Network Layer: Control Plane 2

Question 1

OSPF vs BGP

Answer 1

OSPF: intra, inside an area
BGP: inter, between areas

Question 2

BGP

BGP

Answer 2

• Border Gateway Protocol
• De-facto inter-AS routing protocol (subnets must all have same protocol between each other)
• Allows subnet to advertise its existence and destinations it can reach
• Provides ASs with eBGP and iBGP

Question 3

BGP

eBGP

Answer 3

• External
• Allows AS to obtain subnet reachability info from neighboring ASs

Question 4

BGP

iBGP

Answer 4

• Internal
• Allows AS to propagate reachabilityto all AS-internal routers

Question 5

BGP

Where is eBGP and iBGP ran?

Answer 5

Gateway Routers: iBGP and eBGP
Internal Routers: iBGP
(no routers run only eBGP)

Question 6

BGP

BGP Session

Answer 6

• Two BGP routers exchange BGP messages over semi-permanent TCP connection
• Advertising paths to different destination networks

Question 7

BGP

BGP Advertised Route

Answer 7

Prefix + Attributes
Prefix: destination being advertised
Attributes: ASPATH (list of ASs through which prefix advertisement is passed), NEXTHOP (indicates specific internal-AS router to next-hop AS)

Question 8

BGP

Policy-Based Routing

Answer 8

• Gateway receiving route advertisment uses import policy to accept/decline path
• AS policy also determines whether to advertise path to other neighboring ASs

Question 9

BGP

BGP Path Advertisement

Answer 9

• AS policy might choose to not advertise path
• Gateway router may learn about multiple paths, chooses the best-cost path
• Gateway router only advertises best-cost path to internal routers

Question 10

BGP

BGP Message Types

Answer 10

Exchanged between peers over TCP connection
1. OPEN
2. UPDATE
3. KEEPALIVE
4. NOTIFICATION

Question 11

BGP

BGP Messages: OPEN

Answer 11

Opens TCP connection to remote BGP peer and authenticates sending BGP peer (transport layer)

Question 12

BGP

BGP Messages: UPDATE

Answer 12

Advertises new path (or withdraws old path)

Question 13

BGP

BGP Messages: KEEPALIVE

Answer 13

Keeps connection alive in absence of UPDATES; also ACKs OPEN request

Question 14

BGP

BGP Messages: NOTIFICATION

Answer 14

Reports errors in previous messages; also used to close connection

Question 15

BGP

Why different Intra and Inter routing?

Answer 15

Policy:
* Inter-AS: admin wants control over how its traffic is routed, who routes traffic through its network
* Intra-AS: single admin, so policy is less of an issue

Scale: hierarchical routing saves table size, reduced update traffic

Performance:
* Intra-AS: can focus on performance
* Inter-AS: policy dominates over performance

Question 16

BGP

Hot-Potato Routing

Answer 16

• Choose local gateway that has least intra-domain cost
• Don’t look at inter-domain cost

Question 17

SDN

SDN

Answer 17

• Software Defined Networking
• Remote controller computes and installs forwarding tables in routers

Question 18

SDN

Why a logically centralized control plane? (benefits)

Answer 18

• Easier network management, avoid router misconfigurations, greater flexibility of traffic flows
• Centralized programming is easier (compute once and distribute)
• Distributed programming is harder (each router has to compute and individually distribute)
• Open (non-proprietary) implementation of control plane

Question 19

SDN

Traffic Engineering Abilities

Answer 19

• Traffic rerouting
• Split traffic between different routers
• Categorize different types of traffic and act accordingly

Question 20

ICMP

ICMP

Answer 20

• Internet Control Message Protocol
• Used by routers to communicate network-level information
• Error reporting
• Echo request/reply (used by ping)
• Network layer “above” IP (ICMP messages carried in IP datagrams)

Question 21

ICMP

Error Reporting

Answer 21

Occurs when unreachable host, network, port, or protocol