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CCNP Core > Chap 11 - BGP (part 1) > Flashcards

Chap 11 - BGP (part 1) Flashcards

1
Q

From a BGP perspective what is an Autonomous System?

A
  • A collection of routers under a single organization’s control using one or more IGPs and common metrics to route packets within the AS.
  • If multiple IGPs or metrics are used within the AS then the routing policy must appear consistent to external ASs.
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2
Q

What are the 2 blocks of private AS numbers?

A
  • 64,512 to 65,535
  • 4,200,000,000 to 4,294,967,294
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3
Q

Who assigns public AS numbers?

A

IANA

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4
Q

What 3 items does IANA require before assigning a public ASN?

A
  • Proof of a publicly allocated network range
  • Proof that Internet connectivity is provided through multiple connections
  • Need for a unique routing policy from providers
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5
Q

If an organization cannot provide the 3 items IANA requires to get an AS number what should they do?

A

Use the ASN from their service provider.

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6
Q

What are PA’s?

A

Path Attributes

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7
Q

What are the 4 classes of BGP Prefix Path Attributes?

A
  • Well-known mandatory
  • Well-known discretionary
  • Optional Transitive
  • Optional non-transitive
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8
Q

What is the difference between Well-known Mandatory and Well-known Discretionary Path Attributes?

A
  • Well-known mandatory PA’s must be included with every prefix advertisement
  • Well-known discretionary PA’s may or may not be included
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9
Q

What are 2 characteristics of Optional Path Attributes?

A
  • Recognition by other BGP implementations is optional
  • Can be transitive or non-transitive
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10
Q

What does an NLRI include?

A
  • Prefix length
  • Prefix
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11
Q

What is contained in a BGP Update message?

A
  • Withdrawn routes
  • A single feasible route in the form of an NLRI
  • Path Attributes
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12
Q

Does BGP contain a complete topology of the network?

A

No

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13
Q

How does AS_Path prevent loops?

A

If a router receives a prefix advertisement with its own AS listed in the AS_Path attribute it discards the prefix thinking that it forms a loop

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14
Q

What does AFI stand for and what one thing does it do?

A
  • Address Family Identifier
  • Enables BGP to support specific network protocols such as IPv4 and IPv6 in the same BGP session.
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15
Q

What does MP-BGP stand for and what 2 extensions does it add?

A
  • Multi-protocol BGP
  • Adds AFI and SAFI extensions
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16
Q

What 2 things does a SAFI do?

A
  • Provides additional granularity to MP-BGP
  • Identifies multicast or unicast
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17
Q

What 5 things does MP-BGP do to achieve AFI separation?

A
  • Each AFI has its own config and database
  • Each AFI has its own routing policy
  • Each AFI has its own NLRI
  • 2 additional PA’s are added for granularity
    • MP_REACH_NLRI
    • MP_UNREACH_NLRI
  • AFI’s and SAFI’s are carried inside every BGP Update messages
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18
Q

What port does BGP use?

A

TCP 179

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19
Q

What advantages does TCP bring to BGP?

A
  • TCP allows for fragmentation and sequencing
  • Reliability through acknowledgements and retransmissions
  • Neighbors can be multiple hops away
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20
Q

Do recent implementations of BGP allow fragmentation?

A

No

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21
Q

If fragmentation is not enabled what does BGP do instead?

A

Relies on MTU discovery

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22
Q

What is a BGP session?

A

The established adjacency between 2 BGP routers.

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23
Q

What is required for a BGP session that is multiple hops away?

A

It requires a route to the neighbor in its RIB from static routes or other routing protocols.

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24
Q

Can BGP be thought of as a Control Plane routing protocol or application?

A

Yes because it allows for the exchange of routes with a peer that is multiple hops away.

25
Q

What is the administrative distance of iBGP?

A

200

26
Q

What is iBGP?

A
  • Internal BGP
  • Within the same AS or in the same BGP Confederation
27
Q

What is eBGP?

A
  • External BGP
  • Session established with a router in a different AS.
28
Q

What is the administrative distance of eBGP?

A

20

29
Q

What are 2 times iBGP might be needed?

A
  • When multiple routing policies are needed
  • When transit connectivity is provided through an AS (Company A connects to Company C through Company B)
30
Q

Why is redistributing the BGP table into an IGP not a viable solution?

