BGP

Question 1

AS numbers main purpose

Answer 1

To determine the best path to a prefix

To prevent routing loops between ASs

Question 2

Private BGP AS number range

Answer 2

64512 to 65534

Question 3

BGP peers communicate over which TCP Port

Answer 3

TCP port 179

Question 4

BGP Session States

Answer 4

Idle: Tries to initiate a TCP session with its peer

Connect: Router with the highest ip address manages the connection state

Active: The router with the highest ip (The active router) initiates another TCP connection with the Passive router

OpenSent: Peers begin performing some BGP specific sanity checks

OpenConfirm: The peers wait to receive a keepalive message.

Established: After each peer receives a keepalive

Question 5

Keepalive Message Time and Hold Time

Answer 5

60 seconds. Hold Time is 180 seconds

Question 6

(*) Asterisk Indication in the BGP Table

Answer 6

Indicates the next-hop address is reachable. This means that the IP routing table has the route to reach the next hop.

Question 7

The greater-than symbol (>)

Answer 7

Indicates the best path. BGP only advertises the best path to its peers.

Question 8

Weight

Answer 8

Control which route is chosen as the best path. Locally significant. The default is 0 and 32,768 for self-generated routes. A higher weight is preferred.

Weight in show ip bgp

Question 9

Local Preference

Answer 9

Influences the path traffic takes to prefixes outside an AS. The default value for local preference is 100. A higher local preference is preferred.

LocPrf in show ip bgp

Question 10

Originate

Answer 10

Paths sourced locally are preferred.

Question 11

AS Path Length

Answer 11

The number of autonomous systems in the AS_Path path attribute. Lower AS path lengths are preferred.

Path in show ip bgp

Question 12

Origin Type

Answer 12

Indicates how BGP learned the route. Incomplete and a (?) means the source prefix was redistributed.

IGP (i) preferred over Incomplete (?)

Question 13

Multi-Exit Discriminator (MED)

Answer 13

Reversal of the local preference. Influences how traffic ingresses an AS. A lower MED is preferred.

Metric in show ip bgp

Question 14

Paths

Answer 14

eBGP routes take preference over iBGP routes. eBGP routes have an AD of 20, where iBGP routes have an AD of 200.

Question 15

Oldest Route

Answer 15

BGP prefers the oldest installed route. It will hold onto the existing installed route as the best path

Question 16

r for RIB failure

Answer 16

(r) indicates the BGP wasnt able to install the route in the routing table

Question 17

BGP Redistribution

Answer 17

BGP in OSPF
R1(config)# router ospf 1
R1(config-router)# redistribute bgp 65001 subnets

BGP in EIGRP
R4(config)# router eigrp 100
R4(config-router)# redistribute bgp 65004 metric 100000 1 255 1 1500

Question 18

Modifying Weight

Answer 18

ISP(config)# router bgp 65550
ISP(config-router)# neighbor 203.0.113.1 weight 100

The default weight is 0

Question 19

Advertising Summary Routes

Answer 19

Use Route Aggregation

R1(config)# router bgp 65001
R1(config-router)# aggregate-address 10.0.0.0 255.255.0.0 summary-only

Question 20

Component Routes

Answer 20

Marked with an (s) for suppressed. Component routes are suppressed. They still exist in the RIB and IP routing table but prefixes are not advertised.

Question 21

Route-Map

Answer 21

Have a default deny behavior like ACLs. Use an ip prefix-list

ip prefix-list R4-summary permit 10.0.32.0/19
ip prefix-list R4-summary permit 10.0.23.0/29
ip prefix-list R4-summary deny 0.0.0.0/0 le 32

ISP(config)# route-map R4 deny 10
route-map R4 permit 20
ISP(config-route-map)# match ip address prefix-list R4-summary

ISP(config)# router bgp 65550
ISP(config-router)# neighbor 198.51.100.1 route-map in

set weight 101 gives routes in the route map a higher weight

Question 22

BGP Neighbor Authentication

Answer 22

R1(config)#router bgp 65001

R1(config-router)#neighbor 198.51.100.1 password cisco123

Question 23

Router ID

Answer 23

A tie-breaker, where the route received from a router with the lowest router ID is preferred.