IP/RIP
RIPv1
RIP was originally developed by Xerox and was called Gateway Info (GWInfo)
it then developed into Routed which you will within the Unix environment.
RIP v1 is described in RFC 1058.
RIP is a Distance Vector protocol and uses the Bellmann-Ford algorithm for determining the
best route. Other distance vector routing protocols include IPX RIP, AppleTalk RTMP and IGRP (Inter-Gateway Routing
Protocol). There are also
other routing protocols such as OSPF, IPX NLSP (Netware Link State Protocol and
IS-IS (Intermediate System to Intermediate System) which are known as Link-State routing protocols
i.e. they are concerned with sharing the status of links in their updates rather than complete routing table information.
OSPF tends to be limited to around 1000 routers whereas
IS-IS can scale much larger. Ciscos EIGRP (Enhanced IGRP) is often called a 'hybrid' but is more strictly
a Diffuse Up Algorithm (DUAL). BGP (Border Gateway Protocol) is not really a routing protocol at all, it
is more accurately described as a Path Vector protocol.
In a RIP v1 environment all subnet masks throughout the network must be the same therefore
limiting addressing schemes somewhat. This is because RIP v1 is a Classful routing protocol, it
does not have the ability to transmit
the subnet mask within its updates. RIP v1 imposes the subnet mask on the inbound interface and this
is normally defined by the engineer. Learned routes are entered into the routing table with their
natural mask. As a result there can be a great waste of internet host addresses.
This can be rectified by using RIP v2 (see later)
or OSPF which are able to implement Variable Length Subnet Masks (VLSM).
They can do this because they can transmit the subnet mask within their routing updates and these protocols
are known as Classless routing protocols.
The following diagram shows a restriction on using RIP:
For routing updates being sent from router A about network 131.108.10.0/24,
router B resorts to assigning the natural Class B mask to the network entry in the routing table. Router
B therefore sees a split network since 131.108.0.0 also exists on its own LAN interface.
Similarly:
Although this network will route correctly as it stands in a RIP environment,
the problems arise if the link between A and B fails, because the 131.108.0.0 network will split.
Consider the following scenario:
In a RIP environment, if router C routes the first packet and then switches the rest, then for some conversations
the link to 131.108.0.0 will work very well and for others the link to 131.108.0.0 will fail. This is because
the Class B network 131.108.0.0 is split across two routes even though locally the masks are class C for the subnets.
The networks 222.1.1.0 and 222.1.2.0 are two separate Class C networks anyway. In an IGRP environment, because
IGRP load balances across equal cost paths, the users will have a slower network since TCP will resend the dropped
packets.
Subnets
When creating subnets, the mask determines which portion of the IP address is used for
the subnet address and which is used for the host address. For example, in the mask
255.255.255.0, the first three octets consist of all 1's whilst the last octet contains
0's, the mask is ANDed to the incoming IP address and the host part is separated out in
the AND operation and is considered to be a local interface address. When sizing a
subnet, the formula used is 2n - 2 = number of available subnets or hosts (where
'n' is the number of bits used). For further
discussion on IP addressing, CIDR and VLSM refer to IP
Addressing and CIDR.
RIP v1 Operation
RIP v1 uses UDP port 520 for sending and receiving broadcast messages.
The RIP v1 packet is illustrated below:
The Command field can have one of the following commands:
- Request
- Response
- Traceon (no longer used)
- Traceoff (no longer used)
- Reserved (used by Sun Microsystems)
Each route is identified in the fields from the Address Family Identifier onwards.
There can be up to 25 routes in one RIP packet.
There are two RIP message types Request Messages and Response Messages.
A router requests an update when it first joins a network and all routers respond
with Response Messages containing the routes. Response Messages are sent anyway
every 30 seconds by default. Triggered updates occur when there has been
a change and these triggered updates just contain the changes and help with
quicker network convergence.
IP routing protocols submit their best routes for each destination to the Router Table
Manager (RTM). The RTM then builds the Routing Table and includes all the locally
configured interfaces (i.e. on the router itself) and any static routes that might
have been configured. In addition to the Routing Table, identical copies of which sit on each
slot; there is also a Forwarding Table which acts as a route cache for the IP
datagrams. This Forwarding Table is used to hold the most recent and commonly used datagrams
so saving the need to go through the complete Routing Table every time and this
can be of a configurable size, depending on the memory available.
IP global and interface parameters are maintained within the Management Information
Base (MIB) which is mainly used by external SNMP clients which view and change these
parameters.
The same IP route information can come from different interfaces, routing protocols
or static routes, and since only one entry for a particular destination is allowed,
the RTM makes the decision as to which route goes into the table based on the
Route Preference (which can be configured), or failing that, the Route Weight
(which is not configurable).
RIP is not sensitive to line speed instead it calculates the best routes by way of
'hops' (or 'metric') or 'ticks' (which is time). If the router receives a change
to the reachability of a route and it is worse than the information that it already
has in the Routing Table then it will only update this information if it receives
the information from the original sender.
The maximum number of hops allowed is 15. Once the hop count reaches 16 then the
network becomes unreachable. A directly connected network has a hop of 1.
RIP updates (containing up to 25 entries each) are sent every 30 seconds. Cisco
have introduced a time variance for these updates of between 25.5 and 30 seconds.
This is to stop RIP_JITTER where on Ethernet networks RIP can get sent at the
same time causing update synchronisation so making the network very busy
every 30 seconds.
To help eliminate routing loops and unnecessary RIP updates two techniques are employed.
Split Horizon can be set so that two connected routers do not send each other
routes that have been learned via their connected interfaces, there is no point since
the other router must know these routes. Poisoned Reverse sends the routes
learned but with a hop count of 16 indicating the network to be unreachable via that interface. This
helps defeat larger routing loops.
