Chapters 11 – 14: BGP Exam Answers (CCNPv8 ENARSI)

Explanation: Within the BGP AS 65500, because synchronization is disabled within the AS, IBGP should be used to establish peer relationships to provide routing service within the AS. R2 and R6 represent subnetworks. Static route and default route configuration provide adequate network access.

Explanation: To expand the support for multiple network layer protocols to original BGP-4, RFC 2858 added Multiprotocol BGP (MBGP/MP-BGP) capability by adding an extension called the address family identifier (AFI). An address family correlates to a specific network protocol, such as IPv4 or IPv6, and additional granularity is provided through a subsequent address family identifier (SAFI) such as unicast or multicast.

Explanation: As the BGP prefix is installed into the Loc-RIB table, the following BGP PAs are set, depending on the RIB prefix type:

• Connected network : The next-hop BGP attribute is set to 0.0.0.0, the BGP origin attribute is set to i (IGP), and the BGP weight is set to 32,768.
• Static route or routing protocol : The next-hop BGP attribute is set to the next-hop IP address in the RIB, the BGP origin attribute is set to i (IGP), the BGP weight is set to 32,768, and the MED is set to the IGP metric.

Explanation: To configure EBGP, the router bgp command is followed by the AS number in which the router resides. Conversely, the neighbor command contains the AS number to which the remote router belongs.

Explanation: The address family is initialized by using the BGP router configuration command address-family afi safi . On IOS and IOS XE devices, the default subsequent address family identifier (SAFI) for the IPv4 and IPv6 address families is unicast and is optional.

Explanation: When a company exchanges routes with multiple ISP sites, running IBGP on internal routers provides a better solution with the following features:

• Scalability – The internet has 800,000+ IPv4 network prefixes and it continues to increase in size. IGPs cannot scale to that level of routes.
• Custom routing – Link-state and distance vector routing protocols use one or more specific metrics as the primary method for route selection. IGP protocols always use this routing pattern for path selection. BGP uses multiple steps to identify the best path and to manipulate the path for a specific prefix based on the network layer reachability information (NLRI). The NLRI includes the network prefix, prefix length, and any BGP path attributes (PAs) for a specific route. If the route was considered from the prospective of an IGP, the chosen path could be considered longer or suboptimal than a path chosen by BGP.

Explanation: In BGP configuration, the BGP router configuration command neighbor ip-address remote-as as-number is used to identify the IP address and autonomous system number associated with the BGP neighbor.

Explanation: Route summarization on BGP edge routers via dynamic method is configured by specifying an aggregation network prefix. The aggregate-address command advertises the aggregated route (the supernet) in addition to the specific original component network prefixes. By using the optional summary-only keyword, the component network prefixes in the summarized network range are suppressed. When a BGP router summarizes a route, it does not advertise the AS_Path information from before the aggregation. This effect is indicated by the atomic aggregate attribute. The atomic aggregate attribute indicates that a loss of path information has occurred.

Explanation: One method for conditional matching of routes in BGP is through extended ACLs. When extended ACLs are used for this purpose, the source fields match against the network portion of the route, and the destination fields match against the network mask. The network portion of the matching networks is 10.0.64.0 through 10.0.127.0. The network mask portion is /24 through /26.

Explanation: To restrict traffic to only locally originated traffic, configure the AS path ACL using the regex pattern ^$.

Explanation: There are two types of BGP peer templates:

• Peer session – This type of template involves configuration settings specifically for the BGP session.
• Peer policy – This type of template involves configuration settings specifically for the address family policy.

Explanation: Route summarization on BGP edge routers via dynamic method is configured by specifying an aggregation network prefix. The aggregate-address command advertises the aggregated route in addition to the specific original component network prefixes. By using the optional summary-only keyword, the component network prefixes in the summarized network range are suppressed. When a BGP router summarizes a route, it does not advertise the AS_Path information from before the aggregation. This is indicated by the atomic aggregate attribute. To keep the BGP path information history, the optional as-set keyword may be used with the aggregate-address command. The AS_Path settings from the original prefixes are stored in the AS_SET portion of the AS_Path. The AS_SET, which is displayed within brackets, only counts as one hop, even if multiple autonomous systems are listed.

