3Com 3C13636 Configuration Manual page 886

3Com Router 3000 Ethernet Family
Configuration Guide
Perform the following configuration in the VPN-instance view of MBGP.
Table 3-13 Configure BGP asynchronous with IGP
Configure BGP asynchronous with IGP
By default, asynchronous mode is selected.
Step 5: Permit route loop configuration in Hub&Spoke networking (optional)
Normally, PE-CE configuration can be performed by specifying the AS number of the
peer; other configurations can be performed by keeping the system default value.
In the case of standard BGP, BGP tests routing loop via AS number to avoid generating
routing loop. In the case of Hub&Spoke networking, however, PE carries the AS
number of the local autonomous system when advertising the routing information to CE,
if EBGP is run between PE and CE. Accordingly, the updated routing information will
carry the AS number of the local autonomous system when route update is received
from CE. In this case, PE cannot receive the route update information.
This phenomenon can be avoided by configuring the peer allowas-loop command,
which permits the routing loop to pass through and makes PE still receive the route
update information containing the local AS number from CE.
Perform the following configuration in IPv4 address-family sub-view.
Table 3-14 Allow/disable routing loop
Operation
Allow routing loop.
Disable routing loop
By default, the received route update information is not allowed to generate loop
information.
Step 6: Configure BGP attribute.
IV. Configuring PE-PE route switching
To exchange VPN-IPv4 routing information between PEs, you should configure
MP-IBGP on PEs.
Perform the following configurations in BGP view or PVN instance view.
1) Configure IBGP
These steps are often required.
Step 1: Configure BGP as asynchronous.
Step 2: Configure BGP neighbor.
Operation
peer { group-name | peer-address } allow-as-loop [number]
undo peer { group-name | peer-address } allow-as-loop
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Chapter 3 BGP/MPLS VPN Configuration
Command
undo synchronization
Command