Information Technology Reference
In-Depth Information
event. A converged network implies that all the routers within the same routing domain are
synchronized in their view of the network. In an MPLS VPN, convergence can be assessed
in two areas:
•
Provider backbone convergence
•
VPN site-to-site convergence
Provider Backbone Convergence
If the BGP next hop is unaffected, the convergence of the provider network because of up
or down network events does not cause convergence between two VPN sites. However,
traffic forwarding between the sites might be affected during these events. The convergence
within the backbone can be dependent on a variety of factors, including the following:
•
Physical layer stability
—Physical layer stability can be increased with physical line
protection, such as SONET protection schemes. With SONET Automatic Protection
Switching (APS), for example, line failure is recovered within 50 ms from the point
of failure detection.
•
Circuit or path protection
—With hot-standby circuits or paths, convergence can be
improved. For example, using MPLS Fast Reroute, a feature of MPLS TE, can result
in a failure recovery rate comparable to SONET APS.
•
IGP convergence
—The convergence of an IGP is variable but typically is completed
in tens of seconds. With proper timer tuning, IGP convergence time can be greatly
reduced.
•
LDP/TDP convergence
—Convergence of LDP/TDP is affected by how the labels are
maintained. For example, liberal retention mode (the default for the frame-based
MPLS) allows an LSR to keep all the label mappings received from its neighbors even
though they are not used. When an LDP/TDP session is lost, however, the conver-
gence takes longer.
The next section examines site-to-site convergence.
Site-to-Site Convergence
Site-to-site convergence is perhaps more critical to MPLS VPN convergence than provider
backbone convergence. For a route to be propagated from one site to another site, several
processes are involved. Each can contribute to the convergence.
Site-to-site convergence
is the time it takes for a route to be advertised from a CE to a PE.
This time is dependent on the routing protocol between the PE and the CE. For example,
BGP advertisements are paced differently between eBGP and iBGP. You can change the
default advertisement interval in BGP using the
neighbor advertisement-interval
command.
