7.3 EXTERIOR GATEWAY PROTOCOLS
Exterior Gateway Protocols are utilized for inter-Autonomous System
routing to exchange reachability information for a set of networks
internal to a particular autonomous system to a neighboring
The area of inter-AS routing is a current topic of research inside
the Internet Engineering Task Force. The Exterior Gateway Protocol
(EGP) described in Section [Appendix F.1] has traditionally been the
inter-AS protocol of choice, but is now historical. The Border
Gateway Protocol (BGP) eliminates many of the restrictions and
limitations of EGP, and is therefore growing rapidly in popularity.
A router is not required to implement any inter-AS routing protocol.
However, if a router does implement EGP it also MUST IMPLEMENT BGP.
Although it was not designed as an exterior gateway protocol, RIP
(described in Section [7.2.4]) is sometimes used for inter-AS
7.3.2 BORDER GATEWAY PROTOCOL - BGP
The Border Gateway Protocol (BGP-4) is an inter-AS routing protocol
that exchanges network reachability information with other BGP
speakers. The information for a network includes the complete list
of ASs that traffic must transit to reach that network. This
information can then be used to insure loop-free paths. This
information is sufficient to construct a graph of AS connectivity
from which routing loops may be pruned and some policy decisions at
the AS level may be enforced.
BGP is defined by [ROUTE:4]. [ROUTE:5] specifies the proper usage of
BGP in the Internet, and provides some useful implementation hints
and guidelines. [ROUTE:12] and [ROUTE:13] provide additional useful
To comply with Section [8.3] of this memo, a router that implements
BGP is required to implement the BGP MIB [MGT:15].
To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that an AS advertises to its
neighbor ASs only those routes that it itself uses. This rule
reflects the hop-by-hop routing paradigm generally used throughout
the current Internet. Note that some policies cannot be supported by
the hop-by-hop routing paradigm and thus require techniques such as
source routing to enforce. For example, BGP does not enable one AS
to send traffic to a neighbor AS intending that traffic take a
different route from that taken by traffic originating in the
neighbor AS. On the other hand, BGP can support any policy
conforming to the hop-by-hop routing paradigm.
Implementors of BGP are strongly encouraged to follow the
recommendations outlined in Section 6 of [ROUTE:5].
126.96.36.199 Protocol Walk-through
While BGP provides support for quite complex routing policies (as an
example see Section 4.2 in [ROUTE:5]), it is not required for all BGP
implementors to support such policies. At a minimum, however, a BGP
(1) SHOULD allow an AS to control announcements of the BGP learned
routes to adjacent AS's. Implementations SHOULD support such
control with at least the granularity of a single network.
Implementations SHOULD also support such control with the
granularity of an autonomous system, where the autonomous system
may be either the autonomous system that originated the route,
or the autonomous system that advertised the route to the local
system (adjacent autonomous system).
(2) SHOULD allow an AS to prefer a particular path to a destination
(when more than one path is available). Such function SHOULD be
implemented by allowing system administrator to assign weights
to Autonomous Systems, and making route selection process to
select a route with the lowest weight (where weight of a route
is defined as a sum of weights of all AS's in the AS_PATH path
attribute associated with that route).
(3) SHOULD allow an AS to ignore routes with certain AS's in the
AS_PATH path attribute. Such function can be implemented by
using technique outlined in (2), and by assigning infinity as
weights for such AS's. The route selection process must ignore
routes that have weight equal to infinity.
7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL
It is possible to exchange routing information between two autonomous
systems or routing domains without using a standard exterior routing
protocol between two separate, standard interior routing protocols.
The most common way of doing this is to run both interior protocols
independently in one of the border routers with an exchange of route
information between the two processes.
As with the exchange of information from an EGP to an IGP, without
appropriate controls these exchanges of routing information between
two IGPs in a single router are subject to creation of routing loops.
7.4 STATIC ROUTING
Static routing provides a means of explicitly defining the next hop
from a router for a particular destination. A router SHOULD provide
a means for defining a static route to a destination, where the
destination is defined by a network prefix. The mechanism SHOULD
also allow for a metric to be specified for each static route.
A router that supports a dynamic routing protocol MUST allow static
routes to be defined with any metric valid for the routing protocol
used. The router MUST provide the ability for the user to specify a
list of static routes that may or may not be propagated through the
routing protocol. In addition, a router SHOULD support the following
additional information if it supports a routing protocol that could
make use of the information. They are:
o Subnet Mask, or
o Prefix Length, or
o A metric specific to a given routing protocol that can import the
We intend that one needs to support only the things useful to the
given routing protocol. The need for TOS should not require the
vendor to implement the other parts if they are not used.
Whether a router prefers a static route over a dynamic route (or
vice versa) or whether the associated metrics are used to choose
between conflicting static and dynamic routes SHOULD be
configurable for each static route.
A router MUST allow a metric to be assigned to a static route for
each routing domain that it supports. Each such metric MUST be
explicitly assigned to a specific routing domain. For example:
route 10.0.0.0/8 via 192.0.2.3 rip metric 3
route 10.21.0.0/16 via 192.0.2.4 ospf inter-area metric 27
route 10.22.0.0/16 via 192.0.2.5 egp 123 metric 99
It has been suggested that, ideally, static routes should have
preference values rather than metrics (since metrics can only be
compared with metrics of other routes in the same routing domain,
the metric of a static route could only be compared with metrics
of other static routes). This is contrary to some current
implementations, where static routes really do have metrics, and
those metrics are used to determine whether a particular dynamic
route overrides the static route to the same destination. Thus,
this document uses the term metric rather than preference.
This technique essentially makes the static route into a RIP
route, or an OSPF route (or whatever, depending on the domain of
the metric). Thus, the route lookup algorithm of that domain
applies. However, this is NOT route leaking, in that coercing a
static route into a dynamic routing domain does not authorize the
router to redistribute the route into the dynamic routing domain.
For static routes not put into a specific routing domain, the
route lookup algorithm is:
(1) Basic match
(2) Longest match
(3) Weak TOS (if TOS supported)
(4) Best metric (where metric are implementation-defined)
The last step may not be necessary, but it's useful in the case
where you want to have a primary static route over one interface
and a secondary static route over an alternate interface, with
failover to the alternate path if the interface for the primary
7.5 FILTERING OF ROUTING INFORMATION
Each router within a network makes forwarding decisions based upon
information contained within its forwarding database. In a simple
network the contents of the database may be configured statically.
As the network grows more complex, the need for dynamic updating of
the forwarding database becomes critical to the efficient operation
of the network.
If the data flow through a network is to be as efficient as possible,
it is necessary to provide a mechanism for controlling the
propagation of the information a router uses to build its forwarding
database. This control takes the form of choosing which sources of
routing information should be trusted and selecting which pieces of
the information to believe. The resulting forwarding database is a
filtered version of the available routing information.
In addition to efficiency, controlling the propagation of routing
information can reduce instability by preventing the spread of
incorrect or bad routing information.
In some cases local policy may require that complete routing
information not be widely propagated.
These filtering requirements apply only to non-SPF-based protocols
(and therefore not at all to routers which don't implement any
distance vector protocols).
7.5.1 Route Validation
A router SHOULD log as an error any routing update advertising a
route that violates the specifications of this memo, unless the
routing protocol from which the update was received uses those values
to encode special routes (such as default routes).
7.5.2 Basic Route Filtering
Filtering of routing information allows control of paths used by a
router to forward packets it receives. A router should be selective
in which sources of routing information it listens to and what routes
it believes. Therefore, a router MUST provide the ability to
o On which logical interfaces routing information will be accepted
and which routes will be accepted from each logical interface.
o Whether all routes or only a default route is advertised on a
Some routing protocols do not recognize logical interfaces as a
source of routing information. In such cases the router MUST provide
the ability to specify
o from which other routers routing information will be accepted.
For example, assume a router connecting one or more leaf networks to
the main portion or backbone of a larger network. Since each of the
leaf networks has only one path in and out, the router can simply
send a default route to them. It advertises the leaf networks to the
7.5.3 Advanced Route Filtering
As the topology of a network grows more complex, the need for more
complex route filtering arises. Therefore, a router SHOULD provide
the ability to specify independently for each routing protocol:
o Which logical interfaces or routers routing information (routes)
will be accepted from and which routes will be believed from each
other router or logical interface,
o Which routes will be sent via which logical interface(s), and
o Which routers routing information will be sent to, if this is
supported by the routing protocol in use.
In many situations it is desirable to assign a reliability ordering
to routing information received from another router instead of the
simple believe or don't believe choice listed in the first bullet
above. A router MAY provide the ability to specify:
o A reliability or preference to be assigned to each route received.
A route with higher reliability will be chosen over one with lower
reliability regardless of the routing metric associated with each
If a router supports assignment of preferences, the router MUST NOT
propagate any routes it does not prefer as first party information.
If the routing protocol being used to propagate the routes does not
support distinguishing between first and third party information, the
router MUST NOT propagate any routes it does not prefer.
For example, assume a router receives a route to network C from
router R and a route to the same network from router S. If router
R is considered more reliable than router S traffic destined for
network C will be forwarded to router R regardless of the route
received from router S.
Routing information for routes which the router does not use (router
S in the above example) MUST NOT be passed to any other router.
7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE
Routers MUST be able to exchange routing information between separate
IP interior routing protocols, if independent IP routing processes
can run in the same router. Routers MUST provide some mechanism for
avoiding routing loops when routers are configured for bi-directional
exchange of routing information between two separate interior routing
processes. Routers MUST provide some priority mechanism for choosing
routes from independent routing processes. Routers SHOULD provide
administrative control of IGP-IGP exchange when used across
Routers SHOULD provide some mechanism for translating or transforming
metrics on a per network basis. Routers (or routing protocols) MAY
allow for global preference of exterior routes imported into an IGP.
