Free Juniper JN0-351 Practice Exam with Questions & Answers | Set: 2

A  is correct because RSTP alternate ports block traffic while receiving superior BPDUs from a neighboring switch.  An alternate port is a backup port for a root port, which means it receives better BPDUs from another bridge than the current root port 1 .  However, an alternate port does not forward any traffic, as it is in a discarding state 2 . It only listens to BPDUs and waits for the root port to fail.  If the root port fails, the alternate port can immediately transition to a forwarding state and become the new root port 1 .

C  is correct because RSTP alternate ports provide an alternate higher cost path to the root bridge.  An alternate port is selected based on the same criteria as the root port, which are the lowest bridge ID, the lowest path cost, the lowest sender port ID, and the lowest receiver port ID 3 .  However, an alternate port receives a higher cost BPDU than the root port, otherwise it would be the root port itself 1 . Therefore, an alternate port provides an alternate higher cost path to the root bridge than the root port.

BFD is a protocol that can be used to quickly detect failures in the forwarding path between two adjacent routers or switches. BFD can be integrated with various routing protocols and link aggregation protocols to provide faster convergence and fault recovery.

According to the Juniper Networks documentation, the following protocols support BFD on Junos OS devices 1 :

BGP: BFD can be used to monitor the connectivity between BGP peers and trigger a session reset if a failure is detected.  BFD can be configured for both internal and external BGP sessions, as well as for IPv4 and IPv6 address families 2 .

OSPF: BFD can be used to monitor the connectivity between OSPF neighbors and trigger a state change if a failure is detected.  BFD can be configured for both OSPFv2 and OSPFv3 protocols, as well as for point-to-point and broadcast network types 3 .

LACP: BFD can be used to monitor the connectivity between LACP members and trigger a link state change if a failure is detected.  BFD can be configured for both active and passive LACP modes, as well as for static and dynamic LAGs 4 .

Other protocols that support BFD on Junos OS devices are:

IS-IS: BFD can be used to monitor the connectivity between IS-IS neighbors and trigger a state change if a failure is detected. BFD can be configured for both level 1 and level 2 IS-IS adjacencies, as well as for point-to-point and broadcast network types.

RIP: BFD can be used to monitor the connectivity between RIP neighbors and trigger a route update if a failure is detected. BFD can be configured for both RIP version 1 and version 2 protocols, as well as for IPv4 and IPv6 address families.

VRRP: BFD can be used to monitor the connectivity between VRRP routers and trigger a priority change if a failure is detected. BFD can be configured for both VRRP version 2 and version 3 protocols, as well as for IPv4 and IPv6 address families.

The protocols that do not support BFD on Junos OS devices are:

RSTP: RSTP is a spanning tree protocol that provides loop prevention and rapid convergence in layer 2 networks. RSTP does not use BFD to detect link failures, but relies on its own hello mechanism that sends BPDU packets every 2 seconds by default.

FTP: FTP is an application layer protocol that is used to transfer files between hosts over a TCP connection. FTP does not use BFD to detect connection failures, but relies on TCP’s own retransmission and timeout mechanisms.

BGP is a routing protocol that operates between autonomous systems (AS). An AS is a group of routers under a single administrative control. BGP can be classified into two types: internal BGP (IBGP) and external BGP (EBGP). IBGP is the BGP communication between routers within the same AS, while EBGP is the BGP communication between routers in different AS. BGP uses the AS_PATH attribute to record the AS numbers that the route has passed through, and uses it to prevent routing loops and select the best path.

In this scenario, you want to add a second ISP connection and remove the default route to the existing ISP. This means that you want to have more control over the routing decisions and use BGP to exchange routes with both ISPs. To do this, you need to deploy the BGP protocol in your network and configure both IBGP and EBGP sessions. The correct statements about BGP in this scenario are:

IBGP peers advertise routes received from EBGP peers to other IBGP peers. This is the default behavior of IBGP, as it allows the routers within the same AS to learn the routes from different EBGP peers and select the best exit point. However, IBGP has a rule that it does not advertise routes received from IBGP peers to other IBGP peers, to avoid creating routing loops. Therefore, option B is correct and option C is incorrect. To overcome this rule, IBGP requires a full mesh topology, where every IBGP router is directly connected to every other IBGP router, or a route reflector or confederation design, where some IBGP routers act as intermediaries to reflect or aggregate the routes to other IBGP routers.

EBGP peers advertise routes received from IBGP peers to other EBGP peers. This is the default behavior of EBGP, as it allows the routers in different AS to exchange routes and reachability information. However, EBGP has a rule that it does not advertise routes received from EBGP peers to other EBGP peers, to avoid creating routing loops. Therefore, option D is correct and option A is incorrect. To overcome this rule, EBGP uses the AS_PATH attribute to filter out the routes that contain its own AS number, or uses route maps or policies to control the route advertisement. IBGP does not update the next-hop attribute to ensure reachability within an AS, as the next-hop attribute is preserved by IBGP. Instead, IBGP relies on an underlying IGP (Interior Gateway Protocol) to provide reachability to the next-hop.

A. The routes displayed have traversed one or more autonomous systems. True. The AS path attribute shows AS numbers (e.g., 65401 65520.), indicating the route ' s traversal through various autonomous systems. D. The routes displayed are being learned from an IBGP peer. True. The absence of the ' * > i ' marker before the prefixes indicates they are being learned from an IBGP peer. Therefore, the correct statements are A. The routes displayed have traversed one or more autonomous systems. and D. The routes displayed are being learned from an IBGP peer.

B  is correct because the loops do not exist is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid.  A loop in BGP means that a route has been advertised by the same AS more than once, which can cause routing instability and inefficiency 1 .  To prevent loops, BGP uses the AS-path attribute, which lists the AS numbers that a route has traversed from the origin to the destination 2 .  The receiving router checks the AS-path attribute of the received route and discards it if it finds its own AS number in the list 2 . This way, BGP avoids accepting routes that contain loops.

C  is correct because the next hop is reachable is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid.  The next hop is the IP address of the next router that is used to forward packets to the destination network 3 .  The receiving router checks the next hop attribute of the received route and verifies that it has a valid route to reach it 3 .  If the next hop is not reachable, the received route is not usable and is rejected by the receiving router 3 . This way, BGP ensures that only feasible routes are accepted.

Default BGP keepalive timer in Junos is 30 seconds

The default keepalive time for BGP is 30 seconds, which means that BGP peers send keepalive messages to each other every 30 seconds to verify the liveness of the BGP session. If no keepalive message is received from a peer within the hold time, which is 90 seconds by default, the peer is considered to be unavailable and the BGP session is closed. The keepalive time and the hold time can be configured manually for each BGP peer or peer group, but they must match on both ends of the BGP session. The keepalive time should be one third of the hold time for optimal performance. Therefore, option C is correct and the other options are incorrect. References:  BGP keepalive messages and hold time - Juniper Networks