3Com S7902E, Configuration Manual

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If you start OSPF but do not configure the router ID, the system will automatically elect one.
However, the same election rules produce the same router ID. Therefore, you are recommended to
configure the router ID when starting an OSPF process. For the election rules, refer to OSPF
Configuration in the IP Routing Volume .
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If you configure multiple OSPF VPN instances but do not configure the route tag, the system will
automatically create one based on the AS number configured. If you do not configure BGP, the tag
will be 0. However, the same calculation rule produces the same tag, and hence the same tag will
be created for multiple OSPF VPN instances on the same PE or PEs with the same AS number.
Therefore, you are recommended to configure different tags for different OSPF VPN instance.
Configuring BGP AS Number Substitution
Configuration Prerequisites
Before configuring BGP AS number substitution, complete these tasks:
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Configuring basic MPLS L3VPN
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Configuring CEs at different sites to have the same AS number
Configuration Procedure
When CEs at different sites have the same AS number, you need to configure the BGP AS number
substitution function to avoid route loss.
With the BGP AS number substitution function, when a PE advertises a route to a CE of the specified
peer, if an AS number identical to that of the CE exist in the AS_PATH of the route, it will be replaced
with that of the PE before the route is advertised.
Follow these steps to configure the BGP AS number substitution function:
To do… Use the command… Remarks
Enter system view system-view —
Enter BGP view bgp as-number Required
Enter BGP VPN instance view
ipv4-family vpn-instance
vpn-instance-name
Required
Enable the BGP AS number
substitution function peer { ip-address |
group-name } substitute-as
Required
Disabled by default
For information about the peer { ip-address | group-name } substitute-as command, refer to BGP
Commands in the IP Routing Volume .