Routing Basics 1
Small additions by Dr. Enis Karaarslan, 2014 Purdue - Aaron Jarvis (Network Engineer)
Routing Concepts IPv4 Routing Forwarding Some definitions Policy options Routing Protocols 3
IPv4 Internet uses IPv4 Addresses are 32 bits long Range from 1.0.0.0 to 223.255.255.255 0.0.0.0 to 0.255.255.255 and 224.0.0.0 to 255.255.255.255 have special uses IPv4 address has a network portion and a host portion 4
IPv4 address format Address and subnet mask written as 12.34.56.78 255.255.255.0 or 12.34.56.78/24 mask represents the number of network bits in the 32 bit address the remaining bits are the host bits 5
What does a router do? 7
A day in a life of a router find path forward packet, forward packet, forward packet, forward packet... find alternate path forward packet, forward packet, forward packet, forward packet repeat until powered of 8
Routing versus Forwarding Routing = building maps and giving directions Forwarding = moving packets between interfaces according to the directions 9
IP Routing finding the path Path derived from information received from a routing protocol Several alternative paths may exist best path stored in forwarding table Decisions are updated periodically or as topology changes (event driven) Decisions are based on: topology, policies and metrics (hop count, filtering, delay, bandwidth, etc.) 10
1: How Does Routing Work? Internet is made up of the ISPs who connect to each other s networks How does an ISP in Kenya tell an ISP in Japan what customers they have? And how does that ISP send data packets to the customers of the ISP in Japan, and get responses back After all, as on a local ethernet, two way packet flow is needed for communication between two devices 11
2: How Does Routing Work? ISP in Kenya could buy a direct connection to the ISP in Japan But this doesn t scale thousands of ISPs, would need thousands of connections, and cost would be astronomical Instead, ISP in Kenya tells his neighbouring ISPs what customers he has And the neighbouring ISPs pass this information on to their neighbours, and so on This process repeats until the information reaches the ISP in Japan 12
3: How Does Routing Work? This process is called Routing The mechanisms used are called Routing Protocols Routing and Routing Protocols ensures that the Internet can scale, that thousands of ISPs can provide connectivity to each other, giving us the Internet we see today 13
4: How Does Routing Work? ISP in Kenya doesn t actually tell his neighbouring ISPs the names of the customers (network equipment does not understand names) Instead, he has received an IP address block as a member of the Regional Internet Registry serving Kenya His customers have received address space from this address block as part of their Internet service And he announces this address block to his neighbouring ISPs this is called announcing a route 14
Routing Protocols Routers use routing protocols to exchange routing information with each other IGP is used to refer to the process running on routers inside an ISP s network EGP is used to refer to the process running between routers bordering directly connected ISP networks 15
IGP x EGP An Interior Gateway Protocol (IGP) is a type of protocol used for exchanging routing information between gateways (commonly routers) within an Autonomous System Exterior gateway protocols (EGP) are used to exchange routing information between Autonomous Systems and rely on IGPs to resolve routes within an AS.
What Is an IGP? Interior Gateway Protocol Within an Autonomous System Carries information about internal infrastructure prefixes Two widely used IGPs: OSPF ISIS 17
Why Do We Need an IGP? ISP backbone scaling Hierarchy Limiting scope of failure Only used for ISP s infrastructure addresses, not customers or anything else Design goal is to minimize number of prefixes in IGP to aid scalability and rapid convergence 18
What Is an EGP? Exterior Gateway Protocol Used to convey routing information between Autonomous Systems De-coupled from the IGP Current EGP is BGP 19
Why Do We Need an EGP? Scaling to large network Hierarchy Limit scope of failure Define Administrative Boundary Policy Control reachability of prefixes Merge separate organizations Connect multiple IGPs 20
Interior versus Exterior Routing Protocols Interior automatic neighbour discovery generally trust your IGP routers prefixes go to all IGP routers binds routers in one AS together Exterior specifically configured peers connecting with outside networks set administrative boundaries binds AS s together 21
Interior versus Exterior Routing Protocols Interior Carries ISP infrastructure addresses only ISPs aim to keep the IGP small for efficiency and scalability Exterior Carries customer prefixes Carries Internet prefixes EGPs are independent of ISP network topology 22
Hierarchy of Routing Protocols Other ISPs BGP4 BGP4 and OSPF/ISIS BGP4 IXP Static/BGP4 Customers 23
FYI: Cisco IOS Default Administrative Distances Route Source Default Distance Connected Interface Static Route Enhanced IGRP Summary Route External BGP Internal Enhanced IGRP IGRP OSPF IS-IS RIP EGP External Enhanced IGRP Internal BGP Unknown 0 1 5 20 90 100 110 115 120 140 170 200 255 24
Dynamic Routing Slide
Basics of Dynamic Routing Presented by Aaron Jarvis Network Engineer 26
Agenda Introduction to Dynamic Routing Choosing the Right Protocol Configuring Dynamic Routing How the ITaP Production Data Network uses dynamic routing
Agenda Introduction to Dynamic Routing Choosing the Right Protocol Configuring Dynamic Routing How the ITaP Production Data Network uses dynamic routing
Introduction to Dynamic Routing What is routing? How data is forwarded between subnets Network Layer or Layer 3 in the OSI Model Provides end-to-end reachability
Introduction to Dynamic Routing How are routing decisions made? Forwarded based on the destination IP address Router builds/maintains a routing table Current view of the network Populated either dynamically or manually router#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is 192.5.40.54 to network 0.0.0.0 69.0.0.0/19 is subnetted, 1 subnets O E2 69.51.160.0 [110/1] via 192.5.40.54, 6d21h, Port-channel61 O E2 199.233.104.0/24 [110/1] via 192.5.40.54, 3w4d, Port-channel61 192.5.40.0/24 is variably subnetted, 19 subnets, 4 masks D 192.5.40.238/32 [90/131328] via 192.31.0.13, 2d16h, Vlan4094 [90/131328] via 192.31.0.113, 2d16h, Vlan4084 O IA 192.5.40.225/32 [110/2] via 192.5.40.54, 3w4d, Port-channel61 O IA 192.5.40.227/32 [110/4] via 192.5.40.54, 3w4d, Port-channel61 C 192.5.40.240/28 is directly connected, Vlan898 D 192.5.40.168/29 [90/3072] via 192.31.0.113, 7w0d, Vlan4084 [90/3072] via 192.31.0.13, 7w0d, Vlan4094 Snip
Agenda Introduction to Dynamic Routing Choosing the Right Protocol Configuring Dynamic Routing How the ITaP Production Data Network uses dynamic routing
Choosing the Right Protocol Interior Routing Protocols Used within an autonomous system Used within an area of administrative control Exterior Routing Protocols Used between autonomous systems Used to peer with networks in which you have no administrative control
Choosing the Right Protocol Interior Routing Protocols Static RIP OSPF EIGRP ISIS Exterior Routing Protocols BGP NOTE: This is not an exhaustive list of protocols available but merely a list of those commonly used.
