HSRP stands for Hot Standby Routing Protocol. The following are
members of HSRP group:
1. Virtual router: virtual router is what is seen by the end user device. The virtual router has its own IP and MAC addresses.
2. Active router: Forwards packets sent to the virtual router. An active router assumes the IP and MAC addresses of the virtual router.
3. Standby router: Standby router monitors the state of HSRP by using Hello massages. It assumes the role of Active router, should the current Active router fail.
Image router does not exist and is not a part of HSRP group.
When an Active router fails in HSRP environment, Standby router
assumes the Active router role. This new Active router will remain
as Active router even if the failed Active router comeback to service,
irrespective of the priority levels.
To enable the previous Active router to resume its activity as Active router by taking over the role from a lower priority Active router, use the command:
Rtr(config-if)# standby <group-number> preempt
The hosts served by HSRP router use the IT address of virtual router as the default IP address.
Each router in a standby group can be assigned a priority value.
The range of priority values is between 0 and 255 (including 0 and
255). The default priority assigned to a router in a standby group
is 100. The router with numerically higher priority value will become
Active router in the HSRP standby group.
The command used to set the router's priority in standby group is:
R(config-if)# standby <group-number> priority <priority-value>.
17.1 The following are some of the characteristics of IP multicast traffic:
1. IP multicast traffic uses Class D range of addresses from
126.96.36.199 to 188.8.131.52.
2. A Class D address consists of 1110 as the higher-order bits in the first octet.
3. Multicast packets use User Datagram Protocol. UDP is considered unreliable, because UDP does not support flow control, error recovery functions.
4. 184.108.40.206 represents all hosts on a subnet; 220.127.116.11 represents all routers on a subnet.
5. The addresses in the range 18.104.22.168 - 22.214.171.124 are reserved
for use by network protocols on a local network segment. Packets
with these addresses should never be forwarded by a router. The
time-to-live (TTL) for these packets is always set to a value 1
by the originating router, so that these packets never cross local
6. The address range 126.96.36.199 - 188.8.131.52 are called Administratively Scoped Addresses and analogous to private addresses 10.0.0.0/8.
17.2 Mapping IP Multicast to a MAC address:
The high order 9 bits (out of total of 32 bits) of the IP address are not used for mapping into the MAC address. The lower 23 bits are mapped to lower 23 bits of MAC address.
For example, take a Multicast address: 184.108.40.206. Convert to binary equivalent:
220.127.116.11 = 1110.0000.0000.1100.0000.1001.0000.0100
Take only the last 23 bits = 000.1100.0000.1001.0000.0100 = 0c-09-04
Append the Ethernet Multicast address: 01-00-5E. Note that 01-00-5E always precedes a Multicast MAC address. The bit following this is always a zero.
Therefore, Multicast MAC address for IP address 18.104.22.168 is:
18.0 Unicast Traffic:
The following are true about Unicast traffic:
1. The number of times a packet is transmitted is proportional to the number of hosts that need to receive the packet. If there are N number of hosts that need to receive a packet, the packet is transmitted N times over the network. This increases the Bandwidth consumption and resource consumption on the network.
2. All Unicast traffic need to know the destination address, whereas, Multicast and broadcast messages need not know the destination addresses.
3. Unicasting can work with both TCP and UDP protocols.
19.0 Dense Mode Routing Protocols:
Dense mode routing protocols are suited when all or most of the
routers in a network need to distribute multicast traffic.
Some examples of Dense Mode Routing Protocols are:
1. Distance Vector Multicast Routing Protocol (DVMRP)
2. Multicast Open Shortest Path First (MOSPF)
3. Protocol Independent Multicast Dense Mode (PIM DM)
20.0 Sparse Mode Routing Protocols:
Sparse mode routing is suitable when there the Multicast group
members are sparsely distributed. These protocols put less strain
on network bandwidth, compared to Dense mode protocols.
Some examples of Sparse Mode Routing Protocols:
1. Core-Based Trees (CBT)
2. Protocol Independent Multicast Sparse Mode (PIM SM)