Routing Loops Prevention

smashlizardsNetworking and Communications

Oct 29, 2013 (3 years and 9 months ago)

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Virgil Dobrota
Technical University of Cluj-Napoca, Romania
Virgil.Dobrota@com.utcluj.ro
Internet Protocols
Copyright ©Virgil Dobrota2009, All rights reserved
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21. Routing Loops Prevention
22. Link State Based Routing Protocols:
OSPFv2, OSPFv3
Course 10 -Outline
Copyright ©Virgil Dobrota2009, All rights reserved
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21. Routing Loops Prevention
Copyright ©Virgil Dobrota2009, All rights reserved
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Routing Loops Prevention (I)
Source: Cisco Systems
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Routing Loops Prevention (II)
Routing Loops Prevention Mechanisms:

Maximum number of hops =15

Split horizon

Route Poisoning

Triggered updates

HolddownTimer
Copyright ©Virgil Dobrota2009, All rights reserved
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Maximum number of hops =15
When the packet is forwarded through a router, the number of
hops counter is increased (usually by 1).
If routing loops exist -> count-to-infinity problem
Limit the maximum number of hops to 15 -> 16 is
infinite, i.e. the packet is discarded and an ICMP packet
is sent to the source
(“Destination unreachable”)
Routing Loops Prevention (III)
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Split Horizon
Routing Loops Prevention (IV)
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Route Poisoning
“Poison Reverse”update is sent explicitly advertising that
the (sub-)network is not reachable (distance = 16)
Together with “Triggered Updates”will reduce the
convergence time
Triggered Updates
updates are sent immediately to the neighbouring routers
(at RIP does not have to wait 30 seconds)
Routing Loops Prevention (V)
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HolddownTimer
-> whenever a router advertises an unreachable
route a holddowntimer is started (timeout 180
seconds by default at RIP)
Routing Loops Prevention (VI)
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Algorithm
Routing Loops Prevention (VII)
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Routing Loops Prevention (VIII)
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Timers for RIP
Updates Timer: every 30 seconds (by default)

broadcast updates (RIPv1)

multicast updates (RIPv2)
Invalid Route Timer: after 180 seconds of no updates

the route is not usable but is still stored in the memory

directly connected neighbors are notified that the route has
infinite metric (16) and a “garbage-collection”timer is started.
Garbage—Collection (Flush) Timer: 240 seconds
(by default)

If no updates are received, the routes advertised by the invalidrouter
are finally erased from the routing table

If new updates are received, the routes become valid.
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22. Link State Based Routing Protocols:
OSPFv2, OSPFv3
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OSPF (I)
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OSPF (Open Shortest Path First)

RFC 1247
•Link-state
•Classless routing
•Hierarchical routing
•SPF algorithm, open -> not proprietary
•explicit support for CIDR (Classless InterDomainRouting)
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OSPF (II)
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OSPFv2 (Open Shortest Path First)

RFC 2328
•Point-to-point links between 2 routers
•BMA (Broadcast Multi-Access)networks
examples: LANs
•NBMA (Non-Broadcast Multi-Access)
examples: majority of packet-switched WANs
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OSPF (III)
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OSPFv2 Packet Format
Version =2
Type=1 Hello
=2 Database description
=3 Link State Request
=4 Link State Update
=5 Link State Acknowledgment
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OSPF (IV)
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OSPF (V)
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Layer 3 Multicast Addresses
224.0.0.1: all multicast-capable hosts on the segment
224.0.0.2: all multicast-capable routers on the segment
224.0.0.4: all DVMRP routers on the segment
224.0.0.5: all OSPF routers
224.0.0.6: all OSPF DR (Designated Routers)
224.0.0.9: all RIP-2 routers
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OSPF (VI)
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Multicast MAC Address
01-00-5E + 0 + LS 23 bits of IP address
<-OUI ->
Example:
IP = 224.1.10.10
MAC = 01-00-5E-01-0A-0A
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OSPF (VII)
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Zebra/Quagga
OSPFv32606ospf6d
OSPFv22604ospfd
RIPng2603ripngd
RIPv1, RIPv22602ripd
Routing Protocol
TCPPort Daemon (server)
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OSPF (VIII)
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Experiments
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OSPF (IX)
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Zebra/Quaggaconfigfile for router R1:
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OSPF (X)
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ripngd(a), ospf6d (b) configfiles: