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Nokia BGP for Internet Routing study materials
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Nokia BGP for Internet Routing practice questions

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Question 1BGP Fundamentals and Protocol Operation

In BGP, which transport protocol and port number are used to establish a peering session between two routers?

  1. TCP port 179 Correct answer.
  2. TCP port 520
  3. UDP port 179
  4. UDP port 520

Explanation

BGP runs over TCP and uses well-known port 179 for both the initial connection and the ongoing session. Reliability is delegated to TCP, so BGP itself does not implement retransmission. UDP is never used as the BGP transport.

Question 2BGP Fundamentals and Protocol Operation

Two routers are configured as BGP peers. Which state indicates that the TCP connection is complete and the peers have exchanged and accepted OPEN messages?

  1. Connect
  2. OpenConfirm Correct answer.
  3. Idle
  4. OpenSent

Explanation

OpenConfirm means an OPEN message has been received and accepted and the router is waiting for a KEEPALIVE to move to Established. OpenSent only means our OPEN was sent and we await the neighbor's OPEN. The full session is up only after KEEPALIVE exchange.

Question 3BGP Fundamentals and Protocol Operation

A router receives a BGP UPDATE whose withdrawn-routes field and NLRI field are both empty, while the path attributes include ORIGIN, AS_PATH, and NEXT_HOP. What does this message signal?

  1. A request for the peer to resend its full table
  2. An end-of-RIB marker for the address family Correct answer.
  3. A malformed UPDATE that must be discarded
  4. A route refresh request for the IPv4 unicast table

Explanation

An UPDATE carrying only mandatory attributes and no NLRI or withdrawals is the End-of-RIB marker, indicating that the initial advertisement for that address family is complete. It is valid and is used, for example, to signal graceful-restart completion. It does not request retransmission.

Question 4BGP Fundamentals and Protocol Operation

Which BGP message is sent periodically to keep a session alive and is also used to confirm acceptance of an OPEN?

  1. KEEPALIVE Correct answer.
  2. UPDATE
  3. OPEN
  4. NOTIFICATION

Explanation

KEEPALIVE messages are exchanged at an interval derived from the Hold Time and confirm that the peer is reachable. After OPEN exchange, the first KEEPALIVE moves the session to Established. NOTIFICATION tears a session down rather than maintaining it.

Question 5BGP Fundamentals and Protocol Operation

By default on Nokia SR OS, what is the BGP Hold Time when two peers do not negotiate a different value, and at what interval are KEEPALIVEs sent?

  1. Hold Time 90 seconds, KEEPALIVE every 30 seconds Correct answer.
  2. Hold Time 30 seconds, KEEPALIVE every 10 seconds
  3. Hold Time 180 seconds, KEEPALIVE every 60 seconds
  4. Hold Time 60 seconds, KEEPALIVE every 20 seconds

Explanation

Nokia SR OS defaults to a 90-second BGP Hold Time and a 30-second KEEPALIVE interval. If no KEEPALIVE or UPDATE is received within the Hold Time, the session is reset. The negotiated Hold Time is the lower of the values proposed in the peers' OPEN messages.

Question 6BGP Fundamentals and Protocol Operation

Two peers propose Hold Times of 180 seconds and 90 seconds in their OPEN messages. What Hold Time is used for the session?

  1. 0 seconds, so the session has no hold timer
  2. 180 seconds
  3. 90 seconds Correct answer.
  4. 135 seconds, the average of the two

Explanation

When the proposed Hold Times differ, the smaller value is used for the session. The KEEPALIVE interval is then one third of that negotiated value, here 30 seconds. Averaging is not defined in BGP.

Question 7BGP Fundamentals and Protocol Operation

A BGP speaker originates a route with the ORIGIN attribute set to value 2. What does this value indicate?

  1. The route was learned from an IGP
  2. The route is incomplete, typically redistributed Correct answer.
  3. The route was learned through an EGP
  4. The route is a locally originated network statement

Explanation

ORIGIN value 2 means Incomplete, which is used when the source of the route is unknown, such as a prefix redistributed into BGP. Value 0 is IGP and value 1 is EGP. Path selection prefers IGP over EGP over Incomplete when other attributes tie.

Question 8BGP Fundamentals and Protocol Operation

How does a BGP speaker prepend its own AS number when advertising a route to an eBGP peer?

  1. It adds its AS number once at the end of the AS_PATH
  2. It replaces the existing AS_PATH with its own AS number
  3. It adds its AS number once at the beginning of the AS_PATH Correct answer.
  4. It leaves the AS_PATH unchanged and relies on NEXT_HOP

Explanation

When advertising to an eBGP peer, the speaker prepends its own AS number to the left end of the AS_PATH, which is the most recently traversed AS. This ordering allows loop detection and makes the path length meaningful. The existing path is preserved, not replaced.

Question 9BGP Fundamentals and Protocol Operation

A router receives an eBGP update whose AS_PATH already contains its own AS number. What is the default behavior?

  1. The route is accepted and installed with a penalty
  2. The route is accepted and the AS number is removed
  3. The route is rejected as a loop Correct answer.
  4. The route is accepted but not advertised

Explanation

BGP uses the AS_PATH as its loop-prevention mechanism. If a received AS_PATH contains the local AS number, the route is discarded by default. This is why AS path manipulation must be planned carefully.

Question 10BGP Fundamentals and Protocol Operation

Which statement correctly describes the difference between iBGP and eBGP with respect to the NEXT_HOP attribute?

  1. Neither iBGP nor eBGP modifies the NEXT_HOP attribute
  2. Both iBGP and eBGP always rewrite the next hop to the peer address
  3. eBGP changes the next hop to the peer address, while iBGP normally preserves the next hop learned from outside the AS Correct answer.
  4. eBGP preserves the next hop, while iBGP always rewrites it to the peer address

Explanation

When a route is sent over eBGP, the advertising router sets NEXT_HOP to its own interface address on that session. Within an iBGP session, the NEXT_HOP is normally carried unchanged, which is why next-hop-self or an IGP route to the external next hop is often needed.

Question 11BGP Fundamentals and Protocol Operation

What is the purpose of the BGP synchronization rule in classic BGP implementations?

  1. To ensure that a route learned from iBGP is not advertised to an external peer unless it is also known through an IGP Correct answer.
  2. To ensure that route reflectors forward all routes to their clients
  3. To ensure that KEEPALIVE messages are synchronized across peers
  4. To ensure that all iBGP peers use the same Hold Time

Explanation

The synchronization rule prevents a transit AS from advertising a route learned via iBGP to an external peer until the route is also present in the IGP, avoiding black holes. Modern networks disable it because iBGP is run on all transit routers. It has nothing to do with timers or reflection.

Question 12BGP Fundamentals and Protocol Operation

Why do full-mesh iBGP topologies require every iBGP speaker to peer with every other iBGP speaker?

  1. Because iBGP peers must exchange OPEN messages with all external peers
  2. Because iBGP sessions must share the same AS_PATH length
  3. Because a route learned from one iBGP peer is not re-advertised to another iBGP peer Correct answer.
  4. Because iBGP requires TCP port 179 to be used only in full mesh

Explanation

The iBGP split-horizon rule states that a route learned via iBGP is not forwarded to other iBGP peers. A full mesh or a route reflector is therefore needed to distribute external routes to all routers in the AS. This is a loop-prevention rule specific to iBGP.

Question 13BGP Fundamentals and Protocol Operation

What is the role of a BGP route reflector?

  1. It terminates all eBGP sessions in an AS
  2. It reflects routes learned from one iBGP client to other iBGP clients and non-clients without requiring a full mesh Correct answer.
  3. It replaces the AS_PATH with a cluster list
  4. It converts iBGP routes into eBGP routes for external peers

Explanation

A route reflector relaxes the iBGP split-horizon rule by reflecting routes between clients and non-clients, identified by cluster ID and originator ID. This removes the need for a full iBGP mesh. It does not change the AS_PATH or terminate eBGP sessions.

Question 14BGP Fundamentals and Protocol Operation

Which attribute is used first when a BGP speaker compares multiple paths to the same prefix and the next hops are reachable?

  1. WEIGHT Correct answer.
  2. LOCAL_PREF
  3. MED
  4. AS_PATH length

Explanation

On many implementations, WEIGHT is a vendor-specific attribute evaluated before all standard attributes and is local to the router. After WEIGHT, LOCAL_PREF is compared, then locally originated routes, then AS_PATH length. MED comes later in the decision process.

Question 15BGP Fundamentals and Protocol Operation

Two paths to the same prefix have equal WEIGHT, LOCAL_PREF, and AS_PATH length, but different ORIGIN values: one is IGP and the other is Incomplete. Which path is preferred?

  1. The path with ORIGIN IGP Correct answer.
  2. The path with ORIGIN Incomplete
  3. The path with the higher MED
  4. The path with the lower router ID

Explanation

ORIGIN is compared after AS_PATH length, and the preference order is IGP over EGP over Incomplete. Therefore the IGP path wins before MED or router ID is examined. MED is only considered after origin when earlier attributes tie.

Question 16BGP Fundamentals and Protocol Operation

What does the MULTI_EXIT_DISC attribute influence in BGP path selection?

  1. The number of times a route may be reflected
  2. The priority of KEEPALIVE messages
  3. The preference of a path within the local AS
  4. The preference of an entry point into an AS, typically signaled to a neighboring AS Correct answer.

Explanation

MED is used to influence which entry point a neighboring AS uses to reach a prefix, and it is typically compared only among paths from the same neighboring AS. Lower MED values are preferred. LOCAL_PREF, by contrast, controls preference within the local AS.

Question 17BGP Fundamentals and Protocol Operation

By default, how is MED compared between paths learned from two different neighboring ASes?

  1. MED is not compared unless explicitly configured, so the paths remain tied on this attribute Correct answer.
  2. The lower MED is always preferred
  3. The higher MED is always preferred
  4. MED is compared only if the AS_PATH lengths are equal

Explanation

Standard BGP behavior compares MED only among paths received from the same neighboring AS. Paths from different ASes are considered tied on MED unless the implementation is configured to always compare MED. This prevents MED values from different neighbors from being compared unfairly.

Question 18BGP Fundamentals and Protocol Operation

What is the purpose of the LOCAL_PREF attribute?

  1. To count the number of ASes a route has traversed
  2. To indicate the local preference for an exit point from the local AS Correct answer.
  3. To indicate the preference for an entry point into a neighboring AS
  4. To identify the router that originated a reflected route

Explanation

LOCAL_PREF is exchanged only within an AS and tells iBGP speakers which exit point is preferred for a prefix. A higher LOCAL_PREF value is better. It is not sent to external peers, unlike MED, which signals entry preference.

Question 19BGP Fundamentals and Protocol Operation

A router has an eBGP-learned route and an iBGP-learned route to the same prefix with identical LOCAL_PREF and AS_PATH length. Which route is preferred by the standard decision process?

  1. The route with the lower MED
  2. The eBGP route Correct answer.
  3. The iBGP route
  4. The route with the higher router ID

Explanation

After LOCAL_PREF and locally originated routes, BGP prefers eBGP-learned routes over iBGP-learned routes. This reflects the general preference for routes learned from outside the AS. MED is considered later in the process.

Question 20BGP Fundamentals and Protocol Operation

Which two attributes are considered well-known mandatory in BGP updates for IPv4 unicast?

  1. COMMUNITY and ORIGINATOR_ID
  2. AS_PATH and NEXT_HOP Correct answer.
  3. ATOMIC_AGGREGATE and AGGREGATOR
  4. LOCAL_PREF and MED

Explanation

Well-known mandatory attributes must be present in every UPDATE for a reachable NLRI and are understood by all BGP implementations. For IPv4 unicast these are ORIGIN, AS_PATH, and NEXT_HOP. LOCAL_PREF is well-known discretionary and MED is optional non-transitive.

Question 21BGP Fundamentals and Protocol Operation

What does the NEXT_HOP attribute contain for a route advertised over an eBGP session?

  1. The loopback address of the originating router
  2. The router ID of the receiving peer
  3. The IP address of the advertising router's interface on the peering link Correct answer.
  4. The address of the next AS border router beyond the peer

Explanation

For eBGP advertisements, the sender sets NEXT_HOP to the IP address of its own interface used for the peering session, unless next-hop-unchanged or third-party next-hop behavior applies. The receiver must have a route to that address for the prefix to be usable.

