Principles of Communications Week 3 20-22/10/2026

 

Interdomin routing-  BGP - key 4 slides - 125 127 132 133 in https://www.cl.cam.ac.uk/teaching/2627/PrincComm/slides/routing.pdf

Routing is about choosing paths, if there are alternatives. Choice may be based on local, as well as neighbour and global inputs. BGP mixes this up in interesting ways:

Moving from intra-domain (within one autonomous system/routing domain/internet service provider) to intradomain, the key change is from policy within a domain (as used for steering traffic in centralised routing or in mpls or segment routing) to policy between multiple autonomous (i.e. independent) domains who may have conflicts and often require some level of information hiding (protecting knowledge of their customers' needs from competitors). So while connectivity is the minimum requirement, there's often no shared goal in terms of what is "optimal" (i.e. what routing metric to use) - we'll see that in default cases, for traffic engineering, and various tie breaking reasons, metrics implicitly creep in as an implici part of BGP routing, but not in any way consistently.

Two nice questions in this lecture: Different clients in an AS have different shortest path to egress routers - so how do we use MEDs - this is basically each prefix advertised to the neighbour ASs has attriabutes so we can list each local prefix with a different MED to match its preferred route for traffic inbound towards it!

And what if an AS is very geographically distributed ? how do we configure that? I think ideally one might split into multiple ASs and then use BGP confederations (see slide 123 in routing link above).

The last thing on BGP is going to follow from Traffic Engineering and asks two questions: what really is the model BGP implements, to better understand how it works (and goes wrong)? and What engineering tweaks can we do to make it actually operate better in practice. We'll finish up on this on Oct 27-29th