Different IGPs use different metrics, requiring some translation
technique when introducing information from one protocol into
another protocol with a different form of metric. Some IGPs can
run multiple instances within the same router or set of routers.
In this case metric information can be preserved exactly or
There are at least two techniques for translation between
different routing processes. The static (or reachability)
approach uses the existence of a route advertisement in one IGP to
generate a route advertisement in the other IGP with a given
metric. The translation or tabular approach uses the metric in
one IGP to create a metric in the other IGP through use of either
a function (such as adding a constant) or a table lookup.
Bi-directional exchange of routing information is dangerous
without control mechanisms to limit feedback. This is the same
problem that distance vector routing protocols must address with
the split horizon technique and that EGP addresses with the
third-party rule. Routing loops can be avoided explicitly through
use of tables or lists of permitted/denied routes or implicitly
through use of a split horizon rule, a no-third-party rule, or a
route tagging mechanism. Vendors are encouraged to use implicit
techniques where possible to make administration easier for
8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS
Note that this chapter supersedes any requirements stated under
"REMOTE MANAGEMENT" in [INTRO:3].
8.1 The Simple Network Management Protocol - SNMP
8.1.1 SNMP Protocol Elements
Routers MUST be manageable by SNMP [MGT:3]. The SNMP MUST operate
using UDP/IP as its transport and network protocols. Others MAY be
supported (e.g., see [MGT:25, MGT:26, MGT:27, and MGT:28]). SNMP
management operations MUST operate as if the SNMP was implemented on
the router itself. Specifically, management operations MUST be
effected by sending SNMP management requests to any of the IP
addresses assigned to any of the router's interfaces. The actual
management operation may be performed either by the router or by a
proxy for the router.
This wording is intended to allow management either by proxy,
where the proxy device responds to SNMP packets that have one of
the router's IP addresses in the packets destination address
field, or the SNMP is implemented directly in the router itself
and receives packets and responds to them in the proper manner.
It is important that management operations can be sent to one of
the router's IP Addresses. In diagnosing network problems the
only thing identifying the router that is available may be one of
the router's IP address; obtained perhaps by looking through
another router's routing table.
All SNMP operations (get, get-next, get-response, set, and trap) MUST
Routers MUST provide a mechanism for rate-limiting the generation of
SNMP trap messages. Routers MAY provide this mechanism through the
algorithms for asynchronous alert management described in [MGT:5].
Although there is general agreement about the need to rate-limit
traps, there is not yet consensus on how this is best achieved.
The reference cited is considered experimental.
8.2 Community Table
For the purposes of this specification, we assume that there is an
abstract `community table' in the router. This table contains
several entries, each entry for a specific community and containing
the parameters necessary to completely define the attributes of that
community. The actual implementation method of the abstract
community table is, of course, implementation specific.
A router's community table MUST allow for at least one entry and
SHOULD allow for at least two entries.
A community table with zero capacity is useless. It means that
the router will not recognize any communities and, therefore, all
SNMP operations will be rejected.
Therefore, one entry is the minimal useful size of the table.
Having two entries allows one entry to be limited to read-only
access while the other would have write capabilities.
Routers MUST allow the user to manually (i.e., without using SNMP)
examine, add, delete and change entries in the SNMP community table.
The user MUST be able to set the community name or construct a MIB
view. The user MUST be able to configure communities as read-only
(i.e., they do not allow SETs) or read-write (i.e., they do allow
The user MUST be able to define at least one IP address to which
notifications are sent for each community or MIB view, if traps are
used. These addresses SHOULD be definable on a community or MIB view
basis. It SHOULD be possible to enable or disable notifications on a
community or MIB view basis.
A router SHOULD provide the ability to specify a list of valid
network managers for any particular community. If enabled, a router
MUST validate the source address of the SNMP datagram against the
list and MUST discard the datagram if its address does not appear.
If the datagram is discarded the router MUST take all actions
appropriate to an SNMP authentication failure.
This is a rather limited authentication system, but coupled with
various forms of packet filtering may provide some small measure
of increased security.
The community table MUST be saved in non-volatile storage.
The initial state of the community table SHOULD contain one entry,
with the community name string public and read-only access. The
default state of this entry MUST NOT send traps. If it is
implemented, then this entry MUST remain in the community table until
the administrator changes it or deletes it.
By default, traps are not sent to this community. Trap PDUs are
sent to unicast IP addresses. This address must be configured
into the router in some manner. Before the configuration occurs,
there is no such address, so to whom should the trap be sent?
Therefore trap sending to the public community defaults to be
disabled. This can, of course, be changed by an administrative
operation once the router is operational.
8.3 Standard MIBS
All MIBS relevant to a router's configuration are to be implemented.
o The System, Interface, IP, ICMP, and UDP groups of MIB-II [MGT:2]
MUST be implemented.
o The Interface Extensions MIB [MGT:18] MUST be implemented.
o The IP Forwarding Table MIB [MGT:20] MUST be implemented.
o If the router implements TCP (e.g., for Telnet) then the TCP group
of MIB-II [MGT:2] MUST be implemented.
o If the router implements EGP then the EGP group of MIB-II [MGT:2]
MUST be implemented.
o If the router supports OSPF then the OSPF MIB [MGT:14] MUST be
o If the router supports BGP then the BGP MIB [MGT:15] MUST be
o If the router has Ethernet, 802.3, or StarLan interfaces then the
Ethernet-Like MIB [MGT:6] MUST be implemented.
o If the router has 802.4 interfaces then the 802.4 MIB [MGT:7] MUST
o If the router has 802.5 interfaces then the 802.5 MIB [MGT:8] MUST
o If the router has FDDI interfaces that implement ANSI SMT 7.3 then
the FDDI MIB [MGT:9] MUST be implemented.
o If the router has FDDI interfaces that implement ANSI SMT 6.2 then
the FDDI MIB [MGT:29] MUST be implemented.
o If the router has interfaces that use V.24 signalling, such as RS-
232, V.10, V.11, V.35, V.36, or RS-422/423/449, then the RS-232
[MGT:10] MIB MUST be implemented.
o If the router has T1/DS1 interfaces then the T1/DS1 MIB [MGT:16]
MUST be implemented.
o If the router has T3/DS3 interfaces then the T3/DS3 MIB [MGT:17]
MUST be implemented.
o If the router has SMDS interfaces then the SMDS Interface Protocol
MIB [MGT:19] MUST be implemented.
o If the router supports PPP over any of its interfaces then the PPP
MIBs [MGT:11], [MGT:12], and [MGT:13] MUST be implemented.
o If the router supports RIP Version 2 then the RIP Version 2 MIB
[MGT:21] MUST be implemented.
o If the router supports X.25 over any of its interfaces then the
X.25 MIBs [MGT:22, MGT:23 and MGT:24] MUST be implemented.
8.4 Vendor Specific MIBS
The Internet Standard and Experimental MIBs do not cover the entire
range of statistical, state, configuration and control information
that may be available in a network element. This information is,
nevertheless, extremely useful. Vendors of routers (and other
network devices) generally have developed MIB extensions that cover
this information. These MIB extensions are called Vendor Specific
The Vendor Specific MIB for the router MUST provide access to all
statistical, state, configuration, and control information that is
not available through the Standard and Experimental MIBs that have
been implemented. This information MUST be available for both
monitoring and control operations.
The intent of this requirement is to provide the ability to do
anything on the router through SNMP that can be done through a
console, and vice versa. A certain minimal amount of
configuration is necessary before SNMP can operate (e.g., the
router must have an IP address). This initial configuration can
not be done through SNMP. However, once the initial configuration
is done, full capabilities ought to be available through network
The vendor SHOULD make available the specifications for all Vendor
Specific MIB variables. These specifications MUST conform to the SMI
[MGT:1] and the descriptions MUST be in the form specified in
Making the Vendor Specific MIB available to the user is necessary.
Without this information the users would not be able to configure
their network management systems to be able to access the Vendor
Specific parameters. These parameters would then be useless.
ne 2 The format of the MIB specification is also specified.
Parsers that read MIB specifications and generate the needed
tables for the network management station are available. These
parsers generally understand only the standard MIB specification
8.5 Saving Changes
Parameters altered by SNMP MAY be saved to non-volatile storage.
Reasons why this requirement is a MAY:
o The exact physical nature of non-volatile storage is not
specified in this document. Hence, parameters may be saved in
NVRAM/EEPROM, local floppy or hard disk, or in some TFTP file
server or BOOTP server, etc. Suppose that this information is
in a file that is retrieved through TFTP. In that case, a
change made to a configuration parameter on the router would
need to be propagated back to the file server holding the
configuration file. Alternatively, the SNMP operation would
need to be directed to the file server, and then the change
somehow propagated to the router. The answer to this problem
does not seem obvious.
This also places more requirements on the host holding the
configuration information than just having an available TFTP
server, so much more that its probably unsafe for a vendor to
assume that any potential customer will have a suitable host
o The timing of committing changed parameters to non-volatile
storage is still an issue for debate. Some prefer to commit
all changes immediately. Others prefer to commit changes to
non-volatile storage only upon an explicit command.
9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS
For all additional application protocols that a router implements,
the router MUST be compliant and SHOULD be unconditionally compliant
with the relevant requirements of [INTRO:3].
The Bootstrap Protocol (BOOTP) is a UDP/IP-based protocol that allows
a booting host to configure itself dynamically and without user
supervision. BOOTP provides a means to notify a host of its assigned
IP address, the IP address of a boot server host, and the name of a
file to be loaded into memory and executed ([APPL:1]). Other
configuration information such as the local prefix length or subnet
mask, the local time offset, the addresses of default routers, and
the addresses of various Internet servers can also be communicated to
a host using BOOTP ([APPL:2]).
