H3C Technologies H3C SR8800 - Manual (Page 240)

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Configuring AS number substitution
In MPLS L3VPN, if EBGP is used between PE and CE, sites in different geographical areas should have
different AS numbers assigned to ensure correct route advertisement. If different CEs use the same AS
number, you must configure the relevant PE to replace the AS number of the CE as its own AS number.
This feature is used for route advertisement only.
Figure 89 AS number substitution configuration
As shown in the above figure, CE 1 and CE 2 use the same AS number of 800. If AS number substitution
for CE 2 is configured on PE 2, and PE 2 receives a BGP update sent from CE 1, PE 2 replaces AS number
800 as its own AS number 100. Similar configuration must also be made on PE 1.
To configure AS number substitution for a peer/peer group:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enter BGP view. bgp as-number N/A
3. Replace the AS number of a peer/peer
group in the AS_PATH attribute as the
local AS number.
peer { group-name | ip-address }
substitute-as
Not configured by default
CAUTION:
Improper AS number substitution configuration may cause route loops; use this command with caution.
Removing private AS numbers from updates to a peer/peer group
To remove private AS numbers from updates to a peer/peer group:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enter BGP view. bgp as-number N/A
3. Configure BGP to remove private AS
numbers from the AS_PATH attribute of
updates to a peer/peer group.
peer { group-name | ip-address }
public-as-only By default, BGP updates
carry private AS numbers.
Ignoring the first AS number of EBGP route updates
Typically, BGP checks the AS_PATH attribute of a route update received from a peer. If the first AS number
is not that of the BGP peer, the BGP router discards the route update.
To ignore the first AS number of EBGP route updates: