Information Technology Reference
In-Depth Information
Routes received from a CE must be installed into a VRF and then redistributed into BGP
on the PE. If the PE-CE protocol is eBGP, no redistribution is involved. Next, these routes
are advertised via iBGP to remote PEs or RRs, which might be subjected to the advertise-
ment pacing. iBGP's default advertisement interval is 5 seconds, but you can tune this
between 0 and 600 seconds. The addition of each RR in the path increases the total time to
reach the remote PE.
When the remote PE receives these routes, they are installed into the appropriate VRFs in
each scan-time interval. By default, the VPNv4 import scanner runs every 15 seconds. You
can change this interval to between 5 and 60 seconds using
bgp scan-time import
under
the VPNv4 address family. Note that withdraws are processed immediately and next-hop
deletion is processed at the general BGP scanner interval (60 seconds).
The last two components of site-to-site convergence are the time to advertise VPN routes
from a PE to a CE and the time it takes for a CE to install these routes into its local routing
protocol. Again, the advertisement might be paced if BGP is used.
In summary, the time for site-to-site convergence depends on PE-CE protocols, the number
of hops that routes have to be advertised within the provider networks, and various timers.
Proper timer tuning and testing are important in reducing the total convergence time.
As with all convergence tunings, it is important to note that faster convergence often leads
to less network stability and greater resource consumption. Any change to a default timer
should be carefully evaluated in a simulated environment to discover its impact on
convergence and stability.
NOTE
Case Study: Inter-AS VPN Using Multihop eBGP
Between RRs and IPv4 Labels
This case study demonstrates in detail how an inter-AS scenario works, as discussed
in the section “Multihop Multiprotocol eBGP for VPNv4.” The topology is shown in
Figure 10-32. Inter-AS VPN is needed between AS 100 and AS 200 for VPNa. To reduce
the resource use on ASBRs, two autonomous systems decide to peer between their RRs in
a multihop eBGP for VPNv4 prefixes.
