IK2205 Inter-domain Routing

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1 IK2205 Inter-domain Routing Lecture 5 Voravit Tanyingyong,

2 Outline Redundancy, Symmetry, and Load Balancing Redundancy Symmetry Load balancing Scenarios Controlling Routing Inside the AS Interaction of Non-BGP routers with BGP routers BGP policies conflicting with internal defaults Policy routing

3 Redundancy, Symmetry, Load balancing

4 Redundancy Redundancy is to have multiple paths for the traffic Achieved by having more links connected to one or more providers. Usually in the form of primary and backup links Stockholm POP ISP1 Lund POP ISP2 Stockholm AS4 Lund

5 Default routes A default route is a route in IP forwarding table that is used if a routing entry for a destination does not exist Minimizing the number of routes a router has to learned Provide redundancy in the event of connectivity interruptions Represented by: / or 0/0 Cisco called Gateway of last resort Can be learned dynamically or configured statically

6 Dynamically learned defaults Can be exchanged as a dynamic advertisement The system advertising it is the gateway of last resort for other systems Can be learned via BGP or IGP For redundancy, defaults should be received from multiple sources In BGP, local preference can be used to set degree of prefernece for primary/backup defaults

7 Dynamically learned default (cont d) Single router scenario Set local pref = AS1 Set local pref = primary AS2 backup Set local pref = 100 AS1 IBGP primary AS2 backup Set local pref = Multiple routers scenario Source: Internet Routing Architecture 2nd edition, Cisco Press

8 Statically set defaults Offer more control over routing behaviours Statically set default routes can be set to point to IP address of the next-hop gateway A specific router interface A network number AS1 0/0 towards AS1 E0 0/0 towards Ethernet0 Gateway of last resort is Gateway of last resort is 0/0 via Ethernet0 Source: Internet Routing Architecture 2nd edition, Cisco Press

9 Statically set defaults to network number AS /16 NAP AS / / AS1 0/0 towards /16 Gateway of last resort is /16 Source: Internet Routing Architecture 2nd edition, Cisco Press

10 Symmetry Traffic leaves the AS at a certain exit point returned through the same point Problematic to acheive when having redundancy In practice, it s not a problem to have asymmetrical traffic

11 Load balancing Capability to divide traffic optimally over multiple links Does not mean equal distribution of the load! Two types of traffic in concern Inbound traffic Outbound traffic Applying attributes to outbound routing announcements will affect the behaviour of incoming traffic Applying attributes to inbound routing announcements will affect the behaviour of outgoing traffic

12 Outbound and inbound decision Outbound decision Which way do I send my traffic (outbound) to reach NetA NetA NetA Stockholm Lund Inbound decision I want to receive traffic (inbound) for NetB via Stockholm link and for NetC via Lund link NetB NetC Stockholm Lund Source: Internet Routing Architecture 2nd edition, Cisco Press

13 Scenarios Single-homing Multihoming to a single provider Multihoming to different providers Customers of the same provider with a backup link Customers of different providers with a backup link Inbound traffic, outbound traffic Redundancy, symmetry and load balancing

14 Single-homing Customer Default Outgoing traffic Static ISP Incoming traffic Least expensive and most effective solution Customer point default to provider Provider set static route to customer no redundancy, no load balancing Source: Internet Routing Architecture 2nd edition, Cisco Press IK2205, Fall 2008

15 Multihoming to a single provider Assume that BGP is used as a routing protocol No need to use public ASN Inboound traffic with either MED or AS_PATH Outbound traffic with Local preference AS_PATH = 2 AS_PATH = 2 2 MED = 100 MED=200 Local_pref = 300 Local_pref=200 IK2205, Fall 2008

16 Multihoming to a single provider (cont d) Assume that BGP is used as a routing protocol No need to use public ASN We will discuss the following topics: Default only, one primary link, and one backup link Default, primary, and backup, plus partial routing Default, primary, and backup, plus full and partial routing Automatic load balancing Balancing between two routers sharing multiple paths

17 Default only, primary link and backup link Outbound traffic Follow best 0/0 No load balancing Inbound traffic Closet-exit routing Overrided by using MED No load balancing Redundancy, Symmetry 2001 Cisco Press

18 Default, primary, backup, partial routing 2001 Cisco Press

19 Default, primary, backup, partial routing (cont.) Outbound traffic Traffic to C4 and C5 go over NY link (Loc_pref 300 > 200) Traffic to C2 and C3 go over SF link (Loc_pref 300 > 250) All other outbound traffic go over NY link (Loc_pref 250 > 200) Inbound traffic Traffic to W and Z come over NY link (MED 200 < 300) Traffic X and Y come over SF link (MED 200 < 250) All other inbound traffic come over NY link (MED 250 < 300) Redundancy, Symmetry, load balancing

20 Default, primary, backup, + full and partial routing Customer get full routes on all links or a combination of full routes on one link and default or partial routes on other link Same as previous scenario Inbound traffic controlled by MED Outbound traffic controlled by Local preference

21 Cisco: Automatic load balancing Can be done on identical destinations learned over different links from the same AS. Keep max 6 identical BGP routes to same destination in IP routing table Next-hop will be change automatically when pass on the route to IBGP Does round-robin scheme on a perdestination (host) basis Called BGP multipath 2001 Cisco Press

22 Balancing between two routers sharing multiple paths 2001 Cisco Press 2001 Cisco Press RTA s BGP table Destination Next Hop NetX (best) NetX RTA s IP routing table Destination Next Hop NetX /24 Link1 RTA s BGP table Destination Next Hop NetX RTA s IP routing table Destination Next Hop NetX /24 Link /24 Link2

23 Multihoming to different providers Assume that BGP is used as a routing protocol Inboound traffic with AS_PATH Outbound traffic with Local preference AS_PATH = 2 AS_PATH = 2 2 Local_pref = 300 Local_pref=200

24 Multihoming to different providers (cont d) We will discuss the following topics: Default only, primary, and backup Default, primary, and backup, plus partial routing Default, primary, and backup, plus full and partial routing Customer inbound traffic (AS_PATH manipulation) All cases has the same inbound traffic behaviour

25 Default only, primary, and backup Outbound traffic Follow best 0/0 No load balancing 2001 Cisco Press

26 Default, primary, backup plus partial routing Outbound traffic Some traffic follow partial routes to different providers Rest: follow default 2001 Cisco Press

27 Default, primary, backup plus full and partial routing Outbound traffic For SF and its customer, use SF link Rest: NY link 2001 Cisco Press

28 Customer inbound traffic for multihoming to different providers scenarios Inbound traffic MED is non-transitive manipulate AS_PATH length Path manipulation must influence all the way to the exchange point 2001 Cisco Press

29 Customers of the same provider with a backup link Somtimes, customers have private link to each other to provide internal connectivity as well as backup connectivity 2 scenarios: Private link used as a backup link when an Internet link fails Private link used as primary link for internal traffic and as a backup link in case of an Internet link fails For backup strategy, customers must advertise each others networks to the provider since it has to act as a transit AS for the other customer when the other customer s Internet link fails

30 Private link used as a pure backup AS2 s Outbound traffic All to AS1 Use AS3 when primary link fails AS2 s Inbound traffic All from AS1 Use AS3 when primary link fails 2001 Cisco Press

31 Private link used as primary between AS2 and AS3 AS2 s Outbound traffic Traffic goes over AS3 link to AS3 and its customers Rest: goes over AS1 link AS2 s Inbound traffic Traffic from AS3 and its customer come over AS3 link Rest: comes over AS1 link 2001 Cisco Press

32 Customers of different provider with a backup link The requirement to have provider not use one customer to reach the other customer is more complicated Providers like to minimize configuration as much as possible Providers like to set policy based on ASN than specific networks 2 approach BGP community approach AS path manipulation approach 2001 Cisco Press

33 RFC1998 The community approach Provider set local preference according to communities received in the rouing update This is from ISP1 s perspective, ISP2 should apply similar policy Dynamic mapping of Local preference Pattern Community Local preference Customer loca routes None 100 Customer transit routes 400:40 40 ISP routes 400: Cisco Press

34 The AS path manipulation approach Same as in multihoming to different provides scenario Straightforward and efficient 2001 Cisco Press

35 Controlling routing inside the AS Interaction of Non-BGP routers with BGP routers Injecting BGP into the IGP Following default routes inside the AS BGP policies conflicting with internal defaults Defaults inside the AS: primary/backup BGP policy Defaults inside the AS: other BGP policies Policy routing

36 Interaction of Non-BGP routers with BGP routers Non-BGP routers inside the AS can reach outside world by: Injecting BGP into the IGP Following default routes inside the AS

37 Injecting BGP into the IGP Injecting full BGP routes into IGP is not recommended IGPs were not designed for Internet-sized routing table Injecting partial BGP routes should be done with care Memory CPU resources for calculating paths and processing routing updates Link utilization from routing control traffic Impact on convergence IGP limitation Network topology Some IGP are classful CIDR information will be lost Fluctuations of BGP routes might cause IGP meltdown

38 Following default routes inside the AS More practical than injecting BGP into IGP Border routers injected default route into IGP Internal routers follow the shortest path to the closet border router 2001 Cisco Press

39 BGP policies conflicting with internal defaults Loop can happen if BGP border routers have contradictory BGP policies that result in the traffic s being sent back inside the AS 2 cases how loop might occur and possible solutions Defaults inside the AS in conjunction with a primary/backup BGP policy Defaults inside the AS in conjunction with other BGP policies

40 Defaults inside the AS: primary/backup BGP policy RTA RTB backup T1 RTC SF 0/0 0/0 IBGP RTD NY primary T3 Source: Internet Routing Architecture 2nd edition, Cisco Press

41 Multiple scenarios to aviod loop behavior Manipulating the IGP metric The IBGP path is shorter than the IGP path Running BGP on transit routers Who generates the default, and how is it generated?

42 Manipulating the IGP metric RTA RTB backup T1 RTC SF metric=100 metric=1 0/0 IBGP 0/0 RTD NY primary T3

43 The IBGP path is shorter than the IGP path RTA RTB backup T1 RTC SF 0/0 0/0 IBGP RTD NY primary T3

44 Running BGP on transit routers RTA IBGP RTB IBGP IBGP IBGP IBGP backup T1 RTC SF IBGP RTD NY primary T3

45 Who generates the default, and how is it generated? RTA RTB backup T1 RTC 0/0? 0/0? IBGP RTD SF 0/0 NY 0/0 primary 0/0 T3 0/0

46 RIP- and OSPF- generated deafult Book page RIP-Generated default Manipulating the IGP metric OSPF-Generated default Who generates the default, and how is it generated?

47 Default inside the AS:other BGP policies MED problem Solution Ignore MED Create shorter IBGP path Run IBGP full mesh 2001 Cisco Press

48 Policy routing A means of controlling routes that rely on the source, or source and destination of traffic. Can be used to control traffic both inside and between ASes A glorified form of static routing Used to forced a routing behavior different from what the dynamic routing protocols dictate Drawbacks: Extra static configuration is needed CPU-intensive because it is based on the source IP addresses

49 Policy routing based on traffic source 2001 Cisco Press

50 Policy routing based on traffic source/destination 2001 Cisco Press

51 Policy routing defaults to dynamic routing Important to make sure that if policy-routed traffic cannot be delivered, some other alternative is avaible Cisco Set multiple next hops for policy-routed traffic If none is available, send traffic based on normal dynamic routing 2001 Cisco Press

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