Question 84 - JN0-664 discussion

In the given exhibit, you are observing the BGP routing information for the prefix 10.10.0.0/24. The AS path for this prefix shows `65000 {65137 65224}`. To understand the issue, let's analyze each aspect of the information provided and the options given:

1. **AS Path Analysis**:

- The AS path attribute in BGP is a list of AS numbers that a route advertisement has traversed.

- In this case, the AS path `65000 {65137 65224}` indicates that the route to 10.10.0.0/24 has been advertised through AS 65000, which includes AS 65137 and AS 65224.

2. **Understanding AS Path Prepending**:

- AS path prepending is a technique used by AS administrators to artificially lengthen the AS path.

- By adding their own AS number multiple times into the AS path, they make a route less attractive to others.

- Here, AS 65000 is showing `65137` as part of its AS path, suggesting it might be prepending to manipulate routing decisions.

3. **Options Analysis**:

- **Option A**: 'AS 65000 is pre-pending AS 65137 to route advertisements.'

- This option suggests that AS 65000 is deliberately adding AS 65137 to the AS path. This matches the observed AS path, where AS 65000 has prepended `65137`.

- Therefore, this is the correct explanation.

- **Option B**: 'The local AS is receiving two equal cost routes to 10.10.0.0/24.'

- There is no evidence in the exhibit to support the presence of two equal-cost routes.

- **Option C**: 'The local AS is in the process of withdrawing the route from AS 65137.'

- Route withdrawal would not cause the AS path to include `65000 {65137 65224}`.

- **Option D**: 'AS 65137 is advertising the 10.10.0.0/24 prefix.'

- While AS 65137 is part of the path, it does not explain the presence of AS 65000 in the path, making this option incorrect.

**Conclusion**:

The AS path `65000 {65137 65224}` is indicative of AS 65000 prepending AS 65137. Therefore, the correct answer is:

**A. AS 65000 is pre-pending AS 65137 to route advertisements.**

**Reference**:

- Juniper Networks documentation on BGP AS path manipulation and prepending techniques.

- BGP Best Practices and Routing Policies: [Juniper Networks BGP Best Practices](https://www.juniper.net/documentation/en_US/junos/topics/concept/bgp-best-practices-routing-policies.html)

- RFC 4271, A Border Gateway Protocol 4 (BGP-4): [RFC 4271](https://tools.ietf.org/html/rfc4271) which describes the behavior and attributes of BGP, including AS path prepending.