Question 22BGP Fundamentals and Protocol Operation

Why does BGP use TCP rather than a raw IP protocol for session establishment?

  1. To allow BGP sessions to be encrypted by default
  2. To guarantee that all updates are delivered in less than one second
  3. To rely on TCP's reliable, ordered delivery and retransmission for UPDATE exchange Correct answer.
  4. To avoid the need for any timers on the session

Explanation

BGP delegates reliability to TCP, so lost or corrupted UPDATE messages are retransmitted and delivered in order without BGP implementing its own reliability mechanism. TCP does not provide encryption by default, and BGP still uses Hold Time and KEEPALIVE timers.

Question 23BGP Fundamentals and Protocol Operation

A BGP session remains in Active state for an extended period. What does this state indicate?

  1. The router has detected an error and is closing the session
  2. The session is established and exchanging updates
  3. The router is trying to open a TCP connection and has not succeeded Correct answer.
  4. The router is actively sending UPDATE messages

Explanation

Active means BGP is attempting to establish the TCP connection but has not completed it, often due to reachability, filtering, or configuration problems. It is not a healthy state and does not mean updates are flowing. Established is the state in which peers exchange routes.

Question 24BGP Fundamentals and Protocol Operation

What is the purpose of the BGP NOTIFICATION message?

  1. To request a full routing table refresh
  2. To report an error condition and close the BGP session Correct answer.
  3. To advertise a new prefix to a peer
  4. To confirm receipt of an OPEN message

Explanation

A NOTIFICATION message carries an error code and subcode and is sent when a fatal error is detected, after which the session is closed. It is not used for prefix advertisement or for confirming OPEN messages. Route refresh is handled by a dedicated message.

Question 25BGP Fundamentals and Protocol Operation

In the standard BGP decision process, which comparison is made after AS_PATH length and ORIGIN but before the router ID tiebreaker?

  1. Comparison of the peer's autonomous system number
  2. Comparison of WEIGHT values
  3. Comparison of LOCAL_PREF values
  4. Comparison of MED values Correct answer.

Explanation

After AS_PATH length and ORIGIN, BGP compares MED when the paths come from the same neighboring AS. WEIGHT and LOCAL_PREF are compared earlier, and the router ID is used as a late tiebreaker. The peer AS number is not a separate comparison step.

Question 26BGP Fundamentals and Protocol Operation

Which BGP message type is exchanged first after the underlying TCP session is established, and what is its primary purpose?

  1. The NOTIFICATION message, which acknowledges the TCP handshake before routing data flows
  2. The UPDATE message, which immediately carries the initial table of reachable prefixes
  3. The KEEPALIVE message, which confirms that the peer is reachable and resets the Hold Time
  4. The OPEN message, which negotiates session parameters such as AS number, Hold Time, and capabilities Correct answer.

Explanation

After the TCP connection is up, each speaker sends an OPEN message to introduce itself and negotiate the session parameters: version, its own AS number, the proposed Hold Time, the BGP identifier, and optional capabilities. KEEPALIVEs and UPDATEs only follow once the OPEN exchange is accepted. The OPEN is therefore the first BGP message on a newly formed transport session.

Question 27BGP Fundamentals and Protocol Operation

A router running BGP considers several paths to the same prefix. Which comparison is applied immediately after the LOCAL_PREF attribute in the standard decision process?

  1. The path with the shortest AS_PATH is preferred
  2. The path with the lowest MULTI_EXIT_DISC is preferred
  3. The path with the lowest ORIGIN value is preferred
  4. The locally originated path is preferred over a path learned from a peer Correct answer.

Explanation

The standard decision sequence first prefers the highest WEIGHT (Cisco-specific), then the highest LOCAL_PREF, and then prefers locally originated routes over routes learned from a peer. Only after that does the process compare AS_PATH length, then ORIGIN, then MED. This ordering is why LOCAL_PREF is the main tool for controlling outbound path selection inside an AS.

Question 28BGP Fundamentals and Protocol Operation

Two routers establish an iBGP session over a TCP connection. Which statement accurately describes the TTL value used on packets carrying that session by default?

  1. The TTL is 255 because iBGP sessions are treated as single-hop by default
  2. The TTL is 64 for all BGP sessions regardless of whether they are iBGP or eBGP
  3. The TTL is 1 unless multihop is explicitly configured, so iBGP peers must be directly connected Correct answer.
  4. The TTL is negotiated in the OPEN message and can differ on each side of the session

Explanation

By default, BGP sends packets with a TTL of 1, so an iBGP session can only form between directly connected addresses unless multihop is configured. This default is why iBGP peering across an intermediate router requires an explicit multihop setting. eBGP also uses TTL 1 by default, which is why eBGP peers are normally one hop apart.

Question 29BGP Fundamentals and Protocol Operation

A router receives a BGP UPDATE for a prefix and finds that the NEXT_HOP is not reachable according to its routing table. How does this affect the route?

  1. The route is installed but marked as unreachable until the next hop resolves
  2. The route is installed with a recursive lookup through the default route
  3. The route is retained in the BGP table but is not eligible for best-path selection Correct answer.
  4. The route is immediately discarded and a NOTIFICATION is sent to the peer

Explanation

A BGP route whose NEXT_HOP cannot be resolved is kept in the BGP table but is not considered valid, so it cannot be selected as best path or installed in the routing table. The next-hop reachability check is a precondition for the decision process. Once the next hop becomes reachable, the route can be reconsidered without a new UPDATE.

Question 30BGP Fundamentals and Protocol Operation

What does the BGP identifier field in the OPEN message contain, and how is it normally derived?

  1. It contains a randomly generated 32-bit value that changes with each session
  2. It contains a 32-bit value, usually the highest IPv4 address on the router, used to identify the speaker Correct answer.
  3. It contains the highest IPv4 address configured on the router's loopback interfaces
  4. It contains the autonomous system number of the local speaker

Explanation

The BGP identifier is a 32-bit value carried in the OPEN message and is typically the highest IPv4 address configured on the router. It serves as a stable identifier for the speaker and is used as a tie-breaker in the decision process. It is not the AS number and it does not change per session.

Question 31BGP Session Establishment and Neighbor Configuration

On a Nokia SR OS router, which command displays the operational state of configured BGP neighbor sessions, including whether each session is established?

  1. show router bgp configuration
  2. show router bgp neighbor
  3. show router bgp routes
  4. show router bgp summary Correct answer.

Explanation

The BGP summary command lists every configured peer with its state, uptime, and prefix counters, so it is the standard first check when verifying that sessions have come up. It reports the current FSM state per neighbor, letting an operator confirm which peers reached Established.

Question 32BGP Session Establishment and Neighbor Configuration

Which TCP port does a BGP speaker listen on and connect to when establishing a peering session?

  1. 646
  2. 179 Correct answer.
  3. 89
  4. 520

Explanation

BGP uses TCP as its transport and by convention listens on and connects to port 179. Both peers must be able to reach that port for the session to progress past the TCP connection phase.

Question 33BGP Session Establishment and Neighbor Configuration

Two routers are directly connected and configured as external BGP peers using their physical interface addresses. Which condition must be met before the session can reach the Established state?

  1. Each router must be able to open a TCP connection to port 179 on the other and complete the BGP OPEN exchange Correct answer.
  2. Each router must have the same autonomous system number
  3. Each router must have a route reflector cluster ID configured
  4. Each router must have an identical router ID configured manually

Explanation

A BGP session is built on a working TCP connection, after which the peers exchange OPEN messages and negotiate capabilities. Reachability to port 179 is therefore the prerequisite for any further BGP state progression.

Question 34BGP Session Establishment and Neighbor Configuration

In the BGP finite state machine, which state indicates that the TCP connection is complete and the router has sent an OPEN message but has not yet received one from the peer?

  1. Established
  2. OpenSent Correct answer.
  3. Active
  4. Idle

Explanation

OpenSent means the local speaker has sent its OPEN message and is waiting for the peer's OPEN. The session moves on to OpenConfirm once that message is received and validated.

Question 35BGP Session Establishment and Neighbor Configuration

A BGP neighbor remains in the Active state indefinitely. What does this most directly indicate?

  1. The router is repeatedly attempting the TCP connection but the connection is not completing Correct answer.
  2. The peer has sent a NOTIFICATION message to tear the session down
  3. The session is fully established and exchanging routes
  4. The router has received an OPEN message and is waiting for a KEEPALIVE

Explanation

In the BGP FSM, Active is a connection-attempt state, not a success indicator. A session stuck there usually points to a TCP-level failure such as unreachable addresses, a filtering device, or a missing route to the peer.

Question 36BGP Session Establishment and Neighbor Configuration

You configure an iBGP session between two routers in AS 65000, but the loopback addresses used as update sources are not reachable between them. What is the most likely result?

  1. The session cannot reach Established because the underlying TCP connection fails Correct answer.
  2. The session establishes but only carries eBGP routes
  3. The session establishes and uses the physical interface address automatically
  4. The session establishes but suppresses all updates

Explanation

BGP depends on IP reachability between the configured neighbor addresses before TCP can connect. If the loopback addresses are not advertised through an IGP or static route, the TCP handshake fails and the session never reaches Established.

Question 37BGP Session Establishment and Neighbor Configuration

Which parameter must match on both ends of an external BGP session for the OPEN exchange to be accepted?

  1. The local preference value must be identical
  2. The BGP router priority must be identical
  3. The configured BGP hold time must be compatible so that a valid negotiated value results Correct answer.
  4. The cluster ID must be identical

Explanation

Hold times are carried in the OPEN message and negotiated to the lower of the two values, with specific rules for zero and small values. An unacceptable hold time causes the peer to reject the OPEN with a NOTIFICATION.

Question 38BGP Session Establishment and Neighbor Configuration

What is the purpose of the BGP KEEPALIVE message?

  1. It carries the full table of prefixes during initial convergence
  2. It requests the peer to resend withdrawn routes
  3. It authenticates the TCP connection before the OPEN exchange
  4. It confirms to the peer that the session is still alive without carrying routing information Correct answer.

Explanation

KEEPALIVE messages are sent periodically at roughly one third of the negotiated hold time to prevent the hold timer from expiring. They carry no NLRI and exist purely to maintain the session.

Question 39BGP Session Establishment and Neighbor Configuration

A BGP session has a negotiated hold time of 180 seconds. Approximately how often does each speaker send KEEPALIVE messages?

  1. Every 60 seconds Correct answer.
  2. Every 180 seconds
  3. Every 30 seconds
  4. Every 9 seconds

Explanation

The keepalive interval defaults to one third of the negotiated hold time, so 180 seconds yields roughly 60-second keepalives. This margin allows several keepalives to be lost before the hold timer expires.

Question 40BGP Session Establishment and Neighbor Configuration

Which Nokia SR OS configuration element specifies the autonomous system number under which the router runs BGP?

  1. configure router autonomous-system Correct answer.
  2. configure router bgp local-as
  3. configure router bgp peer-as
  4. configure router bgp group

Explanation

The router-level autonomous-system command sets the AS number the node uses for BGP. Peer AS values are then configured per neighbor or group to identify the remote side.

Question 41BGP Session Establishment and Neighbor Configuration

When configuring an eBGP neighbor on Nokia SR OS, which statement about the peer-as value is correct?

  1. It must always equal the local autonomous system number
  2. It is derived automatically from the router ID of the peer
  3. It must be set to the autonomous system number of the remote peer Correct answer.
  4. It is only required for iBGP sessions

Explanation

For eBGP the peer-as must match the AS number the remote router actually uses, otherwise the OPEN is rejected with a bad peer AS notification. For iBGP the peer-as equals the local AS.

Question 42BGP Session Establishment and Neighbor Configuration

An engineer wants many neighbors to share the same export policy and timers but keep distinct peer addresses. Which Nokia SR OS construct is designed for this?

  1. An import policy applied globally to the router
  2. A route reflector cluster containing the neighbors
  3. A BGP group that holds common parameters with individual neighbors defined inside it Correct answer.
  4. A single neighbor statement listing multiple addresses

Explanation

BGP groups let you define shared settings such as timers, policies, and AS parameters once, while each neighbor inherits them and adds its own address and overrides. This reduces configuration duplication and error.

Question 43BGP Session Establishment and Neighbor Configuration

Two eBGP routers in different AS numbers fail to establish a session, and the log shows a NOTIFICATION with an OPEN error indicating a bad peer AS. What is the most likely cause?

  1. The TCP port on one side is set to 180 instead of 179
  2. The peer-as configured locally does not match the AS number the remote router actually uses Correct answer.
  3. The hold times on the two sides differ
  4. The update-source addresses differ

Explanation

A bad peer AS notification means the AS number in the received OPEN did not match what the local router expected. Correcting the peer-as to reflect the remote router's actual AS resolves it.

Question 44BGP Session Establishment and Neighbor Configuration

Which BGP capability is negotiated during the OPEN exchange to allow multiple address families such as IPv4 unicast and IPv6 unicast over one session?

  1. Four-octet AS number capability
  2. Multiprotocol BGP capability Correct answer.
  3. Graceful restart capability
  4. Route refresh capability

Explanation

The multiprotocol capability advertisement lets peers agree which address families the session will carry. Without it, only the base IPv4 unicast family is assumed.

Question 45BGP Session Establishment and Neighbor Configuration

What does the four-octet AS number capability accomplish when negotiated between BGP peers?

  1. It enables four simultaneous TCP sessions per peer
  2. It increases the maximum prefix length to 32 octets
  3. It compresses the BGP header to four octets
  4. It allows autonomous system numbers larger than 65535 to be carried in the session Correct answer.

Explanation

The original BGP encoding limited AS numbers to two octets, so the capability advertises support for the larger four-octet AS number space. Peers that both support it can exchange the wider values directly.

Question 46BGP Session Establishment and Neighbor Configuration

A router running BGP with a peer that does not support the route refresh capability needs to apply a changed import policy to routes already received. What is the traditional remedy?

  1. Change the router ID and re-establish the session
  2. Issue a route refresh message anyway
  3. Send a KEEPALIVE with the refresh flag set
  4. Reset the session so the peer re-advertises its routes Correct answer.

Explanation

Without route refresh support, the only way to re-run import policy on received routes is to force the peer to resend them, which traditionally means resetting the session. Route refresh capability exists precisely to avoid this disruption.

Question 47BGP Session Establishment and Neighbor Configuration

Which mechanism helps prevent a BGP session from being established with an attacker who spoofs a peer address?

  1. Increasing the hold time to the maximum value
  2. Disabling KEEPALIVE messages
  3. Setting the router ID to match the peer address
  4. TCP MD5 authentication configured identically on both peers Correct answer.

Explanation

TCP MD5 authentication adds a signature to TCP segments so both ends must share the same key before the connection succeeds. A spoofer without the key cannot complete the handshake.

Question 48BGP Session Establishment and Neighbor Configuration

An iBGP session between two routers in the same AS stays in the Connect state. Which underlying issue should be investigated first?

  1. Whether the cluster ID is configured
  2. Whether the export policy includes the peer's loopback
  3. Whether the peer-as value is correct for iBGP
  4. Whether the neighbor address is reachable and permitted through any filtering in the path Correct answer.

Explanation

A session stuck in Connect indicates the TCP handshake is not completing, which points to reachability or filtering problems rather than BGP policy. Verifying the route to the neighbor address and any ACLs along the path is the correct first step.

Question 49BGP Session Establishment and Neighbor Configuration

What is the effect of configuring a BGP neighbor with a passive mode setting on Nokia SR OS?

  1. The router refuses to accept any inbound TCP connection from the peer
  2. The router waits for the peer to initiate the TCP connection instead of initiating it Correct answer.
  3. The router advertises a default route to the peer
  4. The router suppresses all KEEPALIVE messages to the peer

Explanation

Passive mode prevents the local router from actively opening the TCP connection, leaving initiation to the remote peer. This is useful when the far side is expected to connect, for example behind a firewall.

Question 50BGP Session Establishment and Neighbor Configuration

A BGP session flaps repeatedly and the log shows hold timer expired notifications. Which two values should be checked first?

  1. The peer-as and local-as values on both peers
  2. The router IDs and cluster IDs on both peers
  3. The negotiated hold time and the keepalive interval on both peers Correct answer.
  4. The export and import policies on both peers

Explanation

A hold timer expiry means KEEPALIVEs were not received within the negotiated hold time, so the timer and keepalive settings on both sides are the first things to verify. Mismatched or overly aggressive timers are a common cause of repeated expiry.