Following post will present you how Cisco router handles broadcast IP packets.

We have two types of IP broadcast address:

• All subnets broadcast IP (255.255.255.255)
• Directed broadcast – specific subnet broadcast IP (e.g. 10.0.12.255 for 10.0.12.0/24 subnet)

It’s worth to add that all subnets broadcast IP type is not directed broadcast, directed means broadcast sent to all hosts in specific subnets (directed to specific group of hosts).

By default Cisco router does not forward IP packets addressed to any type of broadcast address – router simple drops them or in case it’s ICMP echo to router’s directly connected broadcast subnet respond via echo reply to requestor.

Directed broadcast example

Let’s take a look on the first example. I have generated ping message from R1 to 10.0.23.255. Because R2 is directly connected to the 10.0.23.0/24 subnet will respond to echo via echo reply but will not forward the ICMP packet over Fa0/1 link towards R3 so R3 will never get it.

Here you are debug IP packet from R1 after ping:

R1#ping 10.0.23.255 repeat 1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 10.0.23.255, timeout is 2 seconds:
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 60/60/60 ms
R1#
*Mar 1 00:24:54.467: IP: tableid=0, s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), routed via FIB
*Mar 1 00:24:54.471: IP: s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), len 100, sending
*Mar 1 00:24:54.475: ICMP type=8, code=0
*Mar 1 00:24:54.515: IP: tableid=0, s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), routed via RIB
*Mar 1 00:24:54.519: IP: s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), len 100, rcvd 3
*Mar 1 00:24:54.523: ICMP type=0, code=0

 As you can see R1 gets just R2’s respond.

Let’s add no ip directed-broadcast under Fa0/1 on R2 and see how th debug looks like now on R1:

R2(config-if)#int fa0/1
R2(config-if)#no ip directed-broadcast
R1#ping 10.0.23.255 repeat 1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 10.0.23.255, timeout is 2 seconds:
!
Success rate is 100 percent (1/1), round-trip min/avg/max = 36/36/36 ms
R1#
*Mar 1 00:03:56.839: IP: tableid=0, s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), routed via FIB
*Mar 1 00:03:56.843: IP: s=10.0.12.1 (local), d=10.0.23.255 (FastEthernet0/0), len 100, sending
*Mar 1 00:03:56.847: ICMP type=8, code=0
*Mar 1 00:03:56.863: IP: tableid=0, s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), routed via RIB
*Mar 1 00:03:56.867: IP: s=10.0.12.2 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), len 100, rcvd 3
*Mar 1 00:03:56.871: ICMP type=0, code=0
*Mar 1 00:03:56.931: IP: tableid=0, s=10.0.23.3 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), routed via RIB
R1#
*Mar 1 00:03:56.935: IP: s=10.0.23.3 (FastEthernet0/0), d=10.0.12.1 (FastEthernet0/0), len 100, rcvd 3
*Mar 1 00:03:56.939: ICMP type=0, code=0
As you see R1 now gets response from R2 and R3.

Take a look how it looks like on R2 and R3:

R2#*Mar  1 00:10:16.995: IP: tableid=0, s=10.0.12.1 (FastEthernet0/0), d=10.0.23.255 (FastEthernet0/1), routed via RIB
*Mar  1 00:10:16.999: IP: s=10.0.12.1 (FastEthernet0/0), d=10.0.23.255 (FastEthernet0/1), g=255.255.255.255, len 100, forward directed broadcast
*Mar  1 00:10:17.007:     ICMP type=8, code=0

R3#*Mar  1 00:07:20.491: IP: s=10.0.12.1 (FastEthernet0/1), d=255.255.255.255, len 100, rcvd 2
*Mar  1 00:07:20.495:     ICMP type=8, code=0
*Mar  1 00:07:20.499: IP: tableid=0, s=10.0.23.3 (local), d=10.0.12.1 (FastEthernet0/1), routed via FIB
*Mar  1 00:07:20.499: IP: s=10.0.23.3 (local), d=10.0.12.1 (FastEthernet0/1), len 100, sending
*Mar  1 00:07:20.503:     ICMP type=0, code=0
As you can discovered ip directed-broadcast changes the destination directed broadcast address (10.1.23.255) to all subnet broadcast 255.255.255.255.

