Inter-AS & intra-AS routing protocol info, slide#12

BGP Messages

BGP Messages are exchanged between peers over TCP connection. These messages are: - OPEN - UPDATE - KEEPALIVE - NOTIFICATION

BGP Messages (CONTD)

BGP Messages: - OPEN: Opens TCP connection to peer and authenticates sender - UPDATE: advertises new path (or withdraws old path) - KEEPALIVE: keeps connection alive in absense of UPDATEs, also ACKS OPEN request - NOTIFICATION: Reports errors in previous message; ALSO USED TO CLOSE CONNECTION!!

RIP - Routing Information Protocol: was included with what operating system distribution?

BSD-Unix - 1982 *(intra-AS) reminder

BGP Routing Policy

Slide 12-25 .

What does "Open" refer to in OSPF?

"open" : publicly available

What is a gateway (router)?

* A router at the "edge" of the AS region * Has link to other AS routers(gateways?) * runs inTER-AS protocol

BGP Basics

* BGP Session: two BGP routers(peers) exchange BGP messages - advertising PATHS to different destination network prefixes (path vector protocol) - Exchanged over semi-permanent TCP connections

How/Why are Inter-AS and Intra_AS routing different?

* Policy: *Scale: *Performance:

How/Why are Inter-AS and Intra_AS routing different? (Policy elab)

* Policy: -inter-AS: Admin wants control over how its traffic is routed, who routes through its net(work) - intra-AS: Single admin, so no policy decisions needed

Hierarchical OSPF

* Two lvl Hierarchy: Local Area, backbone - link-state adverts only in area - Each node has detailed area topology; Only knows direction (shortest path) to nets in other areas. * Area border Routers: "Summarize" distances to nets in own area, advert to other Area Border routers. * Backbone routers: run OSPF routing limited to backbone * Boundary Routers (Gateways):Connect it's AS to other AS's

BGP - Distributing Path Information

* Using eBGP sessions between AS gateway routers, one AS sends prefix reachability info to the other AS - The gateway router can then use iBGP to distribute new prefix info to all routers in that local AS * When a router learns of a new prefix, it creates entry for the prefix in its forwarding table.

BGP basics cont

* When AS3 advertises a prefix to AS1: - AS3 "promises"? it will forward datagrams towards that prefix - AS3 can aggregate prefixes in its advertisement

Path Attributes and BGP Routes

*Advertised prefix includes BGP attributes: - prefix + attributes = "route" *two important attributes: - AS-PATH: Contains ASs through which prefix advertisement has passed, eg, AS67, AS17 - NEXT-HOP: indicates specific internal-AS router to next-hop AS. (**maybe be multiple links from current AS to next-hop-AS**) *Gateway router recieving route advertisement uses IMPORT POLICY to accept/decline -"POLICY-BASED" routing -e.g., never route through AS(x)

How/Why are Inter-AS and Intra_AS routing different? (Performance elab)

*Performance: - inter-AS: Policy ~may~ dominate over performance - intra-AS: Can focus on performance

BGP Route Selection

*Router may learn about multiple routers to destination AS, therefore selects route based on: 1 - local preference value attribute(policy decision) 2 - shortest AS-PATH 3 - closest NEXT-HOP router 4 . Additional Criteria... WTF HAAS?

How/Why are Inter-AS and Intra_AS routing different? (Scale elab)

*Scale: - Hierarchical routing saves table size, reduced update traffic

RIP - Routing Information Protocol (intra-AS)

- Exchanges distance vectors (DV's) with neighbors every 30 seconds in response message (aka ADVERTISEMENT) - (which is ADVERTISEMENT? The "exchange" or the "response msg"?) - Max hops = 15, each link has cost 1 - Each Advertisement: List of up to 25 destination SUBNETs (in IP addressing sense)

RIP - Routing Information Protocol - LINK FAILURE & Recovery (part 1)

- If no advertisement is heard after 180 seconds a neighbor/link is declared "dead" . **Then what happens?

inter-AS tasks include:

- Learn which destinations are reachable through other gateways (possibly only neighboring?) NEED CLARIF! - Propagate(via the gateway?) this reachability info to all the routers INSIDE its own AS region.

OSPF - protocol info (pt2)

- OSPF advertisement carries ONE entry PER neighbor - Those advertisements are flooded to ENTIRE AS -- ^ *****Carried in OSPF msgs directly over IP (rather than TCP or UDP)***

RIP - Routing Information Protocol - LINK FAILURE & Recovery (pt2)

- Routes through that neighbor/link are invalidated - New advertisements sent to neighbors - Those neighbors then send out new advertisements in response.. (**IF tables changed**) -Link failure info quickly propagates to entire net - Poison reverse - used to prevent "ping-pong loops" (infinite distance = 16 hops) Recall the MAX was 15

What is OSPF?

- Stands for "Open Shortest Path First" - intra-AS protocol

How are advertisements sent (for RIP protocol) ?

- The advertisements are sent in UDP Packets - The advertisements are periodically repeated (see slide 12-s14)

inter-AS: Forwarding Table entries

- an internal router (i.e. not the gateway) of an AS determines from info propagated to it that its interface L is on the LCP to (the gateway of the AS) to reach subnet X

How is the Forwarding table is configured via the inter-AS and intra-AS routing protocols?

- by BOTH inra-AS & inter-AS routing algorithm - intra-AS: sets entries for internal destinations (DONT get inter/internal confused!!) - inter-AS: sets entries for external destinations. -- intra-AS: ALSO ^^ sets entries for external destinations.

What is HOT POTATO Routing?

- inter-AS - done once router sees multiple paths exist - Sends packets to the closest of two routers(via inter-AS routing info it has) - Enters its own (X,L) entry into the forwarding table

"ping-pong loops" are...?

- mentioned in RIP link failure & recovery - Poison reverse - used to prevent "ping-pong loops" (infinite distance = 16 hops) Recall the MAX was 15 for RIP

Advanced Features of OSPF that are NOT found in RIP? (pt 1)

1) Security: All OSPF messages are authenticated (to prevent malicious intrusions) 2) Multiple same-cost paths (RIP only allowed one) -- ? allowed in table? Was this the hot potato? 3) Hierarchical OSPF in large domains

Two types of routing protocols ____?

1) intra-AS 2) inter-AS

What is a prefix in BGP advertisement?

12-19 ?? BGP message? All nodes on a given network share the same network prefix but must have a unique host number(webopedia)

Advanced Features of OSPF that are NOT found in RIP? (pt 2)

4) For each link, multiple cost metrics for different ToS *** (e.g. satellite link cost set "low" for best effort ToS; or "high" for real-time ToS) 5) Integrated uni-cast and multi-cast support: -- Multicast OSPF(MOSPF) uses same topology database as OSPF

Why is intra-AS protocol used for EXTERNAL destinations (along side inter-AS)? (Recall that intra-AS sets internal destinations, inter-AS has nothing to do with that assignment)

?

What is (AS) - "autonomous systems"

An aggregate (collection) of routers in a "local" region.

(AS) stands for?

Autonomous Systems

What is Broadcast?

Broadcast is the term used to describe communication where a piece of information is sent from one point to all other points. In this case there is just one sender, but the information is sent to all connected receivers.

inter-AS tasks include(pt.2):

Choosing from multiple gateways to forward packets. It will configure forwarding table accordingly, this can be done by a router inside AS1 that isn't necc. the gateway of AS1. I.e., Subnet X can be reached through AS2 as well as AS3. - HOT POTATO ROUTING

inter-AS routing protocol, AKA ______?

Exterior Gateway Protocols (EGP?) e.g. BGP

What is HOT POTATO Routing? (via wikipedia)

Hot potato routing is a routing technique enabling packet routing without storing them in buffers. Instead, this technique continuously transfers data packets until they reach their destination without the packets having to wait or be stored in a buffer.

intra-AS routing protocol, AKA _____ ?

Interior Gateway Protocols (IGP)

OSPF is an Intermediate system to...

Intermediate System Routing. (12-15) - protocol is nearly identical to OSPF -- Intermediate System - Intermediate System Routing -- Is an (IGP) interior gateway routing protocol

What is MOSPF?

Multicast OSPF = MOSPF

What is Multi-Cast?

Multicast is the term used to describe communication where a piece of information is sent from one or more points to a set of other points. In this case there is may be one or more senders, and the information is distributed to a set of receivers (theer may be no receivers, or any other number of receivers).

What BGP message closes a connection?

Notification message (as well as error reporting)

RIP Routing Tables are managed by: ?

RIP Routing Tables are managed by APPLICATION-LEVEL process called ROUTE-D (daemon)

Most common intra-AS routing protocols? (intra-AS == IGP protocols)

RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol ** IGRP is CISCO PROPRIETARY** !

Routers in the same (AS) run the same -_____?

Routing Protocol (as each other.) This is "intra-AS routing protocol.

RIP Table processing

See Slides for Session 12, page 14

What does BGP stand for? What is it?

Short for Border Gateway Protocol, an exterior gateway routing protocol that enables groups of routers (called autonomous systems) to share routing information so that efficient, loop-free routes can be established. BGP is commonly used within and between Internet Service Providers (ISPs). The protocol is defined in RFC 1771.

CIDR - Classless Inter-Domain Routing (from first exam :-/)

Short for Classless Inter-Domain Routing, an IP addressing scheme that replaces the older system based on classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP network prefix.For example: 172.200.0.0/16 The IP network prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses. An IP network prefix of /12,for example, can be used to address 1,048,576 former Class C addresses.

What is eBGP(session)?

Short for External Border Gateway Protocol (BGP), eBGP is the protocol used to transport information to other BGP enabled systems in different autonomous systems (AS). EBGP works opposite to iBGP.

What is iBGP?

Short for Internal BGP (Border Gateway Protocol), iBGP is the protocol used between the routers in the same autonomous system (AS). IBGP is used to provide information to your internal routers.

BGP Routing Policy

Slide 12-24 .

What us Uni-cast?

Unicast is the term used to describe communication where a piece of information is sent from one point to another point. In this case there is just one sender, and one receiver.

OSPF - protocol info (pt1)

Uses Link State algorithm - LS packet dissemination - Topology map at each node -uses Dijkstra's algorithm to compute route

inter-AS: Forwarding Table entries

form: (X, L) - X is a subnet in a different(?) AS - L the router's interface, which is on the L.C.Path. ? (slide 12, 6)