Cisco CCNP BSCI Exam Tutorial Filtering BGP Updates With Prefix Lists
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Cisco CCNP / BSCI Exam Guide: Filtering BGP Updates with Prefix Lists
Summary:
Mastering the skill of filtering BGP updates using prefix lists is crucial for both exam success and real-world application. Learn from Chris Bryant, CCIE
12933.
Keywords:
ccnp, bsci, exam, tutorial, bgp, filter, prefix list, update, routing, certification
---
Filtering BGP routing updates is essential for passing your BSCI and CCNP exams. This guide explores how to effectively use prefix lists for this purpose.
Understanding the Scenario
Router R4 is advertising three networks via BGP. The downstream router, R3, receives these routes and adds them to its BGP table. R3 has two downstream BGP peers, R1 and R2, and advertises itself as the next-hop IP for all BGP routes it sends to these routers.
```plaintext
R4(config)
router bgp 4
R4(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's BGP table reflects these advertisements, and R1 and R2 also have these routes in their respective BGP tables.
BGP Table Overview
For R3:
```plaintext
R3
show ip bgp
*> 21.0.0.0 10.2.2.4 0 0 4 I*> 22.0.0.0 10.2.2.4 0 0 4 I
*> 23.0.0.0 10.2.2.4 0 0 4 I
```
For R2:
```plaintext
R2
show ip bgp
*>i21.0.0.0 172.12.123.3 0 100 0 4 I*>i22.0.0.0 172.12.123.3 0 100 0 4 I
*>i23.0.0.0 172.12.123.3 0 100 0 4 I
```
For R1:
```plaintext
R1
show ip bgp
*>i21.0.0.0 172.12.123.3 0 100 0 4 I*>i22.0.0.0 172.12.123.3 0 100 0 4 I
*>i23.0.0.0 172.12.123.3 0 100 0 4 I
```
Filtering with Prefix Lists
To prevent all routes from being advertised from R3 to R1 and R2, we can use prefix lists. Cisco recommends this method for its ease of use compared to access lists.
Objective:
- Send only the 21.0.0.0 route to R1.
- Send only the 23.0.0.0 route to R2.
Configure R1's filtering:
```plaintext
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 permit 0.0.0.0/0 le 32
R3(config)router bgp 123
R3(config-router)neighbor 172.12.123.1 prefix-list FILTER_R1 out
R3clear ip bgp * soft
```After applying, R1’s BGP table only shows 21.0.0.0:
```plaintext
*>i21.0.0.0 172.12.123.3 0 100 0 4 I
```
Configure R2's filtering:
```plaintext
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 permit 0.0.0.0/0 le 32
R3(config)router bgp 123
R3(config-router)neighbor 172.12.123.2 prefix-list FILTER_R2 out
R3clear ip bgp * soft
```After applying, R2’s BGP table only shows 23.0.0.0:
```plaintext
*>i23.0.0.0 172.12.123.3 0 100 0 4 I
```
View Configured Prefix Lists
Use the following command to check your prefix lists:
```plaintext
R3
show ip prefix-list
```Practice and familiarity with prefix lists are crucial for success in the exam and in real-world networking environments. Embrace their simplicity and power to efficiently manage BGP routes.
You can find the original non-AI version of this article here: Cisco CCNP BSCI Exam Tutorial Filtering BGP Updates With Prefix Lists.
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