BGP Features Overview

BGP Allowas-in

“Allowas-in” allow the BGP to accept the BGP updates even if its own BGP AS number is in the AS-Path attribute.

A Common use case, if the customer has 2 sites and connected via service provider.

LD_Router(config)#router bgp
LD_Router(config-router)#neighb 192.168.10.2 allowas-inCode language: CSS (css)

BGP As-override

A router configured to use BGP AS override on a BGP session monitors outbound routes toward that peer. Whenever a route has the ASN of the peer in its AS-path, all occurrences of this ASN are replaced by the local ASN of the router.

R1(config)#router bgp 100
R1(config-router-af)#neighbor 192.168.10.2 as-overrideCode language: CSS (css)

The it work is that R1 will replace the ASN of neighbor “192.168.10.2” from the BGP UPDATE AS-PATH (if exist) attribute with its own ASN 100 before sending it out to this neighbor.

Note:  When local-as is in use, the ASN specified in the local-as command is used to override the customer ASN

You can read more about local-as in the following article:

Remove Private AS

When a private AS number is allocated to the customer network, the BGP updates from the customer network to ISP will have the private AS number in its AS_PATH list. When the ISP propagates its network information to the global BGP table (Internet), it should not propagate the AS_PATH with the private AS number of the customer to the Internet.

The ISP will remove the private AS number from its AS_PATH list, use the Cisco IOS remove-private-as command.

Private and Private ASN ranges:

• Globally unique AS numbers (1 – 64511)

• Private ASN: 64512 – 65535

The feature remove private AS removes the private AS of routes that are advertised to the configured peer. 

The neighbor x.x.x.x remove-private-as per-neighbor configuration command forces BGP to remove the private AS numbers. You can configure this command for external BGP neighbors. When the outbound update contains a sequence of private AS numbers, this sequence is dropped.

The following conditions apply:

• You can only use this solution with external BGP (eBGP) peers.
• Removes only private ASNs on routes advertised to EBGP peers.
• If the AS-Path for the route has only private ASNs, the private ASNs are removed.
• If the AS-Path for the route has a private ASN between public ASNs, it is assumed that this is a design choice, and the private ASN is not removed

BGP Soft-reconfiguration and Dynamic Inbound Soft Reset

The following is comprehensive article for soft-reconfiguration feature: