Re: Dual CE-PE connection ...

Answered Question
Mar 31st, 2009

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 -

I have this problem too.
0 votes
Correct Answer by Giuseppe Larosa about 7 years 8 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

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

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

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

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

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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