Default Route
A default route, which has a mask of 0.0.0.0 will yield a destination network of
0.0.0.0 when any IP datagram is ANDed with the mask. The result of this is that
if it cannot find a match within the routing table then it uses the next hop address
defined by the 0.0.0.0 destination address. Configuring a default route causes a
certain amount of traffic overhead.
RIP and Subnet Masking
RIP updates are sent to RIP configured updates every 30 seconds and this update
contains all known networks and their hop counts, no mask information is included.
In this RIP packet 8 bytes, out of the possible 16, are unused so 50% of the bandwidth
is wasted. A router will not advertise a subnetwork out of an interface that does not
belong to that network, what it does do is advertise the natural portion of the address
e.g.
In the above diagram router A has two subnetworks, 149.213.128.0 and 149.213.192.0 with a
subnet mask of 255.255.248.0, however router B cannot see these subnetworks because
no mask information is received with RIPv1, instead it advertises a network of
149.213.0.0 with a natural mask of 255.255.0.0. If the advertised network belongs
to the subnetwork of the interface on which it was received, then that interface
mask is associated to that advertised network.
Equal cost paths to the same network are handled by the router using the first one
that it learned.
There are three timers that RIP uses:
- Expiration Timer - This 'Invalid timer' is set when a route is first added to
a routing table and whenever an update arrives for that same route.
- Flush Timer - This 'Garbage timer' starts if the Expiration timer reaches
180 seconds. The Garbage Timer runs for 60 seconds and then deletes the route if
no update occurs.
- Holddown Timer - When an update for a route appears with a higher metric than
the current route in the table, the Holddown Timer prevents changes to the table until
it times itself out thereby maintaining stability in the routing table if routes
are 'flapping'. This stops routes to failed networks being re-introduced by routers that
have yet to find out.
If information about a network is not received within 90 seconds (default value) then
the metric is set to 'U' (in the TI) or '-1' (Site Manager). The network is removed
from the routing table if no information is received after 180 seconds.
Unnumbered IP links
Unnumbered IP interfaces can be configured for use on point to point links as a way
of conserving network address space. An address of 0.0.0.0 is used, along with a mask
of 0.0.0.0, an associate address parameter is used as the source address for anything
transmitted out of that interface. This associate address has to be an IP address of
one of that router's interfaces. No pinging or ARPing is done on or through this
unnumbered interface and RIP decisions are based on the mask of the associated address.
One use of the unnumbered IP interface would be as a link to another manufacturer's
router (such as Cisco) which also has an unnumbered interface.
IP Multinetting
IP multinetting can be achieved by adding subnets to the same interface, normally this is
necessary because a LAN has been inherited and resubnetting is out of the question
due to all the IP address changes that would have to be changed. In an ideal world
multinetting should be avoided because the router interface has to work hard routing
between subnets on the same interface. Using RIP v2 and OSPF allows variable subnets
to be used and, if carefully planned, should allow the configuration of single subnets
with masks allowing enough hosts per subnet.
RIP Accept Policies
RIP Accept Policies can be configured to perform any of the following:
- Listen to RIP updates from certain routers
- Listen only for specific networks
- Alter the advertised metric of a network
- Assign a specific mask to be included with a network in the routing table
- Change the preference metric of a route
An example of a use for an Accept Policy would be to prevent users connected to a router B
from accessing networks off a router A, the Accept Policy would be configured on
router B. The result of this policy would also prevent users on router A from accessing
these networks on router B, this is because, although they could reach the router, they
could not get a response back. RIP Accept Policies are configured globally on the
router and if there are several policies configured a Rule Precedence parameter can
be set so that if a RIP update comes along with contents matching more than one policy,
the policy with the highest precedence takes priority.
RIP Announce Policies
RIP Announce Policies can be configured to do any of the following:
- Pass on certain routes out of particular interfaces
- Automatically create a default route
- Pass on routes learned from particular gateways
- Pass on routes learned from particular interfaces
An Announce Policy could be used, for instance, exclude a couple of subnetworks on router A
from the routing tables of all the other routers within the network, thereby isolating them
from the rest of the network.
When configuring the networks involved in either Accept or Announce Policies, the
match criteria could be 'exact' or 'range'. When range is selected it is worth
noting that the network address, say 153.17.12.0, becomes the lowest address in a
range of addresses limited by the mask. So in our example, if the mask is 255.255.0.0
then the upper limit of the range of addresses would be 153.17.255.255.
Another item worth noting is the Route Source parameter; the hexadecimal value entered
is the SUM of the codes used to represent each routing protocol used for contributing
the route.
RIP v2
The latest RFC for RIP v2 is RFC 1723
and replaces RFC 1388.
The following features are additional to those found within RIP v1:
- Authentication: A simple password of up to 128 characters can be used to
verify legal IP datagrams between RIP v2 configured router interfaces. This helps
in preventing hackers making use of erroneous updates. The Address Family Identifier
is set to a specific value and is read to check authenticity.
- Route Tags
- Subnet Mask: Each network entry within the RIP update has it's subnet mask
included so accurate routing information can be included within the routing tables
and a RIP v2 router advertises all known subnetworks out an interface not belonging to
that subnetwork. Different subnet masks can be configured throughout the whole
network without confusing the routers, address space no longer needs to be wasted as
subnet mask sizes can be adjusted to accommodate different network sizes.
- Multicasting: RIP v2 updates are multicast to 224.0.0.9 (class D) so RIP v2
routers share information independently from other routers.
The RIP v2 packet is illustrated below:
Extensions to RIP are detailed in RFC 2091
and include triggered RIP updates which were designed to allow RIP to run over dial circuits.
Only when a route changes would RIP send a routing update, thereby minimising the number of calls required
to maintain the routing table.
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