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New Member

Multihoming Failover Configuration

Hi Friends,

Need you help in suggesting best options to achieve failover in Multihoming scenario as given in the pic.

Scenario is Single CPE - 2 Service provider .SP1 should be primary for both Internet and MPLS .service should fall back on SP2 incase primary failure.

No Load sharing is required in both cases.

thanks in advance for your help as usual.

thanks

V

9 REPLIES
Cisco Employee

Re: Multihoming Failover Configuration

Without knowing other info such is what routing protocol, I can only suggest you to do floating static

ip route 0.0.0.0 0.0.0.0 x.x.x.x 250

HTH,

jerry

New Member

Re: Multihoming Failover Configuration

Floatin static route is not an option here as last mile is ethernet.Moreover I would like to know how to achieve this with the help of BGP

thanks

Cisco Employee

Re: Multihoming Failover Configuration

Okay, you can try floating static with IPSLA (track).

When you said BGP, are both ISP going to peer with you via BGP and give you a default route?

Silver

Re: Multihoming Failover Configuration

BGP will load share unevenly based on BGP metrics.  If you want to force traffic over a single link while the other link is in a standby mode that can also be done.  BGP is very powerful in route-filtering as well.

### CE1

router bgp 65001
no auto-summary
bgp log neighbor-changes
no syncro
neighbor 1.1.1.1 remote-as 65000
neighbor 2.2.2.2 remote-as 65000
### Add network statements as needed.
### These will be advertised to the SP.
### You can also redistribute from an IGP
### Keep in mind any network statements
### must already exit in the route table.
network 192.168.1.0 mask 255.255.255.0
exit


Chris

New Member

Re: Multihoming Failover Configuration

Hi Vicky

There are a number of way to do this:

  • You could get ISP1 to send you full Internet routes and ISP 2 to send you default only.
  • You could get both ISP to send a default only and use local preference
  • You could get both ISP to send full Internet routes and use local preference
  • Assuming you want to do either of the last two you would need to run BGP. The simplest way to prefer one ISP over the other is use the local preference attribute.I MO

    Your config would look something like this:

    Router bgp 65535

    neighbor x.x.x.x remote-as 100

    neighbor x.x.x.x route-map ISP1-Primary in

    neighbor y.y.y.y remote-as 200

    !

    route-map ISP1-Primary permit 10

         set local-preference 200

    Routes from ISP1 would be treated as more preferable than routes from ISP2 as the local-preference attribute is checked first (from the most used attributes) before AS path length and MED.

    Assuming you have taken an MPLS service from both ISPs and that the connections are over a 802.1Q trunk and the CPE has multiple VRF you can still use the above config only this time include the route maps within the bgp address-families.

    Rgds

    James

    New Member

    Re: Multihoming Failover Configuration

    P.S - There is a great book 'Practical BGP, Print ISBN-10: 0-321-12700-5' on BGP which is well worth a read if you plan to use BGP in any real amount.

    New Member

    Re: Multihoming Failover Configuration

    James ,

    Appreciate your help.since both links are terminating on single router, is  Local prefer is an option ? or weight will help?

    New Member

    Re: Multihoming Failover Configuration

    Vicky

    Weight will be checked before local preference but to be honest I've not used it as it is a Cisco only object.

    There are a few further considerations with local preference to consider when you start running iBGP between multiple routers as LP is forwarded to iBGP neighbors and will override AS-Path selection. This is not normally an issue for enterprise customers and deployments such as the one you are suggesting.

    IMO Local Preference, AS-path and MED are the three big object used for traffic engineering today. More sophisticated policies use BGP communities to manipulate these three objects.

    I hope this helps and good luck!

    Re: Multihoming Failover Configuration

    Hello Vicky,

    Weight is a Cisco-defined attribute that is local to a router. The weight attribute is not advertised to neighboring routers. If the router learns about more than one route to the same destination, the route with the highest weight will be preferred.

    For example in your topology, routes learnt from both SP1 & SP2, the route with the better weight will be preffered. You can also modify that decision so that all routes from SP1 will have a better weight than SP2.



    The local preference attribute is used to prefer an exit point from the local autonomous system (AS). Unlike the weight attribute, the local preference attribute is propagated throughout the local AS. If there are multiple exit points from the AS, the local preference attribute is used to select the exit point for a specific route.


    BGP uses the following criteria, in the order presented, to select a path for a destination:


    ?If the path specifies a next hop that is inaccessible, drop the update.


    ?Prefer the path with the largest weight.


    ?If the weights are the same, prefer the path with the largest local preference.


    ?If the local preferences are the same, prefer the path that was originated by BGP running on this router.


    ?If no route was originated, prefer the route that has the shortest AS_path.


    ?If all paths have the same AS_path length, prefer the path with the lowest origin type (where IGP is lower than EGP, and EGP is lower than incomplete).


    ?If the origin codes are the same, prefer the path with the lowest MED attribute.


    ?If the paths have the same MED, prefer the external path over the internal path.


    ?If the paths are still the same, prefer the path through the closest IGP neighbor.


    ?Prefer the path with the lowest IP address, as specified by the BGP router ID.


    Hope that helps.

    Francisco.

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