Fast switching

First packet to a specific destination IP is process switched. With the first packet the router adds an entry in the fast switch cache, also called route cache. The cache contains the destination IP, data link header information and the next hop. The next packet to the same desination will hit the cache and be fast switched.

Cisco Express Forwarding (CEF)

CEF has a construct called the Forwarding Information Base (FIB) where the best routes from the Routing Information Base (RIB) end up. The FIB is used for forwarding packets. The CEF table is designed as a mtrie which reduces the time needed to lookup a packet. CEF also uses an adjacency table with information needed to create the data link header and trailer and the outgoing interface. The FIB has a pointer to the adjacency table. CEF is enabled globally with the ip cef command. To turn off CEF per interface use the no ip route-cache cef command.

Adjacency table

Uses ARP, inverse ARP and other sources to find out layer tree to layer two mappings. After lookup has been done in FIB the information in the adjacency table is needed to build the header and trailer for the layer two protocol in use.

Inverse ARP

Used with frame-relay. Data Link Connection Identifier (DLCI) is already know but what IP address does the other side have? This is unknown information and is discovered via inverse ARP or statically entered. After receiving a PVC UP message with Local Management Interface (LMI) each router announces its IP over the Virtual Circuit (VC). If LMI is disabled nothing will trigger the inverse ARP process. Point-to-point interfaces ignore InARP information since there is only one way the traffic can be sent on a point-to-point interface.

Performance routing (PfR)

Originally named Optimized Edge Routing (OER) but Cisco added functionality and renamed it PfR. Can take into account the following information:

  • Packet loss
  • Response time
  • Path availabiliy
  • Traffic load distribution

PfR uses a five phase operational model:

Profile – Learn the flows of traffic that have high latency or high throughput
Measure – Passively/actively collect traffic performance metrics
Apply policy – Create low and high thresholds to define in-policy and out-of-policy (OOP) performance categories
Control – Influence traffic by manipulating routing or in conjunction with PBR
Verify – Measure OOP event performance and adjust policy to bring performance in-policy

PfR learns about network performance using IP SLA and Netflow features (one or both). Requirements for running PfR:

  • CEF must be enabled
  • IGP/BGP routing must be configured and working
  • PfR does not support MPLS

Device roles in PfR

Master Controller (MC)

Configured using the oer master command, this device is the decision maker in the cluster of PfR routers. Learns information from the border routers and makes configuration decisions for the network based on this information.

Border Router (BR)

Configured with the oer border command. Provides information to the master and accepts commands from the MC.

It is possible for a router to hold both roles. BR and MC routers mantain communication using keepalives. If keepalives from the MC stops the BR will remove PfR configuration and return to its pre PfR state. More than one MC can be used for failover purposes. PfR traffic classes can be defined by IP address, protocol, port numbers or even DSCP markings

Generic Routing Encapsulation (GRE)

Method for tunneling data from one router to another. Can be used to tunnel multicast and other protocols. The tunnel destination address must be known over something that is not the tunnel itself like a static route.

IP forwarding – notes
Tagged on:                 

Leave a Reply

Your email address will not be published. Required fields are marked *

%d bloggers like this: