Multihoming & MultiSite

Unanswered Question
Mar 23rd, 2009

Hi,

We have two sites LAX and SUV with LAX connecting to ISP A and SUV connecting to ISP A and B. We have two identical Cat-4506 (Cisco 4506

CPU at 800Mhz, Supervisor V-10GE/512 Mb)

on both the sites.

Existing Setup:

Site LAX publishes Network A.A.A.A via ISP A

Site SUV publishes Network C.C.C.C via ISP A and with AS path prepend via B with floating static route for fail over.

No Routes learned via any ISP at present.

We have recently setup the 1G link between the sites and hence we would like to use SUV's ISP A and B as fall back for LAX. Also the following conditions have to be met:

1. Network A.A.A.A will only use the ISPA in LAX when the link is functional likewise the site SUV with Network C.C.C.C via ISP A.

2. In the event of a Link Failure in LAX the ISP A in the SUV should be preferred before ISP B.

Given the Hardware Spec(CPU and Memory limitations) please propose a method to achieve all this.

Any Input is greatly appreciated.

Thanks in advance

subra4u

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Joseph W. Doherty Mon, 03/23/2009 - 10:05

(Assuming both LAX and SUV have at least one public AS [yours] and networks A.A.A.A and C.C.C.C are also public [and yours].)

On LAX add another default route to SUV but with a higher cost. If default fails from LAX to ISP-A, outbound traffic should be forwarded to SUV.

Also on LAX, send network A.A.A.A to SUV.

On SUV, pass along network A.A.A.A to both ISPs as you now do with network C.C.C.C but with one or more extra AS prepend hops. (Depending on how LAX and SUV peer, there might already be an extra AS hop.)

subra4u Mon, 03/23/2009 - 10:19

Hi Joseph,

Thanks for your quick response. We have the same AS for both the Networks A.A.A.A and C.C.C.C. In this Case, I will have to do a iBGP peer between LAX and SUV and in SUV AS prepend will do or should i change the MED values. Please advice.

A quick Is the Hardware good enough to hold a full BGP table.

Thanks in Advance

subra4u

lamav Mon, 03/23/2009 - 12:33

Subra:

You cannot use the MED to influence incoming traffic when you are peering with 2 differet AS numbers - 2 different ISPs. So, AS-path prepend is the way to go.

So, I would:

At LAX:

1. Advertise a.a.a.a to ISP A.

2. Advertise c.c.c.c to ISP A with the AS-path prepended.

3. Configure a default route pointing to ISP A and redistiribute it back to your core.

At SUV:

1. Advertise c.c.c.c to ISP A.

2. Advertise a.a.a.a to ISP A with the AS-path prepended.

3. Advertise a.a.a.a and c.c.c.c to ISP B with the AS-path prepended to a length greater than it was for ISP A.

4. Configure a default route pointing to ISP A with an AD of 220 and redistribute that back to your core.

5. Configure a default route pointing to ISP B with an AD of 240 and redistribute that back to your core.

HTH

Victor

subra4u Fri, 03/27/2009 - 17:21

Hi,

Thanks for your time. I was thinking, if we could influence traffic using MED so that initial failover happens between the same ISP A (incase of a link failure) and use ISP B only as a backup if ISP A is down for some reason. This is mainly because of the commercials involved as we have to pay them(ISP B) based on the utilization. It is just a thought correct me if i am wrong.

cheers

subra

subra4u Mon, 04/06/2009 - 01:42

Hi All,

I am thinking of having the following config. and try. Please give some inputs on this

On SUV

-----------

router bgp 65535

no synchronization

bgp log-neighbor-changes

network c.c.c.c

network a.a.a.a

neighbor ISP A remote-as 65000

neighbor ISP A soft-reconfiguration inbound

neighbor ISP A distribute-list 3 in

neighbor ISP A route-map setMED-ISPA out

neighbor ISP B remote-as 65001

neighbor ISP B distribute-list 3 in

neighbor ISP B route-map prepend out

no auto-summary

access-list 1 permit a.a.a.a

access-list 2 permit c.c.c.c

access-list 3 permit 0.0.0.0

access-list 3 deny any

route-map setMED-ISPA permit 10

match ip address 1

set metric 200

route-map setMED-ISPA permit 20

match ip address 2

set metric 100

route-map prepend permit 10

set as-path prepend 65535 65535 65535

On LAX

---------

router bgp 65535

no synchronization

bgp log-neighbor-changes

network a.a.a.a

network c.c.c.c

neighbor ISP A remote-as 65000

neighbor ISP A distribute-list 30 in

neighbor ISP A route-map setMED-ISPA out

no auto-summary

access-list 1 permit a.a.a.a

access-list 2 permit c.c.c.c

route-map setMED-ISPA permit 10

match ip address 1

set metric 100

route-map setMED-ISPA permit 20

match ip address 2

set metric 200

thanks

subra

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