Dell S4820T Configuration Manual page 199

Figure 27. Before and After AS Number Migration with Local-AS Enabled
When you complete your migration, and you have reconfigured your network with the new information,
disable this feature.
If you use the "no prepend" option, the Local-AS does not prepend to the updates received from the
eBGP peer. If you do not select "no prepend" (the default), the Local-AS is added to the first AS segment
in the AS-PATH. If an inbound route-map is used to prepend the as-path to the update from the peer, the
Local-AS is added first. For example, consider the topology described in the previous illustration. If Router
B has an inbound route-map applied on Router C to prepend "65001 65002" to the as-path, the
following events take place on Router B:
1. Receive and validate the update.
2. Prepend local-as 200 to as-path.
3. Prepend "65001 65002" to as-path.
Local-AS is prepended before the route-map to give an impression that update passed through a router
in AS 200 before it reached Router B.
Border Gateway Protocol IPv4 (BGPv4)
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