Explanation: In troubleshooting BGP, Regular expressions (regex) can be used to parse through the large number of available ASNs. Regular expressions are based on query modifiers used to select the appropriate content. The regex pattern ^300_ indicates to only include the lines that list the exact phrase of 300 as the first AS. Some regex query modifiers are as follows:
_ (underscore) – matches a space
^ (caret) – indicates the start of a string
$ (dollar sign) – indicates the end of a string
. (period) – matches a single character, including a space

Explanation: The prefix restrictions can be put on a BGP neighbor by using the BGP address family configuration command neighbor ip-address maximum-prefix prefix-count [ warning-percentage ] [ restart time ] [ warning-only ]. When a peer advertises more routes than the maximum prefix count, the router moves the neighbor to the Idle (PfxCt) state in the finite-state machine (FSM), closes the BGP session, and sends out the appropriate syslog message.

Explanation: BGP weight is a Cisco-defined attribute and the first step for selecting the BGP best path. Weight is a 16-bit value (0 to 65,535) assigned locally on the router; it is not advertised to other routers.

Explanation: The BGP best-path algorithm uses attributes for the best-path selection. The top 5 attributes, ranked in order of consideration, are the following:
1. Weight.
2. Local preference.
3. Local originated (network statement, redistribution, or aggregation).
4. AIGP.
5. Shortest AS_Path.

BGP weight is a Cisco-defined attribute and is a 16-bit value assigned locally on the router; it is not advertised to other routers. Thus, the weight attributes as specified in RTD and RTE have no effect in RTA for determining the best outbound path. Assuming that the attributes in ranks 2 to 4 are not specified, the shortest AS_Path attribute is used to choose the outbound path when two paths are available.

Explanation: The BGP prefix path attributes are classified as follows:

• Well-known mandatory – supported by all BGP implementations and advertised between autonomous systems
• Well-known discretionary – supported by all BGP implementations and not advertised between autonomous systems
• Optional transitive – not supported by all BGP implementations and advertised between autonomous systems
• Optional nontransitive – not supported by all BGP implementations and not advertised between autonomous systems

Explanation: Local preference (LOCAL_PREF) is a well-known discretionary path attribute and is included with path advertisements throughout an AS. Local preference can be set for specific routes by using a route map or for all routes received from a specific neighbor. If an edge BGP router does not define the local preference upon receipt of a prefix, the default local preference value of 100 is used during best-path calculation. Setting the local preference for specific routes is accomplished by using a route map or route policy with the action set local-preference preference . Setting the local preferences for all routes received by a neighbor can be accomplished by using the BGP address family configuration command neighbor ip-address local-preference preference .

Explanation: EBGP multipathing can be enabled by using the BGP configuration command maximum-paths number-paths . The number of paths indicates the allowed number of EBGP paths to install in the RIB. The command maximum-paths ibgp number-paths sets the number of IBGP routes to install in the RIB. The commands are placed under the appropriate address family.

Explanation: BGP path attributes (PAs) can be either transitive or nontransitive. Transitive PAs are carried from one AS to another AS and those changes could affect downstream routing for other SPs too. Nontransitive PAs influence the routing policy only within the organization. All the BGP path attributes cannot be maintained within IGP protocols. Only BGP can maintain the path attribute as the prefix is advertised from one edge of the AS to the other edge.

Explanation: The BGP multi-exit discriminator (MED) attribute is nontransitive. The purpose of using MED is to influence traffic flows inbound from a different AS. A lower MED is preferred over a higher MED.

Explanation: Some of the reasons that a BGP neighbor relationship might not form are as follows:

• Interface is down : The interface must be up/up.
• Layer 3 connectivity is broken : The IP address for a neighbor must be reachable.
• Path to the neighbor is through the default route : A neighbor must be reachable using a route other than the default route.
• ACLs : An access control list (ACL) or a firewall may be blocking TCP port 179.
• Timers : Timers do not have to match; however, if the minimum holddown from neighbor option is set, it could prevent a neighbor adjacency.

Explanation: Two BGP routers do not have to be directly connected or in the same subnet to form a BGP neighbor relationship; however, Layer 3 connectivity is required. For a router to have Layer 3 connectivity to the neighbor, it needs to have a route in the routing table that points to the neighbor router. The fact that there is no route in the routing table on R5 to reach 2.2.2.2 and R5 can ping R2 with the ping 2.2.2.2 source 5.5.5.5 command indicates that R5 can reach R2 via the default route configured on R5. BGP does not consider the default route a valid route for forming an adjacency.

