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ENARSI > 11. BGP > Flashcards

11. BGP Flashcards

1
Q

What is a BGP session?

A

An esthablished adjacency between 2 BGP routers

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2
Q

What are the 2 types of BGP sessions?

A
  • iBGP (in the same ASN)

- eBGP (not in the same ASN)

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3
Q

What are the 4 BP path attributes (PAs)?

A
  • Well-known mandatory
  • Well-known discretionary
  • Optional transative
  • Optional non-transative
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4
Q

What is the difference between well-known and optional PAs?

A

Well-known PAs need to be recognized by all BGP implementations

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5
Q

What is the difference between well-known mandatory and discretionary?

A

Mandatory must be included in every prefix advertisement, discretionary may of may not

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6
Q

What is the difference between transative and non-transative PAs?

A

Transative stay with the route advertisement. Non-transative cannot be shared from AS to AS.

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7
Q

What is the Network Layer Reachability Information?

A

The NLRI is the routing update that consists of:

  • The network prefix
  • Prefix length
  • Any BGP PAs for that specific route
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8
Q

What type of routing protocol is BGP?

A

A path vector, but behaves as a distance vector.

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9
Q

What is the AS_path attribute?

A

A well-know mandatory that prevents routing loops. If a router receives an advertisement with his AS, it discards

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10
Q

What is AFI?

A

Address family identifier, it correlates to a specific network protocol

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11
Q

What is SAFI?

A

Subsequent address family identifier, such as unicast or multicast

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12
Q

What is the MP_REACH_NRLI?

A

A multiprotocol attribute to allow advertisement of various protocols

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13
Q

What is the MP_UNREACH_NRLI?

A

Used to withdraw routes or removing them from advertisement

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14
Q

What is the BGP port number?

A

179

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15
Q

Can BGP run over multiple hops?

A

Yes, but an underlaying route is needed for this. This can be static or via a routing protocol.

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16
Q

How do BGP neighbors connect?

A

They use the ARP table, or routing table for Multi-hop

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17
Q

What are the 4 BGP messages?

A
  • Open
  • Update
  • Notification
  • Keepalive
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18
Q

What is in the open message?

A
  • BGP version
  • ASN
  • Hold time
  • BGP identifier (RID)
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19
Q

What is the hold time attribute?

A

It sets the hold timer. Upon receipt of an update or keepalive the timer will resets. If the timer reaches 0 the session will be torn down.

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20
Q

What is the default hold time?

A

180 seconds, must be at least 3

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21
Q

What is the BGP identifier?

A

The RID which must be higher than 0

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22
Q

What is in the keepalive message?

A

Messages that are exchanged every 1/3 between peers. Can be turned of by placing the hold timer on 0. KEEPALIVE is also used as ACK.

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23
Q

What is in the update message?

A

The NLRI, routing updates etc

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24
Q

What is the finite-state machine?

A

FSM is used to maintain a table of all BGP peers and their status.

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25
What are the 6 BGP states?
- Idle - Connect - Active - OpenSent - OpenConfirm - Established
26
What happens in the Idle stage?
BGP detects a start event, tries to initiate a TCP connection, and listens for a new connection from another peer
27
What happens if an error causes BGP to go back to the Idle state for the second time?
The ConnectRetry timers is set to 60 seconds and must decrement to 0 before a new connection can be initiated. Further errors wil double the timer in legnth of the previous one
28
What happens in the Connect stage?
BGP initiates the connection. If the three-way handshake completes, the ConnectRetry timer resets, an OPEN message will be send to the neighbor. The status will be changed to OpenSent
29
What happens if the ConnectRetry timer depletes before the Connect stage has completed?
A new TCP connection is attempted, the ConnectRetry timer is reset and the state is set to Active. The neighbor with the higher IP manages the connection
30
What happens in the Active stage?
BGP start a new three-way handshake,. If succesful an OPEN message is send, the hold timer placed 4 minutes and the status changed to OpenSent.
31
What happens if the connection attempt fails in the Active state?
The state moves back to Connect and the ConnectRetry timer is reset
32
What happens in the OpenSent stage?
The router will wait for an open message and will compare: - BGP version - Source IP - AS - RID - Security parameters The hold timer is negotiated (lower wins) and a KEEPALIVE is send (if not set to 0). The state if moved to OpenConfirm
33
What happens when an error is found in the OPEN message during the OpenSent stage?
A notification message is sent, and the state is moved back to Idle
34
What happens when TCP receives a disconnect message in the OpenSent state?
BGP closes the connection and goes back to Active
35
What happens in the OpenConfirm stage?
BGP waits for a KEEPALIVE of NOTIFICATION. If a KEEPALIVE is received the state will move to established. With NOTIFICATION back to idle
36
What happens in the Established stage?
Routes will be exchanged with UPDATE messages.
37
What are the steps to activate BGP?
- Router BGP - RID - Neighbor IP - Neighbor interface (optional) - Authentication (optional) - Modify timers (optional) - Address family - Activate
38
What differs the IPv4 address family from the others?
The IPv4 address family is activated by default
39
What is the command to turn off automatic address family activation?
no bgp default ip4-unicast
40
What is the command to verify BGP sessions?
show bgp afi safi summary
41
What are the three types of tables that BGP uses and how many are there?
- Adj-RIB-in (1 per peer) - Loc-RIB - Adj-RIB-out (1 per peer)
42
What is in the Adj-RIB-in ?
NLRI routes before inbound policies have been applied. The table is cleared after all route policies are processed. All unprocessed routes.
43
What is in the Loc-RIB?
Contains the NLRI routes originated locally or received from other peers. After the validity and next-hop reachability check, BGP best path algorithm selects the best NLRI for a prefix
44
What is in the Adj-RIB-out?
Contains NLRI routes after policies have been applied. All processed routes.
45
What happens if you use the network statement in BGP?
The BGP process will search the global RIB for an exact network prefix. If this is visible it will install the prefix in the Loc-RIB
46
What are the PAs given to a prefix installed in the Loc-RIB?
Connected: Next-hop 0.0.0.0, Origin i for iBGP, weight 32,768 -> no metric Static/protocol: Next-hop RIB, Origin i for iBGP, weight 32,768, MED is the IBP metric
47
What is the process for advertisement to other peers in BGP?
- Verify if the NLRI is valid and that it can be resolved in the RIB - Place in Loc-RIB - Validity check - Process outbound policies - Place in Adj-RIB-out - Advertise
48
What is the BGP process for routes received from other peers?
- Store the route in Adj-RIB-in - Apply inbound policies - Update the Loc-RIB and clear Adj-RIB-in - Validity check (if fails the route remains in the loc-RIB) - Identify best path and pass only the best path and its attributes - Install best path in the global RIB - Apply outbound policies - Place in Adj-RIB-out - Advertise to other peers
49
What command shows the Loc-RIB?
show bgp afi safi
50
What does the asterix indicatie in the Loc-RIB table?
That the route is valid
51
What does the angle bracket indicatie in the Loc-RIB table?
That this is the best path
52
What type of PA is next hop?
Well-known mandatory
53
What type of PA is Multiple-exit discriminator (MED) 'metric'?
Optional non-transative
54
What type of PA is LocPrf (local preference)?
Well-known discretionary
55
With what command can you check the Adj-RIB-out table?
Show bgp afi safi neighbors A.A.A.A advertised routes
56
What is the AD of BGP?
20 for EBGP | 200 for IBGP
57
Why must iBGP require full mesh?
iBGP is not allowed to advertise routes received by other BGP peers. To overcome this BGP peers must establish connections to all BGP peers in the AS
58
What are the reserved 2-octet ASNs?
- 0 - 23456 - 64496 - 64511 - 64512 - 65534 (private) - 65535
59
What is the capability code of the 4-octet ASN?
65
60
What are the reserved 4-octet ASNs?
4,200.000.000 - 4,294,967,295
61
What are the public 4-octet ASNs?
1.16 - 4,199,999,999
62
What is the differene between ASDOT and ASPLAIN notations?
ASPLAIN is decimal and default | ASDOT is binairy
63
What are type of PAs are AS4_PATH & AS4_AGGREGATOR?
Optional transative that provide backwards compatibility
64
Where is the ASN 23456 for?
To provide transition between 2-octet and 4-octet systems
65
Where do the TCP messages FIN, PSH, RST & ECN stand for?
FIN - Finish session PSH - Needs to happen right now RST - Reset ECN - network congestion
66
What is the command to turn on the neighbors received routes?
neighbors NETWORK soft-reconfiguration inbound
67
How can you connect not directly connected peers in eBGP?
- Enable the eBGP multihop | - Disable the connected-check
68
What is the default eBGP TTL?
1

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