2-4 - Troubleshoot BGP

Question 1

Describe the characteristics of BGP (Border Gateway Protocol).

Answer 1

• path-vector routing protocol
• sends updates to its neighbors using reliable transport (TCP)
• highly scalable protocol for the “Internet”
• establishes neighborships
• each administrative domain has a unique AS number

Question 2

What is iBGP (internal BGP)?

Answer 2

when both neighbors are in the same AS

Question 3

What is eBGP (external BGP)?

Answer 3

when both neighbors are in different AS

Question 4

What are the 3 basic parts of BGP configuration?

Answer 4

1. Define the BGP process
2. Establish the neighbor relationship
3. Advertise the networks into BGP

Question 5

IOS-XR command:
show bgp summary

Answer 5

shows the list of BGP neighborships, their current states, amount of received prefixes, and other info

Question 6

IOS-XE command:
show ip bgp summary

Answer 6

shows the list of BGP neighborships, their current states, amount of received prefixes, and other info

Question 7

Describe these fields of the BGP summary table:

BGP route identifier
Neighbor
AS
MsgRcvd
MsgSent
TblVer
InQ
OutQ
Up/Down
State
PfxRcd

Answer 7

BGP route identifier: IP address that all other BGP speakers recognize as the address of this router
Neighbor: IP address of the remote neighbor
AS: AS number of the listed neighbor
MsgRcvd: Number of BGP messages received from neighbor
MsgSent: Number of BGP messages sent to neighbor
TblVer: BGP table version
InQ: number of messages waiting to process from this neighbor
OutQ: number of messages that are queued and waiting to transmit to this neighbor; TCP flow control prevents this router from overwhelming a neighbor with a large update
Up/Down: length of time this neighbor has been in its current state
State: the BGP state; could be established, active, idle, open sent, open confirm, or idle (admin)
PfxRcd: amount of prefixes being received from that neighbor (when session is established)

Question 8

IOS-XR command:
network

Answer 8

in BGP subconfig mode, advertise a network into BGP

Question 9

IOS/IOS-XE command:
network

Answer 9

in BGP subconfig mode, advertise a network into BGP

Question 10

IOS-XR command:
route-policy [route-policy name] [in|out]

Answer 10

in neighbor subconfig mode, applies a route policy to a neighbor in a particular direction

Question 11

IOS-XR command:
show bgp

Answer 11

shows the BGP table and all known prefixes, along with other details

Question 12

IOS/IOS-XE command:
show ip bgp

Answer 12

shows the BGP table and all known prefixes, along with other details

Question 13

Explain the following BGP prefix status codes:

s
d
h
r
S

Answer 13

s: suppressed (e.g. summarization)
d: dampening (route is penalized for going up and down too often, so its not advertised until the penalty expires)
h: history (route is unavailable and probably down, but historic info of the route exists)
r: rib failure (route did not install in the RIB)
S: stale (appears in the NSF-aware router if a route is stale)

Question 14

IOS-XR command:
update-source [interface]

Answer 14

in the neighbor subconfig mode, set the local update-source address to be used for the neighborships

Question 15

IOS/IOS-XE command:
neighbor [x.x.x.x] update-source [interface]

Answer 15

in the BGP subconfig mode, set the local update-source address to be used for the neighborships

Question 16

Full-mesh iBGP is needed because of the BGP split-horizon rule. What two alternatives can be used instead of a full mesh of iBGP neighborships that overcomes BGP split-horizon rule while being more scalable?

Answer 16

• BGP route reflector
• BGP confederations

Question 17

Describe the BGP neighbor states

Idle
Open Sent
Open Confirm
Active
Established

Answer 17

Idle: BGP session is down
Connect: when SYN-ACK packet is received (TCP 3 way handshake)
Open Sent: when the initial BGP Open message is sent
Open Confirm: when the initial BGP Open message is received
Active: router has sent an Open packet and is waiting for a response.
Established: BGP session is up

Question 18

IOS-XR command:
bgp consistency-checker [error-message | auto-repair] [interval (minutes)]

Answer 18

in BGP subconfig mode, configures the consistency checker, which will identify route inconsistencies and take an action to correct it

Question 19

IOS-XR command:
show bgp neighbor

Answer 19

show BGP neighbor details

Question 20

IOS/iOS-XE command:
show ip bgp neighbor

Answer 20

show BGP neighbor details

Question 21

debug ip tcp transactions

Answer 21

find valuable information about failed connection attempts

Question 22

debug ip bgp events

Answer 22

all BGP events will display on the console as they happen

Question 23

debug ip bgp keepalives

Answer 23

get a printout on the console for every keepalive packet that is sent or received

Question 24

debug ip bgp updates

Answer 24

get a printout on the console for every update message that is sent or received

Question 25

debug ip tcp transactions

Answer 25

displays the connect attempts

Question 26

Which of the following statements best describes BGP?

A. A link state protocol where each router advertises its connected interfaces
B. A distance vector protocol that uses the best path based on a metric
C. A path-vector protocl that contains the destination network of the next router
D. An interior gateway protocol that operates within the service provider network

Answer 26

C

Question 27

Which of the following statements is true about BGP?

A. BGP sends routing updates to its neighbors by using an unreliable transport
B. The unreliable transport mechanism that BGP uses is standard UDP
C. BGP computes shortest paths with attributes that attach to reachable IP subnets
D. The sender of the information does not always know if the receiver has the information

Answer 27

C