It is recommended to separate TCP and UDP traffic because of TCP windowing. If they are in the same class and the flows are not broke up in some ways (QoS) there can be TCP starvation / UDP dominance.

Global Synchronization: This is again TCP windowing / flow control. So if many hosts send TCP traffic to a recipient the recipient can ask them to slow the traffic down. But if they slow it down all at once the congestion eases and the hosts try to send traffic faster…
Solution: Weighted Random Early Detection (WRED)

Selective Acknowledgements: When a segment is lost while transmitting, the receiver  tells the sender that he received the 1st segment, so the sender retransmit 2,3,4 segment. To avoid this the receiver adds plus info: I’ve received seg1 and there is seg3 and seg4 out of order. This way the sender will only resend seg2.

ip tcp selective-ack


Direct connections

This means 2 sites that are directly connected (T1,T2 lines…).
They are not flexible, they have low bandwidth and expensive. On the other hand they are very reliable.

Packet switching

Frame relay / ATM ..
This technology has the flexibility to connect multiple sites.

MetroEthernet (dark fiber)

Service Providers install fiber mostly in places with high population.
This technology grant really high speed.

L2 / L3 MPLS

MPLS creates a logical connection between the sites. It is not relevant if the different sites use different technologies ie.: direct connection, frame relay, metroehternet… the packets will be tagged and transmitted to the target site.

L2 MPLS: it’s like the sites are linked with a L2 switch.

L3 MPLS: each site can for example advertise routes to the Service Provider and the SP routes it to other sites’ of the customer.


VPN is getting more and more popular because internet connections are cheap and you receive high bandwidth ( not granted though). With VPN connection you can encrypt your internet connection and this is as secure as a leased line but way more cheaper.


With simple VPN you have to create a full mesh toplogy and with many sites it will get unusable. DMVPN has a Hub router which is in connection with all the spokes. If a spoke wants to speak with another spoke it go to the Hub which tells the router how can it reach the desired router.


  • Unicast
  • Multicast
  • Broadcast


  • Link Local
  • Unicast
  • Multicast
  • Anycast