Information Technology Reference
In-Depth Information
The VPNv4 label distribution is the same as in Figure 10-21. However, the way to make
PE1 (192.168.100.2) and RR1 reachable in AS 200 is different. Within AS 100, these
loopback addresses are reachable via IGP, as in Figure 10-21.
As in the previous example, ASBR1 advertises these loopback addresses in IPv4 with labels
to AS 200 (Figure 10-22 shows only the advertisement of PE1's address). Instead of redis-
tributing these eBGP prefixes into the IGP in AS 200, ASBR2 can advertise them via iBGP
with labels.
When advertising a prefix in iBGP to RR2, ASBR2 can choose to advertise itself as the
BGP next hop by setting
next-hop-self
(as shown in Figure 10-22) or by leaving the next
hop unchanged and redistributing connected routes into the IGP (not shown in Figure 10-22).
In either case, RR2 peers with ASBR2 and PE2 to exchange IPv4 prefixes and labels.
When the BGP next hop is reset on ASBR2, a new BGP label, L4, is assigned for the prefix
192.168.100.2/32 (PE1). Additionally, an IGP label, L5, is assigned to reach ASBR2 itself
(192.168.100.6). PE1's loopback address (192.168.100.2) is reflected in iBGP by RR2 to
PE2, and RR2 advertises ASBR2's loopback address in IGP to PE2, with an IGP label of
L6. Thus, PE2 has a label stack of 3 for the VPN, as follows:
•
The top label is the IGP label to reach ASBR2 (L6).
•
The middle label is the BGP label to reach PE1 via ASBR2 (L4).
•
The bottom label is the VPN label (Lv).
More detailed information on configurations, label exchange, and packet forwarding is
provided in the case study near the end of this chapter.
Non-VPN Transit Provider for VPNv4
Inter-AS VPN can also be provided via a central transit provider that is running non-VPN
MPLS. Figure 10-23 shows such a scenario.
Two client autonomous systems, AS 100 and AS 300, connect to the transit AS,
AS 200. The client autonomous systems provide MPLS VPN, whereas the transit AS is
running only MPLS. VPNv4 information between the two client autonomous systems
is exchanged between RRs using a multihop eBGP.
As discussed previously, PE devices and RRs must have reachability and proper labels
between the two client autonomous systems—namely, end-to-end LSPs from PE to PE. The
client ASBRs can exchange IPv4 prefixes and labels with the ASBRs of the transit AS.
Figure 10-24 demonstrates how PE1's address (192.168.100.2) in AS 100 is distributed
to PE2 in AS 300.
