Study Notes For Cisco Exam 642-661 BGP

1. Overview:
What is BGP, where/when are we gonna use it ?
A) BGP is the successor of EGP [Exterior Gateway Protocol], and currently its
the only EGP deployed. BGP is an Enhanced Distance Vector Protocol used in
routing between Autonomous Systems [AS] “aka Interdomain Routing”,
where an AS is a collection of networks under single administration. We use
BGP in several occasions as Service Providers networks, Multihomed
customers and large enterprise networks, etc…
BGP Basics:
● Only one BGP process per router.
● There is two types of BGP, IBGP & EBGP, if the as-numbers of the
peering routers are the same then its IBGP, if they are different then
its EBGP.
● BGP uses AS numbers [1-64511] as public and [64512-65535] as
private.
● BGP uses TCP as its reliable transport protocol and it runs over TCP
port 179.
● The router with the higher router-id establishes the BGP peering
session.
● BGP uses Keepalive messages to detect the presence of its neighbor,
Keepalive interval value is 60 sec, and Holdtime is 180 sec by default
[1:3 ratio], Holdtime value is exchanged in the Open Message, and
you can only modify the Holdtime value, BGP peers use the lower
Holdtime value configured on either of them.
● BGP uses triggered updates, 5 sec interval for IBGP and 30 sec interval
for EBGP.
● Mandatory well known attributes must exist in each routing update.
● If multiple paths exist for the same network, only one is selected as
the best route and the remaining routes are stored in the memory,
Router propagates best routes only to its neighbors.
● If multi path load sharing is enabled, router can select multiple paths
to a single destination and installs them in the routing table, multiple
path load sharing in BGP supports up to 16 paths.
● Before a route is installed in the routing table, the router checks if its
learned from another routing protocol rather than BGP, if it was
learned from another routing protocol the router compares the
Administrative Distance [AD] and prefers the lower.
● BGP Split Horizon Rule: When a router receives an update it never
sends it back to the source which it received from.
● IBGP Split Horizon Rule: Routes learned from an IBGP neighbor is
never sent to other IBGP neighbors, thus all IBGP routers inside an AS
needs full mesh for consistent routing decisions.

2. ● AS-Path loop prevention mechanism: When a router receives an
update containing its own AS number; it silently ignores the update.
● EBGP peers should be reachable for all BGP speaking routers inside an
AS, this is achieved by either redistributing connected interfaces of the
EBGP peers into IGP, or run IGP over the EBGP peers interface and
make them passive so that they don’t exchange IGP information, or
finally use the “neighbor ip-address next-hop-self” command so that
the edge router announces it self as the next hop for the IBGP peers.
● BGP sessions can be initiated using loopback interfaces, IGP or Static
Routes are used for providing reachability between loopbacks, also the
update source for the BGP session should be modified in order to
successfully establish the session using the “neighbor ip-address
update-source loopback number” command. For EBGP sessions to be
established successfully using the loopback interfaces you will need to
use the “neighbor ip-address ebgp-multihop value“ command.
● IGP is used inside an AS to provide full reachability required for
establishing IBGP sessions, fast convergence in case of physical failure
in one of the multiple paths between IBGP routers, and next hop
resolving “aka recursive look up” for appropriate packet forwarding.
BGP Path Attributes:
● Mandatory Well Known Attributes:
■ Next-hop:ip address of the router sending the updates, by
default it changes when a route is advertised to EBGP
neighbor but not when its advertised to IBGP neighbor.
■ AS-Path:Sequence of ASs path a route has traveled through.
■ Origin: Indicates how BGP learned the route [IGP - EBGP
- ?].
● Discretionary Well Known Attributes:
■ Local Preference:used for consistent routing policy inside an
AS.
■ Atomic Aggregate: informs a neighbor router that the
originating router aggregated the routes.
● Transitive Optional Attributes:
■ Aggregator:Specify the ip address and the AS number of the
router that performed the aggregation.
■ Community: Route tagging mechanism used in filtering or
route selection process.
● Non-Transitive Optional Attributes:
■ Multi-Exit Discriminator [MED]: Discriminate between
multiple exit points within an AS.
■ Cost Community: Used to influence best-path selection for
IBGP and confederations only.

3. ■ Originator-id: Used as a loop prevention mechanism in case
of multiple Route Reflectors.
BGP Session Establishment Process:
● Single BGP process is started on the router using “router bgp as-
number” command.
● Neighbors must be configured manually on both sides using “neighbor
ip-address remote-as as-number“ command.
● It uses TCP port 179 and the session of the router with the higher
Router-id is retained.
● The first state of the BGP session is IDLE which indicates that the
router is currently not attempting any session establishment, for a
router to change its IDLE state; the configured neighbor ip address
should be reachable.
● When peers are correctly configured the state is changes to ACTIVE
which indicates that the router is actively sending connections
attempts to its neighbor.
● When the TCP connection attempt succeed, the router sends an Open
Message containing BGP session information and changes the state to
be OpenSent.The Open Message contains [BGP version number - AS of
local router - Holdtime - Router-ID - Optional parameters].
● If the neighbor router accepts the parameters in the Open Message; it
replies with its own Open Message, the local router receives the Open
Message and changes the state to OpenConfirm, and it verifies the
parameters of the neighbor router, if accepted a keepalive message is
sent as signal of acceptance and then the state is changed to
Established.

4. Route Selection Criteria:
1. Next-hop: If not reachable the route is not installed in the routing
table.
2. Weight: Local to the router.
3. Local Preference: Local within an AS.
4. Originated Routes: Routes originated using the network or summary
commands.
5. AS-Path: Prefers the shortest path.
6. Origin Code: IGP < EGP < ?
7. MED: Prefers the lowest value.
8. EBGP routes over IBGP routes.
9. For IBGP: Prefers path via closest IGP neighbor [Next-Hop with lowest
IGP metric].
10. For EBGP: Oldest path.
11. Lowest BGP Router-id.
Advertising Networks:
There are three ways to announce networks into BGP:
● Network command, Redistribution and Aggregation.
● when either of the three ways is used the AS-Path will appear empty
indicating that the route is locally originated, when the route traverses
through other ASes, the forwarding router prepends its own AS
number to the AS-Path.
● Network command operates differently in BGP; indicates which routes
will be injected in the BGP table not which interface will BGP run over.
● Using a Route-Map with the Network command allows you to alter
Weight, Local Preference, MED and tagging the route.
● When redistributing routes into BGP, they carry an origin of incomplete
“?“. – Conditional Route Injection: is injecting a route into BGP with no
matching route in the routing table, this is achieved by using the “bgp
inject-map map-name exist-map map-name” command.
Summarization & Aggregation:
● Automatic summarization is enabled by default.
● For a router to install a classful network in the BGP table when
Automatic summarization is enabled; A classful network statement
with a classful mask and at least one subnet of this classful network
should exist in the routing table.
● When Automatic summarization is enabled; all redistributed subnets
will be summarized to their classful network.
● When summarization is disabled, an exact match must be found in the
routing table.

5. ● Aggregation is summarization of routes when it is advertised to other
neighbors, and its configured using “aggregate-address ip-address
mask”command.
● For an aggregate route to be advertised to other neighbors; a route
within the range of the aggregate must exist in the BGP table in order
to install the aggregate in the BGP table.
● By default both the aggregate and the specific routes are advertised to
the neighbors, to advertise the aggregate only you will have to use the
“summary-only” keyword with the aggregate command.
Securing BGP Peers:
● MD5 authentication between BGP peers by using the “neighbor ip-
address password password” command.
● TTL-Security: The router compares the TTL value received with the
locally configured hop count value, this option is supported for both
directly connected and multihop EBGP peers. the command for this
option is “neighbor ip-address ebgp-multihop ttl“; where TTL is a
numeric value.
Multihoming:
● Multihoming is a customer being connected to a single ISP with
multiple links or connected to multiple ISP’s.
● Multihomed customers should run BGP with their ISPs using public AS
and provider independent address space.
● Multihomed customers should advertise their own address space only
to their ISPs and do not advertise routes learned from their ISPs do
avoid acting as a Transit-AS between their ISPs.
● For influencing Upstream ISP selection, Weight and Local Preference
can be used inside a Multihomed Customer AS.
● For influencing Downstream ISP selection, MED can be used if the
customer is multihomed to a single ISP as MED doesn’t traverse
through ASes, and AS-path Prepending can be used if the customer is
multihomed to multiple ISPs because AS-path attribute traverses
through ASes.

6. AS-Path Filtering:
● Used to announce or accept prefixes based on AS-Path Attribute.
● It uses Regular Expressions.
● Its implemented on per neighbor basis.
● Use “ip as-path access-list number [permit/deny] as-regular-
expression” & “neighbor ip-address filter-list access-list-number
[in/out]” commands.
Regular Expressions:
● | –> Logical OR.
● [123] [1-4]–> matches any single character from those between
braces.
● dot (.)–> matches any character, even if it was a space.
● ^–> matches beginning of the string.
● $–> matches end of the string.
● Underscore (_)–> matches any delimiter [beginning, end, space, tab,
comma]
● Braces ( ) –> matches two or more AS numbers.
● –> removes the meaning of the special character following it so it
could be matched if found in an AS-Path eg. as () in confederations.
● *–> character is repeated zero times or more.
● ?–> character is repeated zero or one time.
● +–> character is repeated one or more times.
Prefix-List filtering:
● Used to filter announce and accept specific prefixes.
● It has some advantages over IP Access Lists as: Provide flexibility in
editing, inserting and deleting individual lines, Matches based on
subnetmask, etc…
● Its implemented on per neighbor basis.
● An with no Le/Ge matches exactly the specified prefix.
● An entry with Le/Ge matches any route within the range specified.
● Configuration example:
■ “ip prefix-list name seq number [permit/deny] prefix/length
ge value le value” “neighbor ip-address prefix-list name [in/
out]” “redistribute-list prefix-list name out routing-process“.

7. Out Bound Route Filtering [ORF]:
● Its implemented on per neighbor basis.
● Its a BGP feature that allows a router to accept a prefix-list from a
neighbor and apply it to locally configured ORF neighbor.
● A router can install an inbound prefix-list to a peer as an outbound
prefix-list.
● Its used to minimize the number of updates sent between neighbors
and reduce system resources.
● Configuration example:
■ “neighbor prefix-list name [in/out]” “neighbor capability orf
prefix-list [send/receive/both]”
ORF message contains:
● Address Family Information [AFI]/ Subsequent AFI
● ORF types
● When to refresh
● List of ORF entries
ORF Types:
● type 1 –> Network Layer Reachability Information [NLRI]
● type 2 –> Communities
● type 3 –> Extended Communities
● type 128 –> Prefix-List
Route-Map Filtering:
● Route-Map matches: prefix-list/access-list/route originator/next-
hop/origin/AS-path/community/IGP tag/IGP type[internal/external].
● Route-Map can set: origin/next-hop/weight/local
preference/MED/community.
● IP Policy List: is grouping of route-map match clauses then attaching
to route-map.
● Its implemented on per neighbor basis.
● Route Map Continue Cause: its like the match and the set causes of
the route-map, when a match in the route-map is successful continue
clause -if configured- jumps to a pre-specified route-map entry, the
continue clause takes place if a match is successful, if not then it is
ignored.
● If the route-map has no match clause, the continue clause takes place
automatically, if a match is successful the continue clause takes place,
if not then it is ignored.
● Configuration example:
■ “ip policy-list name [permit/deny]match [as-
path/metric/community]route-map name permit seq-
number match policy-map namematch ip address prefix-list

8. namematch ip next-hop prefix-list namematch ip route-
source prefix-list name continue seq-number neighbor ip-
address route-map name[in/out]”
AS-Path Prepending:
● Used to influence other ASes to select a specific return path towards
an AS.
● Used to distribute the load of returning traffic for multihomed
customers, however in this case you will have to monitor the traffic
and prepend AS to path as needed to accomplish the traffic load.
● To avoid BGP AS-Path loop prevention mechanism, use only the AS
number of the sending AS.
● Service Providers use AS-Path filter to allow routes that are originated
from Customers AS only, if the Customer is going to use AS-Path
prepending the Service Provider will have to change their filter to allow
AS-Path containing more than one copy of Customer’s AS number.
● AS-Path prepending is applied using Route-Maps on per neighbor
basis. ”route-map route-map-name permit 10 set as-path prepend as-
no as-no as-no neighbor ip-address route-map route-map-name out”.

9. BGP hide local AS:
● The “neighbor ip-address local-as as-number [no-prepend [replace-as
[dual-as]]]”
■ no-prepend: does not prepend local AS number to any
learned EBGP routes.
■ replace-as: replaces the local AS number with the one set
int the command to the AS-path attribute.
■ dual-as: allows the establishment of EBGP sessions using
either the real AS number or using the AS number set in the
command.
● This usually happens while connecting two different BGP networks with
different AS numbers to not disturb the established peerings [i.e. when
an ISP buys another ISP and merging both networks into only one
network].
● Its drawback : if you configured the above command with an AS
number that already exists for one of the IBGP peers, when this IBGP
receives the route it will detect its own AS number in the AS path and
it will ignore this route considering it as a routing loop.
Multi-Exit Discriminator [MED]:
● MED is used to discriminate between multiple exit points within an AS.
● MED is used to influence path selection in neighbor AS.
● MED doesn’t traverse outside the receiving AS.
● Default value is Zero and in comparison the lower value the better, to
change the default value use “default-metric number” command.
● MED can be set in ways:
■ Using a Route-Map
■ Inherited from an IGP by either using the BGP
Network command or redistributing into BGP.
● MED is compared when different values are received from same AS, if
“bgp always-compare-med” is used MED from different ASes will be
also compared.
● In intra-confederations MED is not compared and to compare it “bgp
bestpath med confed” should be used.
● BGP sets a missing MED value to infinite value, however Cisco IOS
does set it to Zero, to change this behavior of Cisco IOS the “bgp
bestpath med missing-med-worst” command should be used.
● “bgp deterministic-med” allows BGP to compare the MED values after
the AS-Path attribute directly.
Communities:
● Its a mean of tagging routes and used in filtering or route selection.

10. ● By default its stripped in outgoing BGP updates, to enable sending
communities the “neighbor ip-address send-community” should be
used in per-neighbor basis.
● There is no limitation on the number of communities specified for a
route.
● Route-Map is used for setting the community value, it can be applied
with redistribution, network command, neighbor command and
aggregate command.
● In Route-Map configuration, the “additive” keyword prepends new
Community value to the existing Community values, if not used it will
override the existing Community values. “set community value
[value ...][additive]”
● The “ip bgp-community new-format” command is recommended when
the Community value contains AS numbers.
● Community list “ip community-list 1-99 permit|deny value [value ...]”:
■ Values in one line must match to be accepted, if no matches
the list acts as an Access-List and denies the route.
■ Keyword “internet” acts as permit any.
● Extended Community list “ip community-list 100-199 permit|deny
regexp”
■ Matches are based on regular expressions.
■ To match any use “.*” value.
● Named Community list “ip community-list standard|expanded name
permit|deny value|regexp”.
● Sequenced Extended Community List “ip extcommunity-list 100-199|
standard list-name permit|deny regexp” or “ip extcommunity-list
0-99|expanded list-name permit|deny [rt extcom-value][soo extcom-
value]”
■ Allow automatic sequencing or resequencing for BGP
Extended Community List
■ Allow insertion and deletion of lines in the BGP Extended
Community List.
■ Rt: Specifies the Route Target of Extended Community
attribute.
■ Soo: Specifies the Site Of Origin Extended Community
attribute.
● BGP Cost community “set extcommunity cost [igp] community-id cost-
value”
■ Its a non-transitive attribute.
■ Its used to influence best-path selection for IBGP and
confederations only.
■ Default value is 2147483647 and in comparison the lower
value the better.

11. ■ The keyword “IGP”influences the best-path selection at the
POI [point of insertion] which follows the IGP metric
comparison in BGP route selection criteria. In case if the POI
step is not valid the cost community is silently ignored.
BGP Link Bandwidth:
● Used for load balancing over unequal bandwidth links.
● Enabled by using “bgp dmzlink-bw”.
● Routes learned from a directly connected external neighbor propagates
through the IBGP network with the bandwidth of the external link.
● The “neighbor ip-address dmzlink-bw” command is used to advertise
the bandwidth of links used to exit an AS, its configured on the DMZ
interface that connect single hop EBGP neighbor.
Route Reflectors:
● Route Reflectors are router running BGP that are allowed to break the
IBGP loop prevention rules and advertise routes that are received from
IBGP pears.
● Route Reflectors eliminates the need of full mesh IBGP.
● Route Reflector advertises the best routes only.
● When Route Reflector receives a route update from a Route Reflector
Client; it sends the route to all other peers.
● When Route Reflector receives a route update from a Non Route
Reflector Client; it sends the route to all of its Clients and EBGP peers
only.
● When a Route Reflector Client receives an IBGP route update; it sends
it to EBGP neighbors only.
● When a Route Reflector Client receives an EBGP route update; it sends
it to all of its neighbors.
● In case of redundant Route Reflectors; Route Reflector Clusters is used
to prevent routing loops, the Route Reflector adds Cluster-id and
Originator-id to the advertised route updates.
● Originator-id is a non-transitive optional attribute.
● When a Route Reflector receives a route update with its own Cluster-
id; it silently ignores the route update.
● When a Route Reflector Client receives a route update with Originator-
id same as its Router-id; it silently ignores the route update.
● When a Route Reflector receives two IBGP route updates; the non
reflected route update [the one with no Originator-id] is preferred.
● When a Route Reflector receives two IBGP route updates ; the one
with the shortest Cluster-list is preferred.
● Route Reflector configuration:
■ “neighbor ip-address route-reflector-client”
■ “bgp cluster-id cluster-id”

12. Confederations:
● Confederations splits the AS into smaller ASes to reduce the number of
BGP sessions needed for full mesh IBGP.
● Confederations eliminates the need of full mesh IBGP, however its
needed inside each Confederation which can be achieved by setting a
Route Reflector inside the Confederation.
● When communicating to real EBGP neighbors, internal ASes are hidden
and only one external AS is announced to all real EBGP neighbors.
● Intra-Confederation EBGP sessions are used between Member-ASes,
however it is slightly different from the Real EBGP sessions as is
behaves like IBGP in passing BGP attributes as Local-Preference, MED,
Next-Hop.
● Entire Confederation should use same IGP as they all use same Next-
Hop ip- addresses.
● Intra-Confederation AS-Path appears between parentheses ( ).
● To configure Confederations:
■ Start the BGP process with the Member-AS number.
■ Set the external “Real” AS number.
■ List all Member-ASes of the Confederation on each router
with EBGP Session.
■ “router bgp member-as” “bgp confederation identifier
external-as” “bgp confederation peers list-of-member-as”
Peer Groups:
● Used to configure multiple neighbors with similar requirements, also
used as a BGP performance enhancement tool since the router builds a
single update for all Peer Group members which reduces the CPU load.
● IBGP & EBGP neighbors cannot be mixed in one Peer Group.
● Peer Group parameters can be overridden by per-neighbor
configurations on incoming updates only.
● Peer Group configuration: “neighbor group-name peer-group”
“neighbor group-name bgp-parameters” “neighbor ip-address peer-
group group-name”
Route Dampening:
● Used to reduce processing load caused by flapping routes.
● IBGP routes are not dampened.
● When an EBGP route flaps it gets 1000 Penalty Points, when the
Penalty Points exceeds the Suppress Limit the route is dampened. The
Penalty Points decay through the use of a decay algorithm, when it
drops below the reuse limit the route is re-advertised.
● Flapping history of a route forgotten after the Penalty drops below
than half of the Reuse Limit.

13. ● After enabling Route Dampening; routes in the BGP Table are never
removed, the route is kept in the BGP Table and marked as history
“h”.
● To enable Route Dampening, the “bgp dampening [half-life reuse
suppress max-suppress-time] [route-map route-map-name] command
is used.
■ half-time → time for penalty to decrease to half [default
value is 15 minutes].
■ suppress → limit in which penalty of a route exceeds the
route is suppressed [default value is 2000].
■ reuse → limit in which penalty of route drops below, the
route is unsuppressed [default value is 750].
■ max-suppress-time → no route is suppressed longer than
this duration [default value is 60 minutes & maximum us
255 minutes].
● Useful commands:
■ To clear the statistics of routes flaps “clear ip bgp flap-
statistics”.
■ To release Dampened Routes “clear ip bgp dampening”.
■ “show ip bgp dampened-paths”.
■ “show ip bgp flap-statistics [ regexp regexp | filter-list
access-list | ip-address mask [longer-prefix] ]”.