Information Technology Reference
In-Depth Information
Next Hop Carried Unchanged Inside the Receiving AS
With the default behavior, the receiving ASBR does not reallocate a new VPN label for VPNv4
routes from the advertising AS. The address of the advertising ASBR (an IPv4 host route)
can be made reachable in the receiving AS by redistribution (into the IGP) or by iBGP
plus labels. When redistribution is used, these host routes are allocated IGP labels within the
receiving AS. Thus, the remote ASBR becomes the PE devices' BGP next hop, and the BGP
label assigned by that ASBR is used by PE devices. Using BGP to distribute labels for IPv4
prefixes is discussed in the section “Multihop Multiprotocol eBGP for VPNv4.”
Figure 10-17 depicts a scenario using redistribution. When the next hop (NH) is reset at
ASBR1, a new VPN label, Lv2, is assigned. The next hop is carried unchanged in AS 200
(the AS receiving the updates). Thus, Lv2 is still used by PE2 for the same VPN.
Figure 10-17
Prefix and Label Distribution with the Next Hop Unchanged in the Receiving AS
100:100:172.16.0.0/16
NH=PE1, RT=100:1
VPNv4 Label=Lv1
100:100:172.16.0.0/16
NH=ASBR1, RT=100:1
VPNv4 Label=Lv2
100:100:172.16.0.0/16
NH=ASBR1, RT=100:1
VPNv4 Label=Lv2
PE2
PE1
ASBR1
ASBR2
AS 100
AS 200
172.16.0.0/16
NH=CE1
172.16.0.0/16
NH=PE2
AS 65000
AS 65000
VPNa
Site 1
VPNa
Site 2
CE1
CE2
To allow inter-AS connectivity, the receiving VRF of the same VPN must import the same
RT that is exported from the sending VRF.
NOTE
Example 10-21 shows the label stack for 172.16.0.0/16 on PE2. The BGP next hop is
ASBR1 (192.168.56.5), with a VPN label 25 (Lv2). To reach ASBR1, an IGP label 19
is used, with an IGP next hop of 192.168.47.7 (a P router toward ASBR2).
