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

A significant portion of the BSCI and CCNP Examination accomplishment is mastering BGP, and that includes filtering BGP routing updates. With this tutorial, we’ll Look into the best way to filter BGP updates with prefix lists.

R4 is advertising and marketing 3 networks through BGP. The downstream router R3 sees these routes and destinations them into its BGP table as demonstrated below. R3 has two downstream BGP friends, R1 and R2, and is particularly promotion by itself as the next-hop IP tackle for all BGP routes sent to All those two routers.

R4(config)#router bgp 4

R4(config-router)#community 21.0.0.0 mask 255.0.0.0

R4(config-router)#community 22.0.0.0 mask 255.0.0.0

R4(config-router)#community 23.0.0.0 mask 255.0.0.0

R3#demonstrate ip bgp

BGP desk version is four, nearby router ID is three.3.3.3

Standing codes: s suppressed, d damped, h record, * legitimate, > ideal, i

Inside

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

Network Subsequent Hop Metric LocPrf Body weight Path

*> 21.0.0.0 10.2.two.four 0 0 four I

*> 22.0.0.0 10.2.two.4 0 0 four I

*> 23.0.0.0 ten.two.two.4 0 0 four I

R3(config)#router bgp 123

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

R3(config-router)#neighbor 172.12.123.2 subsequent-hop-self

In turn, both equally R1 and R2 have these 3 routes of their respective BGP tables.

R2#display ip bgp

BGP desk Edition is four, regional router ID is 2.two.2.two

*>i21.0.0.0 172.12.123.3 0 100 0 four I

*>i22.0.0.0 172.12.123.3 0 100 0 4 I

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

R1#show ip bgp

BGP desk Variation is four, regional router ID is 19.one.one.one

Community Next Hop Metric LocPrf Bodyweight Path

*>i21.0.0.0 172.12.123.3 0 100 0 four I

*>i22.0.0.0 172.12.123.three 0 100 0 four I

*>i23.0.0.0 172.twelve.123.three 0 a hundred 0 4 I

If we wished R3 to receive all 3 of these routes from R4 although not advertise all of these to R2 and R1, we’ve bought a couple of choices on how to block these routes. Cisco’s recommendation is the usage of prefix-lists, and as you become accustomed to the syntax (which you'll want to do right before using and passing the BSCI), you’ll see they are actually easier to use than access-lists.

In this case, we’re about to configure R3 to mail only the route to 21.0.0.0 to R1 and 23.0.0.0 to R2. However, we do want these two routers to acquire any long run routes that R4 advertises into BGP.

Considering the fact that R1 and R2 will study these routes from an iBGP neighbor, they won't promote the routes to each other.

On R3, we’ll compose a prefix-record that denies 22.0.0.0/eight and 23.0.0.0/eight, but permits all other routes. Immediately after making use of the prefix checklist as revealed, R1 sees just the 21.0.0.0 /8 route.

R3(config)#ip prefix-record FILTER_R1 deny 22.0.0.0/8

R3(config)#ip prefix-list FILTER_R1 deny 23.0.0.0/eight

R3(config)#ip prefix-checklist FILTER_R1 allow 0.0.0.0/0 le 32

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

R3#distinct ip bgp * delicate

BGP desk Variation is 6, nearby router ID is 19.1.1.one

Network Up coming Hop Metric LocPrf Fat Path

The paths to 22.0.0.0/8 and 23.0.0.0/eight are already correctly filtered.

We’ll do https://socialhattori.com/ the identical for R2, besides the route not getting expressly blocked is 23.0.0.0/eight. The road “ip prefix-list allow 0.0.0.0/0 le 32” could be the prefix list equivalent of the “allow any” statement within an ACL.

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

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

R3(config)#ip prefix-record FILTER_R2 allow 0.0.0.0/0 le 32

R3(config-router)#neighbor 172.12.123.2 prefix-listing FILTER_R2 out

R3#crystal clear ip bgp * soft

BGP desk Model is 6, neighborhood router ID is two.2.2.2

Inner

Network Next Hop Metric LocPrf Pounds Path

*>i23.0.0.0 172.twelve.123.3 0 one hundred 0 4 I

The paths to 21.0.0.0/8 and 22.0.0.0/8 happen to be correctly filtered.

To see the prefix lists configured over a route and also the buy of the statements in each record, operate exhibit ip prefix-list.

R3#demonstrate ip prefix-listing

ip prefix-checklist FILTER_R1: three entries

seq five deny 22.0.0.0/eight

seq 10 deny 23.0.0.0/8

seq fifteen permit 0.0.0.0/0 le 32

ip prefix-checklist FILTER_R2: three entries

seq 5 deny 21.0.0.0/8

seq 10 deny 22.0.0.0/8

Get some fingers-on exercise with prefix lists and also you’ll quickly master them. Prefix lists are a crucial Section of working with BGP inside the exam area and output networks, so it’s crucial that you are relaxed working with them.