How to : Subnet advertisement on Multiple Router with Redundancy

[samksam77in]

Hi All,

Need assistance in configuring this Multihome BGP scenario where i want to advertise 201.201.201.0/24 & 110.110.110.0/24 on both upstream for redundancy at POP site and priority for these two subnets should be upstream 2, in case upstream 2 fails traffic will be diverted from upstream 1.

As far as  my understanding I must be having an IBGP peering between IDC and POP routers for the redundancy however with my experiment traffic is getting affected and not able to meet the requirement.

Help on above is highly appreciated.

Have attached the diagram for the reference.

Sam.

[chrihussey]

The simplest way to accomplish this would be to advertise 201.201.201.0/24 & 110.110.110.0/24 normally to ISP #2 and as-prepend the advertisements to ISP #1, probably three times would be sufficient. This way, from the Internet into your routers ISP #2 would be preferred.

In the IGP, on the router connecting to ISP #2, set the local preference of the received routes to something like 150 so it will be the preferred path out to the Internet. 

Hope this helps.

[samksam77in]

Thanks a lor chrihussey.

Would you be able to help me with some sample configuration template to understand the implementation in a better way.

Sam.

[chrihussey · Accepted solution]

Here are some sample configs which should give you a good idea and a URL which gives more detail. Keep in mind with the local preference set, upstream #2 would be preferred for all outbound traffic.

!AS-PATH PREPENDING TO UPSTREAM #1
router bgp 40000
 neighbor 192.168.1.2 remote-as 45000 (UPSTREAM #1)
 !
 address-family ipv4
 neighbor 192.168.1.2 activate
 neighbor 192.168.1.2 route-map PREPEND out
!
!
ip access-list 10 permit 201.201.201.0 0.0.0.255
ip access-list 10 permit 110.110.110.0 0.0.0.255
!
!
route-map PREPEND permit 10
 match ip address 10
 set as-path prepend 40000 40000 40000
route-map PREPEND permit 20 (You only need this if you are advertising other routes)
!
!

!SETTING LOCAL PREFERNCE ON ROUTES RECEIVED BY ISP #2
router bgp 40000
 neighbor 10.108.1.1 remote-as 47000 (UPSTREAM #2)
 neighbor 10.108.1.1 route-map SET-LOCAL-PREF in
!
route-map SET-LOCAL-PREF permit 10
 set local-preference 150
!

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3se/3850/irg-xe-3se-3850-book/irg-prefix-filter.html

[samksam77in]

Thanks a lot again for the efforts you put in to provide with detailed information, would try the same out and would be happy to let you know the outcome.

Sam.