Communities are tag that are attached to the bgp

common property. Communities are tag that are attached to the BGP routes in order to indicate a
specify action to take place.
There are pre-defined communities
I. NO_EXPORT (0xFFFFFF01):- This field tells a router it should propagate prefixes that
belong to this community to IBGP only and not to propagate to EBGP to external autonomous
system.
II. NO_ADVERTISE (0XFFFFFF02):- This field indicates that no prefixes should be advertised
at all.
III. NO_EXPORT_SUBCONFED (0XFFFFFF03):- This is similar to NO_EXPORT but it can
limit the number of IBGP advertisement.
IV. NOPEER (0XFFFFFF04):- It indicates that the prefixes should be advertised to peering
relationships.
Communities are 32 bit values where the first 2 octets determines the AS and the last 2 octets
indicates the locally defined value.
In the wireshark capture 65215,790,340 are the ASes and 1,4,250 are the values associated with
them.
13) Path Attribute - ORIGINATOR_ID:
Originator_ID is a optional non transitive attribute which is used to by Route reflector to prevent
loops. The originator is a 32 bit value. If a BGP router that receives a route from an IBGP neighbor
in the incoming update detects the presence of its own Router-ID in the Originator-ID attribute
May 17, 2017 TDC 563 – HW 3 7

14) Path Attribute - CLUSTER_LIST:
This field includes the IP address of the Route Reflector in order to avoid routing loops. If a BGP
router that receives a route from an iBGP neighbor is configured to operate as a route reflector and
in the incoming update detects the presence of its own Cluster-ID in the Cluster-list attribute it will
reject the update. The Pcap file includes The Router ID of the Route Reflector I.e. 192.168.0.250.
15) Network Layer Reachability Information:
The Network Layer Reachability Information is exchanged between BGP routers using the update
message. It is composed of length and prefix where the lenth is the CIDR value. In the PCAP the
ip address is 172.16.0.0/16 so the length is 16.
h. (4 pts) Consider the second UPDATE message.
i. Why is the ORIGIN different? Igp