Network Layer 7-1

2012 session 1TELE3118: Network

Technologies

Week 7: Network LayerInter-Domain Routing Protocols

Some slides have been taken from:Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross. Addison-Wesley, July 2004. All material copyright 1997-2004. J.F Kurose and K.W. Ross, All Rights Reserved.

Network Layer 7-2

Hierarchical Routing

scale: with 200 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network

Our routing study thus far - idealization all routers identical network “flat”… not true in practice

Network Layer 7-3

Hierarchical Routing in the Internet Internet is organized as Autonomous Systems (AS)

Each AS is an independent administrative domain (e.g. ISP)

Intra-AS routing protocol All routers in an AS run same intra-AS routing protocol Routers in different AS can run different intra-AS routing

protocol

Inter-AS routing protocol Between routers in different AS

Gateway routers: run both intra-AS and inter-AS routing protocols

Network Layer 7-4

Intra-AS and Inter-AS routing

Gateways:•perform inter-AS routing amongst themselves•perform intra-AS routing with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layer

link layer

physical layer

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c

Network Layer 7-5

IGP vs. EGP

Figure 4.5.2-new2: BGP use for inter-domain routing

AS2 (OSPF

intra-AS routing)

AS1 (RI P intra-AS

routing) BGP

AS3 (OSPF intra-AS

routing)

BGP

R1 R2

R3

R4

R5

Intra-area routing protocol also called Interior Gateway Protocol (IGP) Administrator can choose any: RIP, OSPF, ISIS, …

Inter-area routing protocol also called Exterior Gateway Protocol (EGP) Unique: Border Gateway Protocol (BGP)

Network Layer 7-6

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standard

BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate the reachability information to all

routers internal to the AS.3. Determine “good” routes to subnets based

on reachability information and policy. Allows a subnet to advertise its

existence to rest of the Internet: “I am here”

Network Layer 7-7

BGP basics Pairs of routers (BGP peers) exchange routing info over

semi-permanent TCP conctns: BGP sessions Note that BGP sessions do not correspond to physical links. When AS2 advertises a prefix to AS1, AS2 is promising it

will forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Network Layer 7-8

Path attributes & BGP routes

When advertising a prefix, advert includes BGP attributes. prefix + attributes = “route”

Path Vector protocol: similar to Distance Vector protocol each Border Gateway broadcast to

neighbors (peers) entire path (i.e., sequence of AS’s) to destination

• E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z when gateway router receives route advert,

uses import policy to accept/decline.

Network Layer 7-9

BGP operation Point-to-point peering BGP peers explicitly configured

Lack of trust no auto-discovery! BGP session runs over TCP

Reliable Can detect neighbor/link down

4 types of messages: OPEN: opens TCP connection to peer and

authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE keeps connection alive in absence of

UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also

used to close connection

Network Layer 7-10

BGP operation (contd.)

BGP peers exchange route prefixes AS-path Route attributes No cost included!

Route prefixes received from peer are filtered and selected (based on AS-path and route attributes) for installation in RIB

Route prefixes from RIB are sent to peer after filtering and selection

All the complexity is in the use of policies for filtering and selection

Network Layer 7-11

BGP attribute: AS-path Prevents looping!

Prefix 138.39.0.0/16, AS1 AS2: AS-path = AS1 AS2 AS3: AS-path = AS2-AS1 AS3 AS1: AS-path = AS3-AS2-AS1 AS1 detects loop, and can reject the route

AS 1

AS 2

AS 3

138.39.0.0/16

(a)

AS 2

AS 3

138.39.0.0/16

(b)AS 1

Partition healing:rare case where AS1 mayaccept “loop” route:

Network Layer 7-12

BGP attribute: Multi-Exit-Discriminator Used when two AS connect to each other in more than

one place Used by AS to advertise degree of preference of each

link to reach a particular prefix Example:

AS1 and AS2 have 2 BGP sessions: one on each link AS2 advertises prefixes of AS3 to AS1 on both links

• MED advertised on link A better than MED advertised on link B

AS 1 AS 2

AS 3

AS 4

Link A

Link B

Network Layer 7-13

MED (contd.) ISP-1 and ISP-2 connect in New York and San Francisco ISP-1 has customer-1 in San Francisco ISP-2 has customer-2 in New York What happens if:

Case A: Both ISPs set and accept MED? Case B: Both ISP-1 and ISP-2 ignore MED? Case C: ISP-1 accepts MED but ISP-2 ignores MED?

