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IEWB-DYN Lab 6 – BGP Highlights

Posted by Jo on February 9, 2008

I started Lab 6 again from scratch this morning and flew through the core sections in under 3 hours. I guess it was still fresh in my mind from a few days back but still good practice, as I am understanding the concepts a lot more these days! The IGP section hasnt changed from my write up below, but I will add the BGP highlights here.

BGP Best Path Selection

When trying to affect the direction of traffic outbound from an AS you can use the Weight or Local Preference options applied inbound to your AS. This task wanted all traffic destined for AS54 from AS100 to go via BB1. R6 in AS100 has two connections to AS54 – one via BB1 and one via BB3. This task stipulated not to use local preference to accomplish the goal – so the other alternative was to use weight. Weight is locally significant to the router (where local preference is significant within the whole AS), so create a route map setting the weight to a value higher than the default of 0 and apply it inbound on the peering statement to BB1 on R6.

route-map BB1-IN permit 10
set weight 1000

router bgp 100
neighbor 54.1.3.254 route-map BB1-IN in

BGP Filtering

Next there was a task to only accept routes that had a prefix of /20 or under. So, create a prefix-list to match:

ip prefix-list LE20 seq 5 permit 0.0.0.0/0 le 20

Then apply this to inbound route-maps from the BB routers (BB1 and BB3)

route-map BB1-IN permit 10
match ip address prefix-list LE20
!
route-map BB3-IN permit 10
match ip address prefix-list LE20

These route-maps are then applied to the peering sessions for BB1 and BB3.

neighbor 54.1.3.254 route-map BB1-IN in
neighbor 204.12.1.254 route-map BB3-IN in

BGP Summarisation

To advertise sumamry addresses for 191.0.0.0/16 and 150.1.0.0/20 into BGP do the following:

Create summary routes pointing to Null0 for routes to be advertised (2 static routes were permitted in this task)

ip route 150.1.0.0 255.255.240.0 Null0
ip route 191.1.0.0 255.255.0.0 Null0

You can then advertise these into the BGP process. Initially I used the network statements

network 191.1.0.0 mask 255.255.0.0

network 150.1.0.0 mask 255.255.240.0

Which appeared to work fine, but the solutions guide had it accomplished by redistributing statics into BGP

router bgp 200
redistribute static

BGP Stability

You can apply dampening to specific routes by using a route-map to match the prefixes you want to apply dampening to.

First, create the prefix-lists to match the routes:

ip prefix-list AS54 seq 5 permit 112.0.0.0/8
ip prefix-list AS54 seq 10 permit 113.0.0.0/8

Then match them in the route-map and set the dampening parameters:

route-map DAMPEN permit 10
match ip address prefix-list AS54
set dampening 10 1000 2000 30

Finally, under the BGP process apply the route-map to the dampening command:

router bgp 100
bgp dampening route-map DAMPEN

This ensures that only the prefixes matched in the route-map have dampening applied to them.

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