QN09 - Configuring the BGP File

QN09 - Configuring the BGP File
Quick Note 9
Configuring the BGP file (bgp.conf)
UK Support
June 2011
This quick note explains the syntax used in the BGP configuration file in more detail. There are also details
of debug commands for troubleshooting.
1.1 Version
Updated and rebranded
Updated for new GUI
There are two parts to configuring BGP on the TransPort router the first section deals with the configuration
of the BGP daemon via a configuration file. The second section deals with the BGP configuration settings on
the web interface.
2.1 Configuration file
The configuration of BGP on a TransPort router is held in a separate text file, usually named bgp.conf. This
file will need to be manually created using a text editor then placed on to the TransPort router.
Below is an example bgp.conf file with two areas, please note that an interface can only exist in one area. A
‘#’ symbol is used to denote a comment, as can be seen from the example there are three distinct sections
to the config file.
# global configuration
AS 65001
holdtime 180
holdtime min 3
log updates
neighbor {
depend on
set nexthop self
neighbor {
depend on
set nexthop self
neighbor {
depend on
#BGP route Filters
#deny to any prefix prefixlen >= 12
# filter out prefixes longer than 24 or shorter than 8 bits
#deny from any
#allow from any prefixlen 8 - 24
# do not accept a default route
#allow from any prefix
# filter bogus
#deny from any
#deny from any
#deny from any
#deny from any
#deny from any
#deny from any
#deny from any
prefix prefixlen >= 8
prefix prefixlen >= 12
prefix prefixlen >= 16
prefix prefixlen >= 16
prefix prefixlen >= 24
prefix prefixlen >= 4
prefix prefixlen >= 4
The sections are:
Macros – These are user defined variables that set values of parameters used in the rest
of the configuration.
Global configuration – Any of the area or interface configuration option can be set to a
default value here to help simplify the configuration.
Neighbours – At least one area must exist, an area groups interfaces together, any
parameters set in the areas will override any global options. Valid interfaces are ethx, pppx
and tunx, where the x is the interface number.
Filters – The final section contains the filters that are applied to inbound and outbound routing
Please see section 4.0 for a complete list of all available commands and their functions.
2.2 Web interface configuration
There is also a small amount of setup required, in the TransPort router’s web interface, this can be found by
navigating to the following section:
Configuration - Network > IP Routing/Forwarding > BGP
Once you are satisfied that BGP is configured and running correctly the Debug level can be turned off.
To view the debug output, connect to the TransPort router via a serial connection and type bgp 0 debug
on and debug x (where x is the ASY port number e.g. 0,1,2 or 3), or debug t if you are connecting via
Telnet. All of the BGP debug information will now be displayed, to stop the debug information, alter the
debug level back to off and disable and re-enable BGP. To turn off other debug type “debug off”.
There are a number of CLI commands for checking the status of the BGP routing process, the commands
and their functions are listed below
• bgpctl show interface – Lists all of the interfaces on the TransPort router and their status within
BGP, i.e. if they are up or down.
• bgpctl show network – Displays a list of the networks which are connected to the TransPort router
that BGP knows about.
• bgpctl show rib <options> – Displays the Routing Information Base (rib) table, which holds all of
the available routes on the router, there maybe duplicate routes to the same destination in this table.
• bgpctl show fib <connected / static / bgp / nexthop / x.x.x.x>– Displays the Forward Information
Base (fib) table, this table holds the routes which the router will actually use to forward traffic,
therefore there will be not duplicate routes held in this table. The options can be used to look at more
specific routes.
• bgpctl show neighbour – Displays a list of the BGP neighbours that are connected to the LAN
segments which are attached to this router. The current state of each neighbour will also be
displayed in the table.
• bgpctl show ip bgp <options>
• <options> – the options are as follows:
o address / len – Displays information about a specific route, use the detail option as well to
see detailed information.
o as y – Show all entries with as anywhere in the AS path.
o source-as – Show all entries with as as rightmost AS.
o transit-as – Show all entries with as anywhere but rightmost.
o empty-as – Show all entries that are internal routes with no AS's in the AS path.
o summary – This is the same as the show summary command.
o detail – Displays additional detail when used with one of the other options in this list
 Example:
Bgpctl show ip bgp detail as 65001
o memory – Show BGP memory statistics.
• bgpctl neighbor x.x.x.x <Up / Down / Clear> - Allows a neighbour to be set to up or down
manually or cleared which will restart a new session with a neighbour.
• bgpctl fib <Couple / Decouple> - When coupled (default) BGP routes will be placed in the Sroute
table, when Decoupled routes will be learnt and placed in the BGP routing table but not placed in the
Sroute table.
• bgpctl network <options>
o add – Add the specified prefix to the list of announced networks.
o delete – Remove the specified prefix from the list of announced networks.
o flush – Remove all dynamically added prefixes from the list of announced networks.
o show – Show all announced networks.
It is also worth checking the routing table via the web interface, or the command line.
• route print – Will display the entries in the routing table, it will show networks that are attached to
interfaces, static routes but also it will include routes learned dynamically from both BGP and OSPF.
The bgp.conf config file is divided into four main sections.
User-defined variables may be defined and used later, simplifying
the configuration file.
Global Configuration
Global settings for bgp.
Neighbors and Groups
bgpd establishes sessions with neighbors. The neighbor definition and properties are set in this section, as well as grouping
neighbors for the ease of configuration.
Filter rules for incoming and outgoing UPDATES.
With the exception of macros, the sections should be grouped and appear
in bgpd.conf in the order shown above.
Macros can be defined that will later be expanded in context. Macro
names must start with a letter, and may contain letters, digits and underscores. Macro names may not be reserved words (for example, AS,
neighbor, or group). Macros are not expanded inside quotes.
For example:
neighbor $peer1 {
remote-as 65001
Global Configuration
There are quite a few settings that affect the operation of the BGP
daemon globally.
AS as-number
Set the local autonomous system number to as-number. The AS numbers are assigned by local RIRs, or a private as number.
Private AS number are in the range 64512 to 65535
For example:
AS 65001
sets the local AS to 65001.
fib-update (yes|no)
If set to no, do not update the Forwarding Information Base,
a.k.a. the kernel routing table. The default is yes.
holdtime seconds
Set the holdtime in seconds. The holdtime is reset to its initial value every time either a KEEPALIVE or an UPDATE message is
received from the neighbor. If the holdtime expires the session
is dropped. The default is 90 seconds. Neighboring systems negotiate the holdtime used when the connection is established in
the OPEN messages. Each neighbor announces its configured holdtime; the smaller one is then agreed upon.
holdtime min seconds
The minimal accepted holdtime in seconds.
greater than or equal to 3.
This value must be
listen on address
Specify the local IP address bgpd should listen on.
listen on
log updates
Log received and sent updates.
network address/prefix [set ...]
network static [set ...]
network connected [set ...]
Announce the specified network as belonging to our AS. If set to
connected, routes to directly attached networks will be announced. If set to static, all static routes will be announced.
It is possible to set default AS path attributes per network
network set localpref 220
See also the ATTRIBUTE SET section.
nexthop qualify via (bgp|default)
If set to bgp, BGP may use BGP routes to verify nexthops. If
set to default, bgp may use the default route to verify nexthops. By default bgp will only use static routes or routes
added from the Sroute table.
rde med compare (always|strict)
If set to always, the MED attributes will always be compared.
The default is strict, where the MED is only compared between
peers belonging to the same AS.
rde route-age (ignore|evaluate)
If set to evaluate, the best path selection will not only be
based on the path attributes but also on the age of the route,
giving preference to the older, typically more stable, route. In
this case the decision process is no longer deterministic. The
default is ignore.
route-collector (yes|no)
If set to yes, the route selection process is turned off.
default is no.
router-id address
Set the router ID to the given IP address, which must be local to
the machine.
If not given, the BGP ID is determined as the biggest IP address
assigned to the local machine.
BGP establishes TCP connections to other BGP speakers called
neighbors. Each neighbor is specified by a neighbor section, which allows properties to be set specifically for that neighbor:
neighbor {
remote-as 65002
descr "a neighbor"
Multiple neighbors can be grouped together by a group section. Each
neighbor section within the group section inherits all properties from
its group:
group "peering AS65002" {
remote-as 65002
neighbor {
descr "AS65002-p1"
neighbor {
descr "AS65002-p2"
Instead of the neighbor's IP address, an address/netmask pair may be given:
In this case, the neighbor specification becomes a template, and if a
neighbor connects from an IP address within the given network, the template is cloned, inheriting everything from the template but the remote
address, which is replaced by the connecting neighbor's address. With a
template specification it is valid to omit remote-as; bgpd will then
accept any AS the neighbor presents in the OPEN message.
There are several neighbor properties:
announce (all|none|self|default-route)
If set to none, no UPDATE messages will be sent to the neighbor.
If set to default-route, only the default route will be announced
to the neighbor. If set to all, all generated UPDATE messages
will be sent to the neighbor. This is usually used for transit
AS's and IBGP peers. The default value for EBGP peers is self,
which limits the sent UPDATE messages to announcements of the local AS. The default for IBGP peers is all.
announce (IPv4|IPv6) (none|unicast)
For the given address family, control which subsequent address
families (at the moment, only none, which disables the announcement of that address family, and unicast are supported) are announced during the capabilities negotiation. Only routes for
that address family and subsequent address family will be announced and processed.
demote group
Increase the carp demotion counter on the given interface
group, usually carp, when the session is not in state
ESTABLISHED. The demotion counter will be increased as soon as
bgpd(8) starts and decreased 60 seconds after the session went to
state ESTABLISHED. For neighbors added at runtime, the demotion
counter is only increased after the session has been ESTABLISHED
at least once before dropping.
For more information on interface groups, see the group keyword
in ifconfig(8).
depend on interface
The neighbor session will be kept in state IDLE as long as
interface reports no link. This is primarily intended to be used with tunnel interfaces to reduce failover times.
The state of the network interfaces on the system can be viewed
using the show interfaces command to bgpctl.
descr description
Add a description. The description is used when logging neighbor
events, in status reports, for specifying neighbors, etc., but
has no further meaning to bgp.
Do not start the session when bgpd comes up but stay in IDLE.
dump (all|updates) (in|out) file [timeout]
Do a peer specific MRT dump. Peer specific dumps are limited to
all and updates. See also the dump section in GLOBAL
enforce neighbor-as (yes|no)
If set to yes, AS paths whose leftmost AS is not equal to the
remote AS of the neighbor are rejected and a NOTIFICATION is sent
back. The default value for IBGP peers is no otherwise the default is yes.
holdtime seconds
Set the holdtime in seconds.
ration if not given.
Inherited from the global configu-
holdtime min seconds
Set the minimal acceptable holdtime.
configuration if not given.
Inherited from the global
local-address address
When bgp initiates the TCP connection to the neighbor system,
it normally does not bind to a specific IP address. If a localaddress is given, bgp binds to this address first.
max-prefix number [restart number]
Terminate the session after number prefixes have been received
(no such limit is imposed by default). If restart is specified,
the session will be restarted after number minutes.
multihop hops
Neighbors not in the same AS as the local bgp normally have
to be directly connected to the local machine. If this is not
the case, the multihop statement defines the maximum hops the
neighbor may be away.
Do not attempt to actively open a TCP connection to the neighbor
remote-as as-number
Set the AS number of the remote system.
route-reflector [address]
Act as an RFC 2796 route-reflector for this neighbor. An optional cluster ID can be specified; otherwise the BGP ID will be
set attribute ...
Set the AS path attributes to some default per neighbor or group
set localpref 300
See also the ATTRIBUTE SET section. Set parameters are applied
to the received prefixes; the only exceptions are prepend-self,
nexthop no-modify and nexthop self.
softreconfig (in|out) (yes|no)
Turn soft reconfiguration on or off for the specified direction.
If soft reconfiguration is turned on, filter changes will be applied on configuration reloads. If turned off, a BGP session
needs to be cleared to apply the filter changes. Enabling
softreconfig in will raise the memory requirements of bgp because the unmodified AS path attributes need to be stored as
tcp md5sig password secret
tcp md5sig key secret
Enable TCP MD5 signatures per RFC 2385. The shared secret can
either be given as a password or hexadecimal key.
tcp md5sig password mekmidasdigoat
tcp md5sig key deadbeef
ttl-security (yes|no)
Enable or disable ttl-security. When enabled, outgoing packets
are sent using a TTL of 255 and a check is made against an incoming packet's TTL. For directly connected peers, incoming packets
are required to have a TTL of 255, ensuring they have not been
routed. For multihop peers, incoming packets are required to
have a TTL of 256 minus multihop distance, ensuring they have not
passed through more than the expected number of hops. The default is no.
BGP has the ability to allow and deny UPDATES based on prefix or AS
path attributes. In addition, UPDATES may also be modified by filter
For each UPDATE processed by the filter, the filter rules are evaluated
in sequential order, from first to last. The last matching allow or deny
rule decides what action is taken.
The following actions can be used in the filter:
The UPDATE is passed.
The UPDATE is blocked.
Apply the filter attribute set without influencing the filter
The rule parameters specify the UPDATES to which a rule applies. An
UPDATE always comes from, or goes to, one neighbor. Most parameters are
optional, but each can appear at most once per rule. If a parameter is
specified, the rule only applies to packets with matching attributes.
as-type as-number
This rule applies only to UPDATES where the AS path matches. The
as-number is matched against a part of the AS path specified by
the as-type. as-type is one of the following operators:
(any part)
(rightmost AS number)
(all but the rightmost AS number)
Multiple as-number entries for a given type or as-type as-number
entries may also be specified, separated by commas or whitespace,
if enclosed in curly brackets:
deny from any AS { 1, 2, 3 }
deny from any { AS 1, source-as 2, transit-as 3 }
deny from any { AS { 1, 2, 3 }, source-as 4, transit-as 5 }
community as-number:local
community name
This rule applies only to UPDATES where the community path attribute is present and matches. Communities are specified as asnumber:local, where as-number is an AS number and local is a locally significant number between zero and 65535. Both as-number
and local may be set to `*' to do wildcard matching. Alternatively, well-known communities may be given by name instead and
NO_PEER. Both as-number and local may be set to neighbor-as,
which is expanded to the current neighbor remote AS number.
(from|to) peer
This rule applies only to UPDATES coming from, or going to, this
particular neighbor. This parameter must be specified. peer is
one of the following:
group descr
Any neighbor will be matched.
Neighbors with this address will be matched.
Neighbors in this group will be matched.
Multiple peer entries may also be specified, separated by commas
or whitespace, if enclosed in curly brackets:
deny from {,, group hojo }
prefix address/len
This rule applies only to UPDATES for the specified prefix.
Multiple address/len entries may be specified, separated by commas or whitespace, if enclosed in curly brackets:
deny from any prefix {, }
Multiple lists can also be specified, which is useful for macro
good="{,, }"
bad="{, }"
ugly="{, }"
deny from any prefix { $good $bad $ugly }
prefixlen range
This rule applies only to UPDATES for prefixes where the prefixlen matches. Prefix length ranges are specified by using
these operators:
(less than)
(less than or equal)
(greater than)
(greater than or equal)
(range including boundaries)
(except range)
>< and - are binary operators (they take two arguments). For instance, to match all prefix lengths >= 8 and <= 12, and hence the
CIDR netmasks 8, 9, 10, 11 and 12:
prefixlen 8-12
Or, to match all prefix lengths < 8 or > 12, and hence the CIDR
netmasks 0-7 and 13-32:
prefixlen 8><12
prefixlen can be used together with prefix.
This will match all prefixes in the netblock with netmasks longer than 16:
prefix prefixlen > 16
If an UPDATE matches a rule which has the quick option set, this
rule is considered the last matching rule, and evaluation of subsequent rules is skipped.
set attribute ...
All matching rules can set the AS path attributes to some default. The set of every matching rule is applied, not only the
last matching one. See also the following section.
AS path attributes can be modified with set.
set can be used on network statements, in neighbor or group blocks, and
on filter rules. Attribute sets can be expressed as lists.
The following attributes can be modified:
community [delete] as-number:local
community [delete] name
Set or delete the COMMUNITIES AS path attribute. Communities are
specified as as-number:local, where as-number is an AS number and
local is a locally-significant number between zero and 65535.
Alternately, well-known communities may be specified by name:
localpref number
Set the LOCAL_PREF AS path attribute.
If number starts with a
plus or minus sign, LOCAL_PREF will be adjusted by adding or subtracting number; otherwise it will be set to number.
med number
metric number
Set the MULTI_EXIT_DISC AS path attribute. If number starts with
a plus or minus sign, MULTI_EXIT_DISC will be adjusted by adding
or subtracting number; otherwise it will be set to number.
nexthop (address|blackhole|reject|self|no-modify)
Set the NEXTHOP AS path attribute to a different nexthop address
or use blackhole or reject routes. If set to no-modify, the nexthop attribute is not modified. Unless set to self, the nexthop
is left unmodified for IBGP sessions. self forces the nexthop to
be set to the local interface address.
prepend-neighbor number
Prepend the neighbor's AS number times to the AS path.
prepend-self number
Prepend the local AS number times to the AS path.
weight number
The weight is used to tip prefixes with equally long AS paths in
one or the other direction. A prefix is weighed at a very late
stage in the decision process. If number starts with a plus or
minus sign, the weight will be adjusted by adding or subtracting
number; otherwise it will be set to number. Weight is a local
non-transitive attribute and a bgp specific extension.
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