Bgp As Number Substitution; Mpls L3Vpn Configuration Task List - 3Com MSR 50 Series Configuration Manual

1480 C 79: MPLS L3VPN C
HAPTER
BGP AS Number
Substitution
MPLS L3VPN
Configuration Task
List
ONFIGURATION
Since BGP detects routing loops by AS number, if EBGP runs between PEs and CEs,
you must assign different AS numbers to geographically different sites to ensure
correct transmission of the routing information.
The BGP AS number substitution function allows physically dispersed CEs to use
the same AS number. The function is a BGP outbound policy and functions on
routes to be advertised.
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.
n
After you enable the BGP AS number substitution function, the PE re-advertises all
routing information to the connected CEs in the peer group, performing BGP AS
number substitution based on the above principle.
Figure 421 Application of BGP AS number substitution
EBGP_Update:10.1.1.1/32
AS_PATH:800
AS 800
In Figure 421, both CE 1 and CE 2 use the AS number of 800. AS number
substitution is enabled on PE 2 for CE 2. Before advertising updates received from
CE 1 to CE 2, PE 2 finds that an AS number in the AS_PATH is the same as that of
CE 2 and hence substitutes its own AS number 100 for the AS number. In this way,
CE 2 can normally receive the routing information from CE 1.
AS number substitution also applies to a PE connecting multiple CEs through
different interfaces, such as PE 2 in
n
For a multi-homed CE, that is, a CE connected with multiple PEs, the BGP AS
number substitution function must be used in combination with the site-of-origin
(SOO) function. Otherwise, routing loops may appear.
Task
"Configuring VPN Instances" on page 1481
"Configuring Basic MPLS L3VPN" on page 1484
"Configuring Inter-Provider VPN" on page 1491
"Configuring Multi-Role Host" on page 1495
AS 100
PE 1
MPLS backbone
VPNv4_Update:10.1.0.0/16
RD:10.1.1.1/32
AS_PATH:800
CE 1
Figure 421, which connects CE 2 and CE 3.
CE 3
AS 800
PE 2
EBGP_Update:10.1.1.1/32
AS_PATH:100,100
CE 2
AS 800
Remarks
Required
Required
Optional
Configure it as needed
Optional
Configure it as needed