3Com 8800 Configuration Manual page 466

3Com Switch 8800 Configuration Guide
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 will not accept the route update information.
This phenomenon can be avoided by executing the peer allow-as-loop command,
which makes the PE still receives the route update information containing the local AS
number from CE.
Perform the following configuration in IPv4 instance sub-address family view.
Table 37-25 Configure to allow/disable routing loop
Configure to allow routing loop
Configure to disable routing loop
By default, the received route update information is not allowed to generate loop
information.
Step 7: Configure BGP features.
IV. Configuring PE-PE route exchanging
To exchange VPN-IPv4 routing information between PEs, you should configure
MP-IBGP on PEs.
Perform the following configuration in BGP view or PVN instance address family
sub-view.
1) Configure IBGP
These steps are often required.
Step 1: Configure BGP as asynchronous.
Step 2: Configure BGP neighbor.
Note that BGP adjacency is established through loopback interface and the sub-net
mask must be 32 bits.
Step 3: Permit BGP session over any operable TCP interface.
In general, BGP uses the best local address in TCP connection. To keep TCP
connection available even when the interface involved fails, you can perform the
following configuration to permit BGP session over any interface through which TCP
connection with the peer can be set up. The command here is usually executed
together with the Loopback interface.
Operation
37-22
Chapter 37 BGP/MPLS VPN Configuration
Command
peer { group-name
allow-as-loop asn_limit
undo peer { group-name | peer-address }
allow-as-loop asn_limit
| peer-address }