Split View: [컴퓨터 네트워크] 14. 인터넷 라우팅: OSPF와 BGP

Hierarchical Routing Structure
================================

[AS 1: ISP-A]          [AS 2: ISP-B]         [AS 3: Google]
+-----------+          +-----------+          +-----------+
| R1---R2   |          | R5---R6   |          | R8---R9   |
| |    |    |<-------->|  |   |    |<-------->|  |   |    |
| R3---R4   |          | R7        |          | R10--R11  |
+-----------+          +-----------+          +-----------+

Intra-AS: OSPF, RIP, etc. (intra-AS routing)
Inter-AS: BGP (inter-AS routing)

AS Types
=========

1. Stub AS: Connected to only one other AS (enterprises, universities)
   [External] <---> [Stub AS]

2. Multihomed AS: Connected to multiple ASes but no transit traffic
   [AS A] <---> [Multihomed AS] <---> [AS B]

3. Transit AS: AS that allows traffic to pass through (ISPs)
   [AS A] <---> [Transit AS] <---> [AS B]
                    Transit allowed

RIP Characteristics
=====================

- Algorithm: Distance-Vector (Bellman-Ford)
- Metric: Hop count, maximum 15 hops
- Update interval: Sends distance vector to neighbors every 30 seconds
- Timeout: Route invalidated if no update for 180 seconds
- Transport: UDP port 520
- Suitable scope: Small networks (15 hop limit)

OSPF Characteristics
======================

- Algorithm: Link-State (Dijkstra)
- Metric: Bandwidth, delay, etc. configurable by administrator
- Updates: Immediately on change (or 30-minute periodic)
- Transport: IP directly (protocol number 89)
- Authentication: MD5 authentication supported
- Suitable scope: Large networks

OSPF Hierarchical Structure
==============================

             [Backbone Area (Area 0)]
            /         |          \
     [Area 1]    [Area 2]    [Area 3]
     +------+   +------+    +------+
     |R1  R2|   |R4  R5|    |R7  R8|
     |  R3  |   |  R6  |    |  R9  |
     +------+   +------+    +------+

- Backbone Area (Area 0): Central area connecting all areas
- Area Border Router (ABR): Router spanning two areas
- AS Boundary Router (ASBR): Router connected to another AS

OSPF Inter-Area Routing
=========================

Sending packet from R1 in Area 1 to R9 in Area 3:

  R1 (Area 1)
    |
    | Area 1 internal OSPF routing
    v
  ABR1 (Area 1 / Backbone boundary)
    |
    | Backbone (Area 0) OSPF routing
    v
  ABR3 (Backbone / Area 3 boundary)
    |
    | Area 3 internal OSPF routing
    v
  R9 (Area 3)

Within each area: Detailed topology information exchanged
Between areas: Only summarized distance information exchanged

Two Forms of BGP
==================

1. eBGP (External BGP): Between routers of different ASes
   AS1 gateway <---eBGP---> AS2 gateway

2. iBGP (Internal BGP): Between routers within the same AS
   AS1 gateway <---iBGP---> AS1 internal router

eBGP: Exchanges route information between ASes
iBGP: Propagates learned route information within the AS

BGP Route Advertisement Example
==================================

AS2 advertises to AS1:
  Prefix: 138.16.64.0/24
  AS-PATH: AS3 AS2
  NEXT-HOP: 201.44.13.1

Key Path Attributes:
  - AS-PATH: List of ASes the route traverses
    Example: AS3 AS2 --> reaches AS3 via AS2
  - NEXT-HOP: IP address of gateway router toward next AS
  - LOCAL-PREF: Route preference within AS (higher is preferred)
  - MED: Value indicating preferred route to other ASes (lower is preferred)

BGP Route Selection Priority
==============================

1. Highest LOCAL-PREF (policy-based)
2. Shortest AS-PATH
3. Closest NEXT-HOP (Hot-Potato routing)
4. Smallest BGP identifier

Example:
  Route A: LOCAL-PREF=200, AS-PATH=[AS2, AS5], NEXT-HOP=R1
  Route B: LOCAL-PREF=100, AS-PATH=[AS3], NEXT-HOP=R2

  --> Route A selected (higher LOCAL-PREF)

Hot-Potato Routing
====================

Inside AS1:

        iBGP          iBGP
[R1] <-------> [R2] <-------> [R3]
 |              |               |
 | Internal     | Internal      | Internal
 | cost 2       | cost 5        | cost 3
 |              |               |
[GW1]          [GW2]          [GW3]
 |eBGP          |eBGP          |eBGP
 v               v               v
[AS2]          [AS3]          [AS2]

For R2 to reach a prefix in AS2:
  - Via GW1: internal cost 2 (R2->R1->GW1)
  - Via GW3: internal cost 3 (R2->R3->GW3)
  --> GW1 selected (Hot-Potato: nearest exit)

Inter-AS Relationships
========================

         [Tier-1 ISP A] <--Peering--> [Tier-1 ISP B]
          /          \                /          \
    Provider       Provider     Provider       Provider
      /                \          /                \
[Customer AS1]   [Customer AS2] [Customer AS3] [Customer AS4]

- Customers pay providers for traffic delivery
- Peers exchange customer traffic for free
- Providers advertise customer routes elsewhere

BGP Policy Rules
==================

From the perspective of AS X:

1. Routes received from customers: Advertise to everyone (revenue generating)
2. Routes from peers: Advertise only to customers (not to peers or providers)
3. Routes from providers: Advertise only to customers (not to peers or other providers)

Principle: Traffic passing through incurs costs,
           so only actively relay customer traffic that generates revenue

OSPF and BGP Interaction
===========================

Intra-AS:
  - OSPF computes shortest paths between routers within the AS
  - Each router knows routes to all destinations within the AS

Inter-AS:
  - eBGP learns prefix information from external ASes
  - iBGP propagates learned information within the AS
  - Route to NEXT-HOP is provided by OSPF

Forwarding Table Composition:
  Destination Prefix | Next Hop     | Source
  -------------------+--------------+-------
  10.1.0.0/16        | 192.168.1.2  | OSPF
  172.16.0.0/12      | 192.168.1.5  | OSPF
  8.8.8.0/24         | 10.0.0.1     | BGP
  0.0.0.0/0          | 10.0.0.1     | BGP