More on BGP Check out the links on politics: ICANN and net neutrality To read for next time Path selection big example Scaling of BGP

Decision Process Step 1: each border router –Selects which of the paths advertised by its external neighbor can be imported May violate import policy May have routing loop –Picks the best ones – Optionally assign a LOCAL_PREF –Sends its best paths to all its internal neighbors Step 2: All border routers compare all the paths learned through their neighbors and pick the single best path to each prefix

Path Selection Process For each of the paths to the same NLRI –Ignore if next-hop is down –Prefer Higher local preference –If same Prefer locally originated route –If same Prefer Shortest AS path –If same Prefer Lowest origin (IGP better that EGP) –If same Prefer Lowest MED –If same Prefer eBGP over iBGP –If same Prefer Path with lowest IGP cost to next-hop –If same Prefer Lowest router id –If same Prefer Lowest neighbor IP address Different vendors may have some slight variations! –The RFC does not specify all the details (e.g. the preference for shorter AS paths

After path selection Export routes to external neighbors –According to export policy –Add the local AS in the AS_PATH –Remove LOCAL PREF and optionally add MULTI_EXIT_DISC Originate routes inside the domain –Through the appropriate IGP mechanisms –OSPF has AS-external routes –And can use the IGP cost to next-hop to select which one is better…

OSPF external routes Certain routers in OSPF are ASBRs –Inject external routes (type-3 LSAs) –With themselves as the next hops OSPF routers compute next hops for these routes using the cost to reach the ASBR that originated them –And install these in the RIB BUT: clearly I can not inject all the routes in the Internet into OSPF, it will not scale –We will see solutions to this later on

BGP implementation details Uses TCP to talk to neighbors –Provides reliability Important since I have to exchange a lot of information Allows to send only changes and not all the information periodically like EGP –But TCP has its own mechanisms for flow control and connectivity loss detection that may cause surprises

BGP messages Since TCP is a stream protocol I need a way to delimit packets –BGP header Messages –OPEN Used to negotiate parameters when establishing adjacency –UPDATE Contains the reachability information –NOTIFICATION Send when an error occurs –KEEPALIVE To check the health of the adjacency

Paths BGP manages paths Path consists of –Network Layer Reachability Information (NLRI) e.g /24 –A sequence of PATH attributes that give info related to this destination PATH attributes –Each have a Flags field Optional or well known (well known must be supported by all routers) Transitive or local (Transitive gets propagated, local not) Partial or not (partial applies only to part of the path)

Important path attributes ORIGIN (well known) –Is this path learned from IGP, BGP or other AS_PATH –The list of ASes (well known) NEXT_HOP –Next hop to reach the prefix (well known) MULTI_EXIT_DISC (MED) –Helps selection of paths (local, optional) LOCAL_PREF –Helps selection of paths (well known)

Operation Establish a TCP connection with the neighbor Negotiate options and establish adjacency Send all the path info to the neighbor, receive its path info and store it Then only send updates/changes If an update is received that results in a different route process it and update neighbors Send keep-alives to monitor the health of the link –Does not depend on TCP keep-alive mechanism

Prefix aggregation –Need to be able to aggregate prefixes –AS_PATH contains a number of AS_SET and/or AS_SEQUENCE AS_SET: unordered set of ASes traversed AS_SEQUENCE: ordered list of ASes –When aggregating…. Longest common prefix of the AS_SEQUENCE becomes the aggregated AS_SEQUENCE Union of all the rest becomes the new AS_SET –Can repeat recursively –EXAMPLE: why it is good for the network

Scaling Route refresh Route dampening RR

Route Refresh Each time policy changes I need to find out about all the paths from the neighbor –A path that was not valid before (and was discarded) may be valid under the new policy I could do a reset (I.e. re-establish adjacency with peer to relearn all the paths) –Expensive and network disruptive Instead, initiate a route refresh –Tell neighbor to resend me all its paths so I can re- evaluate policy –“Route Refresh” capability, must be supported by both peer routers

Route Reflectors All BRs need to talk iBGP to all others –Full mesh does not scale Use Route Reflectors (RR) –Multiple RRs in an AS –A RR has clients and may be connected (through iBGP sessions) to other RRs –Much fewer iBGP sessions RR computes paths based on information it receives –If path is learned from client, send it to all other clients and RRs –If path is learned from other RR send it only to clients

More on RRs Essentially split the network into clusters –One RR per cluster and multiple clients –All RRs are fully meshed –Use a cluster-id Need to have RR redundancy –Not a good idea to have two RRs per cluster –Each client talks to two RRs

Extensions to BGP over time Communities –Logically group paths together for easier administration –Can apply policies to a community and not to individual paths –Simpler management, no need to add new policy rules for new paths Multi-protocol –Carry different NLRIs –Ipv4, ipv6, multicast –VPNs (we will them later) Graceful restart (we will see it later) 4 bytes ASNs

Issues with BGP BGP dynamics are complex! –We will see it through some papers Understand the structure of the internet and the properties of the topology –How to measure it? –Hot potato routing –Multi-homing and provider selection Understand the convergence properties –Role of policies in convergence –Policy consistency –Effect of the various mechanisms used

Convergence Dynamics 4 types of advertisements –Withdraw, new, longer, shorter –Withdraw and longer converge slower than new and shorter. Why? –BGP explores alternate paths in case of withdraw and longer –EXAMPLE Worse case can be n! steps In theory in fully connected graphs of course –Also in BGP I can have a timer to collect multiple paths before I process them

Route Dampening Route flaps can be a pain –A link that is flapping can cause ripples around the whole Internet Penalize flapping paths –Keep track of updates per peer and per path –If it flaps too much stop announcing it Parameters: additive penalty, half-life, suppress threshold, reuse threshold Can be different on a per prefix/ per peer basis etc It was considered a good idea until 2000 or so –Then people discovered that it was not working that well –A single path withdrawal could appear like a flap as it propagates in the network Makes the path unavailable for long time (10s of minutes)