BGP dual Multi-home

[kazimjhon]

I have two links from different ISP's with two different /24 prefix in one router. Now want to send 3.3.3.0/24 on one ISP 2.2.2.0/24 to the second ISP permanently and also want shift both prefixes automatically on one ISP if the second down. What will be the configuration for this scenario?

[balaji.bandi · Accepted solution]

yes it durable. example look below:

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

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/23675-27.html

Attached good presentation to get overall how it works.

[Georg Pauwen]

Hello,

are the 2.2.2.0/24 and 3.3.3.0/24 networks locally originated ? In that case, policy based routing is probably your best option. The config would look something like below (IP addressing is, obviously, arbitrary):

interface GigabitEthernet0/0
description Uplink to ISP 1
ip address 100.100.100.1 255.255.255.252
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/1
description Uplink to ISP 2
ip address 200.200.200.1 255.255.255.252
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/2
ip address 2.2.2.1 255.255.255.0
ip policy route-map TO_ISP_2
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/3
description Uplink to ISP_1
ip address 3.3.3.1 255.255.255.0
ip policy route-map TO_ISP_1
duplex auto
speed auto
media-type rj45
!
router bgp 1
bgp log-neighbor-changes
network 2.2.2.0 mask 255.255.255.0
network 3.3.3.0 mask 255.255.255.0
neighbor 100.100.100.2 remote-as 2
neighbor 200.200.200.2 remote-as 3
!
ip prefix-list 2-2-2-0 seq 10 permit 2.2.2.0/24
!
ip prefix-list 3-3-3-0 seq 10 permit 3.3.3.0/24
!
route-map TO_ISP_1 permit 10
match ip address prefix-list 3-3-3-0
set ip next-hop 100.100.100.2
!
route-map TO_ISP_1 permit 20
!
route-map TO_ISP_2 permit 10
match ip address prefix-list 2-2-2-0
set ip next-hop 200.200.200.2
!
route-map TO_ISP_2 permit 20

[paul driver]

Hello
You wouldn’t require policy based routing just using bgp attributes should work

you don’t mention if you have a single bgp rtr peering to two isps or dual bgp rtr peering wth dual isps
In any case you would advertise with as-path pre-pend on each prefix to the less preferred isp with a higher asn value but you are still then dependant on the mercy of each isp how they advertise your network.

example:
ip prefix-list 10 permit 3.3.3.3/32
ip prefix-list 11 permit 2.2.2.2/32

route-map prepend1 permit 10
match ip address prefix10
set as-path prepend local asn local asn local asn
route-map prepend1 permit 99

route-map prepend2permit10
match ip address prefix11
set as-path prepend local asn local asn local asn
route-map prepend2 permit 99

router bgp x
neighbour (isp1) route-map prepend1 out
neighbour (isp2) route-map prepend2 out

The above should change the advertisement for those prefixes towards both isps assuming that they are both already receiving both routes!

apologies for the text i am currently stuck in my car in the wilderness!

[Georg Pauwen]

@paul driver 

I (think I) did some extensive testing including what you proposed, but I did not get this to work with locally originated prefixes (as in the sample config I posted). The routing tables on your ISP routers show only one prefix (the one advertised through the route map) ?

Can you post your full running config ?

[paul driver]

Hello

---

@Georg Pauwen wrote:

@paul driver 

I (think I) did some extensive testing including what you proposed, but I did not get this to work with locally originated prefixes (as in the sample config I posted).

---

In your lab it sounds like you dont have ebgp peering between ISP's to simulate wan or internet