What Happens when a router receives a packet..!!..Routing Process of a router..!

 What happens, when a router receives the packet?

Upon receiving the Packet, a router has to follow three  step process before it routes the packets:

-> Routing

-> Forwarding (Switching)

-> Encapsulation

Let’s discuss each one of them in detail

Routing Process: Routing process is nothing but routers control plane. Router records a routing table listing what route should be used to forward a data packet, and through which physical interface connection. Router learns your network routes information either by static configuration or by using dynamically configure routing protocol like IGP (OSPF, EIGRP, RIP, IS-IS) or though Exterior routing protocol like BGP.

When router receives any packet it has to remove Layer 2 header information present on packet(Example:In Ethernet, source and destination Mac address present on L2 header). Once router remove L2 information it looks for Layer 3 information available on packet that is source and destination IP address.

For moving L3 packet between interfaces, router checks destination address and finds longest-prefix match in IP routing table to find outgoing interface. In IPv4 router uses longest mask to identify best routing entry for forwarding packet.

Example: Let’s assume we have configured 3 different static routes with different subnet mask.

Sh ip route 1.1.1.1

ip route 1.1.1.0 255.255.255.0 fa0/2

ip route 1.1.0.0 255.255.0.0 fa0/1

ip route 1.0.0.0 255.0.0.0 fa0/0

In above example when router does route lookup for destination address 1.1.1.1 out of 3 entries router will choose longest-prefix length match entry i.e. 1.1.1.0/24 , because destination address has most common bits matches with selected route and will forward packet out fa0/2.

Destination prefix	Binary Splitting
1.1.1.1	00000001 00000001 00000001 00000001
1St Entry 1.1.1.0/24	00000001 00000001 00000001 00000000
2nd Entry 1.1.0.0/16	00000001 00000001 00000000 00000000
3rd Entry 1.0.0.0/8	00000001 00000000 00000000 00000000

Now for any other destination prefix like 1.1.2.0 longest match is 1.1.0.0/16 and for 1.2.0.0 it would be 1.0.0.0/8

Longest match possible in IPv4 routing is /32 (255.255.255.255) and shortest match possible is default route i.e. 0.0.0.0

->If there are multiple routes with same subnet mask learned via same protocol by router then router chooses lowest metric between them.

For Example: Eigrp use composite “metric” and Ospf uses “Cost” for comparison.

->If there is multiple routes with same subnet mask learn via different protocol on router then router chooses lowest administrative distance (AD).

->Last and important point is recursive lookup: which states that whenever there is route lookup more than once it will be termed as recursive lookup. It has to be done by router till destination address point towards any physical or logical interface.

Example:

We have a network 1.1.1.1 connected somewhere and we are reaching it by interface fa0/0 having next-hop IP address 2.2.2.2.So we can configure static route in two different ways either we can define next-hop IP address i.e.2.2.2.2 or we can mention interface number fa0/0 as gateway shown below.

ip route 1.1.1.1 255.255.255.255 2.2.2.2

ip route 1.1.1.1 255.255.255.255 FastEthernet0/0

Both statements look same although both have different meaning.When you point destination address to next hop as exit interface you don’t need further route lookup as router assume destination address is directly connected to that interface. But when you point destination address to any next hop ip address, we need another route lookup also for next hop ip address is referring as recursive lookup.

To get more information on how static route work when you set gateway as Next-Hop IP address or to Next-Hop interface please refer this document.

Forwarding process: It is also known as switching process. Once router finds outgoing interface, packet move between interfaces by switching process. This is done by process switching, fast switching or cef switching. Forwarding can be done by using adjacency tables reside on the route processor or on interface cards that support switching.

-> Process switching requires the device CPU to be involved for every forwarding decision.

-> Fast switching still uses the CPU for initially packets and to fill cache table in router. Once initial packet has been forwarded, the information about how to reach the destination is stored in a fast-switching cache’s .when another packet going to the same destination, the next hop information can be re-used from the cache and so the router processor doesn’t have to look into it, but if the information is not cached the CPU will have to process entire packets.

