Split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned.
A--B--C
"A" receive route from "B" about "C" (B know how to reach C).
"A" will not send this info back to "B", preventing loop if link between "B" and "C" will fail.
Split-horizon routing with poison reverse is a variant of split-horizon route advertising in which a router actively advertises routes as unreachable over the interface over which they were learned by setting the route metric to infinite (16 for RIP).
The effect of such an announcement is to immediately remove most looping routes before they can propagate through the network. The router is said to have poisoned the network by sending 'false' (infinite) metric values to other devices sharing such updates.
With route poisoning, when a router detects that one of its connected routes has failed, the router will poison the route by assigning an infinite metric to it and advertising it to neighbors. When a router advertises a poisoned route to its neighbors, its neighbors break the rule of split horizon and send back to the originator the same poisoned route, called a poison reverse. In order to give the router enough time to propagate the poisoned route and to ensure that no routing loops occur while propagation occurs, the routers implement a hold-down mechanism.
Routing Protocols Comparison Table
Routing protocols - the software that allow routers to dynamically advertise and learn routes, determine which routes are available and which are the most efficient routes to a destination (RIP, EIGRP, OSPF, BGP)
Routed protocols - is a Network Layer protocol that is used to move traffic between networks (IP, IPX, and AppleTalk)
Non-routable protocols - presume that all computers they will ever communicate with are on the same network (NetBEUI, DLC, LAT, DRP, MOP).
1) Type - exchange routing information within (interior IGP) or between (exterior EGP) an autonomous system (AS).
Autonomous system (AS) - a collection of IP networks and routers under the control of one entity.
2) Convergence - the status of a set of routers having the same knowledge of the surrounding network topology.
3) Protocol Class (Type) - routing algorithms used by varying routing protocols to determine the metric for routing (Distance Vector - Uses hop count, Link State - Uses Shortest Path First, Common View of Network, Hybrid - Distance vector with more accurate update metrics).
4) Administrative distance (AD) - preference of routing protocol - is how a router determines which source of routes it should use if it has two identical routes from different sources. In other words, the router needs to be able to determine which routes to trust if it's receiving the same information from two different sources (which is most trustworthy). The lower the administrative distance - is best
5) Metric - Routers use various metrics and calculations to determine the best route for a packet to reach its final network destination. Each routing protocol uses its own algorithm with varying weights to determine the best possible path (only one from all).
6) Classful routing protocols do not carry subnet mask information on their routing updates the same subnet mask everywhere is needed to avoid routing black holes),
Classless routing protocols include the subnet mask along with the IP address when advertising routing information.
Protocol Administrative distance
Directly connected route 0
Static route out an interface 1
Static route to next-hop address 1
Route Preference (Administrative distance) for different vendors
| Protocol | Cisco | Mikrotik | Juniper | Huawei (H3C), 3Com |
| Directly connected | 0 | 0 | 0 | 0 |
| Static | 1* | 1 | 5 | 60 |
| OSPF internal | 110 | 110 | 10 | 10 |
| OSPF AS external | 110 | 110 | 150 | 150 |
| OSPF NSSA | 110 | 110 | 150 | 150 |
| IS-IS Level 1 internal | 115 | - | 15 | 15 |
| IS-IS Level 2 internal | 115 | - | 18 | 15 |
| IS-IS Level 1 external | 115 | - | 160 | 15 |
| IS-IS Level 2 external | 115 | - | 165 | 15 |
| RIPv1, RIPv2, RIPng | 120 | 120 | 100 | 100 |
| eBGP | 20 | 20 | 170 | 255 |
| iBGP | 200 | 200 | 170 | 255 |
| Unknown | 255 | - | - | 256 |
Neighbor Requirements for EIGRP and OSPF
| Requirements | EIGRP | OSPF |
| Routers MUST be able to send/receive IP packets to one another | YES | YES |
| Interfaces MUST be in UP/UP state | YES | YES |
| Interfaces PRIMARY IP MUST be in SAME subnet | YES | YES |
| Interfaces MUST not be Passive on the connected interface | YES | YES |
| MUST pass authentication (if configured) | YES | YES |
| Router IDs MUST be unique | NO 1 | YES |
| SAME ASN / Process ID | YES | NO |
| SAME Hello / Dead intervals | NO | YES |
| SAME MTU | NO 2 | YES |
| SAME
Area SAME 'Stub' Area Flag |
n/a n/a |
YES YES |
| SAME 'K' Values | YES | n/a |
1 - Duplicate EIGRP RIDs do not prevent routers from becoming neighbors, but it can cause problems when adding external EIGRP routes to the routing table.
