Information Technology Reference
In-Depth Information
Example 10-19
Prefix and Label Information for 172.16.0.0 on PE1
PE1#show ip cef vrf VPNa 172.16.0.0 detail
172.16.0.0/16, version 8, epoch 0, cached adjacency 192.168.23.3
0 packets, 0 bytes
tag information set
local tag: VPN-route-head
fast tag rewrite with Et1/0, 192.168.23.3, tags imposed: {17 21}
via 192.168.100.5, 0 dependencies, recursive
next hop 192.168.23.3, Ethernet1/0 via 192.168.100.5/32
valid cached adjacency
tag rewrite with Et1/0, 192.168.23.3, tags imposed: {17 21}
To avoid the AS_PATH loop detection on CE1, you must configure AS Override on PE1.
NOTE
For the back-to-back VRF approach to work, at least one inter-AS link (an interface or a
subinterface) must be dedicated to each VPN. The same VPN must be configured on both
ends of the link. Because of this restriction, the back-to-back VRF approach is unsuitable
for interconnecting large numbers of VPNs.
Single-Hop Multiprotocol eBGP for VPNv4
You can use multiprotocol eBGP to exchange VPNv4 prefixes and labels across the AS
boundary. No LDP/TDP is needed on the eBGP link. Thus, only VPN labels are exchanged
between the eBGP peers. The inter-AS link is not associated with any VRF and is in the
global table. To reduce memory use, it is recommended that you not configure any VRFs
on the VPNv4 ASBRs.
To allow ASBRs to accept all VPNv4 prefixes, you must disable the default ARF. You do
this by configuring
no bgp default route-target filter
on all VPNv4 ASBRs. Within the
VPN address family, you must activate each VPNv4 ASBR neighbor. You can configure
more granular policy control for each VPNv4 neighbor, as in IPv4 sessions.
All the multiprotocol eBGP peer addresses (IPv4) are installed automatically as connected
/32 host routes in the local IP RIB of the receiving ASBR and thus in the LFIB (even though
LDP/TDP is not configured between the two autonomous systems). A local IGP label is
assigned for a /32 host route on the receiving ASBR. When receiving packets with these
IGP labels, the top label is popped. These host routes are created only under the following
conditions:
•
The session is multiprotocol eBGP.
•
The multiprotocol eBGP neighbor is directly connected.
•
VPNv4 capability is supported on either peer.
