BGP Design Question

Answered Question
Mar 17th, 2010
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Our proposed setup is the following. We are using the same ISP.


Corp RTR---->ISP (20MB)

|

| 100MB Metro LAN

|

DR RTR---->ISP (20MB)


We have been assigned a /24 block from our ISP and I would like to know the best design to load balance and failover to the Metro LAN in the event of a failure.


Am I correct in following the documentation from the link below?

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00800945bf.shtml#conf3


Thanks in advance.

Correct Answer by Giuseppe Larosa about 7 years 1 month ago

Hello Igor,

I see.


I guess you have split the public /24 address block in two or more parts internally in your domain and that edge routers are advertising the /24 as a result of using an aggregate address or a static route to null0 for the /24 IP network.


if the ISP is only one you could try to setup an agreement in order to send also specific /25 routes with them.


the other /25 could be advertised with your own AS prepending


so it could be:


COrp: sends out first /25, aggregate /24 second /25 with As path prepending

Hosting: sends out second /25, aggregate /24, first /25 with AS path prepending


Example:


ip prefix-list firstblock seq 5 permit x.y.z.0/25


route-map CorpToISP permit 10

match ip address prefix firstblock

set as-path prepend yourAS yourAS


route-map CorpToISP permit 20

! second empty block to advertise the other prefixes


router bgp yourAS


neigh ISP-ipaddress route-map CorpToISP out


Hope to help

Giuseppe

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Giuseppe Larosa Wed, 03/17/2010 - 07:06
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Hello Igor,

you are correct in following that link an iBGP session over the metro ethernet link is useful in this case


if DR site should be used only used for failover you could use MED and As path prepending to make the first link the primary link.


route-map fromDRtoISP permit 10

set metric 500

set as-path prepend yourAS yourAS



DR router


router bgp yourAS

neigh ISP route-map fromDRtoISP out


Hope to help

Giuseppe

jgorman1977 Wed, 03/17/2010 - 07:11
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Giuseppe,


I should change DR to Hosting. Our end goal is to host clients and load balance our links as our /24 netblock will be used for clients at the hosting center and for our company at our HQ.  In case of either Internet link failing, we would need traffic to use the other Internet link and the metro link.


Hope this makes sense.


Thanks

Correct Answer
Giuseppe Larosa Wed, 03/17/2010 - 09:27
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Hello Igor,

I see.


I guess you have split the public /24 address block in two or more parts internally in your domain and that edge routers are advertising the /24 as a result of using an aggregate address or a static route to null0 for the /24 IP network.


if the ISP is only one you could try to setup an agreement in order to send also specific /25 routes with them.


the other /25 could be advertised with your own AS prepending


so it could be:


COrp: sends out first /25, aggregate /24 second /25 with As path prepending

Hosting: sends out second /25, aggregate /24, first /25 with AS path prepending


Example:


ip prefix-list firstblock seq 5 permit x.y.z.0/25


route-map CorpToISP permit 10

match ip address prefix firstblock

set as-path prepend yourAS yourAS


route-map CorpToISP permit 20

! second empty block to advertise the other prefixes


router bgp yourAS


neigh ISP-ipaddress route-map CorpToISP out


Hope to help

Giuseppe

jgorman1977 Thu, 03/18/2010 - 08:02
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Giuseppe

Can you explain this command in more detail?


route-map CorpToISP permit 20

! second empty block to advertise the other prefixes

Giuseppe Larosa Thu, 03/18/2010 - 08:29
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Hello Igor,


it is simply an empty route-map clause to be used to advertise the other IP prefixes to eBGP neighbor



route-map CorpToISP permit 20


>> ! second empty block to advertise the other prefixes


this is just a comment sorry if it caused confusion


Hope to help

Giuseppe

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