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ISP- BGP problem


We have two internet service providers ISP1 and ISP2. We have two 1841 series routers on which these links are terminated. Now we are in phase of purchasing "IP address Pool" which is routeable by all IPS's so that it will be routed by both the ISPs and our rosources will be available all the time even if ISP1 or ISP2 link is down. Here is the problem , ISP 1 and ISP 2 are asking us to RUN BGP on our routers. We have 1841 series routers and running BGP on it will be too high expection. Is it possible to run a very samll / limited version of BPG ( similar to OSPF stub area ) on this router ( 128 mb dram , and Adv. Secruity. Enterprise version of IOS is available on router ). What I feel is that can we have a simple static routing done for the same pool from both the service providers and we will use interface tracking to send the traffic either to ISP1 or ISP2.

Purchasing 3845 or 3825 series router will be too costly.

Please share experience.

Thanks in advance



Re: ISP- BGP problem

HI, [Pls Rate if HELPS]

It will be very much possible to run the BGP at the LAST MILE with the Current set of Router facility you have: The Solution can be implemented with Mimimum Resource Utilization of "Router" by controlling the Inbound / Outbound Advertisement:

Your ISP PE Configuration will be as below:

Router#sh run

neighbor remote-as xxxxx

neighbor update-source GigabitEthernet0/

neighbor version 4

neighbor timers 10 30

neighbor activate

neighbor default-originate

neighbor prefix-list CUST-OUT out

neighbor route-map CUST_PRI in

neighbor maximum-prefix 100 90 restart 30

Router#sh ip prefix-list CUST-OUT

ip prefix-list CUST-OUT: 1 entries

seq 5 permit

!! Using the above Prefix List having "CUST-OUT out", the ISP provides the default to the Customer.

All traffic from Customer router will see the default available towards PE of ISP.

Router#sh route-map CUST_PRI

route-map CUST_PRI, permit, sequence 10

Match clauses:

ip address prefix-lists: Cust-in

Set clauses:

local-preference 800

community xxxx:xxxxx xxxx:xxxxx

!! With the above set community values, ISP advertising Customer prefix to the Upstream providers.

!! Since it's a Primary Link, the Local Preference is set to "800"

Router#sh ip prefix-list Cust-in

ip prefix-list Cust-in: 3 entries

seq 5 permit le 24

seq 10 permit le 24

seq 15 permit le 24

!! with the above set Prefix List, ISP receiving the above prefix as /23 & /22 from Customer.

Hope I am Informative.


Best Regards,

Guru Prasad R

Re: ISP- BGP problem


Running BGP on both 1841 with both providers is the optimum case, because this will allow you to control your traffic over internet, but other solutions are also applicable.

Regardless if you have your own AS# or your ISPs will accept peering via private AS, sure you can run BGP with both ISPs with limited resources utilization,

and this can be done by control the inbound and output advertisements.

Inbound: Only allow default-route to be received over both sessions (To save memory Ask your ISPs to make it for you from their side)

Outbound: Advertise your own IP address space.

Downstream / Upstream traffic:

Internally you can use your internal IGP or even static routes to load-balance or load-share or even send your upload over one line, regarding downstream you can use one ISP as primary and the second one as backup or load-balance over both links.

As i said above other solutions are also applicable, for example you can ask your ISP's to advertise your Public Address space on behave you (Make sure your IP address are PA not PI in RIP or your region RIR) but in this case static routes or any other dynamic protocol should be used between you and both service providers, also in this case the links should be use in Main/Backup fashion because downstream load-balance is possible but not easy in this solution (Inconsistent AS).

Best Regards,

Mounir Mohamed

Community Member

Re: ISP- BGP problem

Hi bapatsubodh !!

I think your 1841 can handle a bgp session, but ask your ISP to send to you only a default route, not a full internet table.

With this configuration your equipment will not be overloaded, but there is one problem: You will not have total control over the outbound traffic of your network.

Best Regards,

Alessandro Veras

Re: ISP- BGP problem

Hi alessandro.veras,

I have one comment if you don't mind.

I agree with you upload traffic will not be so much simple like incase if one gear used, but GLBP can do the job.

Best Regards,

Mounir Mohamed

Community Member

Re: ISP- BGP problem

I agree, well remembered.

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