Routing Flashcards
What is an Autonomous System?
- Internet Service Providers (ISPs) offering transit services, such as Deutsche Telekom, Vodafone, DFN
- Organisations at one or more locations, e.g. Universities, companies
- Hosting providers, e.g., Amazon
- Content Delivery Networks, e.g., Akamai, Google
What types of AS exist?
- Transit AS: fwd traffic from one AS to others
- Stub AS: AS connected to only one other
- Multi-Homed AS: AS connected to multiple ASes without offering transit services
What are inter-domain routing protocols and name some
Exchange routing information between ASes (Exterior Gateway Protocols)
Practice only BGP
What is a intra-domain routing protocol and name some
Interior gateway protocols inside an AS
OSPFv2/3, IS-IS, RIP
Eselsbrücke für EGP & IGP?
EGP: Which AS to transfer the packet to?
IGP: Which intra-AS route to use to reach this neighboring AS?
What is a routing information base?
All routing information a router can gather from updates of neighboringrouters
(may contain multiple routes to target)
What is the Forwarding Information Base?
Mapping from a destination IP network address (prefix) to outgoinginterface or next hop router IP address
- Unique for each destination
- Longest prefix matching
What is a Forwarding Decision?
Algorithm uses the FIB to decide how to forward individual packets
Explain how BGP works
- Each AS has a unique AS Number (ASN)
- BGP router exchange their routing table entries
- Which next AS to choose is a policy (business) decision
- Path-Vector: updates contain all ASes on the path towards a destination network (prefix)
What is the difference between iBGP and eBGP
iBGP: Both routers have the same ASN
eBGP: Routers have different ASNs
What message types exist in BGP?
OPEN: Opens a connection between two routers
TEARDOWN: Close the connection
NOTIFICATION: Send error codes
UPDATE: Announce a new route, or un-reachability of an old one
What does an BGP update message contain?
- Destination prefix (a.b.c.d/x)
- AS path - list of ASes
- Next Hop - IP address of the router sending the update
- Origin - Learned via IGP/EGP/other
What is hot potato routing?
Always hand it over as fast as possible
Choosing the “nearest” connection site minimizes the cost
-> Leads to asymetric routing
What types of peering do exist in BGP?
- Private: Direct connection to (frequently large) AS
* Public: Exchanging traffic with other ASes at an Internet Exchange Point (IXP)
How is peering in BPG defined?
- Two ASes peer with each other, if they have some kind of BGP relationship, i.e., two ASes are directlyconnected
- Protocol viewpoint: It is irrelevant if one party pays the other party
- Policy viewpoint: It is very relevant if one party pays the other par
How can private peering be established?
direct network connections between locations
Peering possible at colocation center operated by a carrier
Peering possible at colocation center operated by a carrier-neutral data center provider
Which use cases do exist for private peering?
- Exchange a large amount of traffic with a single AS
- Attractive setup for upstream providers
- Interconnection of, within, and inbetween data centers
Which type of routing has which financial effects?
- Route via a customer (financial gain)
- Route via a peer (no financial gain or loss)
- Route via a provider (financial loss)
Which routes should one AS announce and which not?
- Announce routes that incur financial gain if others use them•Others = costumers
- Announce routes that reduce costs if others use them•Others = peers
- Do not announce routes that incur financial loss(… as long as alternative paths exist)
What are Tier 1 , Tier 2 and Tier 1.5 providers?
Tier-1 / Default-Free-Zone: only peerings, no providers
Tier-2: only peerings, and one or more Tier-1 providers
“Tier-1.5”: almost a Tier-1, but pays money for some links
What is the k-core algorithm?
Remove all nodes of degree k or < k recursively
What are link-state and distance vector algorithms and how do they differentiate?
Distance Vector (RIP, IS-IS): Routers only know the next hop & cost; no global topology; routers exchange cumulative costs Link-State (OSPF): Routers also exchange topology information (complete or partial) -> Shortest path (Djikstra)
What are Area Border Routers?
Routers which are member of area 0 (the backbone) and one or more other areas
- > Can fwd packets from one area to another
- > Each area must have a connection to area 0
What are AS Boundary Routers?
- Routers that inject routes learned from another routing protocol into OSPF
- Those routes may be from a BGP process on the same router, another routing protocol, or even stativroutes redistributed into OSPF
- The term “Autonomous System” in that context does not demand that those routes must lead to anotherAS