MPLS: Autoconfig (enable LDP on all interfaces) only available when using OSPF as IGP.

LDP send discovery packets via UDP to (all routers) port 646. Route-ID is highest loopback but can be forced “mpls ldp route-id x.x.x.x force”. To use the physical connection of the interface (not the loopback due to lack of reachability) use this command on the interface. ” mpls ldp discovery transport-address interface”. Once communications is established, via TCP 646, authentication is verified (MD5 only). After peer is established prefix/label information is exchanged and LFIB is built.


Two Labels: Transport and VPN Label

View Transport label with “sh mpls forwarding-table” and VPN label with “sh ip bgp vpn4 vrf XXX”

OSPF on MPLS VPN: MP-BGP cloud is super area 0 (super backbone), treated as T-3 LSA’s. SAME VPN, SAME DOMAIN_ID (PROCESS ID) T3, different DomainID, T5.

Creating a Sham-Link

Sham-Links allows MPLS network to override backdoor links.
Before you create a sham-link between PE routers in an MPLS VPN, you must:
  • Configure a separate /32 address on the remote PE so that OSPF packets can be sent over the VPN backbone to the remote end of the sham-link. The /32 address must meet the following criteria:
    • Belong to a VRF.
    • Not be advertised by OSPF.
    • Be advertised by BGP.

You can use the /32 address for other sham-links.

  • Associate the sham-link with an existing OSPF area.
EIGRP: Site of Origin – SoO
Used between the PE and CE to prevent route feedback and loops. Could be accomplished with tag and filter but that is too complex. Multi-homed CE’s and CE’s with backdoor links are ideal candidates. Also, used in BGP when the same ASN is used at all remote locations.
CE: Same ASN on both sides will not allow bgp prefixes to be advertised because of BGP’s loop prevention (same asn). You can override on the PE with the neighbor statement and “as-override” command. “Allowas-in” is another option but NOT RECOMMENDED.