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BGP Flashcards

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1
Q

What command is used for dynamic route summarization in BGP?

A

aggregate-address network subnet-mask [summary-only] [as-set]

This command is part of the BGP address family configuration.

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2
Q

Which BGP path attributes are not included in the new BGP aggregate prefix?

A
  • AS_Path
  • Multi-exit discriminator (MED)
  • BGP communities

These attributes are essential for path selection but are omitted in summarization.

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3
Q

What does the atomic aggregate attribute indicate?

A

A loss of path information has occurred

This attribute is used to signal that some details about the original routes are no longer available.

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4
Q

What keyword may be used with the aggregate-address command to keep the component route’s BGP path information?

A

as-set

The as-set keyword allows for the preservation of the original path information when aggregating BGP routes.

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5
Q

What does the regex ^$ match?

A

Empty AS_PATH (routes originated within your local AS)

This regex pattern is used to identify routes that do not have any AS_PATH information.

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6
Q

What does the regex _100$ match?

A

Routes originated in AS 100

100 is the last entry in the path.

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7
Q

What does the regex ^100 match?

A

Matches routes directly received from AS 100

100 is the first entry.

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8
Q

What does the regex 100 match?

A

Routes that passed through AS 100 anywhere in the path

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9
Q

What does regex .* match?

A

Matches any as-path

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10
Q

What command is used to create the AS_Path access list?

A

ip as-path access-list acl-number {deny | permit} regex-query

This command allows for the specification of regular expressions to filter AS paths.

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11
Q

How is the AS_Path access list applied to a neighbor?

A

neighbor ip-address filter-list acl-number {in | out}

This command determines the direction of the filter application on the specified neighbor.

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12
Q

What command is used to initiate a hard reset on a router?

A

clear ip bgp ip-address

This command resets the BGP session for the specified IP address.

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13
Q

How can you clear all the router’s BGP sessions?

A

By using an asterisk (*) in lieu of the peer’s IP address

This command clears all BGP sessions instead of targeting a specific one.

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14
Q

What does performing a soft reset on sessions that support route refresh capability actually initiate?

A

a route refresh

This action allows for the re-establishment of routing information without tearing down the session.

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15
Q

You can perform a soft reset for a specific address family with the command _______.

A

clear bgp afi safi {ip-address | *} soft [in | out]

This command allows for targeted refresh of routing information based on the specified address family.

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16
Q

What command is used to enable communities on a neighbor-by-neighbor basis in BGP?

A

neighbor ip-address send-community [standard | extended | both]

This command configures the BGP address family to send community attributes.

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17
Q

What command is used in IOS XE to display communities in a new format?

A

ip bgp-community new-format

This command makes the communities easier to read.

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18
Q

What is the No_Advertise community in BGP?

A

A community that prevents routes from being advertised to any BGP peer

The No_Advertise community is set with the command set community no-advertise within a route map.

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19
Q

How is the No_Advertise community set in BGP?

A

Using the command set community no-advertise within a route map

This command ensures that the specified routes are not advertised to any BGP peers.

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20
Q

What happens when a route is received with the No_Export community?

A

The route is not advertised to any eBGP peer

The No_Export community is represented by the value 0xFFFFFF01 or 4,294,967,041.

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21
Q

How is the No_Export community set within a route map?

A

set community no-export

This command is used to configure the No_Export community for specific routes.

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22
Q

What is the No_Export_SubConfed community identifier?

A

0xFFFFFF03 (4,294,967,043)

This community is used to control route advertisement within a local AS.

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23
Q

If a router receiving a route with the local AS community is a confederation member, where can the route be advertised?

A

Only within the sub-AS (member AS)

Routes are not advertised between member ASs.

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24
Q

The command used to set the local AS community within a route map is __________.

A

set community local-as

This command is essential for configuring route advertisement behavior.

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25
What is the **configuration syntax** for a community list?
ip community-list {1-500 | standard -list-name | expanded list-name} {permit | deny} community-pattern ## Footnote This syntax is used to define community lists in networking configurations.
26
How is a **community list** referenced in a route map?
match community 1-500 ## Footnote This command is used to apply the community list in routing decisions.
27
What command is used to set a **private BGP community** in a route map?
set community bgp-community [additive] ## Footnote This command allows for the configuration of BGP communities within routing policies.
28
By default, when you set a community in BGP, what happens to existing communities?
They are overwritten ## Footnote This means that only the newly set community will be retained unless specified otherwise.
29
What keyword can be used to **preserve existing communities** when setting a BGP community?
additive ## Footnote Using the additive keyword allows the new community to be added without removing the existing ones.
30
How do you define a **peer group** in BGP?
By using the command **neighbor group-name peer-group** in the global BGP configuration ## Footnote This command establishes a group of peers for BGP configuration.
31
What command is used to configure all BGP parameters for a peer group?
**peer-group group-name** in lieu of **neighbor ip-address** ## Footnote This allows for easier management of multiple BGP peers.
32
What is the first attribute that the **BGP best-path algorithm** uses for selecting the best route?
Prefer the highest weight ## Footnote This is the primary criterion used in the selection process.
33
What does the **BGP best-path algorithm** prefer after the highest weight?
Prefer the highest local preference ## Footnote Local preference indicates the preferred exit point for outbound traffic.
34
What does the **BGP best-path algorithm** consider after the AS_Path?
Prefer the best Origin code ## Footnote The Origin code indicates how the route was learned.
35
What does the **BGP best-path algorithm** prefer regarding the **multi-exit discriminator (MED)**?
Prefer the lowest multi-exit discriminator (MED) ## Footnote MED is used to influence the selection of routes when multiple paths exist.
36
In the **BGP best-path algorithm**, which type of path is preferred over the other?
Prefer an external path over an internal path ## Footnote This preference helps in choosing routes from external BGP over internal BGP.
37
What does the **BGP best-path algorithm** prefer regarding the **IGP neighbor**?
Prefer the path through the closest IGP neighbor ## Footnote Proximity to the IGP neighbor can indicate a more efficient route.
38
What is the **default weight value** for learned routes in Cisco BGP routing?
0 ## Footnote This value is part of a proprietary attribute that influences route preference.
39
True or false: **Locally originated routes** in Cisco BGP have a default weight of 0.
FALSE ## Footnote Locally originated routes have a default weight of 32,768.
40
Which well-known discretionary path attribute is not advertised between eBGP peers and is typically used to influence the next-hop address for outbound traffic?
Local preference (LOCAL_PREF)
41
What is the default local preference value?
100
42
What command enables you to change the default BGP local preference value?
bgp default local-preference default-local-preference
43
Within a route-map, how do you set the local preference?
route-map AS200 permit 10 match ip address prefix-list PRE172 set local-preference 222 route-map AS200 permit 20
44
What is the **third decision point** in the best-path algorithm?
Determine whether the route originated locally ## Footnote Preference is given in the following order: Routes that were advertised locally Networks that have been aggregated locally Routes received by BGP peers
45
What is an optional nontransitive path attribute that is included with advertisements throughout an AS?
Accumulated Interior Gateway Protocol (AIGP) ## Footnote Interior Gateway Protocols typically use the lowest-path metric to identify the shortest path to a destination but cannot provide the scalability of BGP.
46
Within a route-map, how do you prepend paths?
route-map AS200 permit 10 match ip address prefix-list PRE172 set as-path prepend 222 210 route-map AS200 permit 20
47
What is the default **origin** attribute for networks advertised on Cisco routers using the **network statement**?
