bgp attribute config on edge router

[Leftz]

Hi

R1 edge router in client is connected to ISP1 and R2 edge router in this client is connected to ISP2 router. We configure local preference and AS-path on the two routers R1 and R2 respectively to control the bgp load sharing between the two router in client side. so the two bgp attributes in client should be able to control the load sharing in client side. Question is If ISP need to configure some bgp attribute on two IPS router usually? Thanks 

[MHM Cisco World]

There are two directions 

Inbound and Outbound 

The load balance will control the Outbound traffic direction

The control of outbound traffic directions not need any action from ISP side.

[paul driver]

Hello
Nothing to stop the ISP to do so, in fact Id be surprised if they didn't append BGP attributes.

[Leftz]

 local preference impacts inbound routes and as-path impacts outbound routes. so that theoretically these two bgp attribute is enough to control /impact bgp routes. but we like to know in reality ISP configure some attribute? 

[MHM Cisco World]

in reality the ISP can accept or deny any attribute receive from Costumer, such as AS-PATH 

if the ISP remove the private AS then this method of control can not be use for Inbound traffic 
instead you have other option 
The Inbound Traffic Engineering | Gaining control over your inbound traffic (noction.com)

[M02@rt37]

@Leftz 

@MHM Cisco World is correct.

ISP removes the private AS during transit, then the AS path prepending technique may not work as intended for inbound traffic.

One alternative option for controlling inbound traffic is to use BGP communities. Communities are tags that can be attached to BGP routes and allow the ISP to apply certain policies to those routes based on their community value. This allows for more granular control over how traffic is routed to your network.

For example, you could use communities to signal to the ISP which routes should be preferred or avoided, or to control how traffic is routed through different geographic locations.

[M02@rt37]

Hello @Leftz 

Yes, it'spossible for the ISPs to configure BGP attributes on their routers to control the load sharing between the two edge routers in the client's network. The ISPs can configure the following BGP attributes to influence the path selection:

-- AS Path Prepending: The ISP can prepend its own AS number multiple times to the BGP routes that are sent to the client's edge routers. This will increase the length of the AS path and make the routes less preferred by the edge routers in the client's network.

-- Community Values: The ISP can use community values to tag the BGP routes that are sent to the client's edge routers. The client can then configure its edge routers to prefer or ignore certain community values to influence the path selection.

-- Local Preference: The ISP can set a higher local preference value for the BGP routes that are sent to the preferred edge router in the client's network. This will make the routes more preferred by the edge router and ensure that traffic is balanced between the two edge routers.

By configuring these BGP attributes, the ISPs can help the client achieve its load balancing objectives and ensure that traffic is distributed evenly between the two edge routers.

[Leftz]

ISP can send AS path and Community to client, but cannot send Local Preference as LP works only local AS.

And if ISP send AS path to client, the client do not need to Local prefer, otherwise ISP's AS path would not be able to work. right? 

[M02@rt37]

@Leftz 

Local Preference (LP) is a BGP attribute that is used within the local Autonomous System (AS) to control the exit path of outbound traffic. It is not propagated to other ASs, so it cannot be set by the ISP and sent to the client.

On the other hand, the AS path and community attributes are propagated between ASs, so the ISP can send them to the client. The AS path attribute shows the sequence of ASs that a BGP update traverses, while the community attribute is used to tag prefixes with additional information that can be used to control the routing policy of BGP neighbors.

If the ISP sends the AS path to the client, the client can use it to influence its own outbound traffic by choosing a specific path through a particular upstream provider or transit provider. However, the client can still use local preference within its own AS to further influence the exit path of outbound traffic.

[MHM Cisco World]

Not as-path make path prefer than other'

The as-path prepend do that.

The as-path prepend config in siteA and the effect will see in siteB

The LP config in siteA and effect seen in SiteA