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BGP Questions

July 24th, 2017 in ROUTE 300-101 Go to comments

BGP Quick Summary:

Protocol type: Path Vector
Type: EGP (External Gateway Protocol)
Packet Types: Open, Update, KeepAlive, Notification
Administrative Distance: eBGP: 20; iBGP: 200
Transport: TCP port 179
Neighbor States: Idle -> Active -> Connect -> Open Sent -> Open Confirm -> Established
1 – Idle: the initial state of a BGP connection. In this state, the BGP speaker is waiting for a BGP start event, generally either the establishment of a TCP connection or the re-establishment of a previous connection. Once the connection is established, BGP moves to the next state.
2 – Connect: In this state, BGP is waiting for the TCP connection to be formed. If the TCP connection completes, BGP will move to the OpenSent stage; if the connection cannot complete, BGP goes to Active
3 – Active: In the Active state, the BGP speaker is attempting to initiate a TCP session with the BGP speaker it wants to peer with. If this can be done, the BGP state goes to OpenSent state.
4 – OpenSent: the BGP speaker is waiting to receive an OPEN message from the remote BGP speaker
5 – OpenConfirm: Once the BGP speaker receives the OPEN message and no error is detected, the BGP speaker sends a KEEPALIVE message to the remote BGP speaker
6 – Established: All of the neighbor negotiations are complete. You will see a number, which tells us the number of prefixes the router has received from a neighbor or peer group.
Path Selection Attributes: Weight > Local Preference > Self-originates > AS Path > Origin > MED > External > IGP Cost > eBGP Peering > Router ID
Authentication: MD5
BGP Origin codes: i – IGP (injected by “network” statement), e – EGP, ? – Incomplete
AS number range: Private AS range: 64512 – 65535, Globally (unique) AS: 1 – 64511

More information about popular Path Selection Attributes
Weight Attribute:
+ Cisco proprietary
+ First attribute used in Path selection
+ Only used locally in a router (not be exchanged between BGP neighbors)
+ Higher weight is preferred
Weight_BGP_Attribute_Influence.jpg

Local Preference (LocalPrf) Attribute:
+ Sent to all iBGP neighbor (not be exchanged between eBGP neighbors)
+ Used to choose the path to external BGP neighbors
+ Higher value is preferred
+ Default value is 100

LocalPreference_BGP_Influence.jpg

MED Attribute:
+ Optional nontransitive attribute (nontransitive means that we can only advertise MED to routers that are one AS away)
+ Sent through ASes to external BGP neighbors
+ Lower value is preferred (it can be considered the external metric of a route)

MED_BGP_Attribute_Influence.jpg

 

Question 1

Explanation

Private autonomous system (AS) numbers which range from 64512 to 65535 are used to conserve globally unique AS numbers. Globally unique AS numbers (1 – 64511) are assigned by InterNIC. These private AS number cannot be leaked to a global Border Gateway Protocol (BGP) table because they are not unique (BGP best path calculation expects unique AS numbers).

Reference: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13756-32.html

Question 2

Explanation

If MTU on two interfaces are mismatched, the BGP neighbors may flap, the BGP state drops and the logs generate missing BGP hello keepalives or the other peer terminates the session.

For more information about MTU mismatched between BGP neighbors please read: http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/116377-troubleshoot-bgp-mtu.html

Question 3

Explanation

Notice that the Administrative Distance (AD) of External BGP (eBGP) is 20 while the AD of internal BGP (iBGP) is 200.

Question 4

Explanation

Private autonomous system (AS) numbers which range from 64512 to 65535 are used to conserve globally unique AS numbers. These private AS number cannot be leaked to a global BGP table because they are not unique.

Question 5


Explanation

BGP Neighbor states are: Idle – Connect – Active – Open Sent – Open Confirm – Established

Question 6


Explanation

BGP peers are established by manual configuration between routing devices to create a TCP session on (destination) port 179.

Question 7


Explanation

Below is the list of BGP states in order, from startup to peering:

1 – Idle: the initial state of a BGP connection. In this state, the BGP speaker is waiting for a BGP start event, generally either the establishment of a TCP connection or the re-establishment of a previous connection. Once the connection is established, BGP moves to the next state.
2 – Connect: In this state, BGP is waiting for the TCP connection to be formed. If the TCP connection completes, BGP will move to the OpenSent stage; if the connection cannot complete, BGP goes to Active
3 – Active: In the Active state, the BGP speaker is attempting to initiate a TCP session with the BGP speaker it wants to peer with. If this can be done, the BGP state goes to OpenSent state.
4 – OpenSent: the BGP speaker is waiting to receive an OPEN message from the remote BGP speaker
5 – OpenConfirm: Once the BGP speaker receives the OPEN message and no error is detected, the BGP speaker sends a KEEPALIVE message to the remote BGP speaker
6 – Established: All of the neighbor negotiations are complete. You will see a number (2 in this case), which tells us the number of prefixes the router has received from a neighbor or peer group.