9.1.2 BOOTP Relay Agents
In many cases, BOOTP clients and their associated BOOTP server(s) do
not reside on the same IP (sub)network. In such cases, a third-party
agent is required to transfer BOOTP messages between clients and
servers. Such an agent was originally referred to as a BOOTP
forwarding agent. However, to avoid confusion with the IP forwarding
function of a router, the name BOOTP relay agent has been adopted
A BOOTP relay agent performs a task that is distinct from a
router's normal IP forwarding function. While a router normally
switches IP datagrams between networks more-or-less transparently,
a BOOTP relay agent may more properly be thought to receive BOOTP
messages as a final destination and then generate new BOOTP
messages as a result. One should resist the notion of simply
forwarding a BOOTP message straight through like a regular packet.
This relay-agent functionality is most conveniently located in the
routers that interconnect the clients and servers (although it may
alternatively be located in a host that is directly connected to the
A router MAY provide BOOTP relay-agent capability. If it does, it
MUST conform to the specifications in [APPL:3].
Section [5.2.3] discussed the circumstances under which a packet is
delivered locally (to the router). All locally delivered UDP
messages whose UDP destination port number is BOOTPS (67) are
considered for special processing by the router's logical BOOTP relay
Sections [188.8.131.52] and [5.3.7] discussed invalid IP source
addresses. According to these rules, a router must not forward any
received datagram whose IP source address is 0.0.0.0. However,
routers that support a BOOTP relay agent MUST accept for local
delivery to the relay agent BOOTREQUEST messages whose IP source
address is 0.0.0.0.
10. OPERATIONS AND MAINTENANCE
This chapter supersedes any requirements of [INTRO:3] relating to
"Extensions to the IP Module."
Facilities to support operation and maintenance (O&M) activities form
an essential part of any router implementation. Although these
functions do not seem to relate directly to interoperability, they
are essential to the network manager who must make the router
interoperate and must track down problems when it doesn't. This
chapter also includes some discussion of router initialization and of
facilities to assist network managers in securing and accounting for
The following kinds of activities are included under router O&M:
o Diagnosing hardware problems in the router's processor, in its
network interfaces, or in its connected networks, modems, or
o Installing new hardware
o Installing new software.
o Restarting or rebooting the router after a crash.
o Configuring (or reconfiguring) the router.
o Detecting and diagnosing Internet problems such as congestion,
routing loops, bad IP addresses, black holes, packet avalanches,
and misbehaved hosts.
o Changing network topology, either temporarily (e.g., to bypass a
communication line problem) or permanently.
o Monitoring the status and performance of the routers and the
o Collecting traffic statistics for use in (Inter-)network planning.
o Coordinating the above activities with appropriate vendors and
Routers and their connected communication lines are often operated as
a system by a centralized O&M organization. This organization may
maintain a (Inter-)network operation center, or NOC, to carry out its
O&M functions. It is essential that routers support remote control
and monitoring from such a NOC through an Internet path, since
routers might not be connected to the same network as their NOC.
Since a network failure may temporarily preclude network access, many
NOCs insist that routers be accessible for network management through
an alternative means, often dial-up modems attached to console ports
on the routers.
Since an IP packet traversing an internet will often use routers
under the control of more than one NOC, Internet problem diagnosis
will often involve cooperation of personnel of more than one NOC. In
some cases, the same router may need to be monitored by more than one
NOC, but only if necessary, because excessive monitoring could impact
a router's performance.
The tools available for monitoring at a NOC may cover a wide range of
sophistication. Current implementations include multi-window,
dynamic displays of the entire router system. The use of AI
techniques for automatic problem diagnosis is proposed for the
Router O&M facilities discussed here are only a part of the large and
difficult problem of Internet management. These problems encompass
not only multiple management organizations, but also multiple
protocol layers. For example, at the current stage of evolution of
the Internet architecture, there is a strong coupling between host
TCP implementations and eventual IP-level congestion in the router
system [OPER:1]. Therefore, diagnosis of congestion problems will
sometimes require the monitoring of TCP statistics in hosts. There
are currently a number of R&D efforts in progress in the area of
Internet management and more specifically router O&M. These R&D
efforts have already produced standards for router O&M. This is also
an area in which vendor creativity can make a significant
10.2 Router Initialization
10.2.1 Minimum Router Configuration
There exists a minimum set of conditions that must be satisfied
before a router may forward packets. A router MUST NOT enable
forwarding on any physical interface unless either:
(1) The router knows the IP address and associated subnet mask or
network prefix length of at least one logical interface
associated with that physical interface, or
(2) The router knows that the interface is an unnumbered interface
and knows its router-id.
These parameters MUST be explicitly configured:
o A router MUST NOT use factory-configured default values for its IP
addresses, prefix lengths, or router-id, and
o A router MUST NOT assume that an unconfigured interface is an
There have been instances in which routers have been shipped with
vendor-installed default addresses for interfaces. In a few
cases, this has resulted in routers advertising these default
addresses into active networks.
10.2.2 Address and Prefix Initialization
A router MUST allow its IP addresses and their address masks or
prefix lengths to be statically configured and saved in non-volatile
A router MAY obtain its IP addresses and their corresponding address
masks dynamically as a side effect of the system initialization
process (see Section 10.2.3]);
If the dynamic method is provided, the choice of method to be used in
a particular router MUST be configurable.
As was described in Section [184.108.40.206], IP addresses are not
permitted to have the value 0 or -1 in the <Host-number> or
<Network-prefix> fields. Therefore, a router SHOULD NOT allow an IP
address or address mask to be set to a value that would make any of
the these fields above have the value zero or -1.
It is possible using arbitrary address masks to create situations
in which routing is ambiguous (i.e., two routes with different but
equally specific subnet masks match a particular destination
address). This is one of the strongest arguments for the use of
network prefixes, and the reason the use of discontiguous subnet
masks is not permitted.
A router SHOULD make the following checks on any address mask it
o The mask is neither all ones nor all zeroes (the prefix length is
neither zero nor 32).
o The bits which correspond to the network prefix part of the address
are all set to 1.
o The bits that correspond to the network prefix are contiguous.
The masks associated with routes are also sometimes called subnet
masks, this test should not be applied to them.
10.2.3 Network Booting using BOOTP and TFTP
There has been much discussion of how routers can and should be
booted from the network. These discussions have revolved around
BOOTP and TFTP. Currently, there are routers that boot with TFTP
from the network. There is no reason that BOOTP could not be used
for locating the server that the boot image should be loaded from.
BOOTP is a protocol used to boot end systems, and requires some
stretching to accommodate its use with routers. If a router is using
BOOTP to locate the current boot host, it should send a BOOTP Request
with its hardware address for its first interface, or, if it has been
previously configured otherwise, with either another interface's
hardware address, or another number to put in the hardware address
field of the BOOTP packet. This is to allow routers without hardware
addresses (like synchronous line only routers) to use BOOTP for
bootload discovery. TFTP can then be used to retrieve the image
found in the BOOTP Reply. If there are no configured interfaces or
numbers to use, a router MAY cycle through the interface hardware
addresses it has until a match is found by the BOOTP server.
A router SHOULD IMPLEMENT the ability to store parameters learned
through BOOTP into local non-volatile storage. A router MAY
implement the ability to store a system image loaded over the network
into local stable storage.
A router MAY have a facility to allow a remote user to request that
the router get a new boot image. Differentiation should be made
between getting the new boot image from one of three locations: the
one included in the request, from the last boot image server, and
using BOOTP to locate a server.
10.3 Operation and Maintenance
There is a range of possible models for performing O&M functions on a
router. At one extreme is the local-only model, under which the O&M
functions can only be executed locally (e.g., from a terminal plugged
into the router machine). At the other extreme, the fully remote
model allows only an absolute minimum of functions to be performed
locally (e.g., forcing a boot), with most O&M being done remotely
from the NOC. There are intermediate models, such as one in which
NOC personnel can log into the router as a host, using the Telnet
protocol, to perform functions that can also be invoked locally. The
local-only model may be adequate in a few router installations, but
remote operation from a NOC is normally required, and therefore
remote O&M provisions are required for most routers.
Remote O&M functions may be exercised through a control agent
(program). In the direct approach, the router would support remote
O&M functions directly from the NOC using standard Internet protocols
(e.g., SNMP, UDP or TCP); in the indirect approach, the control agent
would support these protocols and control the router itself using
proprietary protocols. The direct approach is preferred, although
either approach is acceptable. The use of specialized host hardware
and/or software requiring significant additional investment is
discouraged; nevertheless, some vendors may elect to provide the
control agent as an integrated part of the network in which the
routers are a part. If this is the case, it is required that a means
be available to operate the control agent from a remote site using
Internet protocols and paths and with equivalent functionality with
respect to a local agent terminal.
It is desirable that a control agent and any other NOC software tools
that a vendor provides operate as user programs in a standard
operating system. The use of the standard Internet protocols UDP and
TCP for communicating with the routers should facilitate this.
Remote router monitoring and (especially) remote router control
present important access control problems that must be addressed.
Care must also be taken to ensure control of the use of router
resources for these functions. It is not desirable to let router
monitoring take more than some limited fraction of the router CPU
time, for example. On the other hand, O&M functions must receive
priority so they can be exercised when the router is congested, since
often that is when O&M is most needed.
10.3.2 Out Of Band Access
Routers MUST support Out-Of-Band (OOB) access. OOB access SHOULD
provide the same functionality as in-band access. This access SHOULD
implement access controls, to prevent unauthorized access.
This Out-Of-Band access will allow the NOC a way to access
isolated routers during times when network access is not
Out-Of-Band access is an important management tool for the network
administrator. It allows the access of equipment independent of
the network connections. There are many ways to achieve this
access. Whichever one is used it is important that the access is
independent of the network connections. An example of Out-Of-Band
access would be a serial port connected to a modem that provides
dial up access to the router.
It is important that the OOB access provides the same
functionality as in-band access. In-band access, or accessing
equipment through the existing network connection, is limiting,
because most of the time, administrators need to reach equipment
to figure out why it is unreachable. In band access is still very
important for configuring a router, and for troubleshooting more
10.3.3 Router O&M Functions
10.3.3.1 Maintenance - Hardware Diagnosis
Each router SHOULD operate as a stand-alone device for the purposes
of local hardware maintenance. Means SHOULD be available to run
diagnostic programs at the router site using only on-site tools. A
router SHOULD be able to run diagnostics in case of a fault. For
suggested hardware and software diagnostics see Section [10.3.3].
10.3.3.2 Control - Dumping and Rebooting
A router MUST include both in-band and out-of-band mechanisms to
allow the network manager to reload, stop, and restart the router. A
router SHOULD also contain a mechanism (such as a watchdog timer)
which will reboot the router automatically if it hangs due to a
software or hardware fault.
A router SHOULD IMPLEMENT a mechanism for dumping the contents of a
router's memory (and/or other state useful for vendor debugging after
a crash), and either saving them on a stable storage device local to
the router or saving them on another host via an up-line dump
mechanism such as TFTP (see [OPER:2], [INTRO:3]).
10.3.3.3 Control - Configuring the Router
Every router has configuration parameters that may need to be set.
It SHOULD be possible to update the parameters without rebooting the
router; at worst, a restart MAY be required. There may be cases when
it is not possible to change parameters without rebooting the router
(for instance, changing the IP address of an interface). In these
cases, care should be taken to minimize disruption to the router and
the surrounding network.
There SHOULD be a way to configure the router over the network either
manually or automatically. A router SHOULD be able to upload or
download its parameters from a host or another router. A means
SHOULD be provided, either as an application program or a router
function, to convert between the parameter format and a human-
editable format. A router SHOULD have some sort of stable storage
for its configuration. A router SHOULD NOT believe protocols such as
RARP, ICMP Address Mask Reply, and MAY not believe BOOTP.
It is necessary to note here that in the future RARP, ICMP Address
Mask Reply, BOOTP and other mechanisms may be needed to allow a
router to auto-configure. Although routers may in the future be
able to configure automatically, the intent here is to discourage
this practice in a production environment until auto-configuration
has been tested more thoroughly. The intent is NOT to discourage
auto-configuration all together. In cases where a router is
expected to get its configuration automatically it may be wise to
allow the router to believe these things as it comes up and then
ignore them after it has gotten its configuration.
10.3.3.4 Net Booting of System Software
A router SHOULD keep its system image in local non-volatile
storage such as PROM, NVRAM, or disk. It MAY also be able to load
its system software over the network from a host or another
A router that can keep its system image in local non-volatile
storage MAY be configurable to boot its system image over the
network. A router that offers this option SHOULD be configurable
to boot the system image in its non-volatile local storage if it
is unable to boot its system image over the network.
It is important that the router be able to come up and run on its
own. NVRAM may be a particular solution for routers used in large
networks, since changing PROMs can be quite time consuming for a
network manager responsible for numerous or geographically
dispersed routers. It is important to be able to netboot the
system image because there should be an easy way for a router to
get a bug fix or new feature more quickly than getting PROMs
installed. Also if the router has NVRAM instead of PROMs, it will
netboot the image and then put it in NVRAM.
Routers SHOULD perform some basic consistency check on any image
loaded, to detect and perhaps prevent incorrect images.
A router MAY also be able to distinguish between different
configurations based on which software it is running. If
configuration commands change from one software version to another,
it would be helpful if the router could use the configuration that
was compatible with the software.
10.3.3.5 Detecting and responding to misconfiguration
There MUST be mechanisms for detecting and responding to
misconfigurations. If a command is executed incorrectly, the router
SHOULD give an error message. The router SHOULD NOT accept a poorly
formed command as if it were correct.
There are cases where it is not possible to detect errors: the
command is correctly formed, but incorrect with respect to the
network. This may be detected by the router, but may not be
Another form of misconfiguration is misconfiguration of the network
to which the router is attached. A router MAY detect
misconfigurations in the network. The router MAY log these findings
to a file, either on the router or a host, so that the network
manager will see that there are possible problems on the network.
Examples of such misconfigurations might be another router with
the same address as the one in question or a router with the wrong
address mask. If a router detects such problems it is probably
not the best idea for the router to try to fix the situation.
That could cause more harm than good.
10.3.3.6 Minimizing Disruption
Changing the configuration of a router SHOULD have minimal affect on
the network. Routing tables SHOULD NOT be unnecessarily flushed when
a simple change is made to the router. If a router is running
several routing protocols, stopping one routing protocol SHOULD NOT
disrupt other routing protocols, except in the case where one network
is learned by more than one routing protocol.
It is the goal of a network manager to run a network so that users
of the network get the best connectivity possible. Reloading a
router for simple configuration changes can cause disruptions in
routing and ultimately cause disruptions to the network and its
users. If routing tables are unnecessarily flushed, for instance,
the default route will be lost as well as specific routes to sites
within the network. This sort of disruption will cause
significant downtime for the users. It is the purpose of this
section to point out that whenever possible, these disruptions
should be avoided.
10.3.3.7 Control - Troubleshooting Problems
(1) A router MUST provide in-band network access, but (except as
required by Section [8.2]) for security considerations this
access SHOULD be disabled by default. Vendors MUST document
the default state of any in-band access. This access SHOULD
implement access controls, to prevent unauthorized access.
In-band access primarily refers to access through the normal
network protocols that may or may not affect the permanent
operational state of the router. This includes, but is not
limited to Telnet/RLOGIN console access and SNMP operations.
This was a point of contention between the operational out of the
box and secure out of The box contingents. Any automagic access
to the router may introduce insecurities, but it may be more
important for the customer to have a router that is accessible
over the network as soon as it is plugged in. At least one vendor
supplies routers without any external console access and depends
on being able to access the router through the network to complete
It is the vendors call whether in-band access is enabled by
default; but it is also the vendor's responsibility to make its
customers aware of possible insecurities.
(2) A router MUST provide the ability to initiate an ICMP echo.
The following options SHOULD be implemented:
o Choice of data patterns
o Choice of packet size
o Record route
and the following additional options MAY be implemented:
o Loose source route
o Strict source route
(3) A router SHOULD provide the ability to initiate a traceroute.
If traceroute is provided, then the 3rd party traceroute
SHOULD be implemented.
Each of the above three facilities (if implemented) SHOULD have
access restrictions placed on it to prevent its abuse by unauthorized
10.4 Security Considerations
10.4.1 Auditing and Audit Trails
Auditing and billing are the bane of the network operator, but are
the two features most requested by those in charge of network
security and those who are responsible for paying the bills. In the
context of security, auditing is desirable if it helps you keep your
network working and protects your resources from abuse, without
costing you more than those resources are worth.
(1) Configuration Changes
Router SHOULD provide a method for auditing a configuration
change of a router, even if it's something as simple as
recording the operator's initials and time of change.
Configuration change logging (who made a configuration change,
what was changed, and when) is very useful, especially when
traffic is suddenly routed through Alaska on its way across town.
So is the ability to revert to a previous configuration.
(2) Packet Accounting
Vendors should strongly consider providing a system for
tracking traffic levels between pairs of hosts or networks.
A mechanism for limiting the collection of this information
to specific pairs of hosts or networks is also strongly
A host traffic matrix as described above can give the network
operator a glimpse of traffic trends not apparent from other
statistics. It can also identify hosts or networks that are
probing the structure of the attached networks - e.g., a single
external host that tries to send packets to every IP address in
the network address range for a connected network.
(3) Security Auditing
Routers MUST provide a method for auditing security related
failures or violations to include:
o Authorization Failures: bad passwords, invalid SNMP
communities, invalid authorization tokens,
o Violations of Policy Controls: Prohibited Source Routes,
Filtered Destinations, and
o Authorization Approvals: good passwords - Telnet in-band
access, console access.
Routers MUST provide a method of limiting or disabling such
auditing but auditing SHOULD be on by default. Possible
methods for auditing include listing violations to a console
if present, logging or counting them internally, or logging
them to a remote security server through the SNMP trap
mechanism or the Unix logging mechanism as appropriate. A
router MUST implement at least one of these reporting
mechanisms - it MAY implement more than one.
10.4.2 Configuration Control
A vendor has a responsibility to use good configuration control
practices in the creation of the software/firmware loads for their
routers. In particular, if a vendor makes updates and loads
available for retrieval over the Internet, the vendor should also
provide a way for the customer to confirm the load is a valid one,
perhaps by the verification of a checksum over the load.
Many vendors currently provide short notice updates of their
software products through the Internet. This a good trend and
should be encouraged, but provides a point of vulnerability in the
configuration control process.
If a vendor provides the ability for the customer to change the
configuration parameters of a router remotely, for example through a
Telnet session, the ability to do so SHOULD be configurable and
SHOULD default to off. The router SHOULD require valid
authentication before permitting remote reconfiguration. This
authentication procedure SHOULD NOT transmit the authentication
secret over the network. For example, if telnet is implemented, the
vendor SHOULD IMPLEMENT Kerberos, S-Key, or a similar authentication
Allowing your properly identified network operator to twiddle with
your routers is necessary; allowing anyone else to do so is
A router MUST NOT have undocumented back door access and master
passwords. A vendor MUST ensure any such access added for purposes
of debugging or product development are deleted before the product is
distributed to its customers.
A vendor has a responsibility to its customers to ensure they are
aware of the vulnerabilities present in its code by intention -
e.g., in-band access. Trap doors, back doors and master passwords
intentional or unintentional can turn a relatively secure router
into a major problem on an operational network. The supposed
operational benefits are not matched by the potential problems.