Click the exhibit. Router R1 is a route reflector with clients R2, R5 and R6. Prefixes advertised by router R5 have a local preference of 200. Router R3 advertises the prefix 192.168.1.0/27 to…

Question

Explanation

BGP Route Reflector – What R5 Sees for 192.168.1.0/27

The Core Concept

This question tests your understanding of BGP Route Reflector (RR) behavior, specifically:

• Which paths an RR selects and reflects
• How ORIGINATOR_ID prevents routing loops
• Why the absence of advertise-external (ADD-PATH) limits what R5 sees

Topology Walkthrough

Setup:

• R1 = Route Reflector; Clients = R2, R5, R6
• R3 = eBGP peer of both R5 and R6 (external, hence the separate AS context)
• R5 applies local-pref 200 to routes received from R3
• R6 applies default local-pref (100) to routes received from R3
• No router uses advertise-external

Step-by-Step Path Propagation

Step 1 – R3 Advertises to Both R5 and R6 (eBGP)

R3 sends 192.168.1.0/27 to both R5 and R6 via eBGP.

• R5 receives it → applies local-pref 200 (per its policy)
• R6 receives it → applies local-pref 100 (default)

Why this matters: Local preference is set when a route enters an AS via eBGP. It's carried within the AS to influence best-path selection.

Step 2 – R5 and R6 Advertise to R1 (the RR) via iBGP

Both R5 and R6 advertise their best path for 192.168.1.0/27 to R1 (their RR).

Step 3 – R1 Runs Best Path Selection

R1 compares both received paths:

1. Same prefix, same AS-PATH length
2. Local preference wins: R5's path (200) > R6's path (100)

R1 selects R5's path as best.

Step 4 – R1 Reflects the Best Path (Without advertise-external)

Without advertise-external (RFC 7911 / ADD-PATH), R1 only reflects a single best path - it does not advertise additional/alternate paths.

R1 reflects R5's path with:

• ORIGINATOR_ID = R5's BGP Router-ID (set by R1 automatically)
• CLUSTER_LIST = R1's cluster-ID

R1 reflects this to all clients except R5 itself (R1 sends it to R2 and R6 only).

Critical rule: When a client receives a reflected route where ORIGINATOR_ID matches its own Router-ID, it discards the route to prevent loops.

Step 5 – What R5 Sees

R5 has:

• Its own direct eBGP route from R3 (local-pref 200)
• R1 does not reflect anything back to R5:
  • The best path has ORIGINATOR_ID = R5 → R5 would reject it anyway
  • R6's path is not the best → R1 won't reflect it (no advertise-external)

Result: R5 sees exactly ONE route - the direct eBGP path from R3, local-pref 200, marked best.

What Goes Wrong If Steps Are Skipped

Memory Tip

"RR reflects ONE best path; ORIGINATOR_ID kills your own echo."

Think of it as shouting into a canyon - the RR reflects your voice (your path) to others, but you don't hear your own echo back. Without advertise-external, the canyon only carries one voice (the best path), not every voice it heard.

Expected show router bgp routes Output on R5

R5 should display one entry for 192.168.1.0/27:

• Learned from R3 (eBGP neighbor)
• Local preference: 200
• Status: best (marked with * or *b in Nokia SR OS notation)
• No alternate/additional paths shown

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