because it has a shorter AS_PATH length.. B is correct because BGP selects the best path based on the shortest AS_PATH length when all other attributes (weight, local preference, origin) are equal. In the output, Path #1 has an AS_PATH of '65050 65100' (2 AS numbers) while Path #2 has '65050 65100 65200' (3 AS numbers), making Path #1 the best path. The 'best #1' annotation confirms this selection.

Answer

Path #2 is the best path because it has a longer AS_PATH, indicating more specific routing.

Answer

Both paths are equally preferred, and BGP uses tie-breaking rules like router ID.

Answer

Path #1 is the best path because it is received from a higher IP address.

Question 14

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast neighbors 10.0.1.2 received-routes
Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.1.0/24   10.0.1.2               0    100      0 65050 i
*> 192.168.2.0/24   10.0.1.2               0    100      0 65050 i
*> 10.10.10.0/24    10.0.1.2               0    100      0 65050 65100 i

Total number of prefixes 3

Based on this output, what can be concluded?

Accepted Answer

R1 has received 3 prefixes from neighbor 10.0.1.2.. The command `show bgp ipv4 unicast neighbors 10.0.1.2 received-routes` displays all BGP routes that have been received from the specified neighbor, regardless of whether they are installed in the routing table. The output shows three prefixes, confirming that R1 has received exactly three routes from neighbor 10.0.1.2. The asterisk (*) and greater-than (>) symbols indicate the route is valid and best, but this does not guarantee installation in the routing table if a better administrative distance route exists.

Answer

All three routes are installed in the routing table of R1.

Answer

R1 is advertising these three routes to neighbor 10.0.1.2.

Answer

The route 10.10.10.0/24 has a weight of 0, meaning it is not preferred.

Question 15

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast neighbors 10.0.1.2 advertised-routes
Network          Next Hop            Metric LocPrf Weight Path
*> 172.16.0.0/16   10.0.1.1               0    100      0 i
*> 172.16.1.0/24   10.0.1.1               0    100      0 i

Total number of prefixes 2

Based on this output, what can be concluded?

Accepted Answer

R1 is advertising 2 prefixes to neighbor 10.0.1.2.. The command 'show bgp ipv4 unicast neighbors 10.0.1.2 advertised-routes' specifically displays the routes that Router R1 is sending to the BGP neighbor at 10.0.1.2. The output shows two prefixes (172.16.0.0/16 and 172.16.1.0/24) with a next hop of 10.0.1.1 (R1's own interface), confirming these are routes R1 is advertising. Therefore, option B is correct.

Answer

R1 is receiving 2 prefixes from neighbor 10.0.1.2.

Answer

Neighbor 10.0.1.2 is advertising these routes to R1.

Answer

The routes are being advertised with a next hop of 10.0.1.2.

Question 16

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast 192.168.1.0/24
BGP routing table entry for 192.168.1.0/24, version 15
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     1
  Refresh Epoch 1
  65050, (received & used)
    10.0.1.2 from 10.0.1.2 (10.0.0.2)
      Origin IGP, metric 0, localpref 100, weight 0, valid, external, best
      rx pathid: 0, tx pathid: 0x0

Based on this output, what can be concluded?

Accepted Answer

The route is from AS 65050 and is the best path.. The output shows the BGP table entry for 192.168.1.0/24 with path 65050, and the line 'valid, external, best' confirms that this route is from AS 65050 and is selected as the best path. The 'best' keyword in the status flags directly indicates that this path is the best among all available paths for this prefix.

Answer

The route was learned from an internal BGP peer.

Answer

The route has a local preference of 0.

Answer

The route is not valid because it is external.

Question 17

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast neighbors 10.0.1.2
BGP neighbor is 10.0.1.2, remote AS 65050, external link
  BGP version 4, remote router ID 10.0.0.2
  BGP state = Established, up for 00:23:45
  Last read 00:00:15, last write 00:00:10, hold time is 90, keepalive interval is 30 seconds
  Neighbor sessions:
    1 active, is not multisession capable
  Neighbor capabilities:
    Route refresh: advertised and received(new)
    Four-octets ASN: advertised and received
    Address family IPv4 Unicast: advertised and received
    Enhanced Refresh: advertised and received
    Multisession: advertised
  Message statistics:
    InQ depth is 0
    OutQ depth is 0
                         Sent       Rcvd
    Opens:                  1          1
    Notifications:          0          0
    Updates:                5          3
    Keepalives:            47         48
    Route Refresh:          0          0
    Total:                 53         52
  Default minimum time between advertisement runs is 30 seconds

Based on this output, what can be concluded?

