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Re: Dual CE-PE connection ...

Answered Question
Mar 31st, 2009
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Hi,

I would like to confirm if I have two 3845 CE routers peering with say two 7206 routers.


7206-1 7206-2

| \/ |

ebgp /\ ebgp

| / \ |


3845-1 --ibgp-- 3845-2

In the above setup, the 3845-1 would be the primary router that sends and receive BGP routes from and to the site if it uses a higher loopback address. (bgp selection process)


This would mean that 3845-2 would be the secondary router in this scenario.


Pls advice,


Cheers,

- internetB -

Correct Answer by Giuseppe Larosa about 8 years 4 months ago

Hello Shiva,


1) yes


2) no it is not enough to be sure incoming traffic will be sent to C3845-1

again the lowest BGP router-id is preferred not the highest: the BGP router-id is chosen as the highest loop ip address in the device. But even if C3845-1 has the lowest BGP router-id if in the SP network someone sends traffic to C7206-2 for any reason traffic will be sent to C3845-2.

Without using MED to indicate your preferred incoming point the SP can make different choices: this can be acceptable or not.


Hope to help

Giuseppe


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Giuseppe Larosa Tue, 03/31/2009 - 02:30
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Hello Shiva,

you may need to set MED for outbound routes and to set local-preference on received routes in order to achieve your objective.


set a lowest MED like 50 out 3845-1 and an higher metric 100 out 3845-2


route-map rise_metric1 permit 10

match ... ! acl here if needed

set metric 50


neigh 7206-1.ipaddr route-map rise_metric1 out


on c3845-2:


route-map rise_metric2 permit 10

match ... ! acl here if needed

set metric 100


neigh 7206-2.ipaddr route-map rise_metric2 out


set a local-preference higher then default 100 on c3845-1 for prefixes learned by c7206-1


neigh 7206-1.ipaddr local-preference 200


note: a lowest BGP router-id is preferred.


see


Prefer the route that comes from the BGP router with the lowest router ID


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml


Hope to help

Giuseppe


InternetB Tue, 03/31/2009 - 02:34
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Hi Giuseppe,

I have seen this document before. My question was rather if I do not set any attribute, which would be the default router that the routes take for inbound/outbound traffic assuming that the bgp router-id is higher on 3845-1 ?


Thank you,


Cheers,

InternetB.

Giuseppe Larosa Tue, 03/31/2009 - 02:37
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Hello Shiva,

I was modifying my first post.


I've provided there an example of what I would do to have C3845-1 to be preferred exit point and ingress point for traffic.


the MED is the right tool to influence how traffic is sent to you.


the local-preference helps to enforce C3845-1 as the preferred exit point


If using HSRP or GLBP having C3845-1 as the active router can be enough for the outbound direction.


Hope to help

Giuseppe


InternetB Tue, 03/31/2009 - 02:55
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Hi Gieseppe,

1) So for outboung traffic, it depends which router is active on HSRP/GLBP and if we load balance the vlans on 3845-1 and -2 for active vlans, i guess we can load balance the outbound traffic right ?


2) Now for inbound traffic, would it all come to 3845-1 if no attributes are set assuming it has the higher router-id ?



Pls advice,


- InternetB -

Correct Answer
Giuseppe Larosa Tue, 03/31/2009 - 04:06
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Shiva,


1) yes


2) no it is not enough to be sure incoming traffic will be sent to C3845-1

again the lowest BGP router-id is preferred not the highest: the BGP router-id is chosen as the highest loop ip address in the device. But even if C3845-1 has the lowest BGP router-id if in the SP network someone sends traffic to C7206-2 for any reason traffic will be sent to C3845-2.

Without using MED to indicate your preferred incoming point the SP can make different choices: this can be acceptable or not.


Hope to help

Giuseppe


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