Cisco CCNP BSCI Exam Tutorial The BGP Attribute MED
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Cisco CCNP/BSCI Exam Guide: Understanding the BGP Attribute MED
Overview
To excel in the BSCI exam and achieve your CCNP certification, a strong grasp of BGP attributes is essential. This includes mastering the Multi-Exit Discriminator (MED) attribute. If you're aiming for the CCIE, understanding these attributes is crucial.
What is the MED Attribute?
The MED attribute helps routers in different Autonomous Systems (AS) determine the best path for sending data. It serves as a suggestion from one AS to another, indicating which path should be preferred.
Example Scenario
Consider Router R1 in AS 1 and Routers R2, R3, and R4 in AS 234. R4 advertises a loopback address through BGP. R1 has two potential paths to reach this loopback: through R2 (172.12.123.2) or R3 (172.12.123.3). Let’s analyze which path R1 selects.
```plaintext
R1
show ip bgp 4.4.4.4
BGP routing table entry for 4.4.4.4/32, version 8
Paths: (2 available, best
2, table Default-IP-Routing-Table)
Flag: 0x208
Advertised to non-peer-group peers:
172.12.123.3
234
172.12.123.3 from 172.12.123.3 (3.3.3.3)
Origin IGP, localpref 100, valid, external
234
172.12.123.2 from 172.12.123.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, best
```
In this case, R1 selects 172.12.123.2 as the next-hop for reaching AS 234.
Configuring MED
By default, R1 could continue to choose the same next-hop for all routes from AS 234. However, by configuring different MED values, R1 can be influenced to choose the preferred path. The router with the lower MED value will be selected. Let’s set a MED of 200 on R2 and 100 on R3.
```plaintext
R2(config)
route-map SET_MED_200 permit 10
R2(config-route-map)set metric 200
R2(config-route-map)router bgp 234
R2(config-router)neighbor 172.12.123.1 route-map SET_MED_200 out
R3(config)
route-map SET_MED_100 permit 10
R3(config-route-map)set metric 100
R3(config-route-map)router bgp 234
R3(config-router)neighbor 172.12.123.1 route-map SET_MED_100 out
```After clearing R1's BGP table, R1 will recognize both paths as valid but will prefer the one through R3 due to its lower MED.
Key Takeaway
Remember, the MED is a metric used to determine path preference, with lower values being favored. Mastering this concept will bring you closer to acing the BSCI exam and earning your CCNP certification!
You can find the original non-AI version of this article here: Cisco CCNP BSCI Exam Tutorial The BGP Attribute MED.
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