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In a series of blog posts, I will go over each of the Cisco’s official blueprint sections for CCIE R&S 400-101 V5.1 written exam and call out the very essential topics and knowledge base that are must-know for passing the exam. Please note that all of our CCIE Written Exam Cert Study Guides include similar exam essentials at the end of each chapter to reinforce key concepts for our readers.

I hope that you find them useful!

  • IOS XE allows development of data plane ASICs outside the IOS instance and have them program to a set of standard APIs which in turn enforces Control Plane and Data Plane processing separation. It accomplishes Control Plane / Data Plane separation through the introduction of the Forwarding and Feature Manager (FFM) and its standard interface to the Forwarding Engine Driver (FED)
  • Hardware based switching platforms use Content Addressable Memory (CAM) for storing the CEF related information. These tables are finite and can fill up to exhaustion, which would cause forwarding to fall back to software. Once those entries are filled up it switches to software forwarding with an error message similar to “C4K_L3HWFORWARDING-2-FWDCAMFULL”.
  • The number of load balanced paths used is limited by the number of entries the routing protocol puts in the routing table, the default in IOS is 4 entries for most IP routing protocols with the exception of BGP, where it is one entry. The maximum number that can be configured is 6 different paths.
  • CEF polarization occurs when traffic uses per destination load balancing and the same algorithm is used throughout the network which causes traffic to not be load balanced after the first distribution. You can use universal algorithm to avoid polarization where a 32 bit value is added to the hashing algorithm
  • Configuration of portfast on all end-device ports (such as printers, PCs, servers, and so on) should limit TCNs to a low amount
  • The primary reasons for unicast flooding behavior include asymmetric routing, STP topology changes (as evident with repeated TCNs), and MAC forwarding table overflow.
  • Using per-packet load balancing to share the traffic load across available paths to a given destination can cause out-of-sequence packets in a particular data flow. This can result in unsatisfactory data transmission for video and voice streaming traffic.
  • Micro-bursting is a behavior where rapid bursts of data packets are sent in quick succession, leading to periods of full line-rate transmission that can overflow packet buffers of the network stack, both in network endpoints and routers and switches inside the network.
  • If the TTL field reaches zero before the datagram arrives at its destination, then the datagram is discarded and an ICMP error datagram (11 – Time Exceeded) is sent back to the sender
  • PMTUD is only supported by TCP
  • Global synchronization of TCP hosts, for example, can occur because packets are dropped all at once. Global synchronization manifests when multiple TCP hosts reduce their transmission rates in response to packet dropping, then increase their transmission rates once again when the congestion is reduced.
  • When TCP flows are combined with UDP flows within a single class and the class experiences congestion, TCP flows continually lower their transmission rates due to congestion control, potentially giving up their bandwidth to UDP flows that are oblivious to drops. This effect is called TCP starvation or UDP dominance
  • FIB and LFIB are data plane related tables
  • “ip forward-protocol spanning-tree” command is supported over Advanced Research Projects Agency (ARPA)-encapsulated Ethernet, FDDI, and High-Level Data Link Control (HDLC) encapsulated serials, but is not supported on Token Rings. As long as the Token Rings and the non-HDLC serials are not part of the bridge group being used for UDP flooding, turbo flooding will behave normally.
  • IPv4 option type field is 8-bit long and is divided into three sub-fields namely Copied (1-bit), Option Class (2-bit) and Option number (5-bit)
  • IOS-XE can host applications outside the IOS context
  • Only IOS-XE control and data planes can utilize multi core processors
  • IOS XE provides additional system functions that run as multiple separate processes in the OS.
  • “Copy” and “request platform” are two CLIs that can be used to update various IOS-XE sub-packages
  • Because of modularity, Cisco IOS XE allows for platform independent code separation and abstraction
  • IOS-XE provides supports for data plane ASICs outside the OS
  • CEF adjacency and forwarding tables can be built independently
  • With BGP PIC enabled, CEF recursion is disabled when next-hop is learned via /32 mask or next-hop is directly connected


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Evolving Technologies V1.1 is now part of all CCIE/CCDE Written exams.
All of our study material is already updated with V1.1 material!
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