Tag: MTU

Review/Perspective: Switch Administration

2.1.a Implement and troubleshoot switch administration

  • 2.1.a [i] Managing MAC address table
  • 2.1.a [ii] errdisable recovery
  • 2.1.a [iii] L2 MTU


Managing MAC address table

The MAC Address table found in layer 2 IOS contains the MAC address of every known device on a network, and the port their traffic is coming in on.  As a frame enters the switch, it examines the ethernet frame and records the MAC address and the port it comes in on and maintains a database of MACs:ports so it knows where to forward frames as needed.  Otherwise if a destination MAC is missing from the MAC table, switches will flood that frame out all ports in a last ditch effort to find the host the frame is destined for.

Commands to view the MAC address table can be found in the BeThePackets Wiki at:



errdisable recovery

Should a port kick into errdisable, you can recover it typically by bouncing the port using the shut/no shut commands.




MTU is the Maximum Transmission Unit which defines the largest size of frames an interface can transmit without the need to fragment.  there are three types of MTU that are recognized when configuring a switch:

  • 10/100Mbps switch interfaces
  • 1000Mbps switch interfaces
  • Routed and SVI interfaces


MTU mismatches can occur if the interface MTU configuration is lower than the originally sized ethernet frame.  You can adjust this by configuring the offending interface with the correct size MTU.





Review/Perspectives: IP Operations part 2

1.1.d Explain IP operations

  • 1.1.d [iii] IPv4 and IPv6 fragmentation
  • 1.1.d [iv] TTL
  • 1.1.d [v] IP MTU1

IP Fragmentation – is an IP process that breaks datagrams into smaller pieces so packets may be formed small enough to pass through a link with a small MTU than the original datagram size.  Fragments are reassembled by the receiving host.

In IPv4 If the size of the PDU is larger than the next hops MTU the device has two options:

  • Drop the packet and send back an ICMP message to indicate packet is to big
  • Fragment the packet and send it over the link with smaller MTU.

IPv6 hosts are required to determine the optimal path MTU before sending packets, and guarantees that any IPv6 packet smaller or equal to 1280 bytes must be deliverable.

IPv4 routers will fragment data where IPv6 routers do not fragment, but rather drop packets larger than their MTU.

IPv4 and IPv6 even though the headers are different between the two, they both contain fields necessary to determine if fragmentation is needed.

TTL – The IP header contains a field for the TTL counter, for each hop a packet traverses the TTL value starts at 255 and decrements by 1 for each successive hop.  Once it reaches 0 the path to get to the destination is considered unreachable and dropped.  the routing device which dropped the packet will send an ICMP message back to the sender informing them of the unreachable status of the destination due to TTL expiry.

TTL behavior is handled slightly differently on MPLS label switched networks.  When a packet enters an MPLS cloud the IP TTL value is copied after being decremented to the MPLS TTL values of the labels pushed onto this traffic.  Once the traffic has reached its MPLS destination the TTL value is decremented by 1 as the label is taken off.


If  the label on  the packet must be swapped in transit, the TTL of the incoming label is copied to the swapped label. the TTL is copied to all top level labels pushed onto the packet. If the operation is to pop a label the TTL is decremented by 1 and copied to the newly exposed label unless the value is greater than the TTL of the new label in which the copy does not happen.


TTL Expiring and Labels – when a LSR receives a label switched packet that decrements its TTL to 0 it will discard the frame and send back an ICMP message like a normal router does, however the LSr may not have an IP path to the source of the packet.  In this case the ICMP message is forwarded along the LSP the original packet was following.  In general P routers on an MPLS backbone do not house all VPN routing information, which is the reason why the message is forwarded back on the same LSP path the original packet was taking in hopes that the originating LSP router upon receiving the ICMP packet will forward the message to the ultimate originator.

This operation is only performed if the MPLS payload is IPv4 or IPv6 traffic, any other type of transport protocol used is dropped.



IP MTU – The MTU is  the max length of data that can be transmitted by a protocol in one instance.  Typically for Ethernet is set to 1500 bytes by default and this is the largest number of bytes that can be carried within an Ethernet frame.


You can set this value globally requiring a restart of the device or you can set it per interface.  Jumbo frames with MTU size up to 9000 is supported for Gig+ links.

TCP MSS is the the MTU subtracting the number of bytes required for IP and TCP/other headers where needed.  To manipulate the value of the MSS field, use the interface configuration ip tcp adjust-mss command and set the value.

PPPoE: Point to Point Protocol over Ethernet

Point to Point Protocol over Ethernet (PPPoE) visualizes Ethernet multiple point to point sessions between client hosts and an access concentrator….essentially turning a broadcast Ethernet domain into a point to multi-point environment.

The PPPoE client feature in IOS allows the router as opposed as an end user host workstation to serve as the client in the network.  This allows entire LANs to connect to the internet over a single PPPoE connection terminated to a router.

PPP interface IP addresses are assigned using an upstream DHCP server and the IP Configuration Protocol (IPCP), which is a sub protocol of PPP.  IP address negotiation must be enabled on the dialer interface in the router for it to obtain an IP address.

PPPoE also introduces an additional 8byte transport overhead, (2 bytes for the PPP header and 6 bytes for PPPoE)  in order to adjust for this in the 1500 byte MTU, you have to decrease the MTU to 1492 bytes so the entire encapsulated packet fits within the 1500 byte Ethernet frame.  For TCP sessions the Max segment size is reduced to 1452 this allows for 40 bytes in TCP and IP headers and 8 bytes in the PPPoE totaling 1500 bytes that fit into an ordinary Ethernet frame.

MTU mismatches can prevent a PPPoE connection from establishing or carrying large datagrams, so this is a good place to check when troubleshooting connections.