What in case we would still send directed broadcast to subnet IP? We can use broadcast-address command for this propose.

R2#show run int fa0/1
interface FastEthernet0/1
 ip address 10.0.23.2 255.255.255.0
 ip broadcast-address 10.0.23.255
 ip directed-broadcast

Now R3 gets ICMP packet directed to subnet broadcast 10.0.23.255.

R3#*Mar  1 00:41:35.391: IP: s=10.0.12.1 (FastEthernet0/1), d=10.0.23.255 (FastEthernet0/1), len 100, rcvd 3
*Mar  1 00:41:35.395:     ICMP type=8, code=0

Here you are diagram that shows above tests.

 

 

All subnets broadcast example

In the following example I will show you how router handles typical broadcast packets. The best example is the DHCP address allocation process (more about it you can read here). The first message called as DHCP Discovery is sent to 255.255.255.255 broadcast address. By default router will ignore this packet and drop it. To properly handle it and send as unicast IP toward final destination we have to use ip helper-address command under fa0/0 interface on R2, exactly under interface that receives broadcast packets.

Please check following scheme and take a look on the mentioned post. Enjoy 😉

  

 

Here you are our most popular NetContractor’s post about QoS fields mystery.

QoS Classification is done mainly based on two fields: in Ethernet it’s CoS field and in IP header it’s ToS. Naming convention for specific fields in IP header has developed over years from the CS and IPP to DSCP. The main reason for that was not enough naming class to classify traffic. BTW, today once we classify traffic and would send it over provider’s MPLS cloud we have to properly map our classes to provider classes to take advantage from the QoS features that have been purchased. What is interesting that MPLS frame uses  3-bits long EXP field that can only address up tp 8 classes of traffic so marking more classes (from the client perspective) have no sense when we would push it over MPLS.

But let’s back to the naming. Due to demand for more classes naming has changed. At the begining just first 3 bits of 8-bits ToS was used to name and mark traffic, it would be enough even until now. Then QoS fetures and class naming has changed due to fast grow of VoIP. QoS has been popular and key significant to achieve better voice quality. Engineers tried to involve more bits to mark more classes. Finally we have still 8-bits long ToS field with few class names depends on what part of the field we take.  For someone that is just starting with QoS it maybe confusing so I thought to share with you the QoS Values Calculator that I have created and used during my CCIE study.

I’ve added ToS in HEX to the QoS Values Calculator v2 . These values are useful when you would like to generate IP traffic with specific ToS/DSCP value by ping command from the IOS CLI. Ping with ToS is very helpful during QoS configuration test. You can easily generate test ICMP traffic with specific value in ToS field  and see if it matches rigth QoS class.

Be aware that during extended ping from IOS CLI, TOS HEX value has to be defined in the 0xHH format where HH is HEX value.

To be honest this is the most popular post of this blog.

Please take a look, any feedbacks are more then welcome. Enjoy.

QoS Values Calculator v3 in PDF format here.

Here you are my fast notes regarding EIGRP.

• IP Protocol: 88, Uses Multicast IP: 224.0.0.10
• Protocol Dependent Modules (IP, IPX, Appletalk)

Determining Loop Free Path

• Feasibility Condition (AD<FD) must be meet
• Split-Horizon – never advertise a route out of the interface through which you learned it

Reliable Transport Protocol (RTP)

• Packets (reliable delivery and packets will be delivered in order – waits for ACK)

Guaranteed delivery > reliable multicast and confirmation reply as unicast ACK

Ordered delivery > 2 sequence number in EIGRP packet (incremented seq each pack. and last received seq)

