BGP Community - No-export

I am continuing from my previous post on BGP communities. Since the topology is the same we will skip the interface and BGP configurations and dive right into what is no-export community, how to configure it and see how it affects routing updates.

Same topology as my previous blogtorial BGP Communities No-Advertise 

Complete configs can be downloaded here.

BGP Community No-Advertise

Much like a community in real life -- a group of "people" (usually) with common beliefs, A BGP community "is a group of destinations which share some common attribute" - RFC 1997.

There are numerous BGP communities, but basically it is translated and interpreted as a 32bit value. BGP communities are optional transitive attributes meaning they are not required and they can be passed on to other AS only if the router(s) decides to do so. 

These are the well-known communities. More being suggested and drafted in other RFCs.

INTERNET - By default all destinations are assigned this community. 

NO_EXPORT - Do not send the route to any eBGP neighbors. 

NO_EXPORT_SUBCONFED - Do not send the route to any sub-confederations. 

NO_ADVERTISE - Do not send the route to any neighbors (iBGP or eBGP).

LOCAL-AS - Do not send to anyone other than your LOCAL-AS (So this will prevent the router from sending to sub-confederations). 

ip multicast helper-map - Converting Broadcast and Multicast

If you work in the financial industry you are bound to encounter a situation where you will have an application that publishes udp data as broadcast and you will need to convert it to multicast -- maybe even back to broadcast if needed. This is very similar to the "ip helper address" because "ip helper-address" converts DHCP (udp) to unicast packets and sends it to the DHCP server.

Couple of very important roles of the routers in this setup are FHR and LHR. First-hop router that is connected directly to the source which is broadcasting the data and the Last-hop router that is connected to the destination subnet which can convert the multicast back to broadcast. 

Consider this simple topology and let's get started. 

Complete configs can be found here.

Understanding TCP SLOW START

In my opinion, TCP Slow Start is a rudimentary concept that should be mastered by any Network Engineer. It maybe a good idea to read through my previous post about Window Size and Scaling before continuing. To understand TCP Slow Start there are a few terms that we must be familiar with. 

TCP Slow Start - Congestion control mechanism which controls the growth of the sending rate.  

IW - Initial Window 

CWND - Congestion Window (The send window of the sender)

RWND - Receive Window (The window size of the receiver) 

SMSS - Sender Maximum Segment Size (Maximum amount of bytes that can be stuffed into a packet on the sender side) 

ISST - Initial Slow Start Threshold

SST - Slow Start Threshold

FlightSize - Amount of unacknowledged data that can be on the wire. This is usually set to the CWND however the actual formula is min(cwnd, rwnd). In most cases the CWND will be smaller than the RWND.