Choosing the Right Protocol Static Routing May be suitable on small networks Administration intensive as changes have to be made on each router Commonly used for default routing 0.0.0.0/0 Next Hop Router
Choosing the Right Protocol Dynamic Routing Protocol Types Distance Vector Routing Information Protocol(RIP) Interior Gateway Routing Protocol(IGRP) Enhanced Interior Gateway Routing Protocol(EIGRP) Link State Open Shortest Path First(OSPF) Intermediate System to Intermediate System(ISIS) Path Vector Border Gateway Protocol(BGP)
Choosing the Right Protocol Routing Information Protocol(RIP) RFC 1058(RIPv1), 1988 Classful, no support for VLSM No support for authentication RFC 2453(RIPv2), 1998 Classless, support for CIDR Support for authentication Uses hop count as routing metric Slow to converge Not very scalable Limited to 15 hops
Choosing the Right Protocol Interior Gateway Routing Protocol(IGRP) Invented by Cisco to overcome limitations of RIP Allows for hop count up to 255 Allows for multiple route metrics Bandwidth Delay Load MTU Reliability Classful, no support for VLSM
Choosing the Right Protocol Enhanced Interior Gateway Routing Protocol(EIGRP) Replaced IGRP Maintains a Topology table Successors, feasible successors Allows for multiple route metrics Classless, support for CIDR Very fast to converge Maintains neighbor relationships Difusing Update Algorithm(DUAL) Not as CPU intensive as OSPF
CIDR (Classless Inter-Domain Routing, sometimes known as supernetting) is a way to allocate and specify the Internet addresses used in inter-domain routing more flexibly than with the original system of Internet Protocol (IP) address classes.
Choosing the Right Protocol Open Shortest Path First(OSPF) RFC 2328(OSPFv2), 1998 Maintains neighbor relationships Concept of Areas Diferent areas can be used to control flooding of routing information Classless, supports VLSM Fast to converge CPU Intensive Dijkstra Algorithm Designing can be complicated
Choosing the Right Protocol Intermediate System to Intermediate System(ISIS) RFC 1142, 1990 Dijkstra Algorithm Mainly used by large service providers Does not use IP to carry routing information Uses ISO addresses Level Concept Level 1 or Intra Area Level 2 or Inter Area Level 1/2 or Both Classless, supports VLSM
Choosing the Right Protocol Border Gateway Protocol(BGP) RFC 4271(BGPv4), 2006 Peers manually defined Used typically for multi-homing to ISP(s) Very scalable Makes decisions based upon AS Path Lots of policy options Very granular control
Agenda Introduction to Dynamic Routing Choosing the Right Protocol Configuring Dynamic Routing How the ITaP Production Data Network uses dynamic routing
Configuring Dynamic Routing How to configure dynamic routing? Choose a protocol that meets your needs Each vendor has specific commands but should have a configuration guide available to assist Plan well to ensure a functional network Create a diagram Consider high availability technologies HSRP VRRP
HSRP - Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy protocol for establishing a fault-tolerant default gateway, The Virtual Router Redundancy Protocol (VRRP) is a computer networking protocol that provides for automatic assignment of available Internet Protocol (IP) routers to participating hosts. This increases the availability and reliability of routing paths via automatic default gateway selections on an IP subnetwork.
Configuring Dynamic Routing Best Practices Only enable a routing protocol for interfaces you intend on using If compromised can cause a lot of problems Enable authentication(if available) Use authentication to ensure that the remote routers are valid peers
Configuring Dynamic Routing Best Practices Cont Control Network Advertisements Only allow networks to be advertised that should be Private Address Space Use only within your organization Private AS Numbers Strip before sending to ISP
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IP route lookup Based on destination IP address longest match routing More specific prefix preferred over less specific prefix Example: packet with destination of 10.1.1.1/32 is sent to the router announcing 10.1/16 rather than the router announcing 10/8. 49
IP route lookup Based on destination IP address Packet: Destination IP address: 10.1.1.1 R1 10/8 announced from here R3 R2 10/8 R3 10.1/16 R4 20/8 R5 30/8 R6.. R4 10.1/16 announced from here R2 s IP routing table 50
IP route lookup: Longest match routing Based on destination IP address Packet: Destination IP address: 10.1.1.1 R1 10/8 announced from here R3 R2 10.1.1.1 && FF.0.0.0 10/8 R3 vs. Match! 10.1/16 10.0.0.0 && FF.0.0.0 R4 20/8 R5 30/8 R6..IP routing table R2 s R4 10.1/16 announced from here 51
