AS path manipulations

1.5.d i Local AS, allowas-in, remove private as

local-AS

General information on “BGP local-AS”:

• When a BGP peer changes its AS number, every neighbor connected to it needs to change its BGP configuration
• To overcome this issue, the local-as command was introduced
• With local-as it’s possible for one physically connected AS to appear as two separate AS’es the outside world
• Problem: ISP-A (AS 100) buys ISP-C (AS 300). ISP-B (AS 200) is unable to change its configuration.
• Solution: ISP-C (AS 300), even now physically integrated into ISP-A (AS 100), can use the local-as feature to still pretend being AS 300 to ISP-B.

“BGP local-AS” CLI configuration commands:

## Configuring the BGP local-as feature
Router(config)# router bgp <asn>
Router(config-router)# neighbor <ip> local-as <asn>

allowas-in

General information on “BGP allowas-in”:

• By default, incoming routes which include the own AS in the AS_PATH are discarded
• This is because BGP uses the AS_PATH as a loop prevention mechanism
• As soon as the own AS is seen in the AS_PATH, BGP thinks it’s a loop
• This can cause problems in special situations
• Problem: Two sites of the same customer are connected via MPLS (VPNv4) and use the same AS number. As soon as the routes of site A reach site B they will get discarded because site B sees his own AS number in the AS_PATH.
• Solution: Configure allowas-in per neighbor on both sides to allow the installation of routes which include the local AS number. Important: Configured on the customer site!
• Important: Configured on the customer site!

“BGP allowas-in” CLI configuration commands:

## Configuring the BGP allowas-in feature
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor <ip> allowas-in

as-override (not on blueprint)

General information on “BGP as-override”:

• Fixes the same problem like allowas-in but is applied on the provider site
• Should always be used together with the SoO (Site of Origin) feature
• Difference to allowas-in: as-override overrides the AS_PATH attributes of the originating AS with his own AS number.
• Important: Configured on the provider site!

“BGP as-override” CLI configuration commands:

## Configuring the BGP as-override feature
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor <ip> as-override split-horizon

remove-private-as

General information on “BGP remove-private-as”:

• Private AS numbers should never be leaked onto the internet (just like private IP address ranges)
• If connected to a single(!) ISP, a private AS can be used to conserve public AS numbers
• However, the ISP must remove the private AS before sending route updates to the global BGP mesh (internet)
• Important: The remove-private-as command must be applied on the ISP connecting to the global BGP mesh and not on the customer site!

“BGP remove-private-as” CLI configuration commands:

## Configuring the BGP remove-private-as feature
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor <ip> remove-private-as

1.5.d ii Prepend

General information on “BGP Prepend”:

• Normally an outbound policy but can also be used inbound
• Affects the neighbor where the route is advertised to
• Can be used to manipulate the AS Path and therefor influence the inbound traffic on the router
• Example: Neighbor X receives the same route from two peer A and B with the same AS_PATH. If you prepend several AS’es to the path from peer A the neighbor X he won’t consider the route in the best path selection process anymore since the AS_PATH is longer.

“BGP Prepend” CLI configuration commands:

## Configuring a route-map for AS Path prepending
Router(config)# route-map [NAME] permit
Router(config-route-map)# match [argument]
Router(config-route-map)# set as-path [prepend] [asn1 asn2 asn... | last-as]


## Applying the AS Path prepending route-map to a BGP neighbor
Router(config)# router bgp <asn>
Router(config-router)# address-family [ipv4 | ipv6 | vpnv4 | ...]
Router(config-router-af)# neighbor [IP | peer-group] route-map [ROUTE-MAP-NAME] out

1.5.d iii Regexp

General information on “BGP Regexp”:

• Regexp = Regular Expressions
• Used to match one/several string/s depending on the syntax

Regexp characters:

Regexp examples: