0

How does a Cisco Router (IOS-XE) decide if a Packet will be MPLS Label switched or ip routed?

In a Juniper MX MPLS Pathes have an own Administrative Distance (or Preference in Junipers wordings) So Static Routes are preferred over LDP MPLS Pathes but IS-IS Targets will Label switch because LDP has a better preference than IS-IS.

How does Cisco (IOS-XE) handle this?

In my understanding LFIB entries are always preferred above RIB routing Topology information so the FIB will always contain available LSPs and is suppressing RIP Routes.

2

The router determines the type of frame received by the Ethertype field.

A "standard" IP packet has an Ethertype value of x0800. An MPLS frame has an Ethertype of x8847.

This picture may be helpful:

enter image description here

3
  • Thanks for this answer, but my question was more like: How does a Cisco IOS-XE Device decide if it will mace a forwarding decision based on the MPLS Label or the IP Header? – Andreas Schaefer Feb 21 at 14:51
  • Based on the Ethertype as shown above. – Ron Trunk Feb 21 at 15:34
  • Sorry i will provide an example to clarify my question: An IP packet from a customer's site enters the ingress LER, this IP packet is encapsulated in an ethernet frame with ethertype 0x0800. The IGP has determined the best IP-Routing Path, LDP has created Label switch Path to the LER. on basis on which criterion does the ingress LER decide if this IP Packet will be forwarded towards the Egress PE based on the information in the IP Routing table (simple IP-Routing), or will be label switched based on the information in the MPLS Forwarding table? – Andreas Schaefer Feb 22 at 4:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.