• HELLO – multicast, unreliable
• ACK – (it Hello packet with no data in them), unicast, unreliable
• UPDATE – include route info, multicast/unicast, reliable
• QUERY – manage DUAL computation, multicast or unicast, reliable
• REPLAY – manage DUAL computation, unicast, reliable
• If packet is reliable/multicast and ACK is no received from the neighbor
  • Then packet is retransmitted as a unicast to unresponding neighbor
  • If ACK is not received after 16 unicast retransmission > neighbor is dead
  • Timers – calculated based on the Smooth Round Trip Time (SRTT)>average time between transmission of packet to the neighbor and the receipt of an ACK
  • Multicast Flow Timer – time to wait for ACK before switching from multicast to unicast
  • Retransmission Timeout (RTO) – time between subsequent unicast packets

Neighbor Discovery/Recovery

• Hello – 5/60 seconds – ip hello-interval eigrp
• Hold-Time – 15/180 – ip hold-time eigrp

DUAL – Diffusing Update Algorithm

• Feasible Distance – lowest calculated metric to the destination
• Successor – router (next-hop) with the lowest (best) metric to the destination
• Feasible Successor – one of the backup of Successor that meets Feasibility Condition

Before DUAL compute the metric following have to take place

1. Establish adjacency between neighbors
2. Updates exchange
3. DUAL calculates metric based on the received Advertised Distance from the neighbor + cost to the neighbor
4. Lowest calculated metric is Feasible Distance (FD), router that advertised this metric is Successor
5. Successor’s router with the best metric is RIB
6. If the Feasibility Condition is meet when AD<FD (FD of current successor) [Loop Free condition]
7. If neighbor’s AD to the destination meets the FC neighbors become a Feasible Successor. FS can be elected as Successor when current Successor goes down and if has the lowest metric to the destination with comparison with other Feasible Successors

DUAL Finite State Machine

1. If FS can’t be found in the Topology Table, then router begins a Diffusing computation and route is Active
2. Router sends Query to all of it’s neighbors
3. If Neighbor has one or more Feasible Successors for the destination it will send reply to the questioning router
4. If Router doesn’t receive reply to query in Active time, route is declared Stuck-In-Active (SIA)
5. Neighbors that didn’t reply will be removed from the neighbor table

 

METRIC (BW, Delay, Load, Reliability)

M = (10 000 000 /minBW + DLY/10 [in 10 micro sec])*256

traffic-share balanced

• Bandwidth— A value represented as the smallest bandwidth between the source and destination
• Delay— The collective delay of interfaces along the path
• Reliability— The lowest (worst) reliability along the network path
• Load— Represented by the worst load on a connection between the source and destination, in bps
• MTU— The smallest maximum transmission unit value in the path
• K1 = bandwidth
• K2 = load
• K3 = delay
• K4 = reliability
• K5 = MTU

Default EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

Stuck-In-Active (SIA)

• timers active-time 3
• timers active-time disabled
• Stub and Summarization allows to reduce sending of Query to neighbors
• show ip eigrp topology all-links – shows routes that are not Feasible Successors

Default routing is very important feature and can be find in each network as last resort mechanism to route packets out of organization to unknow destination. Default origination has few configuration dependency on routing protocol and these will be presented in this post.

OSPF

Let’s start from the most popular IGP protocol. In OSPF default prefix (0/0) can be propagated in two different ways:

• Explicitly with default-information originate
• Stub Area Border Router (ABR)

To originated 0/0 explicitly we have to issue following command under OSPF process:

R1(config-router)#default-information originate

Once above command has been issued OSPF router will act as Autonomous System Boundary Router (ASBR). Default prefix will not appear in ASBR’s LS database and will not be originated to peers until 0/0 prefix exist in routing table.

To get default network in the routing table we have two options:

    Redistribute 0/0 from the another routing protocol (RIP, EIGRP, BGP)
    Add static route for 0/0

Default-information originate command has optional keyword – “always” which means originate 0/0 even if no default prefix in routing table exist.

By default network will be propagated as E2 type with metric 1, of course it can be adjusted using metric or metric-type command option.