A
  • Scalability (too many routes)
  • Custom Routing - BGP uses PA’s to calculate path, IGPs use a metric. A BGP path might be suboptimal when compared with the IGP path
  • An IGP can’t understand Path Attributes
31
Q

What is one viable solution to providing connectivity in an AS using iBGP?

A

Establishing a full mesh where all routers are neighbors with each other

32
Q

How many BGP message types are there?

A

4

33
Q

What are the 4 BGP message types?

A
  • OPEN - establishes adjacency
  • UPDATE - advertises, updates, or withdraws routes
  • NOTIFICATION - indicates an error condition to a BGP neighbor
  • KEEPALIVE - ensures that the BGP neighbor is still alive
34
Q

What 5 things are in an OPEN message?

A
  • BGP version number
  • Originating router’s ASN
  • Hold time
  • BGP Router ID
  • Other optional parameters for session capabilities
35
Q

What 3 routing changes happen by the local host if the neighbor’s Hold Timer reaches 0?

A
  • BGP session is torn down
  • Routes from that neighbor are removed
  • An UPDATE route withdraw message sent to other BGP neighbors
36
Q

How is the Hold Timer negotiated between two neighbors?

A

Both routers propose a Hold Time in the OPEN message. The shortest of the two is agreed upon.

37
Q

What are the 2 criteria for Hold times?

A
  • Must be at least 3 seconds
  • Must be 3 times the Keepalive interval
38
Q

What are 4 characteristics about the BGP Identifier?

A
  • It is the router ID.
  • 32-bit UNIQUE number
  • Must be non-zero or neighborships won’t establish
  • Can be set manually or dynamically
39
Q

What is a heartbeat mechanism that ensures a neighbor is up and healthy?

A

The Hold timer.

40
Q

How often are BGP KEEPALIVE messages sent?

A

By default they are exchanged every 60 seconds or 1/3 of the hold time.

41
Q

When are 4 times when a NOTIFICATION message is sent?

A
  • An error in the BGP session is detected
  • Hold Timer expired
  • Neighbor capabilities changed
  • BGP session reset was requested
42
Q

What does SAFI stand for?

A

Subsequent Address Family Identifier

43
Q

What 2 facts are true about Well-known Mandatory attributes?

A
  • Must be recognized by all BGP speakers
  • Must be included in all update messages
44
Q

What kind of BGP attribute is AS_PATH?

A

Well-known mandatory

45
Q

What is contained in the AS_PATH attribute?

A

The list of all autonomous systems the prefixes contained in this update have passed through.

46
Q

How does the AS_PATH attribute prevent loops?

A

If a router receives an NLRI with that router’s own AS listed the router assumes a loop and drops the UPDATE.

47
Q

What happens if the HOLD timer is set to 0?

A

Keep-alives are disabled.

48
Q

What does NLRI stand for?

A

Network Layer Reachability Information

49
Q

What 3 things does an UPDATE message do?

A
  • Advertises feasible routes
  • Withdraws previously advertised routes (prefix only)
  • Can function as a keep-alive to reduce bandwidth
50
Q

What is the default HOLD timer interval?

A
  • 180 seconds (3 times the KEEP-ALIVE value)
51
Q

What is the HOLD timer?

A

A countdown timer for each neighbor.

52
Q

What 2 things can reset the HOLD timer?

A
  • An UPDATE message
  • A KEEP-ALIVE
53
Q

What is the purpose of the HOLD timer?

A

To make sure the neighbor is alive.

54
Q

How can the Router ID be used to detect loops?

A

The Router ID appears in all UPDATE messages. If a router receives an UPDATE message with its own Router ID in it then there’s a loop.

55
Q

How is eBGP different from iBGP in regards to their BGP peers?

A

eBGP router expects its peer to be in a different AS. iBGP router expects its peer to be in the same AS.

56
Q

How is eBGP different than iBGP when it comes to TTL and what effect does it have?

A
  • eBGP TTL is 1, iBGP TTL is 255
  • eBGP’s TTL of 1 prevents neighborships over multiple hops.
57
Q

What does an eBGP router do to prevent loops that an iBGP router does not?

A
  • eBGP router prepends its own AS to any route it advertises.
  • When the eBGP router receives a route that has its own AS number in the AS Path it discards it.
58
Q

What does a BGP NOTIFICATION message do?

A

It causes the BGP session to close.

59
Q

How many NLRIs and withdrawn routes in an update message?

A

Only 1 NLRI per update message but it can have multiple withdrawn routes

CCNP Core (46 decks)

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