TCP IPv4 and IPv6 (P)MTU Maximum Transmission Unit (MTU) is the largest size Protocol Data Unit (PDU) that can be sent over the network. Higher MTU leads to a more efficient network as you can move more in a single packet/frame. In addition, Path MTU is the largest packet size along a given route from source to destination that can be sent without fragmentation. By setting the DF flag (or always in IPv6), routers that receive a packet that is larger than it’s MTU drops the packet and replies with an ICMP Unreachable (Datagram too big) message, also containing it’s MTU. The host will resend smaller packets until it reaches the destination.
Category: CCNP R&S
Internet Control Message Protocol Internet Control Message Protocol (ICMP) is part of the IP protocol suit. ICMP is used primarily for network management. Like TCP and UDP, ICMP is a protocol of it’s own. Unlike TCP and UDP, ICMP isn’t for sending data for applications to use. You’ve probably heard of ping right? Ping is Control Message type 0. The two Control Messages we’ll look at today are ICMP Unreachable and ICMP Redirect. These are Control Message Types 3 and 5 respectively.
Asymmetric Routing occurs when the route the packets take to the destination is different to the return route. Causes of Asymmetric Routing are FHRPs (VRRP, HSRP, GLBP) and hot potato routing. Other causes include static routes like in this demonstration:
Out-of-order packets are received in a different order to which they were sent. This can be because of asymmetric routing, load balancing, route changes during transmission or other variables. TCP includes sequence numbers in packets so the receiver can put the packets back in to order. Even with TCP sequencing packets, out-of-order packets can cause issues performance or reliability issues on the network.
Unicast Flooding occurs when a frame enters the switch destined to a MAC address unknown to the CAM table on the VLAN. The switch will flood the unicast frame out of every port on the respective VLAN expect the one it entered on. It’s normal for some traffic on the network to be Unicast Floods as it’s part of the learning process to fill switches CAM tables. However some situations may result in excessive unicast flooding traffic and result in a negative impact on performance.
Cisco Express Forwarding (or CEF for short) is a layer 3 switching technology propriety to Cisco. CEF allows hardware level routing (switching) to happen without using CPU cycles. Due to this the router is able to perform routing at much lower latency and higher throughput while maintaining low CPU utilization. CEF is enabled/disabled with the (no) ip cef configuration command.