i (for IGP) ## Footnote This indicates that the network is part of an Interior Gateway Protocol.
48
What origin attribute is assigned to **redistributed networks** on Cisco routers?
? (incomplete) ## Footnote This indicates that the origin of the network is not fully known.
49
List the **origin preference order** from most preferred to least preferred.
* IGP Origin (Most) * Exterior Gateway Protocol (EGP) Origin * Incomplete Origin (Least) ## Footnote This order determines the preference for routing decisions based on the origin of the route.
50
What command is used to modify a prefix’s **Origin attribute** on a route map?
set origin {igp | incomplete} ## Footnote This command allows for the adjustment of the Origin attribute for routing protocols.
51
What is an optional non-transitive attribute used to suggest a preferred ingress point to an adjacent Autonomous System (AS) when multiple connections exist?
Multi-Exit Discriminator (MED)
52
Is a higher or lower MED value preferred?
A lower MED value is preferred
53
What is the default MED value?
0
54
How does **BGP** set the **MED**?
Automatically to the IGP path metric ## Footnote This occurs during network advertisement or redistribution.
55
The MED value is also referred to as?
The metric
56
How do you set the MED value (metric) within a route-map?
route-map AS200-R3 permit 10 match ip address prefix-list PRE172-01 set metric 80 route-map AS200-R3 permit 20
57
What does the **always-compare-med** feature allow for in BGP?
Comparison of MED regardless of the AS_Path ## Footnote This feature is useful for ensuring consistent path selection in BGP.
58
How do you enable the **always-compare-med** feature in BGP?
Using the BGP configuration command **bgp always-compare-med** ## Footnote This command is part of the BGP configuration settings.
59
Are iBGP paths more desirable than eBGP paths?
Np
60
Are confederation member AS paths more desirable than iBGP paths?
yes
61
What are the **BGP path attributes** that need to match when configuring **BGP multipath**?
* Weight * Local preference * AS_Path length * AS_Path content * Origin * MED ## Footnote Confederations can contain a different AS_CONFED_SEQ path.
62
True or false: When configuring BGP multipath, the **advertisement method** must match for iBGP and eBGP to be considered equal.
TRUE ## Footnote If the prefix is learned from an iBGP advertisement, the IGP cost must match.
63
Fill in the blank: The **AS_Path content** can differ in the case of __________.
confederations ## Footnote Confederations can contain a different AS_CONFED_SEQ path.
64
What command is used to enable **eBGP multipath**?
maximum-paths number-paths ## Footnote This command specifies the allowed number of eBGP paths to install in the RIB.
65
What does the command **maximum-paths ibgp number-paths** do?
Sets the number of iBGP routes to install in the RIB ## Footnote This command is used to configure iBGP multipath settings.
66
What is the first tool evaluated in the **inbound policy order** for BGP?
Prefix-list ## Footnote This is the initial step in determining which routes are accepted from a BGP neighbor.
67
What is the second tool evaluated in the **inbound policy order** for BGP?
Distribute-list (if present) ## Footnote This tool is used to control the redistribution of routes into BGP.
68
What is the third tool evaluated in the **inbound policy order** for BGP?
Route-map (if present) ## Footnote Route-maps can modify attributes of routes based on defined conditions.
69
What is the fourth tool evaluated in the **inbound policy order** for BGP?
Filter-list (AS-PATH filters) ## Footnote This tool is used to filter routes based on their AS-PATH attributes.
70
Which of the following are reasons a BGP neighbor relationship might not form? (Choose two.) The BGP timers are mismatched. The BGP packets are sourced from the wrong IP address. The neighbor is reachable using a default route. The network command is misconfigured.
A BGP neighbor relationship will not form if packets are sourced from the wrong IP address. In addition, the neighbor must be reachable from a route in the routing table other than the default route.
71
What is the BGP state of a neighbor if a TCP session cannot be formed? Open Idle Active Established
If a TCP session cannot be formed, the BGP state will be Idle.
72
73
Do **BGP timers** have to match between neighbors?
No ## Footnote BGP uses the lowest timers set between the two neighbors.
74
What's the administrative distance of eBGP routes?
20
75
