Information Technology Reference
In-Depth Information
All VPNv4 routes that need to be exchanged with the remote AS are stored in the BGP
VPNv4 table and LFIB but not in the IP RIB or FIB. The in/out labels in the LFIB shown
for these prefixes are top labels. The out label is the BGP (VPN) label if the route is from
the remote AS; it is the IGP label (to reach the PE) if the route is from the local AS. To see
the entire label stack, use show mpls forwarding detail . To see the BGP VPNv4 labels
only, use show ip bgp vpnv4 all labels .
Compared to the back-to-back VRF method, no VRFs are configured on the ASBRs using
this method. Thus, the ASBRs do not function as traditional PEs.
NOTE
Example 10-20 shows a sample LFIB on a VPNv4 ASBR. The two host routes are peer
addresses. The first host route is of a PE in the local AS. The second host route is of a
VPNv4 ASBR of the remote AS. There are two VPNv4 routes. The first VPNv4 route is
from the PE in the local AS. The second one is from the VPNv4 ASBR from the remote AS.
The out label 26 is the IGP label to reach the PE, and the out label 22 is the VPN label for
the VPNv4 prefix.
Example 10-20 Sample LFIB on a VPNv4 ASBR
VPN-ASBR#show mpls forwarding
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 Pop tag 192.168.10.4/32 0 Fa0/0 192.168.11.4
17 Pop tag 192.168.110.2/32 0 Fa3/0 192.168.110.2
22 26 10:1:10.1.2.0/24 3846 Fa0/0 192.168.11.4
23 22 1:1:10.1.1.0/24 1180 Fa3/0 192.168.110.2
When multiprotocol eBGP is used across the autonomous systems, the next hop is reset at
the AS border, as in IPv4. As such, VPN labels are rewritten. The label assignment and
exchanges are different, depending on how the next hop is set or reset.
As in IPv4, the BGP next hop for VPNv4 can be made reachable in the AS receiving the
updates in the following two ways:
The next hop is the advertising ASBR (of the AS that advertises the updates) and is
carried unchanged inside the receiving AS. This is the default behavior. The
advertising ASBR must be reachable within the receiving AS.
The receiving ASBR resets the BGP next hop to itself when advertising to its iBGP
neighbors. This is configured using the per-neighbor next-hop-self setting under the
VPNv4 address family.
 
Search WWH ::




Custom Search