R4 advertising on the three networks BGP. The router downstream R3 sees these routes and submit them to the table BGP, as shown below. R3 has two downstream BGP, R1 and R2, and as advertising itself after skipping the IP address & 39; BGP for all routes of these two routers.
R4 (config) bgp 4
R4 Router (config - router) network 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 show ip bgp
BGP the table version 4, Number local router is 3.3. 3.3
Status codes: the oppressed, steamed d, h * valid history & gt; best, i - Internal
Origin codes: i - IGP, e - from & 39; EEA? -- Incomplete
Network Next Hop Metric LocPrf weight Path
* & gt; 21.0.0.0 10.2.2.4 0 0 4 * I
& gt; 22.0.0.0 10.2.2.4 0 0 4 I
& gt; 23.0 .0.0 10.2.2.4 * 0 0 4 I
R3 (config) bgp 123
R3 router (config-router) 172.12.123.1 nearest neighbour-hop self
R3 (config-router) 172.12.123.2 nearest neighbour-hop self
In twice R1 and R2 of these three lines in the BGP tables.
R2 show version Ip bgp
BGP Table 4, local router is 2.2.2.2
Status codes & 39; identification: the oppressed, steamed d, h * History & valid amp; gt; best, i - Internal
Origin codes: i - IGP, e - from & 39; EEA? -- Incomplete
Network Next Hop Metric LocPrf weight Path
* & gt; i21.0.0.0 172.12.123.3 01000 4 * I
& gt; i22.0.0.0 172.12.123.3 01000 4 * I
& GT; i23.0.0. 0172 .12.123.3 01000 4 I
R1 show ip bgp
BGP table of version 4, local router is 19.1.1.1
Status codes & 39; identification: the oppressed, steamed d, h * valid history & gt; best, i - Internal
Origin codes: I - IGP, e - EEE -? -- Incomplete
Network Next Hop Metric LocPrf weight Path
* & gt; i21.0.0.0 172.12.123.3 01000 4 * I
& gt; i22.0.0.0 172.12.123.3 01000 4 * I
& GT; i23.0.0. 0172 .12.123.3 01000 4 I
If we wanted to get all three of these R3 lines R4, but not all of them advertise R1 and R2, we have a number of & 39; options, as shown in these lines block. Cisco & 39, the recommendation is to use the prefix & 39; lists, and if you become accustomed to the syntax (what you should do before you start to take and dissemination of the OIOS), and & 39; you will see that they are actually easier to use than the & 39; access Lists.
In this case, the configuration R3 dispatched only of & 39; route to 21.0.0.0 and 23.0.0.0 for R1 R2. But we want these two routers d & 39; get that all future roads R4 campaign BGP.
Since R1 and R2 are & 39; more information on these roads iBGP d & 39; a neighbor, they are not advertise connections other.
On R3, we write a prefix of the list, which denies 22.0.0.0 / 8 and 23.0.0.0 / 8, but with all the other lines. After & 39; application of the prefix of the list, as indicated, the R1 sees only 21.0.0.0 / 8 route.
R3 (config) ip prefix list FILTER R1 deny 22.0.0.0/8
R3 (config) ip prefix list FILTER R1 deny 23.0.0.0 / 8
R3 (config) ip prefix list FILTER R1 allow 0.0.0.0 / 0 the 32
R3 (config) bgp 123
R3 Router (config-router) 172.12.123.1 neighbours prefix list FILTER R1 out
R3 clear * ip bgp soft
R1 show ip Bgp
BGP table version 6, local router is 19.1.1.1
Status codes & 39; identification: the oppressed, steamed d, h * valid history & gt; best, i - Internal
Origin codes : i - IGP, e - from & 39; EEA? -- Incomplete
Network Next Hop Metric LocPrf weight Path
* & gt; i21.0.0.0 172.12.123.3 01000 4 I
The paths d & 39; access to 22.0.0.0 / 8 and 23.0.0.0 / 8 was a success , filtered.
We & 39; ll do Even for R2, with the exception of & 39; n & 39; line is not locked, it is explicitly 23.0.0.0 / 8. The line & quot; ip list prefix allow 0.0.0.0 / 0 the 32 & quot; & 39; is the equivalent of & 39; a prefix of the list & quot; & A ; amp; quot; instruction in a ACL.
R3 (config) ip prefix list 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 list FILTER R2 allow 0.0. 0.0 / 0 the 32
R3 (config) bgp 123
R3 Router (config-router) 172.12.123.2 neighbours prefix list FILTER R2 out
R3 clear ip bgp soft
R2 * show ip bgp
BGP table version 6, is 2.2.2.2
Status router local codes d & 39; identification: the oppressed, steamed d, h History, valid * &; Gt; best, i - Internal
Origin codes: i - PGI, E - from & 39; EEE, ? -- Incomplete
Network Next Hop Metric LocPrf weight Path
* & gt; i23.0.0.0 172.12.123.3 01000 4 I
The paths d & 39; access to 21.0.0.0 / 8 and 22.0.0.0 / 8 was a success , filtered.
To prefix to the list, see the configuration, a distance & 39; as in the order of statements in each list, run show ip-prefix list.
R3 show ip prefix list
ip-prefix FILTER R1 the list: 3 entries
seq 5 deny 22.0.0.0/8
seq 10 deny 23.0.0.0/8
seq 15 allow 0.0 .0.0 / 0 prefix 32
ip list FILTER R2: 3 entries
seq 5 deny 21.0.0.0/8
seq 10 deny 22.0.0.0 / 8
seq 15 permit 0.0.0.0 / 0 few 32
Get hands-on practice with lists of prefix And you will quickly you started. Prefix lists are an important element of cooperation with BGP in & 39; examination of the chamber and production networks, and it is essential that you can work comfortably with them.
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