Newer protocols like TRILL and SPB can handle topologies with loops
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Routing Table
Global routing
▶
netstat -r ▶
▶ ▶
Not directly connected networks need a gateway (next hop) Recursive lookup ▶ ▶ ▶
Route flags
In practice this is only one level deep Hence the gateway needs to be directly connected In theory a deeper recursion level would be possible and useful
Flag G H S
Set route needs gateway route to host route added statically (mostly by admin)
Unset route is directly connected route to network route added dynamically (by a protocol)
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arp command
route command
▶ ▶
▶
Syntax varies between systems On Linux systems1 ▶
▶
Used to interact with the arp table Read the arp table ▶
▶
route add default
Adding a static host route ▶
▶
▶
Adding a static default route ▶
▶
Adding a static network route
▶
Exercise: Rewrite the first two entries as instances of the last
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arp -s
arp only works for IPv4 ▶
1
arp -d
Add a static entry to the arp table ▶
route add -net netmask gw
arp
Delete an entry from the arp table ▶
route add -host gw
▶
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is the traditional command to show the routing table
For IPv6 use the ip [-6] command
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ip subcommands
ip command
ip . . . ▶
Introduced in the Linux iproute2 package
▶
General interface to kernel addressing and routing
▶
Replaces ifconfig, route and arp (almost) completely
▶
Has support for IPv6, routing policies and multiple routing tables
Subcommand link address neighbor route rule
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Mathematical representation
Route selection
▶
▶
▶
Longest prefix match ▶
Graphs (undirected, labeled) ▶
Host routes are preferred over network routes
▶
Default route has shortest prefix ▶ ▶
Nodes are routers and hosts Edges are (point to point) connections ▶
Route of last resort Cannot be used in the Internet core ▶
▶ ▶
Why?
▶
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Network nodes are not necessary because of this restriction
Labels represent “cost” of using an edge in your route Undirected edges imply a restriction to ▶
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Meaning layer 2 interface settings (MAC) layer 3 interface settings (IP) arp cache data routing table data routing table selection
two-way communication paths with the same cost in both directions
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Broadcast networks (square)
Broadcast networks (triangle)
R
R
R
R
R
N
R.
R
R
N
R.
R
R
R
R
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Internet Routing (1)
Node reduction
▶ ▶
LAN with O(N) routers or hosts
▶
Full mesh of O(N2 ) edges
▶
Reduced with only 1 extra node to O(N) edges Works for LANs, but also for NBMA networks
▶
▶ ▶
.
R
▶
Based on top level structure defined by Autonomous Systems (AS) Each AS has administrative control over a collection of ▶ ▶
Routers (and hosts) Networks
Definition (AS — Autonomous System (from RFC 1930))
In many situations But not always (take care)
An AS (Autonomous System) is a connected group of one or more IP prefixes run by one or more network operators which has a single and clearly defined routing policy.
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Internet Routing (3)
Internet Routing (2)
▶
Edge routers inside an AS can be directly connected to edge routers in another AS ▶ ▶
▶
Used for inter-AS routing Using an exterior routing (or gateway) protocol (EGP) ▶ ▶
▶ ▶
Example: BGP4 No other protocols in use (except variants of BGP)
Intra-AS routing Using an interior routing (or gateway) protocol (IGP) ▶
Examples: RIP, OSPF, IS-IS
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Distance Vector Routing
Routing Protocol Classification
▶
Static ▶ ▶ ▶
▶
▶
A “human” protocol Explicit commands like “route add” or “ip route add” Side effect of a script (“semi-automatic/semi-human”)
▶ ▶
Distance vector algorithm ▶ ▶
Dynamic (automatically by a routing protocol) ▶
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Routers within the boundary of a single AS communicate with each other to provide
Distance Vector (for instance RIP) Path Vector (for instance BGP) Link State (for instance OSPF or IS-IS)
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Bellman-Ford (1957) Distributed shortest path
▶
Original ARPANET routing algorithm
▶
Decentralised
▶
Asynchronous
▶
No choice of root like in STP
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Link State Routing
Path Vector Routing
▶
▶
▶
Instead of the distance to the destination the complete path – on AS level – is specified
▶
Decentralised. . . ▶
▶
Link state algorithm ▶
The algorithm is still ▶
▶
▶
Like distance vector routing, but. . .
. . . but there is an informal notion of tiers
Asynchronous
The algorithm may depend on explicit or implicit policies
.
.
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Dijkstra (1959) Single source shortest path
▶
Complete knowledge is distributed to all nodes in an area
▶
Knowledge about the local network topology is flooded to all participants in an area
▶
Every node executes the shortest path algorithm and draws the same conclusions