Explanation: BGP peers are the foundation of BGP information sharing. Without peers, BGP will not learn BGP routes. Some common reasons that BGP routes might be missing from either the BGP table or the routing table include the following:

• Missing or bad network mask command : An accurate network command is needed to advertise routes.
• Next-hop router not reachable : To use a BGP route, the next hop must be reachable.
• BGP split-horizon rule : A router that learns BGP routes through an IBGP peering does not share those routes with another IBGP peer.
• Better source of information : If the same network is learned from a more reliable source, it is used instead of the BGP-learned information.
• Route filtering : A filter might be preventing a route from being shared with neighbors or learned from neighbors.

Explanation: The origin is a well-known mandatory BGP path attribute used in the BGP best-path algorithm. A value of i represents an IGP, e indicates EGP, and ? indicates a route that was redistributed into BGP.

Explanation: Cisco routers review BGP attributes in the following ranked order when deciding which path is the best-path:

• Prefer the highest weight .
• Prefer the highest local preference .
• Prefer the route originated by the local router.
• Prefer the path with the shorter Accumulated Interior Gateway Protocol (AIGP) metric attribute.
• Prefer the shortest AS_Path .
• Prefer the lowest origin code.
• Prefer the lowest multi-exit discriminator (MED).
• Prefer an external path over an internal path.
• Prefer the path through the closest IGP neighbor .
• Prefer the oldest route for EBGP paths.
• Prefer the path with the lowest neighbor BGP RID .
• Prefer the path with the lowest neighbor IP address .

The first path attribute to be checked is the weight. A higher value is better. In this case, no weight is listed because both routes are using the default value 0. Setting the weight higher than 0 will make the route the best-path.

Explanation: Two BGP routers do not have to be directly connected or in the same subnet to form a BGP neighbor relationship; however, Layer 3 connectivity is required. For a router to have Layer 3 connectivity to another, it needs to have a route in the routing table that points to the neighbor router. If no route to the neighbor exists, a neighbor relationship cannot form. When the show bgp ipv4 unicast summary command to review/verify BGP states is being used, the Idle state under the State/PfxRcd field indicates that the local router is not able to make a TCP connection with the neighbor.

Explanation: The show bgp ipv6 unicast command can be used to verify the IPv6 unicast routes that have been learned from all neighbors. The route 2001:db8:2::/64 is the route that was learned from the neighbor at 2.2.2.2. Because the next hop is not reachable, route 2001:db8:2::/64 is not installed in the routing table. The address of ::FFFF:2.2.2.2 is a dynamically generated next hop that was created to replace the original next hop of 2.2.2.2. This occurs because an IPv6 route cannot have an IPv4 next-hop address. To solve this issue, a route map can be created that changes the next hop to a valid IPv6 address and attach it to the neighbor statement. This needs to be done on the router that advertises the route, not on the router receiving the route.

Explanation: IBGP peers do not prepend their ASN to AS_Path because the NLRI would fail the validity check and would not install the prefix into the IP routing table. No other method exists for detecting loops with IBGP sessions and RFC 4271 prohibits the advertisement of an NLRI received from an IBGP peer to another IBGP peer. RFC 4271 states that all BGP routers in a single AS must be fully meshed to provide a complete loop-free routing table and prevent traffic from being dropped.

Explanation: Private BGP communities follow a convention where the first 16 bits represent the AS of the community organization, and the second 16 bits represent a pattern defined by the originating AS. A private BGP community pattern can vary from organization to organization, does not need to be registered, and can signify geographic locations for one AS while signifying a method of route advertisement in another AS. A private BGP community is set in a route map with the command set community bgpcommunity [ additive ].

Explanation: The local preference attribute indicates the preference for exiting the AS to the destination network. The local preference is not advertised between eBGP peers and is typically used to influence the next-hop address for outbound traffic (that is, leaving an autonomous system). Local preference can be set for specific routes by using a route map or for all routes received from a specific neighbor. A higher value is preferred over a lower value.

Explanation: The State/PfxRcd indicates the current state of the BGP peer or the number of prefixes received from the peer. A value of 0 or another number indicates that the adjacency is successfully formed. The value indicates the number of prefixes received from the neighbor. Messages received and sent between routers are indicated under the MsgRcvd and MsgSent fields. BGP uses TCP to communicate. If there is Layer 3 connectivity, the state value is Idle. If Layer 3 connectivity exists and a three-way TCP handshake is complete, an open message is sent. However, if there is no response to the open message, the state value is Active.

Explanation: The fourth hexadecimal of the IPv6 network 2001:db8:0:35::/64 is 0x35 that is equivalent to binary notation of 00110101. Thus 6 bits is the least needed for the purpose of route summarization.