EIGRP or OSPF – Which should I use?
Kevin Delgadillo, PLM, IP Routing, NSSTG Ernie Mikulic, PM, OSPF, PfR, SAF
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Which routing protocol is better?
Which routing protocol should I use in my network? Should I switch from the one I’m using?
IPv4 Ends Merge RST-3210 IPv6 11048_05_2005_X2© 2008 Cisco Systems, © 2005 Cisco Systems, Inc. All rights reserved. Inc. All rights reserved. Cisco Public
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The Questions Is one routing protocol better than any other protocol? Define “Better!”
Uses less resources? Easier to troubleshoot?
Both are good choices Cisco offers full-featured implementations of both today Cisco EIGRP/OSPF deployment in the enterprise is ~50/50 today
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Converges faster?
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Easier to configure? Scales to a larger number of routers, routes, or neighbors? More flexible? …
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The Questions The answer is yes if: The network is complex enough to “bring out” a protocol’s specific advantages You can define a specific feature (or set of features) that will benefit your network tremendously…
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The Questions But, then again, the answer is no! Every protocol has some features and not others, different scaling properties, etc. Let’s consider some specific topics for each protocol....
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EIGRP or OSPF: Which Routing Protocol? Link State & Distance Vector Convergence Speed Topology and Heirarchy Summary
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Link State & Distance Vector Link state • OSPF is an example • Each router tells the world about its neighbors • All information passed is connectivity related • Each node in the network constructs a connectivity map of the network
• Each node keeps identical link-state database from which routing table is derived • More complex than distance vector protocols
Distance vector • EIGRP is an example (but does not behave like a “pure” DV protocol) • Each router tells its neighbors about its world • Each node shares its routing table with its neighbors • Simpler than link state protocols
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Convergence Speed Equal Cost Convergence OSPF Convergence EIGRP Convergence Convergence Summary
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Convergence Speed Which protocol converges faster? OSPF verses EIGRP Is DUAL faster, or Dijkstra SPF?
Rules of Thumb The more routers involved in convergence, the slower convergence will be The more routes involved in convergence, the slower convergence will be
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Convergence Speed Three steps to convergence Detect the failure Calculate new routes around the topology change Add changed routing information to the routing table
The first and third steps are similar for any routing protocol, so we’ll focus on the second step
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Equal Cost Start with B>C>E and B>D>E being equal cost
A
If C fails, B and E can shift from sharing traffic between C and D to sending traffic to D only Number of routers involved in convergence: 2 (B and E)
B
C
D
Convergence time is in the milliseconds E
F
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OSPF C fails B and E flood new topology information
A
All routers run SPF to calculate new shortest paths through the network
B
SPF
B and E change their routing tables to reflect the changed topology Number of routers involved in convergence: 2 (B and E)
C
SPF
D
E
F
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OSPF Within a single flooding domain (OSPF area) Convergence time depends on flooding timers, SPF timers, and number of nodes/leaves in the SPF tree What happens when we cross a flooding domain boundary?
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OSPF E floods topology changes to C and D
A
C and D summarize these topology changes and flood it to B B builds a summary from the summary flooded to B, and floods it into area 2
Area 2
B Area 0 C
D
A calculates a route to B, then recurses C onto B E
Convergence time is dependent on the network design
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Area 1 F
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OSPF– Convergence Data 2.500 2.000
Convergence time with default timers and tuned timers
Time
IPv4 OSPF 1.500 IPv6 OSPF 1.000
Linear (IPv4 OSPF) Linear (IPv6 OSPF)
0.500
IPv4 and IPv6 IGP convergence times are similar - The IPv6 IGP implementations
0.000 0
500
1000
1500
2000
2500
3000
Number of Prefixes
Time
might not be fully optimized yet - Not all Fast Convergence optimizations might be available
0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0
IPv4 OSPF IPv6 OSPF Linear (IPv6 OSPF) Linear (IPv4 OSPF) 0
500
1000
1500
2000
2500
3000
Number of Prefixes
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All specifications subject to change without notice
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OSPF Within a flooding domain The average convergence time, with default timers, is on the order of seconds With optimal SPF/LSA timers, the convergence time can be in the milliseconds
Outside the flooding domain Network design and route aggregation are the primary determining factors of convergence speed
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EIGRP
A 10
DUAL works on a simple geometric principle: If my neighbor’s cost (RD) to reach a given destination is less than my best cost (FD), then the alternate path (FS) cannot be a loop
30
35
B 10
15 C
D
10
10 20
E
B>D>E>F is 35 10
B>C>E>F is 30 D>E>F is 20, which is less than the best path, 30, so B>D>E>F cannot be a loop FC Rule: Choose FS for path where RD
