eigrp-or-ospf-which-should-i-use
.pdf
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
A
B
C |
D |
E
F
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11 |
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)
A
SPF
B
C |
D |
SPF
E
F
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12 |
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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13 |
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
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A |
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Area 2 |
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B |
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Area 0 |
C |
D |
E
Area 1
F
© 2008 Cisco Systems, Inc. All rights reserved. |
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14 |
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
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2.500 |
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2.000 |
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IPv4 OSPF |
Time |
1.500 |
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IPv6 OSPF |
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1.000 |
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0.500 |
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0.000 |
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0 |
500 |
1000 |
1500 |
2000 |
2500 |
3000 |
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Number of Prefixes |
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0.5 |
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0.45 |
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0.4 |
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0.35 |
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IPv4 OSPF |
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Time |
0.3 |
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0.25 |
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IPv6 OSPF |
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0.2 |
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0.15 |
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0.1 |
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0.05 |
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0 |
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0 |
500 |
1000 |
1500 |
2000 |
2500 |
3000 |
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Number of Prefixes |
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All specifications subject to change without notice |
15 |
|
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
© 2008 Cisco Systems, Inc. All rights reserved. |
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16 |
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
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A |
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10 |
30 |
35 |
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B |
10 |
15 |
C |
D |
10 |
10 |
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20 |
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E |
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10 |
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F |
FD = Feasible Distance
RD = Reported Distance
FC = Feasibility Condition
FS = Feasible Successor
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17 |
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)
|
A |
|
10 |
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B |
10 |
15 |
C |
D |
10 |
10 |
E 10
F
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18 |
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
A
B
C |
D |
E
F
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19 |
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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