COMMON BGP CONFIGURATION IN ISP ENVIRONMENT

If you have studied BGP, you may be wondering which attribute to use where? which are the common attribute used the most? what is the common configuration when it comes to BGP while setting up a PE-CE link?

Let me share with you the two most common scenario in ISP environment for PE-CE configuration.

First, when the customer have only one link with ISP.

CPE Configuration looks like as follows:


interface FastEthernet0/0
description "Wan Interface toward ISP"
 ip address X.X.X.1 255.255.255.252  (Point to Point IP between ISP and Customer)
exit


interface FastEthernet0/1
description "Lan Interface"
 ip address Y.Y.Y.1 255.255.255.240
exit


router bgp UU   (Customer AS number. Could be private if the customer doesn't have its own AS No.)
 no synchronization
 bgp log-neighbor-changes
 network Y.Y.Y.0 mask 255.255.255.240  (Customer Public N/W)
neighbor X.X.X.2 remote-as ZZ  (EBGP Peering with ISP)
no auto-summary
exit

PE Configuration:-

interface FastEthernet 0/0
description "Toward Customer"
 ip address X.X.X.2 255.255.255.252  (Point to Point IP between ISP and Customer)
exit


router bgp ZZ (ISP AS number)
 no synchronization
 bgp log-neighbor-changes
neighbor X.X.X.1 remote-as UU (EBGP Peering with Customer)
neighbor X.X.X.1 prefix-list customer-routes in  (Only accepting the routes assigned to customer
neighbor X.X.X.1 prefix-list default-route out (Only sending default route to customer)
no auto-summary
exit

ip prefix-list default-route seq 5 permit 0.0.0.0/0
ip prefix-list customer-routes seq 5 permit Y.Y.Y.0/28

As you see in the configuration, most of the ISP accepts only the network assign to you just to prevent the customers from corrupting the routing table by false advertisements.
Also, ISP will send only the default route towards you just to prevent the CPE from crashing due to huge number of internet routes.

Second scenario is same as first but in this scenario customer has two links (for redundancy) towards ISP rather than one.

For this kind of setup, ISP uses one of the BGP attribute to prefer one path over another.

The ISP (PE) side configuration looks like this.


router bgp ZZ (ISP AS number)
 no synchronization
 bgp log-neighbor-changes
neighbor X.X.X.1 remote-as UU (EBGP Peering with Customer's Primary Link)
neighbor X.X.X.1  prefix-list customer-routes in  (Only accepting the routes assigned to customer
neighbor X.X.X.1 prefix-list default-route out (Only sending default route to customer)

neighbor X.X.Y.1 remote-as UU (EBGP Peering with Customer's Backup Link)
neighbor X.X.Y.1 route-map customer-routes in  (Only accepting the routes assigned to customer
neighbor X.X.Y.1 prefix-list default-route out (Only sending default route to customer)

no auto-summary
exit

route-map customer-routes permit 10
match ip address prefix-list customer-routes
set weight 0
exit

In above configuration, BGP attribute "weight" is used to influence the path. You can use any attribute to do it. If the link are terminated in different PE router, then weight can't be used. In this case, local-preference or AS- Path prepending is used.
In the next post i will explain how the configuration is done when you have two links from two different ISP (Multi-Homing).