Why does mpls need cef

In Example , the out label on PE2 to reach This is correct. Two labels are found in the FIB output. The IGP label is 16 in this output.

The next case verifies whether this IGP label is correct. PE2 uses PE1's loopback address to reach On the P router, the local tag to reach Therefore, for traffic destined for When the P router receives traffic toward Here are the latest Insider stories.

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Even though the server responded OK, it is possible the submission was not processed. This site uses Akismet to reduce spam. Learn how your comment data is processed. Skip to content Process Switching Process switching is the slowest of all switching methods.

Fast Switching The first packet for a destination that arrives is process switched. The command to verify which path a labeled IPv4 packet will take in the case of per-destination load balancing is show mpls forwarding-table labels label exact-path ipv4 source-address destination-address Following are the general rules for load balancing labeled packets on a non-IPv6-capable Cisco IOS router: If the MPLS payload is an IPv4 packet, the load balancing is done by hashing the source and destination IP address of the IPv4 header.

The debug command debug ip cef drops [ access-list ] tells you if IP packets are dropped on the ingress PE router. Share this! Author: Mo Moghaddas Building zeeg. Leave a Reply Cancel reply Your email address will not be published. Let me give you an illustration:. The control plane is responsible for exchanging routing information using routing protocols, building a routing table and ARP table.

The data plane is responsible for the actual forwarding of IP packets. What is recursive routing? Let me give you an example:. In the example above I have three routers. R3 has a loopback interface that we want to reach from R1. I will use static routes for reachability:. The second static route is required to reach network R1 has to check the routing table 3 times before it knows where to send its traffic. Doing multiple lookups to reach a certain network is called recursive routing.

Most of the time all incoming and outgoing IP packets will be processed and forwarded by the data plane but there are some exceptions, first let me show you this picture:.

Most of the IP packets can be forwarded by the data plane. The control plane can forward outgoing IP packets to the data plane or use its own forwarding mechanism to determine the outgoing interface and the next hop IP address. An example of this is local policy based routing. Our multilayer switch has many more steps to take than the layer 2 switches so theoretically it should be slower right?

One reason that multilayer switches are able to forward frames and packets at wirespeed is because of special hardware called ASICs in the dataplane. Table lookups are fast! This way it is able to quickly lookup information in the CAM table. When we look for a MAC address we always require an exact match.

We require the exact MAC address if we want to forward an Ethernet frame. For example an IP packet with destination address Information like the routing table are stored in a TCAM table for this reason. We can decide whether all or some bits have to match. If we want to match IP address There is nothing that matches In this case there is an entry that matches The example above applies to routing table lookups, access-lists but also quality of service, VLAN access-lists and more.

There are different switching methods to forward IP packets. Here are the different switching options:. When using process switching the router will remove the header for each Ethernet frame, look for the destination IP address in the routing table for each IP packet and then forward the Ethernet frame with the rewritten MAC addresses and CRC to the outgoing interface.

Everything is done in software so this is very CPU-intensive. If you like to keep on reading, Become a Member Now!