BGP

Question 1

What is the command to show the summary of BGP running on a router?

Answer 1

Router1# show ip bgp summary

Question 2

What is meant by a well-known BGP attribute?

Answer 2

This means that it is an industry standard according to RFC 4271 and that all routers that run BGP will have these attributes.

Question 3

What are the three well-known, mandatory BGP path attributes?

Answer 3

1.) AS-Path
2.) Next-Hop
3.) Origin

Question 4

What are the two well-known, discretionary BGP path attributes?

Answer 4

1.) Local Preference
2.) Atomic Aggregate

Question 5

What is meant by a BGP attribute that is transitive?

Answer 5

It means that the router will pass the attribute along to another router if received, even if that router does not support the attribute.

Question 6

What is meant by a BGP attribute that is optional?

Answer 6

The attribute is not compliant with RFC 4271 industry standard.

Question 7

What types of simple messages do BGP routers send to each other?

Answer 7

Open, Keepalive, Update, Notification

Question 8

What are the 6 progressive states of BGP peering?

Answer 8

Idle
Connect
Active
OpenSent
OpenConfirm
Established

Question 9

Briefly describe what is happening in each of the BGP peering states

Answer 9

Idle - nothing is going on. Could mean a neighbor relationship has failed and returned to Idle

Connect - a TCP three way handshake has been sent.

Active - the BGP router is actively trying to create a neighbor relationship

OpenSent - an “Open” message has been sent and awaiting a confirmation message

OpenConfirm - a BGP neighbor has sent back an OpenConfirm message

Established - a BGP neighbor relationship has been formed.

NOTE: You will not see OpenSent, Open Confirm, or Established when trying to view BGP neighbor states.

Question 10

What is the Cisco BGP Best Path Algorithm in order of attribute preference?

Answer 10

1. Weight
2. Local Preference
3. Locally Originated
4. Accumulated IGP (AIGP)
5. Shortest AS-Path
6. Origin Type
7. Lowest Multi-Exit Discriminator (MED/Metric)
8. eBGP over iBGP
9. Oldest Route
10. Lower Peer Router-ID (RID)
11. Lowest Cluster List Length
12. Lowest Next Hop Address

Question 11

What command shows the BGP routing table?

Answer 11

Router# show ip bgp

Question 12

What command would you use to see bgp neighbor information?

Answer 12

Router# show ip bgp summary

Question 13

What command(s) might you need to ensure that an iBGP neighbor gets a BGP route into its routing table?

Answer 13

Router(config)# router bgp <AS>
Router(config-router)# neighbor <ip> remote-as <AS>
Router(config-router)# neighbor <ip> next-hop-self</ip></AS></ip></AS>

Question 14

What command would you use to ‘reset’ all bgp neighbors? Why do you need to be careful about using that command?

Answer 14

Router# clear ip bgp *

This command will break all connections to BGP peers until the neighbor states are reestablished.

Question 15

What is the command you would use to see all the routes being sent out from the router you are on?

Answer 15

Router1# show ip bgp neighbors <ip> advertised-routes</ip>

Question 16

When looking at a BGP routes table, what does an asterisk (*) in front of the route mean?

Answer 16

The route is valid

Question 17

When looking at a BGP routes table, what does a sideways carat (>) in front of the route mean?

Answer 17

This is the best route for this network

Question 18

What command(s) would you use to do a route summary for all network advertisements in the 199.88.77.0 /24 range of addresses?

Answer 18

Router(config)# router bgp <AS>
Router(config-router)# aggregate-address 199.88.77.0 255.255.255.0 summary-only</AS>

NOTE: on some versions of IOS you need to use the address-family ipv4 command first

Question 19

Define ASBR

Answer 19

Autonomous System Boundary Router - a type of router that runs multiple interior gateway protocols and serves as a gateway to other autonomous systems operating with interior gateway protocols.

Question 20

What are the steps to configure an iBGP peer relationship between Router1 and Router2?

Answer 20

Router1(config)# router bgp 60001
- (where 60001 is the AS number)
Router1(config-router)# bgp router-id 1.1.1.1
Router1(config-router)# neighbor 200.0.0.2 remote-as 60001
Router1(config-router)#neighbor 200.0.0.2 next-hop-self

Router2(config)# router bgp 60001
Router2(config-router)# bgp router-id 2.2.2.2
Router2(config-router)# neighbor 200.0.0.1 remote-as 60001
Router1(config-router)# network 201.0.2.0 mask 255.255.255.0

Question 21

What general steps are needed to create route filters for BGP route distribution?

Answer 21

1. Create a Access-list to “permit” or capture the network that needs to be filtered
2. Create a route-map and ‘match ip address’ of the ACL created
3. Under BGP config mode, use the redistribute command to target the routing protocol and route-map

Question 22

Describe the specific steps to redistribute OSPF routes into BGP and vice versa

Answer 22

FISH1(config)# ip access-list extended OSPF_ROUTES

FISH1(config-ext-nacl)# permit ip 203.0.113.0 0.0.0.255 any

FISH1(config-ext-nacl)# deny ip any any

FISH1(config)# route-map OSPF_FILTER

FISH1(config-route-map)# match ip address OSPF_ROUTES

FISH1(config)# router bgp 64777

FISH1(config-router)# redistribute ospf 1 route-map OSPF_FILTER

FISH1(config)# ip access-list extended BGP_ROUTES

FISH1(config-ext-nacl)# permit ip 1.0.0.0 0.0.0.255 any

FISH1(config-ext-nacl)# deny ip any any

FISH1(config)# route-map BGP_FILTER

FISH1(config-route-map)# match ip address BGP_ROUTES

FISH1(config)# router ospf 1

FISH1(config-router)# redistribute bgp 64777 subnets route-map BGP_FILTER metric 50