BGP Multihoming with two upstream (ISP) on a single router

[Md. Mohashin Khan]

Hi,

I need to configure BGP multi-homing with two upstream provider using only one router which will be acting both as a Core and Edge Router.

Consider the following scenario:

Our ASN - 64512
Public IP Prefixes - 1.1.1.0/24, 2.2.2.0/24 and 3.3.3.0/24, 4.4.4.0/24

Upstream 1:
ASN - 100
IP: 100.100.100.0/30

Upstream 2:
ASN - 200
IP: 200.200.200.0/30

Our requirement:

Both the route must be active at the same time pass 1.1.1.0/24, 2.2.2.0/24 inbound and outbound traffic via Upstream 1 and 3.3.3.0/24, 4.4.4.0/24 inbound and outbound traffic via Upstream 2.
Incase of failure of any one upstream traffic must be automatically rotued via other one.

Diagram Like:

Our network----------------gi0/1|router|tg1/1---------------Upstream1

                                                                  |tg1/2---------------Upstream2   

Regards

Md. Mohashin Khan

[Mark Malone]

Hi

There's is a particular guide shows you how to do this

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.html#anc22

see section

Load Sharing When Multihomed to Two ISPs Through a Single Local Router

[Georg Pauwen]

Hello,

in addition to Mark's post, have a look at the link below (especially BP Topology 4):

Sample Cisco BGP Configuration by topology

http://showipbgp.com/bgp-configurations/cisco.html

[paul driver]

Hello

As you using one rtr you can utilize weight path attribute to manipulate what path is chosen to get to certain prefixes.

As for return path as-path prepending can be used, but you haven't stated what networks you wish to prepend so i have included a made up prefix in example below for completion.

Weight PA
access-list 10 permit 1.1.1.0 0.0.0.255
access-list 10 permit 2.2.2.0 0.0.0.255

access-list 11 permit 3.0.0.0 0.255.255.255
access-list 11 permit 4.0.0.0 0.255.255.255

route-map AS100-Weight permit 10
match ip address 10
set weight 40000

route-map AS100-Weight permit 99


route-map AS200-Weight permit 10
match ip address 11
set weight 50000

route-map AS200-Weight permit 99

As-path PA
10.0.0./24 is to be preferred to by ISP200
20.0.0./24 is to be preferred to by ISP100

access-list 1 permit 10.0.0.0 0.255.255.255
route-map ASPREPEND100 permit 10
 match ip address 1
 set as-path prepend 64512 64512 64512

route-map ASPREPEND100 permit 99

access-list 2 permit 20.0.0.0 0.255.255.255
route-map ASPREPEND200 permit 10
 match ip address 2
 set as-path prepend 64512 64512 64512

route-map ASPREPEND200 permit 99

router bgp 64512
neighbor x.x.x remote-as 100
neighbor x.x.x.x route-map AS100-Weight in
neighbor x.x.x.x route-map ASPREPEND100 out
neighbor x.x.x remote-as 200
neighbor x.x.x.x route-map AS200-Weight in
neighbor x.x.x.x route-map ASPREPEND200 out 
 

res

Paul

[Md. Mohashin Khan]

Hi Paul Driver,

Thanks for your significant reply. But dear I want to forward any destinations at a time both upstream peers. Like when  1.1.1.0/24, 2.2.2.0/24 any request go through upstream-1 and 3.3.3.0/24, 4.4.4.0/24 any request go through upstream-2.

Regards

Md. Mohashin Khan

[paul driver]

Hello
The weight PA would do that as in the example provided

Res
Paul

[Md. Mohashin Khan]

Hi Paul,

My mention any destinations means my users can sent any request at any destinations. I don't want to mention but the condition is if 1.1.1.0/24 users sent any request that must be routed through Upstream-1 and if 3.3.3.0/24 users sent any request that must be routed through Upstrream-2. As I know it is possible by IP SLA with PBR.

Regards

Md. Mohashin Khan

[paul driver]

Hello

Weight PA is locally significant to the rtr for routes it receives from it bgp peers as such you apply a higher value to the most preferred route

As-Path PA would advertised multiple ASNs in its as sequence to neighboring ebgp peers for mprefered return path for specific prefixes

If the 1.1.1.1 and 3.3.3.3 are not destination address but source address, In that case PBR would be the viable way to go, but you only need to PBR on the subnet that you wish to take the secondary path as by default all other traffic would use the primary path.

Basic PBR
access-list 1 permit host 1.1.1.1

route-map PBR
match ip address 1
set ip next-hop x.x.x.x

int x/x
description LAN
ip policy route-map PBR

res

Paul