© 2010 Cisco Systems, Inc. All rights reserved.Cisco Public
1
Kevin Delgadillo, PLM, IP Routing, NSSTG
Ernie Mikulic, PM, OSPF, PfR, SAF
EIGRP or OSPF – Which should I use?
© 2008 Cisco Systems, Inc. All rights reserved. Cisco Public 2
Which routing protocol is better?
Which routing protocol should I use in my network?
Should I switch from the one I’m using?
2© 2005 Cisco Systems, Inc. All rights reserved.
RST-3210
11048_05_2005_X2
IPv4 EndsMergeIPv6
© 2008 Cisco Systems, Inc. All rights reserved. Cisco Public 3
The Questions
Is one routing protocol better than any other protocol?
Define “Better!”
Converges faster?
Uses less resources?
Easier to troubleshoot?
Easier to configure?
Scales to a larger number of routers, routes, or neighbors?
More flexible?
…
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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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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A
B
C D
F
E
Equal Cost
Start with B>C>E and B>D>E being equal cost
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)
Convergence time is in the milliseconds
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A
B
C D
F
E
OSPF
C fails
B and E flood new topology information
All routers run SPF to calculate new shortest paths through the network
B and E change their routing tables to reflect the changed topology
Number of routers involved in convergence: 2 (B and E)
SPF
SPF
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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
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
A calculates a route to B, then recurses C onto B
Convergence time is dependent on the network design
A
B
C D
F
E
Area 1
Area 0
Area 2
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OSPF– Convergence Data
Convergence time with default timers and tuned timers
IPv4 and IPv6 IGP convergence times are similar
- The IPv6 IGP implementations
might not be fully optimized yet- Not all Fast Convergence optimizations might be available
0.000
0.500
1.000
1.500
2.000
2.500
0 500 1000 1500 2000 2500 3000
Number of Prefixes
Tim
e
IPv4 OSPF
IPv6 OSPF
Linear (IPv4
OSPF)Linear (IPv6
OSPF)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 500 1000 1500 2000 2500 3000
Number of Prefixes
Tim
e
IPv4 OSPF
IPv6 OSPF
Linear (IPv6
OSPF)Linear (IPv4
OSPF)
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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A
B
C D
F
E
EIGRP
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
B>D>E>F is 35
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<FD
10
10 15
10 10
10
30 35
20
FD = Feasible Distance
RD = Reported Distance
FC = Feasibility Condition
FS = Feasible Successor
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A
B
C D
F
E
EIGRP
B will install the path through C, and mark the path through D as a Feasible Successor (FS) in the topology table
When C fails, B looks for alternate loop free paths (FS)
Finding one, it installs it
Local repair, no flooding
Convergence time is in the milliseconds
Number of routers involved in convergence: 2 (B and E)
10
10 15
10 10
10
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A
B
C D
F
E
EIGRP
If the second path cannot be proven loop free
B and E detect the failure, and have no alternate path
B queries A and D
A replies that it has no path
D replies with its alternate path
E queries D and F
F replies that it has no path
D replies with its alternate path
Hop-by-hop queries; no flooding
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EIGRP
For paths with feasible successors, convergence time is in the milliseconds
The existence of feasible successors is dependent on the network design
For paths without feasible successors, convergence time is dependent on the number of routers that have to handle and reply to the query
Query range is dependent on network design
Good design is the key to fast convergence in an EIGRP network
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Convergence Summary
We can sort typical convergence times into three groups:
EIGRP with a feasible successor
OSPF with modified SPF/LSA throttle timers
EIGRP without a feasible successor and good design
OSPF with default timers
EIGRP without a feasible successor without good design
Good
Best
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Convergence Summary
It’s possible to converge in under one second using either protocol, with the right network design
Rules of Thumb:
More aggregation tends towards better performance for EIGRP
Less aggregation tends towards better performance for OSPF
If you’re going to use OSPF, tune the SPF/LSA timers
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Topology
Hub and Spoke
Full Mesh
Support for Hierarchy
Topology Summary
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OSPF Hub and Spoke
OSPF relies on every router within a flooding domain to have the exact same view of the network’s topology (link state database) to calculate loop free paths
OSPF flooding rules have implications for scaling and design in hub and spoke networks
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OSPF Hub and Spoke
Although B can only reach C through A, it still receives all of C’s routing information
As the number of remote sites increases, the amount of information each remote site must process and store also increases
This limits scaling in link state hub and spoke networks
B
A
C
D
reachability
only
through A
all link state
information
is flooded
to B
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OSPF Hub and Spoke
Controlling route distribution
There’s no way to allow C and D to receive information about 10.1.1.0/24, and not E and F
BA
10.1
.1.0
/24
C
D
E
F
Area 0
Area 1
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EIGRP Hub and Spoke
Controlling query range
If A loses its connection to 10.1.1.0/24, it builds and transmits five queries: one to each remote, and one to B
Each of the remote sites will query B
B must process and reply to five queries
BA
10.1
.1.0
/24
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EIGRP Hub and Spoke
If these spokes are remotes sites, they have two connections for resiliency, not so they can transit traffic between A and B
A should never use the spokes as a path to anything, so there’s no reason to learn about, or query for, routes through these spokes
BA
10.1
.1.0
/24
Don’t Use
These Paths
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EIGRP Hub and Spoke
To signal A and B that the paths through the spokes should not be used, the spoke routers can be configured as EIGRP stubs
BA
10.1
.1.0
/24
router#config t#
router(config)#router eigrp 100
router(config-router)#EIGRP stub connected
router(config-router)#
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EIGRP Hub and Spoke
Marking the spokes as stubs allows them to signal A and B that they are not valid transit paths
A simply will not query the remotes, reducing the total number of queries in this example to 1
BA
10.1
.1.0
/24
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EIGRP Hub and Spoke
Marking these remotes as stubs also reduces the topological complexity (meshiness) of the network
Without stub configuration on spokes, B believes it has five paths to 10.1.1.0/24, so it has to maintain five topology table entries
BA
10.1
.1.0
/24
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EIGRP Hub and Spoke
Routers which are configured as EIGRP stubs will only advertise locally connected or redistributed destinations
These remotes will not pass A’s advertisement of 10.1.1.0/24 to B
B will only have one path to 10.1.1.0/24
BA
10.1
.1.0
/24
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Full Mesh
Full mesh topologies are complex:
2 routers = 1 link
3 routers = 3 links
4 routers = 6 links
5 routers = 10 links
6 routers = 15 links
…
Adjacencies = links(links-1)/2
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OSPF Full Mesh
Flooding routing information through a full mesh topology is also complicated
Each router will, with optimal timing, receive at least one copy of every new piece of information from each neighbor on the full mesh
OSPF uses notion of Designated Router (DR) to improve scalability in mesh networks
New Information
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EIGRP Full Mesh
Routes must be advertised between every pair of peers in the mesh so each router has the correct next hop and routing information
Number the links so they can be summarized to a single advertisement at the edge
Good for smaller mesh networks, summarization more important for larger mesh networks
Summarize
Summarize
Summarize
Summarize
Summarize
Summarize
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OSPF Support for Hierarchy
OSPF requires a hierarchical design
Summarization and filtering occur at flooding domain borders
Summarization and filtering can also be configured at routers redistributing routes into OSPF
In a two layer hierarchy, the flooding domain border naturally lies on the aggregation/core boundary
area 0
Su
mm
ari
za
tio
n
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EIGRP Support for Hierarchy
EIGRP does not require a heirarchical design
Auto-summarization enabled by default at classful network boundaries
EIGRP enables you to summarize at any desired boundary
Proper network design is still needed!
Distribution
Access
Core
Su
mm
ari
zati
on
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Topology Summary
Rules of Thumb
EIGRP performs better in large scale hub and spoke environments
OSPF perform better in large full mesh environments, if tuned correctly
EIGRP tends to perform better in more strongly hierarchical network models, OSPF in flatter networks
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Other Considerations - 1
EIGRP forms adjacencies and exchanges routing updates with neighbors
OSPF forms adjacencies with DR/BDR
OSPF can be more efficient than EIGRP for large meshed networks
EIGRP uses metric based on bandwidth and delay
OSPF uses interface cost (inversely proportional to bandwidth)
EIGRP may provide more flexibility in selecting best path
EIGRP by default limits usage to at most 50% of link bandwidth in worst case
OSPF uses 100% of link bandwidth when required
EIGRP may be better suited for lower bandwidth WAN applications
EIGRP provides feature velocity, but is Cisco-proprietary
OSPF is an Internet RFC standard
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Other Considerations - 2
EIGRP sends hop-by-hop queries only when Feasible Successor cannot be found
OSPF regularly syncs LSA database and floods network with topology change
EIGRP can be more efficient by minimizing routing information exchanged
EIGRP is a conceptually simpler routing protocol
OSPF’s rules for different types of areas and LSAs can be conceptually more difficult to understand
Some customers believe EIGRP is easier to implement, but both are feature-rich and scalable
EIGRP supports automatic summarization
OSPF’s requires manual summarization
Care is needed in either case to ensure proper summarization!
EIGRP supports both equal and unequal cost load sharing
OSPF only supports equal cost load sharing
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Summary
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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?
Did we answer these questions???
42© 2005 Cisco Systems, Inc. All rights reserved.
RST-3210
11048_05_2005_X2
IPv4 EndsMergeIPv6
© 2008 Cisco Systems, Inc. All rights reserved. Cisco Public 43
Summary
There is no “right” answer!
“IT DEPENDS…”
Consider:
Your business requirements
Your network design & topology
Convergence time requirements dictated by your applications
Other intangible factors
EIGRP and OSPF are generally pretty close in capabilities and development (GR, BFD, IPv4/IPv6)
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Summary
EIGRP
LargeMesh
Hub and Spoke
Flat Aggregated
Flat Hierarchical
OSPF
Rules of Thumb
Complex Simpler
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