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Convergence times
Convergence is the time that it takes from the instant a failure occurs in the network until a new
path through the network is discovered, and all routers or switches are aware of the new path.
Convergence times vary, based on the complexity and size of a network.
convergence times (single link failure)
a single link failing in a relatively simple network. They reflect update and/or hello timers
expiring. Dialup "convergence" times reflect the time that it takes to dial, connect, and
authenticate a connection. These times do not take into account LAG, fast spanning tree, or
multipath routing, which speed up convergence. This table shows the importance of carefully
planning for fail-over in a network. For example, both OSPF and EIGRP (Layer 3) protocols
converge faster than spanning tree (Layer 2). When designing a highly available data network, it
is more advantageous to use Layer 3 protocols, especially link-state (OSPF) or hybrid (EIGRP)
protocols, than Layer 2 (spanning tree).
Table 82: Sample convergence times (single link failure)
Protocol
EIGRP (Cisco)
OSPF
RIP
IGRP (Cisco)
Spanning tree (Layer 2)
ISDN dialup (connect +
authentication)
56-k dialup (connect +
authentication)
lists some sample convergence times that are based on
Approximate convergence
time (seconds)
2
6 to 46
210
400
50+
2
60
Convergence times
Table 82: Sample
Issue 3.4.1 June 2005
355
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