A major part of your BSCI and CCNP exam success is mastering BGP, and that includes filtering BGP routing updates. In this tutorial, we'll take a look at how to filter BGP updates with prefix lists.
Laptop Battery R4 is advertising three networks via BGP. The downstream router R3 sees these routes and places them into its BGP table as shown below. R3 has two downstream BGP peers, R1 and R2, and is advertising itself as the next-hop IP address for all BGP routes sent to those two routers.
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Thinkpad R4(config)#router bgp 4
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Microsoft R4(config-router)#network 21.0.0.0 mask 255.0.0.0
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Laptop Computers R4(config-router)#network 22.0.0.0 mask 255.0.0.0
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Laptop Computer R4(config-router)#network 23.0.0.0 mask 255.0.0.0
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Desktop Computer
R3#show ip bgp
Notebooks BGP table version is 4, local router ID is 3.3.3.3
Lenovo Status codes: s suppressed, d damped, h history, * valid, > best, i -
Internal
Hard Drive Origin codes: i - IGP, e - EGP, ? - incomplete
Travelstar Network Next Hop Metric LocPrf Weight Path
Gateway *> 21.0.0.0 10.2.2.4 0 0 4 I
Laptop Parts *> 22.0.0.0 10.2.2.4 0 0 4 I
Software *> 23.0.0.0 10.2.2.4 0 0 4 I
Hard Drives R3(config)#router bgp 123
Electronics R3(config-router)#neighbor 172.12.123.1 next-hop-self
Canon R3(config-router)#neighbor 172.12.123.2 next-hop-self
Desktop Pc In turn, both R1 and R2 have these three routes in their respective BGP tables.
Desktop Computers R2#show ip bgp
Think Pad BGP table version is 4, local router ID is 2.2.2.2
Repair Status codes: s suppressed, d damped, h history, * valid, > best, i -
Internal
Data Recovery Origin codes: i - IGP, e - EGP, ? - incomplete
Cisco Network Next Hop Metric LocPrf Weight Path
Keyboard *>i21.0.0.0 172.12.123.3 0 100 0 4 I
Monitor *>i22.0.0.0 172.12.123.3 0 100 0 4 I
Desktop *>i23.0.0.0 172.12.123.3 0 100 0 4 I
Infosys
R1#show ip bgp
Refurbished Laptops BGP table version is 4, local router ID is 19.1.1.1
Wipro Status codes: s suppressed, d damped, h history, * valid, > best, i -
Internal
Lap Top Origin codes: i - IGP, e - EGP, ? - incomplete
Refurbished Network Next Hop Metric LocPrf Weight Path
Memory *>i21.0.0.0 172.12.123.3 0 100 0 4 I
Intel *>i22.0.0.0 172.12.123.3 0 100 0 4 I
As400 *>i23.0.0.0 172.12.123.3 0 100 0 4 I
Averatec If we wanted R3 to receive all three of these routes from R4 but not advertise all of them to R2 and R1, we've got a couple of options on how to block these routes. Cisco's recommendation is the use of prefix-lists, and once you get used to the syntax (which you should do before taking and passing the BSCI), you'll see they are actually easier to use than access-lists.
Hardware In this case, we're going to configure R3 to send 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 get any future routes that R4 advertises into BGP.
Dual Xeon Since R1 and R2 will learn about these routes from an iBGP neighbor, they will not advertise the routes to each other.
Storage On R3, we'll write a prefix-list that denies 22.0.0.0/8 and 23.0.0.0/8, but permits all other routes. After applying the prefix list as shown, R1 sees only the 21.0.0.0 /8 route.
Seagate R3(config)#ip prefix-list FILTER_R1 deny 22.0.0.0/8
Computer Sales R3(config)#ip prefix-list FILTER_R1 deny 23.0.0.0/8
Computer Hardware R3(config)#ip prefix-list FILTER_R1 permit 0.0.0.0/0 le 32
Printers R3(config)#router bgp 123
Technology R3(config-router)#neighbor 172.12.123.1 prefix-list FILTER_R1 out
Mainframe R3#clear ip bgp * soft
Samsung R1#show ip bgp
Computer Repair BGP table version is 6, local router ID is 19.1.1.1
Used Computers Status codes: s suppressed, d damped, h history, * valid, > best, i -
Internal
Network Origin codes: i - IGP, e - EGP, ? - incomplete
Digital Cameras Network Next Hop Metric LocPrf Weight Path
Desktops *>i21.0.0.0 172.12.123.3 0 100 0 4 I
Cognos The paths to 22.0.0.0/8 and 23.0.0.0/8 have been successfully filtered.
Hosting We'll do the same for R2, except the route not being expressly blocked is 23.0.0.0/8. The line "ip prefix-list permit 0.0.0.0/0 le 32" is the prefix list equivalent of a "permit any" statement in an ACL.
Netfinity R3(config)#ip prefix-list FILTER_R2 deny 21.0.0.0/8
Internet R3(config)#ip prefix-list FILTER_R2 deny 22.0.0.0/8
Cheap Computer R3(config)#ip prefix-list FILTER_R2 permit 0.0.0.0/0 le 32
Digital Camera R3(config)#router bgp 123
Printer R3(config-router)#neighbor 172.12.123.2 prefix-list FILTER_R2 out
Xseries R3#clear ip bgp * soft
Maxtor R2#show ip bgp
Data Storage BGP table version is 6, local router ID is 2.2.2.2
Hitachi Status codes: s suppressed, d damped, h history, * valid, > best, i -
Internal
Rational Origin codes: i - IGP, e - EGP, ? - incomplete
Websphere Network Next Hop Metric LocPrf Weight Path
Battery *>i23.0.0.0 172.12.123.3 0 100 0 4 I
It Support The paths to 21.0.0.0/8 and 22.0.0.0/8 have been successfully filtered.
Western Digital To see the prefix lists configured on a route as well as the order of the statements in each list, run show ip prefix-list.
Music R3#show ip prefix-list
Networks ip prefix-list FILTER_R1: 3 entries
Toner seq 5 deny 22.0.0.0/8
Cheap Laptops seq 10 deny 23.0.0.0/8
Wholesale seq 15 permit 0.0.0.0/0 le 32
Brother ip prefix-list FILTER_R2: 3 entries
Netvista seq 5 deny 21.0.0.0/8
Camera seq 10 deny 22.0.0.0/8
Networking seq 15 permit 0.0.0.0/0 le 32
Sharp Get some hands-on practice with prefix lists and you'll quickly master them. Prefix lists are an important part of working with BGP in the exam room and production networks, so it's vital that you are comfortable working with them.
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