1. 2011 session 1 TELE3118: Network Technologies Week 7: Network Layer Inter-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 All material copyright J.F Kurose and K.W. Ross, All Rights Reserved.
Network Layer

2. Hierarchical Routing Our routing study thus far - idealization
all routers identical
network “flat”
… not true in practice
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
Network Layer

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

4. Intra-AS and Inter-AS routing
C.b
Gateways:
perform inter-AS routing amongst themselves
perform intra-AS routing with other routers in their AS
B.a
A.a b
A.c c a a C b a B d c A b
network layer
inter-AS, intra-AS routing in
gateway A.c
link layer
physical layer
Network Layer

5. IGP vs. EGP
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

6. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
BGP provides each AS a means to:
Obtain subnet reachability information from neighboring ASs.
Propagate the reachability information to all routers internal to the AS.
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. 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 1c 2a
AS3
AS1
AS2 1a 2c 2b 1b 3c
eBGP session
iBGP session
Network Layer

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

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

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

11. BGP attribute: AS-path
Prevents looping!
Prefix /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
Partition healing:
rare case where AS1 may
accept “loop” route:
AS 3
AS 1
AS 3
(b)
AS 1
AS 2
(a)
AS 2
/16
/16
Network Layer

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
Link A
AS 2
AS 3
AS 4
Link B
Network Layer

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?
Cust 2
Case A:
ISP 2
ISP 1
Cust 1
Network Layer

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 to AS2 and AS3
AS4 receives the prefix from both, but chooses the AS3-AS1 path since it has better local-pref
AS 3
AS 1
AS 4
AS 2
/16
Network Layer

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

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
AS2
BGP session
ISP
AS1
customer R1 R2
/23
Network Layer

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)
ISP-1
ISP-2
customer
Network Layer

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
ISP R1 R2 R3
customer
138.39/16
204.70/16
ISP R1 R2 R3
customer
138.39/16
204.70/16
Network Layer

19. Multi-homing to multiple providers
ISP3
ISP1
ISP2
138.39/16
204.70/16
customer
Options for customer address space:
Exclusively from ISP1 (or from ISP2)
E.g.: customer uses /24 and advertises this prefix to ISP2
ISP3 gets prefixes /16 from ISP1 and 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 /24 and /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!
Network Layer

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

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)
C.b
B.a
A.a b
A.c c a a C b a B d c A b
Network Layer

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
LAN
/28 R1 R2
ISP
LAN
/28
LAN
/24
LAN
/28
server
farm
Network Layer
new PC

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?
LAN
/28 R1 R2
ISP
LAN
/28
LAN
/24
LAN
/28
server
farm R3
LAN
/24
new PC
Network Layer

24. Summary Policy: Scale: Performance:
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

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!
Network Layer