IP route lookup: Longest match routing Based on destination IP address Packet: Destination IP address: 10.1.1.1 R1 10/8 announced from here R3 R2 R4 10/8 R3 10.1/16 R4 20/8 R5 30/8 R6.. 10.1/16 announced 10.1.1.1 && FF.FF.0.0 Match as well! from here vs. 10.1.0.0 && FF.FF.0.0 R2 s IP routing table 52
IP route lookup: Longest match routing Based on destination IP address Packet: Destination IP address: 10.1.1.1 R1 10/8 announced from here R3 R2 R4 10/8 R3 10.1/16 R4 20/8 R5 30/8 R6.. 10.1/16 announced from here 10.1.1.1 && FF.0.0.0 Does not match! vs. 20.0.0.0 && FF.0.0.0 R2 s IP routing table 53
IP route lookup: Longest match routing Based on destination IP address Packet: Destination IP address: 10.1.1.1 R1 10/8 announced from here R3 R2 R4 10/8 R3 10.1/16 announced 10.1/16 R4 from here 10.1.1.1 && FF.0.0.0 20/8 R5 vs. Does not match! 30/8 R6 30.0.0.0 && FF.0.0.0.. R2 s IP routing table 54
IP route lookup: Longest match routing Based on destination IP address Packet: Destination IP address: 10.1.1.1 R1 10/8 announced from here R3 R2 R4 10/8 R3 10.1/16 R4 20/8 R5 30/8 R6.. 10.1/16 announced Longest match, 16 bit netmask from here R2 s IP routing table 55
IP Forwarding Router decides which interface a packet is sent to Forwarding table populated by routing process Forwarding decisions: destination address class of service (fair queuing, precedence, others) local requirements (packet filtering) Forwarding is usually aided by special hardware 56
Routing Information Base (RIB) Forwarding Information Base (FIB) Routing Tables Feed the Forwarding Table BGP 4 Routing Table OSPF Link State Database Connected Routes Static Routes 57
RIBs and FIBs FIB is the Forwarding Table It contains destinations and the interfaces to get to those destinations Used by the router to figure out where to send the packet Careful! Some people still call this a route! RIB is the Routing Table It contains a list of all the destinations and the various next hops used to get to those destinations and lots of other information too! One destination can have lots of possible nexthops only the best next-hop goes into the FIB 58
Packet-switched networks Packet-switched networks are built on mesh topologies in which multiple paths to a destination exist. The links in the mesh are point-to-point links joined by routers.
Packet-switched networks A path to a destination may go through any number of routers, and the path may change at any time due to traffic problems or failed links. In this environment, there are two possible packet-routing methods: - Hop-by-hop, destination-based routing - Explicit routing
Hop-by-hop, destinationbased routing This scheme is like getting directions along the way. A packet has a destination address. Each router looks at the address and makes a routing decision about how to forward the packet. Thus, decisions are made on a hop-by-hop basis in the network until the packet reaches its destination.
Explicit routing This scheme relies on a network made of switch routers or ATM switches. A predefined path is specified in advance for a packet. This is a virtual circuit in the ATM world. Since the path is predefined, the packet is switched at each node, thus eliminating the need to make routing decisions at every node along the path.
Explicit routing Explicit routing is useful for traffic engineering, QoS (Quality of Service), and the prevention of routing loops. It requires path setup in advance, something that can be done in IP networks with MPLS (Multiprotocol Label Switching). Source routing is a form of explicit routing in which end systems discover a path through the network in advance of sending packets. Constraint-based routing is a related technique that builds paths based on various constraints programmed into the network, such as bandwidth requirements for specific types of traffic.
Explicit versus Default Routing Default: simple, cheap (cycles, memory, bandwidth) low granularity (metric games) Explicit (default free zone) high overhead, complex, high cost, high granularity Hybrid minimise overhead provide useful granularity requires some filtering knowledge 64
Egress x Ingress Egress Traffic: Network traffic that begins inside of a network and proceeds through its routers to a destination somewhere outside of the network. Ingress Traffic: Network traffic that originates from outside of the networks routers and proceeds toward a destination inside of the network.
Egress Traffic How packets leave your network Egress traffic depends on: route availability (what others send you) route acceptance (what you accept from others) policy and tuning (what you do with routes from others) Peering and transit agreements 66
Ingress Traffic How packets get to your network and your customers networks Ingress traffic depends on: what information you send and to whom based on your addressing and AS s based on others policy (what they accept from you and what they do with it) 67
BGP and AS Internet is a network of interconnected networks. Border Gateway Protocol (BGP) is widely used for routing in the backbone of the Internet. BGP identifies networks which are under a common management as Autonomous Systems (AS). Each AS uses a unique Autonomous System Number (ASN) in BGP routing. According to CAIDA dataset, there are 45658 different (2014) ASN worldwide.
Autonomous System (AS) AS 100 Collection of networks with same routing policy Single routing protocol Usually under single ownership, trust and administrative control 69
Caida As-rank http://as-rank.caida.org/
NSP IXP - Tier1 Network Service Providers (NSP) are organizations which provides direct access to the Internet. The biggest transit-free NSPs that can reach all other networks are called Tier-1. Tier-1 networks peer with every other Tier-1 network. Internet Exchange Points (IXP) are the aggregation points where networks peer with each other and high rate of network traffic is present. As an example, LINX is an IXP which has aggregated traffic up to 1.194 Tbps. There are 125 known IXPs in Europe.
Definition of terms Neighbours AS s which directly exchange routing information Routers which exchange routing information Announce send routing information to a neighbour Accept receive and use routing information sent by a neighbour Originate insert routing information into external announcements (usually as a result of the IGP) Peers routers in neighbouring AS s or within one AS which exchange routing and policy information 76
Routing flow and packet flow packet flow AS 1 accep announce routing flow t announce accep t packet flow AS 2 For networks in AS1 and AS2 to communicate: AS1 AS2 AS2 AS1 must must must must announce to AS2 accept from AS1 announce to AS1 accept from AS2 77
Routing flow and Traffic flow Traffic flow is always in the opposite direction of the flow of Routing information Filtering outgoing routing information inhibits traffic flow inbound Filtering inbound routing information inhibits traffic flow outbound 78
Routing Flow/Packet Flow: With multiple ASes AS 1 AS 34 N1 AS16 AS 8 N16 For net N1 in AS1 to send traffic to net N16 in AS16: AS16 must originate and announce N16 to AS8. AS8 must accept N16 from AS16. AS8 must announce N16 to AS1 or AS34. AS1 must accept N16 from AS8 or AS34. 79 For two-way packet flow, similar policies must
Routing Flow/Packet Flow: With multiple ASes AS 1 AS 34 N1 AS16 AS 8 N16 As multiple paths between sites are implemented it is easy to see how policies can become quite complex. 80
Routing Policy Used to control traffic flow in and out of an ISP network ISP makes decisions on what routing information to accept and discard from its neighbours Individual routes Routes originated by specific ASes Routes traversing specific ASes Routes belonging to other groupings Groupings which you define as you see fit 81
Routing Policy Limitations red red Internet green AS99 green packet flow AS99 uses red link for traffic to the red AS and the green link for remaining traffic To implement this policy, AS99 has to: Accept routes originating from the red AS on the red link Accept all other routes on the green link 82
Routing Policy Limitations red red Internet AS99 AS22 green green packet flow AS99 would like packets coming from the green AS to use the green link. But unless AS22 cooperates in pushing traffic from the green AS down the green link, there is very little that AS99 can do to achieve this aim 83
Routing Policy Issues Late May 2012: 410000 prefixes Not realistic to set policy on all of them individually 42000 origin AS s Too many to try and create individual policies for Routes tied to a specific AS or path may be unstable regardless of connectivity Solution: Groups of AS s are a natural abstraction for filtering purposes 84
Routing Basics End 85