The second way to originate default is to configure stub area, then ABR will generate 0/0. Please look at OSPF Area Types and LSA Propagation post for details here. Keep in mind that ABR router does not originated 0/0 to standard Not-So-Stubby (NSSA) area, default-information originate or no-summary keyword is needed then.

EIGRP

With EIGRP protocol we have 4 options to generate default route, via:

    network 0.0.0.0
    redistribution
    summarization
    ip default-network

First option is similar to OSPF. Default route needs to exist in routing table and then will be propagated once network 0.0.0.0 command is added under EIGRP process.

R1(config)#router eigrp 1
R1(config-router)# network 0.0.0.0
R1(config-router)#ip route 0.0.0.0 0.0.0.0 null 0

 

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
1.0.0.0/32 is subnetted, 1 subnets
D 1.1.1.1 [90/409600] via 10.0.12.1, 00:06:32, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
D* 0.0.0.0/0 [90/281600] via 10.0.12.1, 00:05:51, FastEthernet0/0

R2 sees default route as EIGRP internal (AD=90) route with star. Star means default – last resort route will be used if no specific route exist to the specific destination.

Second option is to use redistribute command and take default based on static route or from another routing protocol.

R2#show ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
D*EX 0.0.0.0/0 [170/281600] via 10.0.12.1, 00:00:07, FastEthernet0/0

In this case peers will see default as EIGRP external (AD=170) route with star.

Third option of default route generation is based on the summarization. In EIGRP routes’ summarization is done per interface. It’s very handy option and can be find just in EIGRP.

R1(config)#int fa0/0
R1(config-if)#ip summary-address eigrp 1 0.0.0.0 0.0.0.0

Peers will see default route as EIGRP internal (AD=90).

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
D* 0.0.0.0/0 [90/307200] via 10.0.12.1, 00:00:15, FastEthernet0/0

The last option is using ip default-network command in global configuration mode; additionally prefix needs to be added under EIGRP process. Prefix needs to be classfull network. Of course local interface on router needs to exist and be in up state.

R1(config-if)#int lo1
R1(config-if)#ip add 1.0.0.1 255.0.0.0
R1(config-if)#router eigrp 1
R1(config-router)#network 1.0.0.0

R2#sh ip route
Gateway of last resort is not set
D* 1.0.0.0/8 [90/156160] via 10.0.12.1, 00:00:02, FastEthernet0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0

R2 sees 1.0.0.0 subnet as candidate default route and 10.0.12.1 peer will be used as default gateway.

RIP

With RIP protocol we have 4 options to generate default route, via:

• network 0.0.0.0
• default-information originate
• redistribution
• ip default-network

First option propagates default route without need to exist in routing table.

R1(config)#router rip
R1(config-router)#no auto
R1(config-router)#network 0.0.0.0

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
R* 0.0.0.0/0 [120/1] via 10.0.12.1, 00:00:02, FastEthernet0/0

Second option is propagates default route the same like default-information originate always in OSPF – prefix does not need to exist in routing table.

R1(config)#router rip

R1(config-router)#version 2
R1(config-router)#no auto
R1(config-router)#network 10.0.0.0
R1(config-router)#default-information originate

R2#sh ip route>
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
R* 0.0.0.0/0 [120/1] via 10.0.12.1, 00:00:02, FastEthernet0/0

Third option is simply redistribution.

R1(config)#ip route 0.0.0.0 0.0.0.0 Null0
R1(config)#router rip
R1(config-router)# redistribute static metric 5

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
R* 0.0.0.0/0 [120/5] via 10.0.12.1, 00:00:01, FastEthernet0/0

The last option is similar to ip default-network in EIGRP but interesting thing – does not need add classfull network under RIP configuration process.

R1(config)#int lo1
R1(config-if)# ip add 1.0.0.1 255.0.0.0
R1(config-if)# ip default-network 1.0.0.0

The output of show ip route command is also different – instead of classful network with star showing pure 0.0.0.0/0

R2#*Mar 10 23:39:27.912: RIP-DB: redist 0.0.0.0/0(metric 1, last interface FastEthernet0/0) to RIP
*Mar 10 23:39:27.912: RIP-DB: network_update with 0.0.0.0/0 succeeds
*Mar 10 23:39:27.912: RIP-DB: adding 0.0.0.0/0 (metric 1) via 10.0.12.1 on FastEthernet0/0 to RIP database
*Mar 10 23:39:27.912: RIP-DB: add 0.0.0.0/0 (metric 1) via 10.0.12.1 on FastEthernet0/0
*Mar 10 23:39:27.916: RIP-DB: Adding new rndb entry 0.0.0.0/0
*Mar 10 23:39:27.916: RIP-DB: Created rip ndb summary entry for 0.0.0.0/0
*Mar 10 23:39:27.916: RIP-DB: Adding new rndb entry 0.0.0.0/0
*Mar 10 23:39:31.113: RIP-DB: network_update with 0.0.0.0/0 succeeds
*Mar 10 23:39:31.113: RIP-DB: adding 0.0.0.0/0 (metric 1) via 10.0.12.1 on FastEthernet0/0 to RIP database

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
R* 0.0.0.0/0 [120/1] via 10.0.12.1, 00:00:06, FastEthernet0/0

BGP

We have covered all IGP protocols. Let’s take a closer look at BGP.

With BGP protocol we have 3 options to generate default route, via:

    default-information originate
    network 0.0.0.0
    default-originate to specific neighbor

First option is similar to OSPF and EIGRP but with one difference. Besides 0/0 needs to exist in routing table additionally has to be redistributed to BGP routing from static or any other dynamic routing protocol. Just one important note – 0/0 prefix is not visible in BGP table until default-information originate command will be issued, strange but true. Let’s test it.

R1(config)#ip route 0.0.0.0 0.0.0.0 Null0
R1(config)#ip route 2.2.2.2 255.255.255.255 Null0
R1(config)#router bgp 1
R1(config-router)# redistribute static
R1(config-router)#exit
R1#sh ip route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
2.0.0.0/32 is subnetted, 1 subnets
S 2.2.2.2 is directly connected, Null0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 is directly connected, Null0

R1#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 2.2.2.2/32 0.0.0.0 0 32768 ?

As you can see no 0/0 prefix in BGP table, let’s add key command.

R1(config)#router bgp 1

R1(config-router)#default-information originate

R1(config-router)#do sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 0.0.0.0 0 32768 ?
*> 2.2.2.2/32 0.0.0.0 0 32768 ?

Here we are! Confirmed that R2 is getting route.

R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
2.0.0.0/32 is subnetted, 1 subnets
B 2.2.2.2 [200/0] via 10.0.12.1, 00:00:19
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
B* 0.0.0.0/0 [200/0] via 10.0.12.1, 00:00:05

Second option, use of network 0.0.0.0 under BGP requires 0/0 prefix in routing table too – the same like with first one but network command assure existence default network in the BGP table and propagation to all neighbors, so no need to redistribute into BGP table.

R1(config)#router bgp 1
R1(config-router)#network 0.0.0.0
R1(config-router)#ip route 0.0.0.0 0.0.0.0 null 0

R2#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*>i0.0.0.0 10.0.12.1 0 100 0 i
R2#sh ip route
Gateway of last resort is 10.0.12.1 to network 0.0.0.0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.12.0 is directly connected, FastEthernet0/0
B* 0.0.0.0/0 [200/0] via 10.0.12.1, 00:01:08

Third option is usfull and allows to select to which neighbors to send 0/0 prefix without need of filtering. This option does not need to have 0/0 in routing table to originate default.

R1(config)#router bgp 1
R1(config-router)# neighbor 10.0.12.2 default-originate
R2#sh ip bgp
Network Next Hop Metric LocPrf Weight Path
*>i0.0.0.0 10.0.12.1 0 100 0 i

As you see there is some dependency in default route generation. It’s good to know it.