Question 8

Question 9

Question 10

Comments
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  1. Sorlags
    November 15th, 2018

    New question : How activate the Multiprotocol for BGP on a router?
    I don’t rembember the exacly answer and how many are right. But I can try

    A) no bgp default ipv4-unicast
    B) bgp default ipv4-unicast
    C) bpg-mp enable
    D) no bgp default ipv6-nexthop
    E) bgp default ipv6-nexthop
    F) ???
    D) ???

    Answer : A

    Cisco info on MP-BGP : https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_l3_vpns/configuration/15-mt/mp-l3-vpns-15-mt-book/mp-bgp-mpls-vpn.html#GUID-BE5E2865-5D2B-45AE-830D-65CACC35320A

  2. Sidhu
    November 29th, 2018

    Answer ,A, is correct while By default, neighbors that are defined using the #neighbor remote-as# command in router configuration mode exchange only unicast address prefixes. so we disable the default configuration. To exchange other address prefix types,
    such as multicast and VPNv4, neighbors must also be activated using the neighbor activate
    command in address family configuration mode.

  3. Dany1
    December 2nd, 2018

    Question 1 -> wrong explanation
    Question 4-> wrong answer. Correct answer is C. 64512-65534 (private AS numbers)

    According to rfc6996, section 5:” IANA Considerations

    IANA has reserved, for Private Use, a contiguous block of 1023
    Autonomous System numbers from the “16-bit Autonomous System Numbers”
    registry, namely 64512 – 65534 inclusive.

    IANA has also reserved, for Private Use, a contiguous block of
    94,967,295 Autonomous System numbers from the “32-bit Autonomous
    System Numbers” registry, namely 4200000000 – 4294967294 inclusive.

    So, that is true about AS numbers:
    AS range:
    1-64.495: public AS numbers.
    64.512 – 65.534: private AS numbers.
    Reserved :
    0:-reserved
    64.496 – 64.511 – reserved to use in documentation.
    65.535 – reserved.

    Digitaltut.com please correct your answers according to RFC6996

  4. Dany1
    December 2nd, 2018

    Question 8: none of the answers are correct (in my opinion)
    Hypothesis: “influence traffic from AS200 and AS300 so that it uses link to reach AS100”

    My translation is : multi-home (AS100 with links) to several ISPs(AS 200 and AS 300). One of the solution in that scenario to influence traffic is named AS_Path_Prepending

    MED is solution for multi-home (AS 100 with links) to only one ISP (AS 300, for example)

  5. Dany1
    December 2nd, 2018
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    December 3rd, 2018

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  7. Hank
    January 14th, 2019

    Where is the describe question ?

  8. Sorlags
    February 3rd, 2019

    @Dany1

    The asnwer of Q4 is correct. 65535 is reserved but part of private ASNs.

    https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13756-32.html

    “Private autonomous system (AS) numbers which range from 64512 to 65535 are used to conserve globally unique AS numbers”

  9. $$$$$$
    April 5th, 2019

    Please answer ?

    what is the maximum number of equal metric path BGP uses for load balancing…?

  10. Anonymous
    July 26th, 2019

    Which two options are benefits of BGP peer groups?(Choose two)
    A. A Configuration change can be applied simultaneously to all peers in the peer group
    B. They cam optimize backbone routers
    C. They can be updated via multicast
    D. Each neighbor in a peer group can have different inbound BGP policies
    E. The use soft update to minimize bandwidth consumption
    F. They support groups of paths

    could someone provide correct answer as there conflicts in dumps answers

  11. trueteller
    September 8th, 2019

    Which two options are benefits of BGP peer groups?(Choose two)
    A. A Configuration change can be applied simultaneously to all peers in the peer group
    B. They cam optimize backbone routers
    C. They can be updated via multicast
    D. Each neighbor in a peer group can have different inbound BGP policies
    E. The use soft update to minimize bandwidth consumption
    F. They support groups of paths

    A D

  12. trueteller
    September 8th, 2019

    A is obvious.
    For D -> You can customize the inbound update policy for any member of a peer group.
    https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13755-29.html
    Section “Requirements of Peer Groups”

  13. 300-101
    December 2nd, 2019

    Q4
    @digitaltut & all others who referenced cisco website for Q4

    kindly read RFC 7300
    https://tools.ietf.org/html/rfc7300

    Notable words–> “Although these “Last ASNs” border on Private Use
    ASN [RFC6996] ranges, they are not defined or reserved as Private Use
    ASNs by [IANA.AS].”

    so answer should be 64512-65534

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