Routing Issue

eandrcisco007 · ‎09-12-2014

Basically, I have two routers (Primary/backup) each connected to a different ISP with 100/50 mbps directly connected to L3 MPLS, the backup router is mainly used as a backup router and both routers redistributing routes between bgp/eigrp and vica versa, so basically I need to figure it out why asymmetric routing is happening and network traffic takes the 50 mbps route instead of 100 mbps (default route) despite its current configuring which I assume it may have some type of misconfiguration.

The interface on the backup router (pointing to MPLS) is currently down and a quick fix has to be proposed asap.

I wonder if anyone could give me a hint or suggestion.

Thanks.

Akash Agrawal · ‎09-15-2014

Hi,

Please clarify which traffic is taking 50mb link instead of 100mb link, forward traffic or reverse traffic. If it is forward traffic, while redistributing from BGP to EIGRP, you can manipulate eigrp cost. If it is reverse traffic, while advertising BGP prefixes, you can do as-path prepend on backup router.

If you have further query, please share config of both routers along with follow up query.

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Regards,

Akash

eandrcisco007 · ‎09-15-2014

Hi Akash,

Thank you for your feedback.

It is happening on the reverse traffic, despite having as path prepend configured on the back up router.

Below are the BGP configuration on both routers If it could shed some light on the issue.

Primary Router : (100 mbps)

router bgp 11111

network aa.aa.aa.aa mask bb.bb.bb.bb

network xx.xx.xx.xx mask xx.xx.xx.xx

bgp log-neighbor-changes

redistribute static

redistribute eigrp 1 route-map EIGRP2BGP

neighbor xx.xx.xx.xx remote-as 1

neighbor xx.xx.xx.xx default-originate

neighbor xx.xx.xx.xx soft-reconfiguration inbound

neighbor xx.xx.xx.xx distribute-list 5 in

neighbor xx.xx.xx.xx route-map LOCALPREF in

neighbor xx.xx.xx.xx remote-as 11111

neighbor xx.xx.xx.xx update-source Loopback0

Backup Router : (50 mbps)

router bgp 11111

bgp log-neighbor-changes

no synchronization

network aa.aa.aa.aa mask bb.bb.bb.bb

redistribute eigrp 1 route-map EIGRP2BGP

neighbor xx.xx.xx.xx remote-as 1

neighbor xx.xx.xx.xx soft-reconfiguration inbound

neighbor xx.xx.xx.xx route-map AS-PREPEND out

neighbor xx.xx.xx.xx remote-as 11111

neighbor xx.xx.xx.xx update-source Loopback0

route-map BGP-POLICY-BACKUP-OUT permit 10

match ip address PERMIT-ALL

set as-path prepend 11111 11111

set community 11111:0000

route-map AS-PREPEND permit 20

match ip address prefix-list AS-PREPEND

set as-path prepend 11111 11111

route-map AS-PREPEND permit 30

route-map BGP-POLICY-BACKUP-IN permit 10

set as-path prepend 11111 11111

interface GigabitEthernet0/2/3

description Level3 MPLS : 000000

bandwidth 50000

ip address xx.xx.xx.xx xx.xx.xx.xx

load-interval 30

duplex full

speed 1000

jubair151 · ‎09-16-2014

Hi eandrcisco007

One workaround i can give you is to only advertise the supernet subnet from the backup router and advertise the specific routes from primary router. Or else You can use suppress and un-suppress map concept in BGP if you are using contiguous network

AS prepend will only work, if you have connected to same service provider in normal case . ( I hope MPLS what we are referring is your intranet WAN).

Regards,

Jubair.S

Regards,

Jubair.S

Akash Agrawal · ‎09-16-2014

Hi Jubair,

I agree with your workaround with advertising supernet on backup link but as-path prepend should also work and as-path length is not limited to immediate ISP.

Regards,

Akash

Akash Agrawal · ‎09-16-2014

Hi ,

This is network related issue and need to take few outputs from service provider also why they are preferring backup path. But on configuration side, you have to just check if your prefixes for which are checking reverse path should be included in the prefix-list AS-PREPEND, otherwise configuration is correct.

Regards,

Akash

jubair151 · ‎09-16-2014

Dear Akash Agrawal ,

Yes, As prepend should work without any issue if it is connected one service provider. As per the first statement, backup link is connected another ISP, not in the same ISP. If the links what we are talking about is internet link, then he can speak with both ISP provider to make the as prepend useful. But if it is used to route intranet WAN, then I think probably ISP won't have any roles in this. One more suggestion I can provide is as below.

If this location is brach location, then to reach the branh location over 100 Mbps primary link from data center location, you can tweak the metric of the branch location routes over the corresponding ISP provider in data center WAN router to be good metric. Then, any traffic which is coming from the data center to your branch will take the your primary ISP only. In this way, you can ensure that all the return traffic from the data center to your branch location will take only primary ISP.

Regards,

Jubair.s

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eandrcisco007 · ‎09-16-2014

Hi Jubair,

Great , Thank you but would you be more specific about tweaking the metric please?

jubair151 · ‎09-16-2014

dear eandrcisco007 ,

Attached the picture to understand the concept, kindly let me know if this clears your doubt.

Regards,

Jubair.S

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