-> When CEF mode is enabled it build the CEF FIB and adjacency tables reside on the route processor, and the route processor performs the express forwarding.

In switching process device do actual packet link load balancing depending on the methodology we use.

Encapsulation process: L3 header will remain intact unchanged except for nating, vpn etc. layer 2 headers keep changing on hop by hop basis, depending on transmission media. For transmitting L3 packet on wire router need to find out l2 information for packets and it’s depending on the type of media we are using for transmission.

To explain encapsulation process in bit detail, I have created a small topology shown as below in diagram.

As discussed above, depending on the transmission media (In this example transmission media is Ethernet) MAC address in layer 2 headers will keep changing on hop by hop basis.

To generate some traffic, Lets ping from R3 to R2 interface address.As soon as R1 receives the packet from R3, It will remove the L2 information sent by R3 and check the L3 information that is source (20.1.1.2) and destination address (10.1.1.1) available on packet. Then it will look into its routing table to find out going interface i.e. fa0/0 in above example. Once router identify outgoing interface it will attach L2 header before putting the packet on the wire. So now R1 will attach its own interface Mac address as source and R2’s as destination mac address.

Address resolution protocol (ARP) table on R1:

To get closer packet level overview, I have also attached some packet capture taken on R1's interfaces.

Packet capture on R1’s Fa0/1:

Packet capture on R1’s Fa0/0:

Multipoint Broadcast Interfaces, Routing, and ARP

 

When the router needs to route a packet which matches an entry in the routing table with a next-hop value, it performs Layer 3 to Layer 2 resolution for the next-hop address. If it matches an entry in the routing table with just the outgoing/exit local interface, without a next-hop value, it performs Layer 3 to Layer 2 resolution for the final destination of the IP packet.

From a design perspective, the ideal solution for this problem is to never configure a static route to point out a multipoint interface. Static routes should either point to the next-hop value of the neighbor on the multipoint interface or point to an interface only if it is point-to-point, such as a GRE tunnel, PPP or HDLC link.

 

When you configure a static route to use an interface attached to a broadcast media (e.g. ethernet), a Cisco router expects that the network is directly attached. As a result it has to ARP for anything that falls within the scope of your static route. Consider the following topology:

Chesterton# ip route 1.2.3.4 255.255.255.255 eth0/0

 

In this configuration, router Chesterton has to make an ARP request for 1.2.3.4/32 and broadcast it via Ethernet0/0. He is now totally reliant on one of two possiblities:

1.    A Static ARP entry

2.    Vegas will “proxy-arp” his request

 

 

If neither of these conditions exists, he won’t be able to reach his destination. The topology presented is a minor case, and as a result there’s no real problem with it. If we were to increase the load, we begin to see a greater set of problems.

Chesterton# no ip route 1.2.3.4 255.255.255.255 eth0/0

Chesterton# ip route 0.0.0.0 0.0.0.0 eth0/0

 

Now that we’ve added a little more scope for router “Chesterton” to look for, we have a higher possibility for impact. If he attempts to reach 8.8.8.8, 180.0.123.12, and 5.4.3.2 we will see arp entries for each address (all of which will have the MAC address of router Vegas’ e0/0 interface). If there is a lot of traffic from Chesterton to the internet, we have the potential to fill up the arp-cache; thus, causing memory problems that will lead to forwarding problems.

Bottom Line (TL;DR version):

Yes, you can do it and it shouldn’t be a big deal on a small deployment; However, it’s bad practice and could really backfire in a big network.

 

 

When configuring a static route, the following options are available:

 

1-specify only the next-hop value; route is valid as long as a route exists for the next-hop value.

2-Specify only the local outgoing interface; route is valid as long as the interface is in the UP/UP state.
3-Specify both next-hop value and local outgoing interface.

 

When the third option is selected, the local outgoing interface behaves like a condition for the next-hop value and should be read like: this static route is valid only if the configured next-hop value is reachable over the configured interface, which means as long as the interface is in the UP/UP state and has nothing to do with IP/ARP/NHRP functionality with the next-hop.