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Cisco CCNP / BSCI Test Tutorial: Filtering BGP Updates With Prefix Lists

A serious portion within your BSCI and CCNP Examination good results is mastering BGP, and that features filtering BGP routing updates. In this particular tutorial, we’ll take a look at how you can filter BGP updates with prefix lists.

R4 is marketing a few networks by means of BGP. The downstream router R3 sees these routes and areas them into its BGP table as revealed down below. R3 has two downstream BGP peers, R1 and R2, which is promotion alone as another-hop IP deal with for all BGP routes sent to People two routers.

R4(config)#router bgp 4

R4(config-router)#community 21.0.0.0 mask 255.0.0.0

R4(config-router)#network 22.0.0.0 mask 255.0.0.0

R4(config-router)#network 23.0.0.0 mask 255.0.0.0

R3#display ip bgp

BGP table version is 4, neighborhood router ID is three.three.3.three

Position codes: s suppressed, d damped, h historical past, * valid, > most effective, i

Inner

Origin codes: i – IGP, e – EGP, ? incomplete

Community Following Hop Metric LocPrf Fat Route

*> 21.0.0.0 10.2.2.4 0 0 4 I

*> 22.0.0.0 ten.two.2.four 0 0 4 I

*> 23.0.0.0 10.2.2.four 0 0 four I

R3(config)#router bgp 123

R3(config-router)#neighbor 172.twelve.123.one future-hop-self

R3(config-router)#neighbor 172.12.123.two subsequent-hop-self

Subsequently, the two R1 and R2 have these 3 routes in their respective BGP tables.

R2#display ip bgp

BGP table Model is 4, regional router ID is two.two.two.2

*>i21.0.0.0 172.12.123.3 0 a hundred 0 4 I

*>i22.0.0.0 172.12.123.3 0 one hundred 0 4 I

*>i23.0.0.0 172.twelve.123.three 0 100 0 four I

R1#demonstrate ip bgp

BGP table Edition is 4, regional router ID is 19.one.1.one

Network Following Hop Metric LocPrf Body weight Route

*>i21.0.0.0 172.12.123.three 0 a hundred 0 https://socialhattori.com/ four I

*>i22.0.0.0 172.12.123.3 0 one hundred 0 4 I

*>i23.0.0.0 172.12.123.three 0 100 0 four I

If we wished R3 to get all a few of these routes from R4 but not advertise all of them to R2 and R1, we’ve bought a few alternatives regarding how to block these routes. Cisco’s recommendation is the use of prefix-lists, and as you get used to the syntax (which you must do prior to taking and passing the BSCI), you’ll see they are literally much easier to use than accessibility-lists.

In this instance, we’re going to configure R3 to deliver only the route to 21.0.0.0 to R1 and 23.0.0.0 to R2. Having said that, we do want these two routers to get any long run routes that R4 advertises into BGP.

Because R1 and R2 will study these routes from an iBGP neighbor, they won't market the routes to one another.

On R3, we’ll generate a prefix-list that denies 22.0.0.0/8 and 23.0.0.0/8, but permits all other routes. Following making use of the prefix record as proven, R1 sees just the 21.0.0.0 /8 route.

R3(config)#ip prefix-listing FILTER_R1 deny 22.0.0.0/eight

R3(config)#ip prefix-checklist FILTER_R1 deny 23.0.0.0/8

R3(config)#ip prefix-list FILTER_R1 permit 0.0.0.0/0 le 32

R3(config-router)#neighbor 172.12.123.1 prefix-checklist FILTER_R1 out

R3#very clear ip bgp * gentle

BGP desk Edition is six, area router ID is 19.1.one.one

Community Next Hop Metric LocPrf Excess weight Path

The paths to 22.0.0.0/8 and 23.0.0.0/8 have been effectively filtered.

We’ll do the same for R2, except the route not staying expressly blocked is 23.0.0.0/8. The line “ip prefix-list permit 0.0.0.0/0 le 32” will be the prefix listing equal of the “allow any” assertion within an ACL.

R3(config)#ip prefix-list FILTER_R2 deny 21.0.0.0/8

R3(config)#ip prefix-listing FILTER_R2 deny 22.0.0.0/8

R3(config)#ip prefix-checklist FILTER_R2 permit 0.0.0.0/0 le 32

R3(config-router)#neighbor 172.twelve.123.2 prefix-checklist FILTER_R2 out

R3#apparent ip bgp * comfortable

BGP table Model is six, area router ID is two.2.2.2

Inside

Network Next Hop Metric LocPrf Bodyweight Route

*>i23.0.0.0 172.12.123.3 0 one hundred 0 4 I

The paths to 21.0.0.0/8 and 22.0.0.0/8 are correctly filtered.

To begin to see the prefix lists configured on a route together with the buy on the statements in Every listing, operate demonstrate ip prefix-listing.

R3#exhibit ip prefix-listing

ip prefix-listing FILTER_R1: 3 entries

seq 5 deny 22.0.0.0/eight

seq 10 deny 23.0.0.0/8

seq 15 permit 0.0.0.0/0 le 32

ip prefix-listing FILTER_R2: 3 entries

seq 5 deny 21.0.0.0/8

seq 10 deny 22.0.0.0/eight

Get some hands-on apply with prefix lists and you simply’ll swiftly master them. Prefix lists are a very important A part of dealing with BGP inside the Examination home and production networks, so it’s vital that you are relaxed working with them.