ISP 1

ISP 2

Cust 2

Cust 1

Case A:

Network Layer 7-14

BGP attribute: Local-Pref Most commonly used attribute Determines local (i.e. within AS) preference of use of

received route E.g.: say AS3 provides better service than AS2 to AS4

AS4 can configure local-pref of routes from AS3 to be higher (better) than those heard from AS2

AS1 advertises prefix 138.39.0.0.16 to AS2 and AS3 AS4 receives the prefix from both, but chooses the AS3-

AS1 path since it has better local-pref

AS 1

AS 2

AS 3

138.39.0.0/16

AS 4

Network Layer 7-15

BGP policies Can be complex, yet are key to flexibility

and control of inter-AS routing Examples:

Avoid using competitor’s network• avoid routes with AS-n in AS-Path

Avoid transit service, i.e. do not carry any traffic that does not have source or destination within AS

• Do not advertise any non-local routes to peers Let another ISP carry most cross-country load

• Use of MED was shown earlier More examples in subscriber-ISP connection next

Network Layer 7-16

Subscriber connection: singly-homed Easy case! Possible options:

Static configuration: easiest• Customer has default route via R2• ISP configures static route to customer’s prefix

Include customer in ISP’s IGP (too risky!) Run a small IGP (say RIP) on R1-R2 link, leak that into BGP Run a single BGP session

• customer will still likely use a default route or a small set of filtered routes and not absorb the entire Internet routing table

customer ISPR1 R2

AS1

AS2

138.39.2.0/23

BGP session

Network Layer 7-17

Multi-homed subscriber Multiple customer links to one or more ISPs Why?

Reliability (redundancy) Performance (load-sharing)

Challenging Static routing often doesn’t suffice (why?) Want to minimize routing prefixes injected into customer

network BGP configuration requires thought and planning, taking into

account both traffic directions (to and from the customer)

customer

ISP-2ISP-1

Network Layer 7-18

Multi-homing to a single provider

Example 1: same router in ISP, different routers in customer ISP to customer traffic:

customer sets MED Customer to ISP traffic: 2

default routes!

Example 2: different routers in ISP, same router in customer ISP to customer traffic: as

before Customer to ISP traffic:

customer may have to get BGP prefixes from ISP

138.39/16

R1ISP

R3

customerR2

204.70/16

138.39/16

R1ISP

R3

customer

R2

204.70/16

Network Layer 7-19

Multi-homing to multiple providers

Options for customer address space: Exclusively from ISP1 (or from ISP2)

• E.g.: customer uses 138.39.1/24 and advertises this prefix to ISP2• ISP3 gets prefixes 138.39/16 from ISP1 and 138.39.1.24 from ISP2• ISP3 traffic to customer will go via ISP2 (longest prefix match)• Aggregation is pushing traffic away?!

From both ISP1 and ISP2• E.g.: customer uses 138.39.1/24 and 204.70.1/24• Good load-sharing if traffic to these prefixes is about the same

Independently from address registry• Can manipulate load-sharing better, but bad for aggregation!

Bottom line: it all depends on the traffic patterns!

ISP1

customer

ISP3

ISP2138.39/16 204.70/16

Network Layer 7-20

Interaction among routing protocols Every routing protocol is computing its own

routes: how does it all fit? Question: do they interact with each other? Yes! Question: which route is inserted in the forwarding

tables? If conflict, priority mechanism is used

Question: how does IGP fill its routing table? Direct routes: directly-connected interfaces Static routes: user configured

Question: How does BGP fill it routing table? Learns AS local networks from IGP

Network Layer 7-21

E-BGP vs. I-BGP Question: How do BGP routes get propagated within AS?

E.g.: how does B.b learn about routes from AS-A and AS-B? Inject BGP routes into IGP? bad idea – IGPs don’t scale Preferred way of distributing externally learnt prefixes

within an AS:• Internal-BGP (I-BGP): full-mesh within AS

Our earlier discussion on BGP peering between different AS• Technically correct to call it External-BGP (E-BGP)

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c

Network Layer 7-22

Configuring routing In your organization you have to install a new PC in a

server-farm. The PC is multi-homed on two LANs. What static routes do you need to configure on the PC for shortest-path routing to all destinations? Assume: The PC is not routing between LANs The PC is not running any routing protocols Pick any IP addresses for the router interfaces consistent with

the LAN subnets

LAN193.1.1.32/28

LAN202.1.1/24

LAN193.1.1.0/28

LAN193.1.1.16/28

ISPR1 R2

serverfarm

new PC

Network Layer 7-23

Configuring routing (contd.) Now suppose your organization gets a second link to the

ISP via a new router R3. Your PC now has 3 LAN interfaces, and your organization has two links to the Internet. Can you suggest ways of load-balancing traffic to/from your organization?

LAN193.1.1.32/28

LAN202.1.1/24

LAN202.1.2/24

LAN193.1.1.0/28

LAN193.1.1.16/28

ISPR1 R2

R3

serverfarm

new PC

Network Layer 7-24

Summary Hierarchical routing: intra-AS versus inter-AS Policy:

Inter-AS: admin wants control over how its traffic routed, who routes through its net.

Intra-AS: single admin, so no policy decisions needed

Scale: hierarchical routing saves table size, reduced update

traffic

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

Network Layer 7-25

Summary (contd.) Principles of BGP operation

Path-vector Configuration driven Route attributes (AS-Path, MED, Local-Pref,

…) Policies dictate everything! How does a customer connect to ISP? Examples of single and multi-homing

Interaction between routing protocols How does it all fit?

Design examples Finished with IP routing - whew!