Information Technology Reference
In-Depth Information
Figure 7-18
Prefix Propagation in Step 2
R1
R1
RR
RR
NEXT_HOP:
R2
NEXT_HOP:
R4
NEXT_HOP:
R3
NEXT_HOP:
R3
NEXT_HOP:
R3
Withdraw
R2
R4
R2
R4
Client
Client
Client
Client
R3
R3
Client
Client
Table 7-3
Initial Paths in R1
Path
BGP Next Hop
AS_PATH
MED
RID
1
R4
300 400
6
172.16.67.6
2
R2
200 400
10
192.168.45.4
3
*
R3
300 400
5
192.168.45.5
Using the steps described in Chapter 2, R1 selects the path via R3 as the
best path. Here is what happens:
(a)
R1 compares the top two paths.
(b)
Because the paths are from different neighboring
autonomous systems, MED is not compared.
(c)
The tiebreaker is the router ID (RID). Because R4 has a
lower RID, it is determined to be the better of the first two.
(d)
R1 compares Path 1 and Path 3. Path 3 is determined to be
the better path because it has a lower MED.
(e)
R1 reflects its best path to its clients.
(f)
Because the best path is via R3, R1 sends a withdraw
message to R3 for the route.
(g)
Routers R2 and R4 receive the path with R3 as the BGP
next hop.
