CCNP ENCOR v8 Chapters 11 – 12: BGP Test Online

Hint

BGP routers exchange messages reliably over TCP using port 179.

Hint

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.

Hint

A BGP open message is used to establish a BGP adjacency. Both peer sides negotiate session capabilities before BGP peering is established.

Hint

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.

Hint

Statically configuring the BGP router ID (RID) is a best practice to ensure RID stability. If the RID is not statically configured, the dynamic RID allocation logic uses the highest IP address of any active loopback interfaces. If there is not an active loopback interface, then the highest IP address of any active interfaces becomes the RID when the BGP process initializes.

Hint

For a BGP session to initiate, one address family for a neighbor must be activated. On Cisco routers the IPv4 address family is activated by default; however, it may cause confusion when working with other address families. The BGP router configuration command no bgp default ip4-unicast disables the automatic activation of the IPv4 AFI.

Hint

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.

Hint

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.

Hint

The BGP configuration rules in IPv4 apply to IPv6, except that the IPv6 address family must be initialized, and the neighbor is activated. Routers with only IPv6 addressing must statically define the BGP RID to allow sessions to form.

Hint

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.

Hint

IGP routing protocols such as EIGRP and OSPF are used within an organization to exchange routes. An EGP routing protocol such as BGP is used to route traffic originating within one AS to the rest of the Internet (other individual autonomous systems).

Hint

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.

Hint

It is appropriate to use BGP when an autonomous system is multihomed, or has more than one connection to another autonomous system or to the Internet. BGP is not appropriate for single-homed autonomous systems. Autonomous system 65003 is a single-homed AS because it is only connected to one other autonomous system, 65002.

Hint

If an enterprise uses BGP to connect with more than one service provider, it runs the risk of its autonomous system (AS) becoming a transit AS. Because BGP uses path cost to select best path, a path through the internal BGP routers might appear with a better path cost, hence the risk of local AS providing transit routing to everyone in the internet. Transit routing can be avoided by applying outbound BGP route policies that only allow for local BGP routes to be advertised to other autonomous systems.

Hint

A route map uses the command syntax route-map route-map-name [ permit | deny ] [ sequence-number ]. The rules that apply to route map statements are as follows: If a processing action is not provided, the default value permit is used. If a sequence number is not provided, the sequence number is incremented by 10 automatically. If a matching statement is not included, an implied all prefixes is associated with the statement. Processing within a route map stops after all optional actions have processed and after matching a conditional matching criterion.

Hint

Multihomed environments should be configured so that branch routers cannot act as transit routers. In most designs, transit routing of traffic from another branch is undesirable, as WAN bandwidth may not be sized accordingly. If transit behavior is required, it should be restricted to the data centers or specific locations with these design criteria: Proper routing design can accommodate outages. Bandwidth can be sized accordingly. The routing pattern is bidirectional and predictable.

Hint

Prefix lists are configured with the global configuration command ip prefix-list prefix-listname [ seq sequence-number ] { permit | deny } high-order-bit-pattern / high-order-bit-count [ ge ge-value ] [ le le-value ]. Because prefix lists cannot be re-sequenced, it is advisable to leave enough space for insertion of sequence numbers at a later time.

Hint

The prefix matching logic works the same for IPv6 networks as for IPv4 networks. The fourth hexadecimal group of the match specification is 0x0030, which is 00000000 00110000 in binary. To match /60 prefix length, the fourth hexadecimal group of a network must be between 00000000 00110000 and 00000000 00111111, that is, between 0x0030 and 0x003F. The prefix length must be between /61 and /63.

Hint

A route map has four components:

• Sequence number: Dictates the processing order of the route map.
• Conditional matching criteria: Identifies prefix characteristics (network, BGP path attribute, next hop, and so on) for a specific sequence.
• Processing action: Permits or denies the prefix.
• Optional action: Allows for manipulations, depending on how the route map is referenced on the router. Actions can include modification, addition, or removal of route characteristics.

Hint

In route map configuration, if there are multiple variables (ACLs, prefix lists, tags, and so on) configured for a specific route map sequence, only one variable must match for the prefix to qualify. The Boolean logic uses an OR operator for this configuration. The processing action is performed only after a match occurs.

Hint

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.

Hint

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.

Hint

Prefix lists provide another method of identifying networks in a routing protocol. A prefix list identifies a specific IP address, network, or network range and allows for the selection of multiple networks with a variety of prefix lengths by using a prefix match specification. A prefix match specification contains two parts: a high-order bit pattern and a high-order bit count, which determines the high-order bits in the bit pattern that are to be matched. The high-order bit pattern is also known as the network address and the high-order bit count as the mask length. The network portion of the matching networks is 10.224.0.0 and higher. The mask length of the matching networks must be greater than or equal to /24 and less than or equal to /28.

Hint

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 100_ indicates to only include the lines that contain the exact phrase of 100. 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

Hint

Private BGP communities follow a particular convention where the first 16 bits represent the AS of the community origination, 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.

Hint

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.

Hint

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.

Hint

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

Hint

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.

Hint

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

Hint

A Notification message is sent when an error is detected with the BGP session, such as a hold timer expiring, neighbor capabilities change, or a BGP session reset is requested. This causes the BGP connection to close.