Accepted Answer

The BGP session is up and functioning normally.. Option C is correct because the BGP state is 'Established', the hold time (90 seconds) and keepalive interval (30 seconds) are standard, and the last read (15 seconds ago) and last write (10 seconds ago) are well within the hold time, indicating the session is stable and functioning normally. The InQ and OutQ depths are 0, confirming no queued updates or issues.

Answer

The BGP session is in the 'Idle' state because the last read was 15 seconds ago.

Answer

The neighbor is an internal BGP peer because the remote AS is 65050.

Answer

The neighbor has sent 5 updates and received 3 updates.

Question 18

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast 10.0.0.0/8
BGP routing table entry for 10.0.0.0/8, version 25
Paths: (2 available, best #2, table default)
  Advertised to update-groups:
     1
  Refresh Epoch 1
  65050 65100
    10.0.1.2 from 10.0.1.2 (10.0.0.2)
      Origin IGP, metric 0, localpref 100, weight 0, valid, external
      rx pathid: 0, tx pathid: 0x0
  65050
    10.0.1.3 from 10.0.1.3 (10.0.0.3)
      Origin IGP, metric 0, localpref 100, weight 0, valid, external, best
      rx pathid: 0, tx pathid: 0x0

Based on this output, what can be concluded?

Accepted Answer

Path #2 is the best path because it has a shorter AS_PATH.. BGP selects the best path based on a set of well-defined tie-breaking rules. After comparing weight, local preference, and origin, the next step is AS_PATH length. Path #2 has an AS_PATH of '65050' (one AS), while Path #1 has '65050 65100' (two ASes). Since a shorter AS_PATH is preferred, Path #2 is chosen as the best path, making option B correct.

Answer

Path #1 is the best path because it has a longer AS_PATH, indicating more specific routing.

Answer

Both paths are equally preferred, and BGP uses tie-breaking rules like router ID.

Answer

Path #2 is the best path because it is received from a higher IP address.

Question 19

router bgp 65000
 bgp router-id 10.0.0.1
 neighbor 10.0.0.2 remote-as 65001
 neighbor 10.0.0.2 route-map SET_COMMUNITY in
!
route-map SET_COMMUNITY permit 10
 set community 100:200
!
What is the effect of this configuration?

Accepted Answer

Routes received from 10.0.0.2 are tagged with community 100:200, but the community is not sent to other BGP peers unless send-community is configured.. The route-map SET_COMMUNITY is applied to inbound updates from neighbor 10.0.0.2, so routes received from that neighbor are tagged with community 100:200. However, BGP does not propagate communities to other peers unless the neighbor is explicitly configured with the send-community command. Without send-community, the community attribute is stripped from outbound updates, so the community is not sent to other BGP peers.

Answer

Routes received from 10.0.0.2 are tagged with community 100:200 and automatically sent to all neighbors.

Answer

The route-map is applied to outbound updates to 10.0.0.2, setting community on routes sent to that neighbor.

Answer

The configuration is invalid because the community must be a string, not a numeric value.

Question 20

router bgp 65000
 bgp router-id 10.0.0.1
 neighbor 10.0.0.2 remote-as 65000
!
Which statement about this configuration is true?

Accepted Answer

This is an iBGP session because the remote AS matches the local AS.. This configuration establishes an iBGP session because the local AS number (65000) matches the remote AS number (65000). In BGP, when both routers share the same AS, the session is classified as internal BGP (iBGP), regardless of the IP addressing scheme used.

Answer

This is an eBGP session because the neighbor IP is in a different subnet.

Answer

The router will automatically set next-hop-self for routes sent to this neighbor.

Answer

The configuration is invalid because iBGP requires a loopback interface.