Introduction to routing

Transcription

1 DD2490 p Introduction to routing Olof Hagsand KTH/CSC

2 Network example: KTH Intranet

3 Levels of abstraction The Internet is huge Necessary to divide the routing problem into sub-problems. There are several layers of abstractions The Internet is partitioned into Autonomous systems (AS) An independent administrative domain Routing between AS:s is called inter-domain routing / External routing Based on commercial agreements Policies, Service-levelagreements Routing within an AS Routing inside an AS: Intra-domain routing / Internal routing Best path based on hop/bw metrics

4 Autonomous systems - RFC1930 An Autonomous system is generally administered by a single entity. Operators, ISPs (Internet Service Providers) An AS contains an arbitrary complex sub-structure. Each autonomous system selects the routing protocol to be used within the AS. Policies or updates within an AS are not propagated to other AS:s. An AS-number is (currently) a 16-bit unique identifier Interconnection between AS:s Service Level Agreements (SLA:s) Internet Exchange Points (IX:s)/ Network Access Points (NAPs) Direct connections

5 US university AS:s AS-paths to US universities from NORDUnet Daniel Åman, KTHNOC 2006

6 Internet structure Ideally, there is a well-defined hierarchy in the Internet a tree. 1 A few large Tier 1 backbone providers the core of the Internet (Sprint, Level3, Telstra,...) 2 Tier 2 regional ISPs, or NSPs (Network Service Providers) 3 Smaller ISPs 4 Customers A well-defined hierarchy is nice for address aggregation > smaller IP tables However, the hierarchy has broken down due to market forces: Provides transit for everyone else Peering at IXs, direct connections. The Internet structure is now more in the form of a graph --> larger routing tables

7 AS graph and peering relations Tier 1: Full Internet connectivity AS1 AS2 Transit Peer NSPs ISPs AS3 AS5 AS4 Customer Stubs/ Customers AS6 AS7 AS8 AS9

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10 IGP/EGP ISP Customer EGP IGP EGP IGP IGP Exterior Gateway Protocol. Interior Gateway Protocol. Runs between networks/domains (inter-domain) Runs within a network/domain (intra-domain) Examples: BGP, static routing Examples: RIP, OSPF, IS-IS.

11 Static vs dynamic routing Static routing Manually configure routing table Typically for small networks Single-homed, default route Hosts are (almost) always statically routing Dynamic routing As soon as the network is non-trivial, it is too difficult to manually configure a network (see lab1) Need dynamic routing protocol Only routers participate in dynamic routing

12 The routing table Currently, backbone IP tables are more than entries. The RIB may be much larger Virtual private networks (many customer routing tables) the tables are even larger Also, a routing table is actually many datastructures: Many different protocols Forwarding information base (FIBs) Routing information base (RIBs)

13 Announced networks From Geoff Huston,

14 Load balancing The routing protocol gives several routes to a network Either select the best Or load-balance between several links Unequal-cost multi-path Equal-cost multi-path (ECMP) The forwarding decides how to balance actual traffic: random (but this break TCP flows) load balance per flow load balance per address pairs

15 Example: load-balancing IS-IS/OSPF load balancing with two 3ms paths, one slow 20 ms path. Hosts from the same LAN (or different flows from same host) may take different routes. 3 ms 3 ms 20 ms

16 Aggregation Also called summarization The netid part of IPv4 addresses can be aggregated (summarized) into shorter prefixes. Summarization is often done manually Leads to smaller routing tables (fewer prefixes) Threats: multi-homing and load-balancing / / / / /24

17 Asymmetric Routing A rule rather than an exception: Hot-potato routing To- traffic and from- traffic take different paths Send traffic out of your AS as soon as possible Cold-potato Try to keep your traffic as long as possible.

18 Fault detection An important feature in a routing protocol is how to detect link or node/router failures. In many cases, a node can directly detect a link failure: Directly connected copper Ethernet (loss of signal) FIber (loss of light) But more often indirect methods must be used Switched network (failure >1 switch away), Node failure Partially broken link Routing protocols have timers / hello protocol seconds-10s of seconds For faster detection use BFD Bidirectional Forwarding Detection Send many 'pings' and detect losses Down (and below) 100ms.

19 Protection switching After is failure is detected, the riuter should reroute around the failures Next-hop is changed, so all routes depending on the next-hop need to be re-computed The control-plane recomputes the routes and pushes them out to the forwarding plane Even though the number of next-hops is small, the number of routes with a specific next-hop can be very large, for example in BGP. Protection switching time is therefore failure detetcion + time for computing and pushing out new forwarding entries Small forwarding tables are therefore faster, or routing that does not need large tables. (In bridging, failure detection and protection switching is done with the spanning tree protocol)

20 Black-holing Black-holing: announce prefix, but traffic to the prefix is dropped (not delivered) Loops: circular announcements causing packet loops TTL is decremented until packet drops -> same symptom as black-holing Reasons: Transient errors due to long convergence (see countto-infinity in RIP) Misconfigurations Attacks (DOS, man-in-the-middle) Response to attacks: create a black-hole for attacked prefixes which removes DOS traffic

21 Metrics A fundamental functionality in a dynamic routing protocol: Find the best path to a destination But what is best path? Interior routing: typically number of hops, or bandwidth Exterior routing: business relations peering Metrics Number of hops (most common) Bandwidth, Delay, Cost, Load, Policies

22 Routing algorithms How does a router find a best path? Most solutions based on SPF (Shortest Path First) algorithms that are well known in graph theory. Bellman-Ford Dijkstra Apart from that, there are also other algorithms in Multicast routing Ad-hoc routing Sensor networks Delay-tolerant networks

23 Routing protocol classes Almost all unicast routing protocols can be classified into one of two groups: Link-State protocols (OSPF, IS-IS) Distance-Vector protocols (RIP, IGRP, BGP) They are also classified into Exterior (Inter-domain) routing protocols Between autonomous systems Interior (Intra-domain) routing protocols Within an autonomous system

24 Popular Unicast Routing Protocols Routing Protocols Interior RIP OSPF IGRP (cisco) Exterior IS-IS BGP EGP

25 Routes may come from many protocols Direct Local Configured static routes Aggregate Example: Static Networks on directly connected interfaces Manually aggregated routes RIP, OSPF, ISIS, BGP, RSVP,...

26 Route preference / Administrative distance Several protocols may include the same prefix. How do you decide which route to install in your routing table? Default preference (on Juniper) is: Direct > Local > Static > OSPF > ISIS > RIP > Aggregate > BGP Can be changed or overridden with policies

27 Redistribution of routing information If several protocols are running on the same router E.g., an OSPF as interior and BGP as exterior E.g. static routes into dynamic routing protocol The router can distribute routes from one protocol to another Interior routes need to be advertized to the Internet Exterior routes may need to be injected into the interior network Typically these routes are aggregated But only a subset the backbone tables are very large Necessary for domain carrying transit traffic Not necessary for a domain using only a default route Typically, redistributed routes are filtered in different ways due to routing policies

28 The routing process Routing protocol 1 RIB Routing protocol 2 Routing protocol 3 RIB RIB Routing Information Base Routing Process CPU Linecards FIB FIB FIB Forwarding Information Base

29 Routing instances and tables Routing Instance: main RIBs Routing Instance: other RIBs inet.0 Routing protocol 3 RIB inet.0 IPv4 unicast routes inet6.0 IPv6 unicast routes inet.1 IPv4 multicast forwarding cache inet.2 IPv4 multicast RPF table inet.3 IPv4 routes learnt from MPLS-TE path exploration inet.4 MSDP routes mpls.0 MPLS label-switch table Example: main.inet.0 juniper_private1.inet.0 Logical routers, VPNs, virtual routers, etc, use routing instances.

30 Routing policies Neighbours Neighbours Import RIB Export Protocols Protocols FIB Note: Export policies may be applied only to active routes! Protocol Default import action Default export action direct and static accept all N/A RIP accept all RIP routes reject all BGP accept all BGP routes export all active BGP routes IS-IS accept all IS-IS routes reject all (IS-IS uses LSAs) OSPF accept all OSPF routes reject all (OSPF uses LSAs) MPLS accept all MPLS routes export all active MPLS routes

31 Example routing policy: Redistribution In JunOS, policies are made up match/action pairs Example, announce an aggregated prefix routes in BGP Note: First declare policy, then export policy-statement MYNETWORK { term 1 { from { # match protocol aggregate; route-filter /24 exact; } then accept; #action } } protocols bgp { export MYNETWORK; # Apply policy }

32 Routing policy: syntax and flow Changing the default routing policy Syntax: policy-options { policy-statement name { term term-name { from { match; } then { action; } } } } accept route term reject Policy 1 term1 term2 term3 Policy 2 term1 term2 term3 default policy next

33 Applying policies protocol bgp { export p0; import p1; group external-peers { type external; export p2; import p3; neighbor { export p4; import p5; } } } Global properties Group properties Peer properties Export policy evaluation order: p4->p2->p0 If verdict (accept, reject) policy chain is terminated Side-effects may still apply

34 More match statements You can specify more matches (logical and): metric route-filter (next slide) Protocol family as-path community local-preference neighbor next-hop origin preference prefix-list...

35 Route-filters Route-filter match types route-filter /16 exact; route-filter /16 orlonger; route-filter /16 longer; route-filter /16 upto /24; route-filter /16 through /20; route-filter /16 prefix-length-range /20-/24; /16 /20 /24 /32

36 More actions Combined with accept: accept reject next policy next term trace as-path-expand as-path-prepend community color external load-balance per-packet local-preference metric next-hop origin preference

37 The CLI Two major modes: Operational mode: Monitor and troubleshoot, network connectivity, hardware Configure mode: Configuration of interfaces, routing protocols, authentication, logging, etc. Completion and query Line editing As you would expect, <TAB> and <?> Emacs operations: <ctrl-b>, <ctrl-f>, <ctrl-a>, <ctrle>, <ctrl-p>, <ctrl-n>,... On-line help: help reference help topic

38 Operations commands show show system storage help help topic help reference request request system reboot restart restart routing gracefully set set cli monitor clear test ping traceroute start shell show system users show chassis hardware detail show interfaces show route show route protocol direct show route table inet.0 show route receive-protocol show route advertising-protocol show log configure file file list file compare

39 Extending commands Level of detail: Pipe commands terse compare brief count detail display extensive except find match resolve save Example: > show route ospf extensive trim Example: > show route display xml > show route match 10.0 > show route save output # show compare rollback 0

40 Configure mode: Tree-based editing protocols { bgp { export default; group external { family inet { unicast; } } } ospf { area { interface lo0.0; interface fe-0/0/0.0; } } } top protocols bgp export default # set protocols bgp group external family inet unicast [edit] # edit protocols bgp group external [edit protocols bgp group external] # set family inet unicast [edit protocols bgp group external] # show family inet { unicast; } # top [edit] # ospf group external area family inet interface lo0.0 unicast up top interface fe-0/0/0

41 More configuration Alternative output (set): # show display set set protocols bgp export default_route set protocols bgp group external family inet unicast set protocols ospf area interface lo0.0 set protocols ospf area interface fe-0/0/0.0 set policy-options policy-statement default then accept Loading from file load override load merge load relative load override terminal load set

42 Commiting configurations Changing the state of the router candidate configuration. Commit semantics you need to explicitly commit for changes to take effect. Variants: commit confirmed and-quit commit check comment <string> commit sync You can make rollbacks to previous commits rollback 0 the state before editing rollback 1 previous commit Comparing changes show compare show compare rollback 2 Displaying configurations in different formats show display set show display xml