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Using a Single AS Number for All CE Sites
If you want to use the same AS number for all of your CE sites, you can substitute a
PE router’s autonomous system number for that of a neighbor by specifying the
neighbor’s IP address in the
neighbor as-override command. If you fail to do this,
the CE router recognizes its AS in the AS path of received routes and determines it
has discovered a routing loop; the routes are rejected.
Example
In the following example, the router’s AS number of 777 overrides the neighboring
router’s AS number of 100.
host1:vr1(config)# router bgp 777
host1:vr1(config-router)# neighbor 172.16.20.10 remote-as 100
host1:vr1(config-router)# neighbor 172.16.20.10 update-source loopback0
host1:vr1(config-router)# address-family ipv4 vrf vpn1
host1:vr1(config-router-af)# neighbor 172.25.14.12 remote-as 100
host1:vr1(config-router-af)# neighbor 172.25.14.12 as-override
neighbor as-override
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Use to enable the use of the same AS number for all CE sites by substituting the
current router’s AS number in routing tables for that of the neighboring router.
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If you specify a BGP peer group by using the peer-group-name argument, all the
members of the peer group inherit the characteristic configured with this
command. You cannot override the characteristic for a specific member of the
peer group.
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New policy values are applied to all routes that are sent (outbound policy) or
received (inbound policy) after you issue the command.
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To apply the new policy to routes that are already present in the BGP routing
table, you must use the
clear ip bgp command to perform a soft clear or hard
clear of the current BGP session.
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Behavior is different for outbound policies configured for peer groups for which
you have enabled Adj-RIBs-Out. If you change the outbound policy for such a
peer group and want to fill the Adj-RIBs-Out table for that peer group with the
results of the new policy, you must use the
clear ip bgp peer-group command
to perform a hard clear or outbound soft clear of the peer group. You cannot
merely perform a hard clear or outbound soft clear for individual peer group
members because that causes BGP to resend only the contents of the Adj-RIBs-Out
table.
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Example 1
host1:vr1(config-router)# neighbor 192.168.255.255 as-override
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Example 2
host1(config-router)# neighbor 192.168.1.158 as-override
Configuring BGP VPN Services
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Chapter 5: Configuring BGP-MPLS Applications