BGP - Weight

[s.kanth]

Hi ,

CR1#

router bgp 65457

no synchronization

bgp log-neighbor-changes

network 10.52.232.248 mask 255.255.255.248

aggregate-address 10.52.232.0 255.255.248.0 summary-only

redistribute static

neighbor 147.225.59.241 remote-as 65000

default-metric 100000

no auto-summary

CR1#show ip route 10.50.252.0

Routing entry for 10.50.252.0/24

  Known via "bgp 65457", distance 20, metric 0

  Tag 65000, type external

  Last update from 147.225.59.241 04:41:40 ago

  Routing Descriptor Blocks:

  * 147.225.59.241, from 147.225.59.241, 04:41:40 ago

      Route metric is 0, traffic share count is 1

      AS Hops 2

      Route tag 65000                                             

CR1#show ip bgp 10.50.252.0

BGP routing table entry for 10.50.252.0/24, version 765439

Paths: (1 available, best #1, table Default-IP-Routing-Table)

  Not advertised to any peer

  65000 65000, (aggregated by 65457 10.50.252.254)

    147.225.59.241 from 147.225.59.241 (159.24.197.199)

      Origin IGP, localpref 100, valid, external, atomic-aggregate, best

Remote Site A is having MPLS connection and advertising 10.50.252.0/24. I wanted CR1 (in data center) to advertise 10.50.252.0/24 into our full mesh MPLS\BGP cloud when remotesite A MPLS is down. This is because I have ASA beneath running a s2s tunnel to ASA located at the remote site A.

I planned to apply this configuration. So please review and let me know if this is the right method

ip route 10.50.252.0 255.255.255.0 10.52.237.250

router bgp 65457

neighbor 147.225.59.241 route-map increase_weight in                    

access-list 10 permit 10.50.252.0 0.0.0.255

route-map increase_weight permit 10

match ip address 10

set weight 40000

route-map increase_weight permit 20

[Giuseppe Larosa · Accepted solution]

Hello S.kanth,

your configuration template looks like correct in the most:

with a weight of 40000 the route learned in eBGP is preferred over the locally generated route (that has a weight of 32768), and this allows for a correct restore when the eBGP route becomes again available after a failure.

Correctly the route-map has a second empty clause to accept unchanged all other routes that can be received from eBGP neighbor.

The static route must be a floating static route with AD 220 so that it cannot be preferred over eBGP route ( AD=20) when this is present otherwise it would be preferred all the time to the eBGP route ( static default AD is 1 < 20)

ip route 10.50.252.0 255.255.255.0 10.52.237.250 220

you need to add an AD to the static route to make it floating.

The value of AD to be used is actually free I prefere to use a value greater then 200 because 200 is the iBGP AD.

Hope to help

Giuseppe

[s.kanth]

Hi Giuseppe,

Should I use AS path prepend also? I ask this because

Imagine Scenairo:

My remote site A  MPLS is down..Now CR1(in DC) will advertise this route into BGP cloud.

My remote site A MPLS is UP.. will other site learn from CR1 or the remote site MPLS ?

Thanks

Sri

[Giuseppe Larosa · Accepted solution]

Hello Sri,

adding AS path prepending to the locally generated route can be considered, but it is not strictly necessary as the locally generated route will be removed when the remote eBGP route comes back (weight 40000)

It could help improve transition between the two routes but it should be a second order effect.

Hope to help

Giuseppe

[s.kanth]

Thanks for the suggestion.

We will test and let you know how this goes..