Loop prevention in BGP is done by verifying the AS number in the AS Path. If the receiving router sees its own AS number in the AS Path of the received BGP packet, the packet is dropped. The receiving Router assumes that the packet was originated from its own AS and has reached the same place from where it originated initially.
The feature could be a disaster if customers are using same AS number along the various sites and disallows customer sites having identical AS numbers to be linked by another AS number. In such a scenario, routing updates from one site will be dropped when the other site receives them.
To override this feature, AS-Override function causes to replace the AS number of originating router with the AS number of the sending BGP router. The command is neighbor ip-address as-override and can only be executed under the VPNv4 address-family.
Here are the steps to illustrate the As-Override process
Router TAURUS_Site-A advertises route 10.3.3.3 with AS100.
Router PE-1 propagates this as an internal route to PE2 as AS100.
PE2 prepends 10.3.3.3 with AS 121 and replaces 100 in the AS-Path to 121 and propagates the prefix.
Router TAURUS_Site-B accepts 10.3.3.3 update.
Understanding of MPLS Technology
Understanding of MBGP
Understanding of OSPF routing protocol
In this Topology, router PE-1 and PE-2 forms the Service Provider MPLS Cloud. The two routers are connected via fast Ethernet interface 0/0 and are running OSPF (Area 0) routing protocol. MPLS is configured on physical links (Fast Ethernet 0/0) of the SP network. Tagging is done via LDP and the labels are assigned in the range 100-199 on PE1 and 200-299 on PE2.
TAURUS and CINDY are the two customers with multiple sites (Site-A and Site-B). Customer TAURUS is operating under AS 100 and customer CINDY is operating under AS 200.
VPNv4 neighbor relationship is configured between (vrf TAURUS and vrf CINDY)
PE-1 & TAURUS_Site-A
PE-1 & CINDY_Site-A
PE-2 & TAURUS_Site-B
PE-2 & CINDY_Site-A
Routes from each site are advertised to PE routers within EBGP session. These routes are further propagated to next PE routers which are then forwarded to respective Customer-Site.
Note: All configurations are tested on Cisco 3700 series router with IOS 12.4.