2 - May allow the other router to be listed in the show ip ospf neighborcommand, but the MTU mismatch will prevent proper operation of the topology exchange
Link Neighbor Requirements for EIGRP and OSPF 1
Link Neighbor Requirements for EIGRP and OSPF 2
Subnetting
Subnets Calculator - http://jodies.de/ipcalc
192.168.5.63/28
a) bitmask /28 =...240 : 32-28=4, 2^4=16 -> 256-16=240
b) range ..5.48-63 : nets 5.0 , 5.16, 5.32, 5.48 (+16)
c) subnets 2^m =16 : blocksize - host bits = 8 - 4 =4
d) hosts 2^n -2 =14: 2^4-2 =14
My simple method for mask calculation:
1) dotted (long to short )
1.1) greater than /24 (>24)
255.255.255.192 = /26
1) 256-192 = 64
2) 64 = 2^6
3) 32 bit length - 6 bit = 26
1.2) less than /24 (>24)
255.255.240.0 = /20
0) last block .0 = minus 8 bits
1) 256-240 =16
2) 16 = 2^4
3) 32 bit length - 4 bits - 8 bits = 20
2) CIDR (short to long)
2.1) greater than /24 (>24)
/27 = 255.255.255.224
1) 32 - 27 = 5
2) 2^5 = 32
3) 256 - 32 = 224
2.2) les than /24 (<24)
For network (ISP)
1 users (/32) ~ 313 kbps (dec 2011)
1 users (/32) ~ 445 kbps (feb 2014, or +2.2 years)
| CIDR | Nr of /24 | ~Traffic (mbps) | Subnet Mask | Addresses | Wildcard | Usable Addresses |
|---|---|---|---|---|---|---|
| /32 | - | 0.25 | 255.255.255.255 | 1 | 0.0.0.0 | none |
| /31 | - | 255.255.255.254 | 2 | 0.0.0.1 | none | |
| /30 | - | 255.255.255.252 | 4 | 0.0.0.3 | 2 | |
| /29 | - | 255.255.255.248 | 8 | 0.0.0.7 | 6 | |
| /28 | - | 255.255.255.240 | 16 | 0.0.0.15 | 14 | |
| /27 | - | 255.255.255.224 | 32 | 0.0.0.31 | 30 | |
| /26 | - | 255.255.255.192 | 64 | 0.0.0.63 | 62 | |
| /25 | - | 255.255.255.128 | 128 | 0.0.0.127 | 126 | |
| /24 | 1 | 62.5 | 255.255.255.0 | 256 | 0.0.0.255 | 254 |
| /23 | 2 | 125 | 255.255.254.0 | 512 | 0.0.1.255 | 510 |
| /22 | 4 | 250 | 255.255.252.0 | 1 024 | 0.0.3.255 | 1 022 |
| /21 | 8 | 500 | 255.255.248.0 | 2 048 | 0.0.7.255 | 2 046 |
| /20 | 16 | 1000 (200) | 255.255.240.0 | 4 096 | 0.0.15.255 | 4 094 |
| /19 | 32 | 2000 | 255.255.224.0 | 8 192 | 0.0.31.255 | 8 190 |
| /18 | 64 | 4000 | 255.255.192.0 | 16 384 | 0.0.63.255 | 16 382 |
| /17 | 128 | 8000 | 255.255.128.0 | 32 768 | 0.0.127.255 | 32 766 |
| /16 | 256 | 16000 | 255.255.0.0 | 65 536 | 0.0.255.255 | 65 534 |
| /15 | 512 | 32000 | 255.254.0.0 | 131 072 | 0.1.255.255 | 131 070 |
| /14 | 1024 | 64000 | 255.252.0.0 | 262 144 | 0.3.255.255 | 262 142 |
| /13 | 255.248.0.0 | 524 288 | 0.7.255.255 | 524 286 | ||
| /12 | 255.240.0.0 | 1 048 576 | 0.15.255.255 | 1 048 574 | ||
| /11 | 255.224.0.0 | 2 097 152 | 0.31.255.255 | 2 097 150 | ||
| /10 | 255.192.0.0 | 4 194 304 | 0.63.255.255 | 4 194 302 | ||
| /9 | 255.128.0.0 | 8 388 608 | 0.127.255.255 | 8 388 606 | ||
| /8 | 255.0.0.0 | 16 777 216 | 0.255.255.255 | 16 777 214 | ||
| /7 | 254.0.0.0 | 33 554 432 | 1.255.255.255 | 33 554 430 | ||
| /6 | 252.0.0.0 | 67 108 864 | 3.255.255.255 | 67 108 862 | ||
| /5 | 248.0.0.0 | 134 217 728 | 7.255.255.255 | 134 217 726 | ||
| /4 | 240.0.0.0 | 268 435 456 | 15.255.255.255 | 268 435 454 | ||
| /3 | 224.0.0.0 | 536 870 912 | 31.255.255.255 | 536 879 910 | ||
| /2 | 192.0.0.0 | 1 073 741 824 | 63.255.255.255 | 1 073 741 822 | ||
| /1 | 128.0.0.0 | 2 147 483 648 | 127.255.255.255 | 2 147 483 646 | ||
| /0 | 0.0.0.0 | 4 294 967 296 | 255.255.255.255 | 4 294 967 294 |
Decimal To Binary
| Subnet Mask | Wildcard | ||
|---|---|---|---|
| 255 | 11111111 | 0 | 00000000 |
| 254 | 11111110 | 1 | 00000001 |
| 252 | 11111100 | 3 | 00000011 |
| 248 | 11111000 | 7 | 00000111 |
| 240 | 11110000 | 15 | 00001111 |
| 224 | 11100000 | 31 | 00011111 |
| 192 | 11000000 | 63 | 00111111 |
| 128 | 10000000 | 127 | 01111111 |
| 0 | 00000000 | 255 | 11111111 |
Classful Network Architecture
| A | 0.0.0.0 – 127.255.255.255 |
| B | 128.0.0.0 – 191.255.255.255 |
| C | 192.0.0.0 – 223.255.255.255 |
| D | 224.0.0.0 – 239.255.255.255 |
| E | 240.0.0.0 – 255.255.255.255 |
IANA-reserved private IPv4 network ranges
| 24-bit Block (/8 prefix, A) | 10.0.0.0 – 10.255.255.255 |
| 20-bit Block (/12 prefix, B) | 172.16.0.0 – 172.31.255.255 |
| 16-bit Block (/16 prefix, C) | 192.168.0.0 – 192.168.255.255 |
thanks... useful info
ReplyDelete