What's the administrative distance of iBGP routes?
200
76
To enable **BGP authentication**, what command should be placed under the neighbor session parameters?
neighbor ip-address password password ## Footnote neighbor 10.12.1.1 password CISCOBGP
77
What is the default **hold timer** duration for maintaining a BGP session?
3 minutes (180 seconds) ## Footnote A packet must be received every 3 minutes to keep the BGP session active.
78
By default, BGP sends a **KEEPALIVE** message to a BGP neighbor every _______.
60 seconds ## Footnote This message helps maintain the BGP session by confirming the connection.
79
What command is used to modify the **BGP timers** for a session?
neighbor ip-address timers keepalive holdtime [minimum-holdtime] ## Footnote neighbor 10.12.1.2 timers 10 40 (10 is hello time and 40 is hold time)
80
What is the **TTL** on **eBGP packets**?
1 ## Footnote This setting limits the lifespan of eBGP packets to a single hop.
81
What is the **TTL** on **iBGP packets**?
255 ## Footnote This allows for multi-hop sessions in iBGP configurations.
82
What do **iBGP peers** do with the **next-hop address** if it is not 0.0.0.0?
They do not modify the next-hop address ## Footnote This behavior is important for maintaining the integrity of routing information in BGP.
83
What happens when an **RR** receives an **NLRI** from a **non-RR client**?
The RR advertises the NLRI to an RR client ## Footnote It does not advertise the NLRI to a non-RR client.
84
If an **RR** receives an **NLRI** from an **RR client**, what does it do?
It advertises the NLRI to RR clients and non-RR clients ## Footnote This ensures that both types of clients receive the information.
85
What is the action of an **RR** when it receives a **route** from an **eBGP peer**?
It advertises the route to RR clients and non-RR clients ## Footnote This allows for broader dissemination of routing information.
86
What command is used under the **neighbor address family configuration** to designate a route reflector client?
neighbor ip-address route-reflector-client ## Footnote This command is essential for configuring BGP route reflectors in a network.
87
What is the purpose of the **Originator** attribute in BGP?
To set the value to the RID of the router that injected/advertised the prefix into the AS ## Footnote This attribute is created by the first route reflector.
88
True or false: The **Originator** attribute can be overwritten if it is already populated on a route.
FALSE ## Footnote If Originator is already populated, it should not be overwritten.
89
What happens when a router receives a route with its **RID** in the **Originator** attribute?
The route is discarded ## Footnote This is a safety mechanism to prevent routing loops.
90
What is the **Cluster List** in BGP?
A non-transitive BGP attribute updated by the route reflector ## Footnote This attribute is crucial for managing route reflection in BGP.
91
How does the route reflector update the **Cluster List** attribute?
It appends its cluster ID to the attribute ## Footnote The cluster ID is by default the BGP identifier.
92
What happens if a route reflector receives a route with its **cluster ID** in the Cluster List attribute?
The route is discarded ## Footnote This mechanism prevents routing loops in BGP.
93
True or false: The **Cluster List** attribute is a transitive BGP attribute.
FALSE ## Footnote The Cluster List is a non-transitive attribute.
94
What is a **confederation** in the context of autonomous systems?
A confederation consists of sub-ASs known as member ASs that combine into a larger AS known as an AS confederation ## Footnote This structure allows for the organization and management of multiple autonomous systems under a unified framework.
95
What is the first step to configure a **BGP confederation**?
Initialize the BGP process with the global command router bgp member-asn | router bgp 65001 (confederation sub-asn) ## Footnote This command starts the BGP configuration for the specified member Autonomous System Number (ASN).
96
What command is used to identify the **BGP confederations**?
bgp confederation identifier as-number | bgp confederation identifier 200 (non confederation asn) ## Footnote This command sets the identifier for the BGP confederation.
97
On routers that directly peer with another member AS, what command identifies the **peering member AS**?
bgp confederation peers member-asn ## Footnote This command specifies the member AS that the router is peering with.
98
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