Information Technology Reference
In-Depth Information
All configurations directly under
router bgp
apply to IPv4 unicast only. Alternatively, these
configurations can be made under the IPv4 address family in certain IOS releases using
address-family ipv4 unicast
. These configurations are still required even if only VPNv4
prefixes are exchanged, because the BGP neighbor relationships are still established over
IPv4. If no IPv4 prefixes are to be exchanged for all peers, the IPv4 session can be disabled
using the command
no bgp default ipv4-unicast
. Alternatively, IPv4 sessions can be
disabled per neighbor using
no neighbor activate
.
To exchange VRF-specific routing information with a CE, configurations are needed under
address-family ipv4 vrf
. In this example, RIP routes from the CE are redistributed into BGP.
To activate the VPNv4 prefix exchange with other PEs, an address family for VPNv4 is
needed. Each neighbor must be activated individually. Configurations are made in this
segment to attach extended communities to the VPNv4 prefixes.
VPNv4 Route and Label Propagation
VPNv4 prefixes and VPN labels are propagated by multiprotocol BGP from one PE to
another PE. As with IGP labels, BGP labels are distributed from a downstream PE to an
upstream PE.
Consider Figure 10-7 for MPLS VPN route and label propagation. The prefix 172.16.0.0/16
can be reached on PE2 via a VPN static route. The VRF VPNa is associated with the prefix.
The VPNv4 prefix 100:100:172.16.0.0/16 and its label (Lv) are advertised to PE1, with the
BGP next hop (NH) set to itself. PE2's reachability (192.168.100.5) is advertised in the IGP
and LDP. The link between PE2 and CE2 (192.168.56.0/24) is also redistributed into VPNa
as a connected route (not shown in Figure 10-7).
Figure 10-7
MPLS VPN Route and Label Propagation
AS 100
100:100:172.16.0.0/16
NH=PE2, RT=100:100
VPNv4 Label=Lv
172.16.0.0/16
CE1
PE1
PE2
CE2
VPNa
192.168.100.5/32
NH=P
IGP Label=L2
192.168.100.5/32
NH=PE2
IGP Label=L1
VPNa
P
